ACID ETCHING
This Library Dissertation is submitted to Sri Ramachandra Dental College and Hospital Sri Ramachandra Medical College and Research Institute (Deemed University) in partial fulfillment of the regulation for the Degree of Master Dental Surgery in Conservative Dentistry and Endodontics Examination held in 2003
Dr. M. SHANMUGARAJ
Under the Guidance of
Prof. Dr. S. Balagopal, M.D.S.,M.Sc., Professor and Head Department of Conservative Dentistry and Endodontics Sri Ramachandra Dental College and Hospital Porur, Chennai – 600 116. 1
CERTIFICATE This is to certify that the library dissertation entitled “ACID ETCHING” here with submitted by Dr. M. SHANMUGARAJ, in partial fulfillment for the degree of Master of Dental Surgery in Conservative Dentistry and Endodontics, is a bonafide work carried out by him under my supervision and guidance.
Dr. S. Balagopal, M.D.S., M.Sc., Professor and Head, Department of Conservative Dentistry and Endodontics Sri Ramachandra Dental College and Hospital Porur, Chennai – 600 116.
Forwarded by
Dr. M.J. Ramakrishnan, M.D.S., Principal Sri Ramachandra Dental College and Hospital Porur, Chennai – 600 116. 2
ACKNOWLEDGEMENT I would like to express my deepest sense of gratitude to my guide Dr. S. Balagopal, M.D.S., M.Sc., Professor and Head, Department of Conservative Dentistry and Endodontics, Sri Ramachandra Dental College and Hospitals, for his endless hours of enthusiastic involvement, expert guidance and his caring support given to me throughout this library dissertation, it has been a remarkable learning experience all the way.
I
would
like
to
extend
my
heartfelt
gratitude
to
Dr. M.J. Ramakrishnan, M.D.S., Principal, Sri Ramachandra Dental College and Hospitals, for permitting me to use the scientific literature and research facilities of the college.
My sincere thanks to Dr. Arathi Ganesh, M.D.S., Associate Professor, for her guidance, untiring support and valuable suggestions and for sharing her extensive experience with me during the entire course.
I would be unfair to claim reward individually and not acknowledge the untiring efforts of the team comprising of Professors, Dr. Mruthyunjaya M.D. 3
S., Dr. Singaravel Pandiyan, M.D.S., and Senior Lecturer Dr. Lakshmi Balaji M.D.S., Lecturer Dr. Priscilla Solomon, M.D.S., who supported me at every juncture.
I would also like to give credit to my Co-PGs for their constant support and constructive criticism at every step.
My endearing thanks to Mr. Baskaran, Annai Net Park, Porur, Chennai for providing technical support for the study.
I would like to thank my Parents for their constant support and encouragement in pursuing a career in dentistry. I like to thank my beloved wife for being a pillar of support during the course, for understanding and bearing me during the different ups and downs of postgraduate life.
4
CONTENTS Page no
INTRODUCTION HISTORY ENAMEL Formation of enamel Composition of enamel Structure of enamel DENTIN Physical properties Composition of dentin Structure of dentin ACID ETCHING OF ENAMEL Steps in acid etching technique Effects of etching on enamel Role of fluorides in etching of enamel Factors affecting etching of enamel Variation in acid etching methodologies ACID CONDITIONING Of DENTIN Goals of acid conditioning of dentin Effects of acid conditioning of dentin Factors affecting dentin conditioning Conditioners on dentin surface BIOCOMPATIBILITY VARIOUS ACID CONDITIONERS SELF ETCHING PRIMERS EFFECT OF ACID ETCHING ON PRIMARY TEETH APPLICATIONS OF THE ACID CONDITIONERS FACTORS TAKEN INTO CONSIDERATION ACID CONDITIONERS FOR GI CEMENTS BIBLIOGRAPHY 5
1 4 10 10 11 12 23 24 24 24 41 41 45 47 50 52 55 56 58 68 77 79 86 95 98 100 102 103 105
INTRODUCTION The possibility of bonding restorative materials to the hard dental tissues intrigued the dental professionals for many years. The development and regular use of adhesive materials has begun to revolutionize many aspects of restorative and preventive dentistry. Attitudes towards cavity preparation are altering since, with adhesive materials, it is no longer necessary to produce large undercuts in order to retain the filling.
These techniques are
therefore responsible for the conservation of large quantities of sound tooth structure, which would otherwise be victim of the dental bur.
Buonocore (1955) was the first to report the positive effects of application of 85% phosphoric acid to enamel for the retention of acrylic resin restorations.
Gwinnett, Matsui and Buonocore (1969),
further explored the effect of acid solutions and this came to be accepted as an integral part of any direct tooth coloured restorative technique.
Bonding of restorative materials to hard dental tissues would be impossible without the use of acid solutions. It is the effect of these various acid solutions and pretreatments that results in the hard dental
tissues
being
characterized
by
numerous
microscopic
porosities, which allows the resin to readily wet the surface and penetrate into these micro porosities. Once the resin penetrates into these micro porosities it can be polymerized to a form a mechanical bond to the hard dental tissues.
6
The success of acid etching of enamel led Buonocore et al (1956) to try to acid etch dentin using 7% HCL for one minute.
Unlike
enamel when dentin is etched, surface becomes mineral poor, protein rich, and it tends to become wetter (Brannstrom and Norden Vall 1977).
Unfortunately, success with dentin was never realized,
because the relatively cured resin materials that were available at that time would not wet dentin very well. Buonocore, however, was very much aware of the requirements for good bonding. The term conditioner or etchant is used to describe agents that are washed off the dentin.
The word “etchant” has, until recently,
been taboo in western dentistry, for describing the action of various acidic materials on dentin. Etching of the dentin can be defined as “any alteration to the dentin done after the creation of dentin cutting debris, termed the smear layer” (Eick et al 1970). One of the objectives of dentin etching is to create a surface capable of micro-mechanical bonding to a dentin-bonding agent. researched as dentin etchants.
Several acids have been
These include hydrochloric acid,
oxalic and pyruvic acid in addition to the better known acid such as phosphoric, maleic, citric and nitric acid. Fusayama et al (1979) were the first to report the successful use of Phosphoric acid to remove the smear layer, etch the dentin and restore with adhesive composite resin. Acid etching of dentin is used by many bonding systems to remove the smear layer and permit bonding directly to the dentin matrix.
Although early animal studies indicated that acid etching
caused moderate to severe pulpal reactions, there is a high probability that the pulpal irritation may have been due to micro leakage of bacteria and their products. As these reactions are not seen following acid etching of dentin bonding systems. 7
It is clear that one can acid etch dentin if, and only if, one can seal the dentin with subsequently placed bonding systems. Because acid etching increases dentin permeability and dentin wetness, successful bonding of adhesive resins to acid etched dentin requires the use of hydrophilic resins that bond equally well to both peri tubular and inter tubular dentin.
The trend seems to be toward
lowering both the concentration of acids and the time of etching of dentin.
While all bonding systems should be carefully scrutinized
prior to marketing, the future looks very promising for the use of adhesive resins on both enamel and dentin through the effective use of acid-etch technique.
8
HISTORY There are names in the lexicon of dental adhesion that those of us in the field should always remember to acknowledge because it was on their shoulders that we stood as we grappled with our own research problems. Dr. Michael Buonocore was certainly one of such best-known pioneers in adhesive bonding of resins to teeth. He found that lightly etching enamel created a micro porous surface into which direct filling liquid resins could flow polymerize and make a micro mechanical attachment. He thereby achieved his primary objective of bonding, a conservative means of sealing developmental pits and fissures. One of Dr. Buonocore’s contemporary Dr. George Newman developed similar methods to bond orthodontic brackets directly to the enamel of teeth. Another distinct advantage of effective acid etching technique and its resultant adhesive bonding to dentin is the prevention of removal of healthy dentin for mechanical retention of composite restorations, a process that is painful without an anesthetics. The mechanism of enamel bonding is well understood and involves a micro-mechanical union between enamel and the resin, which occupies tissue microspores enlarged by the action of an acidic conditioning agent. However, acid etching of the pulp dentin complex and its resultant bonding has undergone a number of changes over the last thirty years and both the bond strength values and biocompatibility to pulp dentin organ have tremendously improved.
9
Brannstrom et al (1984) suggested that on a number of occasions, they inadvertently acid etched teeth with small pulp exposures at the base of deep cavities.
These were not often
discovered until subsequent histo-pathological examination of the extracted teeth. Unless there was concomitant infection there was no particular damage or inflammation to the pulp. However, they noticed that when restorations leaked, and bacteria colonized the cavity surface, the teeth that had been etched exhibited more severe pulpal responses, than those that were not etched. Many attempts have been made to synthesize different coupling agents for tooth surfaces. One of the earliest successful compounds tested was NPG-GMA, the reaction product of N-phenyl-glycine and glycidyl methacrylate (Bowen 1965). The use of this surface-active comonomer alone improved the water resistant bonding between resins and enamel and dentin to a degree that was statistically but not clinically significant (Bowen 1965). Removal of the structurally weak smeared layer, pellicle, or other superficial layers of the tooth surface by use of acidic (Fusayama & others, 1979, Fusayama 1980) or chelating agents might reduce the availability of calcium ions on dentin surfaces for interaction with a chelating surface active co-monomer like NPG-GMA or other coupling agent
with,
supplement
preferably, calcium
ion
multiple-bonding sites
for
ligand
improved
groups.
bonding,
To certain
appropriate metal cations were evaluated for use on tooth surfaces. Experiments indicated that the most effective agent might be ferric oxalate, primarily because of the iron ion’s high tendency to be bound strongly by denting and enamel and its high chelate stability constants with molecules that have linked groups similar to those of NPG-GMA (Bowen 1978).
10
Furthermore, the oxalate would form an insoluble precipitate with calcium ions, which, together with insoluble ferric phosphate, would seal the dental tubules to provide pulp protection and desensitization. Nakabayashi (1982) introduced the concept of hybridization. The technique consists of applying an acid, ranging in concentration from 10% to 30% to the surface of dentin. Within 15 minutes the acid selectively dissolves away the inorganic component of the dentin to a depth of 5 to 10 microns. It then flows in to the dentinal tubule for upto 100 microns at which point it diffuses laterally into the peritubular dentin for up to 10 microns. As in the previous case the calcium component is selectively eliminated. Then these spaces are replaced
by
an
insoluble
resin
component
that
completely
encapsulates all exposed collagenous fiber. It was then discovered that the additional use of a relatively hydrophilic monomer containing two free carboxyl groups in addition to two polymerizable groups on each molecule dramatically improved bond strengths to levels of clinical significance (Bowen and others, 1982). This monomer was called “PMDM� (the reaction products of pyromellitic dianhydride and hydroxyethyl-methacrylate). There was a synergistic interaction between the NPG-GMA and the PMDM (Bowen and others, 1984). The original adhesive system developed was a sequential application of aqueous acidic ferric oxalate, followed by an acetone solution of NPG-GMA or NTG-GMA (the reaction product of Nptolyglycine and glycidyl methacrylate), and then an acetone solution of PMDM. This system was effective only if placed in the described sequential order utilizing all three components.
The acidic ferric
oxalate solution was removing the original smear layer, the disturbed surface layer caused by mechanical abrasion in preparing a 11
restoration site (Bowen and others, 1984), and laying down a layer of precipitation product that was plugging up the lumina of dentinal tubules. The latter function significantly reduced tooth sensitivity to the subsequent procedure. The NTG-GMA was necessary to induce polymerization of the PMM, but the exact mechanism of this free radial initiation is still not clear. During subsequent experimentation, it was discovered that the smear- removing capabilities of ferric oxalate were due primarily to the presence of small amounts of nitric acid left over from the synthesis of the oxalate (Cobb and others, 1989).
Controlled additions of nitric
acid to the aqueous oxalate solution were made to determine the optimum acid concentration for this solution (Blosser and Bowen, 1988). A small increase in the concentration of nitric acid to about 2.5% HNO3 by weight also improved the simultaneous etching of instrumented enamel. However, an adverse side effect of the application of the ferric oxalate solution was discovered; the occasional appearance of black staining at the adhesive interface in early animal trails (Stanley, Bowen and Cobb, 1988). This could be reproduced in the laboratory by applying a sodium sulfide solution to ferric oxalate treated dentin. The cause of this staining in vivo is probably (although not proven to be) the reduction of ferric to ferrous ions by sulfide forming anaerobic microorganisms resulting in the formation of black ferrous sulfide pigments. To eliminate this, acidic aluminum oxalate was substituted, and it produced no staining on dentin.
Aqueous solution of aluminum
oxalate and nitric acid were then applied, and no staining problems occurred in animal trails (Blosser and others 1989).
12
There was some evidence in vitro that aluminum oxalate did not produce as much of the reaction products plugging the dentin tubules, as has ferric oxalate. Eventually, the first successful transfer of the adhesion technology developed by scientists at the ADA Health Foundation’s Paffenbarger Research Center involved the development of a product that incorporated aluminum oxalate in a conditioning solution. In the continuing research, it was found that the aluminum oxalate could be eliminated entirely from the experimental system without loss in adhesion, if the dilute nitric acid solution was retained.
None of the other acids evaluated, in a wide range of
concentrations, were as good or better than the dilute nitric acid (which should be distinguished from concentrated nitric acid, a strong oxidizing agent).
It was then surprisingly discovered that NPG (N-
phenyl-glycine) could be substituted for NPG-GMA or NTG-GMA. The experimental system was then reduced to the three components of ♦
Dilute nitric acid
♦
NPG acetone solution
♦
PMDM acetone solution The three components still had to be applied individually in
sequence to adhesive adhesion, and efforts were concentrated on ways to simplify application. It was then suspected and verified that NPG would be soluble in the dilute aqueous nitric acid solution.
This
permitted a simplification of the procedure to the application of two solutions. ♦
An acidic NPG solution
♦
PMDM acetone solution
13
However, preparation and storage of the first solution was difficult because of the reactivity of the NPG molecule to atmospheric oxygen. Storage times were very short if the acid NPG solution was exposed to air.
If the solution was used shortly after mixing, adhesion was
effective.
Methods were then developed for preparing the solution
under an inert atmosphere and protecting it from subsequent oxygen exposure. These protected solutions were solutions were stable under normal storage conditions. Some commercial products are currently based on this two-solution system. Current experimentation with the system is focusing on optimizing the individual components.
Nitric
acid concentrations will continue to be refined to yield optimal treatment of both dentin and enamel. Different analogues of the NPG molecules are being synthesized toward improving effectiveness, storage, stability and ease of synthesis (Johnson, Asmussen and Bowen 1989). PMDM is being investigated to isolate more effective linking agents between tooth surfaces and the overlying restorative resins.
Many years of experience in etching
enamel with phosphoric acid have shown bonding by this method to be most reliable clinically. However, it is noteworthy that the use of the chemically functional and more hydrophilic dentin bonding agents, significantly increases bond strengths to acid etched enamel at least in laboratory tests. A number of bonding systems already available to practioners are
beneficial
for
increased
versatility
performance of restorative materials.
toward
improving
the
And, given the high tensile
strength of dentin (Bowen and Rodriguez 1962), the progress made in the last decade, and the currently recognized need for dentin as well as enamel bonding, it is reasonable to expect that before the end of this decade the intensive and extensive research efforts will succeed in providing
clinicians
with completely satisfactory materials and
methods for preventive and restorative dentistry by way of adhesive bonding to both dentin and enamel through the reliable use of acid treatments on both enamel and dentin. 14
ENAMEL Enamel is the most highly calcified and hardest tissue of the body.
Enamel contains 96% inorganic portion and 4% organic
portion. Unlike dentin, cementum and bone, cells of ectodermal origin. In the human tooth, the enamel normally forms a covering layer for the whole of the crown, but varies considerably in thickness in different parts of the crown (FIG .1). Enamel is a composite material consisting of two phases: 1: Mineral. 2: Organic. The mineral phase, an apatite calcium phosphate, is the major component and accounts for the hardness of the tissue. The properties of the mineral phase are modulated dramatically because it is divided into microscopic whiskers or fibers known as crystals. The crystals are cemented together by the organic phase, which is a matrix of protein polymer. The composite resists brittle fracture far better than does crystalline apatite alone.
Formation of Enamel The long, thin, lathe like crystals that compose enamel are oriented roughly perpendicular to its surface. These crystals grow in a gel of protein matrix, which disappears to a large extent as the crystals grow within it. Eventually, the protein matrix takes the form of extremely thin layers, which both glue and separate the enamel crystals. The basic orientation of the enamel crystals is perpendicular to the tooth surface.
15
This
orientation
results
from
their
tendency
to
grow
perpendicularly to the surface on which they develop. The developing surface is not simply flat, but is pitted by the secretory poles of the ameloblasts. A good three-dimensional picture of the sub-microscopic structure
of
enamel
can
be
obtained
by
visualizing
crystals
perpendicular to this peculiarly shaped, pitted surface. However, it is probably of more significance and greater interest to understand the discontinuities in the enamel structure, which develop at the sharp concavities of the boundaries, or floors and walls, of these pits. It is the arrangement of the crystals at the developing surface that causes the discontinuities in crystal orientation, which we know as the prism boundaries or junctions. These locations acquire a more concentrated organic matrix during maturation and in the adult tissue are distinguished by the name “prism sheaths�.
Composition of Enamel The enamel consists mainly inorganic material (96%) and only a small amount of organic substance and water (4%). The inorganic material is apatite. The nature of the organic constituents of enamel is incompletely understood. In development and histological staining reactions the enamel matrix resembles keratinizing epidermis. More specific methods have revealed sulfydryl groups and other reactions suggestive of keratin. (FIG. 2) However chemical analysis of the matrix of mature enamel indicate that the amino acid composition is not closely related to the keratin and is distinctly different from collagen. Proteins can be isolated in several different fractions; they generally contain high percentages of serine, glutamic acid and glycine. 16
Roentgen-ray diffraction studies reveal that the molecular structure is typical of the group of proteins called cross-beta proteins. In addition histochemical reactions have suggested that the enamelforming cells of developing teeth also contains a polysaccharideprotein complex and that an acid mucopolysaccharide enters the enamel itself at the time when calcification becomes a prominent feature. Tracer studies have indicated that the enamel of erupted teeth of rhesus monkeys can transmit and exchange radioactive isotopes originating from the saliva and the pulp. Considerable investigation is still required to determine the normal physiologic characteristics and the age changes that occur in the enamel. Brudevold et al (1960) reported the inorganic components inorganic components of enamel are principally apatite in its hydroxy, fluoride or carbonate ions. Minor variations occur in composition in which aluminium, barium, magnesium, strontium, radium and vanadium among others can be found in the lattice.
Structures of Enamel Enamel Prism or Rod The prism or rod is the fundamental structural unit of enamel, each prism extends from its site of origin at the DEJ to the outer enamel surface crystals of hydroxyapatite (FIG. 3). All enamel with few exceptions (eg: very thin enamel) is made up of super assemblies of these structures, combined with varying amounts of interprismatic material. Changes in the orientation of the crystals, relative to each other, mark the boundaries of the prisms. In the human enamel, the 17
boundary of the prism body is incomplete cervically. Here the prism is continuous with a wedge-shaped ‘tail’, which comparative studies (BOYDE 1965) show to be interprismatic enamel. The combined shape of the prism body and the tail is that of a keyhole (FIG. 4 and 5). The body of the prism is approximately 5micron meter wide and the prism plus tail keyhole is approximately 9micron meter long (FIG. 6). The apatite crystals are most closely packed in the prism bodies, which occupy 60-65% v/v of enamel (Shellis 1984) when considered (FIG. 7). The configuration of enamel crystals is related to the organization of the ameloblast and it’s tome’s processes. The forming Fig 1. Distribution of enamel (A. Dental surface of enamel consists of B. pits, each defined by a wall made up of enamel covering anatomical crown, Fig 2.active Composition of enamel Dentinoenamel C. Cemento newly formed junction, interprismatic enamel. During secretion, each of by volume percentage enamel junction) these walled pits is occupied is occupied by a tomes processes. The inter prismatic walls are formed slightly earlier than the prism enamel, which constitutes the floors of the pits and are formed by secretion sites at the ameloblasts peripheries. The presumptive prism boundary is defined by the position of the junction between the pit wall and the floor. In human enamel the Fig 4. Orientation pit is at it’s deepest occlusally, and rises to become confluentofwith the crystals in forming Fig 3. Individual enamel wall cervically, thus eliminating the boundary rod in head this &region. Each tail rods inter digitizing with neighboring rods region) is formed as a cooperative effort by wall (inter prismatic adjacent secretory ameloblasts. Based on current knowledge of enamel formation, it is clear that each ameloblast is responsible for the formation of one prism at its central secretary site and a portion of the surrounding inter prismatic region at its cooperative peripheral sites. Inter prismatic enamel contains more enamel protein than the prism bodies, because the crystals meet at different angles and thus cannot be packed as tightly together.
18
Fig 5. Orientation of enamel rods
19
The consistent arrangement of the inter prismatic enamel, with its greater protein content, accounts for the fish scale appearance observed in ground sections. Due to its ultra-structural organization, enamel despite it’s hardness and density- has appreciable porosity. The pore affects the mechanical and optical properties of enamel; the formation of carious lesions is strongly influenced by the pathways for diffusion and by electro chemical effects arising from the charge on the pore wall. The prism junctions or boundaries, which are the sites where crystals of the tail region of one prism meet with those in the body of another, are sites where there is an abrupt change in the crystal orientation. Consequently, prism junctions have enlarged pores, filled with matrix and hence increased porosity. (Hamilton et al 1973). In human enamel the incomplete prism junctions form laminar pores with curved cross-section running from the dentinoenamel junction to the outer surface. In outer enamel the prism junctions tend to separate, and thus exist as independent channels, whilst those in inner enamel (especially in molars) interconnect to form a three-dimensional network of laminar spores (Boyde 1989, Shellis 1996). Enamel mineral is composed of relatively small crystals, the arrangement of which results in internal pores that are small and variable in form, orientation and distribution. Chromium soleplate demineralization
has
been
used
to
provide
ultra
structural
information on the distribution of matrix (Sundstrom and Zelander 1968) used this technique, and reported individual crystals with a coating of matrix. Matrix is more apparent in the tail region than in the body region. 20
The material at prism junctions has a raised solubility (Shellis 1996), which may be due to the deposition of the mineral with increased magnesium and carbonate content during amelogenesis, leading to the formation of sites with defective, more soluble apatite (Shellis 1996). The increased solubility at the prism junctions, combined with faster diffusion in this region, accounts for the demineralization pattern observed in advancing carious lesions. At such lesions sites demineralization occurs preferentially via these prism junctions and then spreads laterally into the inter prismatic regions. While the largest pores in enamel are associated with the prism junctions, they only contribute in a small way to the total porosity, most of which is associated with prism bodies and tails. Here, the pores exist as very narrow gaps between closely packed crystals but some, while small, are elongated and tubule like and may communicate with the prism junction pores only through narrow inter crystalline pores. Enamel rods follow a wavy, spiraling course, producing an alternating arrangement for each group or layer of rods as they change direction in progressing from the dentin toward the enamel surface where they end a few micrometers short of tooth surface (FIG. 8) Enamel rods rarely run a straight radial course because it appears there is an alternating clockwise and counterclockwise deviation of the rods from the radial course at all levels of the crown. They initially follow a curving path through one third of the enamel next to the dentino-enamel junction. After that, the rods usually follow a more direct path through the remaining two thirds of the enamel to the enamel surface. Boyde (1976) stated that the keyholes shape of the prisms in cross section tends to prevent slip across prism boundaries under lateral shear.
The keyhole configuration results from the unique
shape of the typical pit produced on the development surface by ameloblast. 21
Gnarled Enamel There are groups of enamel rods that may entwine with adjacent groups of rods, and they follow a curving irregular path towards the tooth surface. These comprise gnarled enamel, which occurs near the cervical regions and the incisal and occlusal areas (FIG. 5) Gnarled enamel is not subject to cleavage as is regular enamel. This type of enamel formation does not yield readily to pressure of bladed, hand cutting instruments in tooth preparation (FIG. 9) Hunter Schreger Bands The changes in the direction of the enamel prisms that minimize
cleavage
in
the
axial
direction
produce
an
optical
appearance called Hunter Schreger bands (FIG.10 and 11). These bands appear to be composed of alternate light and dark zones of varying widths that are slightly different permeability and organic content.
These bands are found in different areas of each class of
teeth. Since the enamel rod orientation varies in each tooth, Hunter – Schreger bands also have a variation in the number present in each tooth. In the anterior they are located near the incisal surface. They increase in the number and areas of the teeth from, the canines to the premolars in the molars the bands occur from near the cervical region to the cusp tips. The orientation of the enamel rod heads and tails and gnarling of the enamel rods provide strength by resisting, distributing, and dissipating impact forces. In the inner one-half to two thirds of the enamel, curvature of the prisms is responsible for the formation of HUNTER-SCHREGER BANDS. Each band consists of 10-13 prisms, which in alternate bands are sectioned approximately longitudinally or approximately transversely. However the transition between alternate bands is gradual.
22
Fig 7. Enamel rods in cross section
Fig 6. Key hole shaped enamel rods
Fig 8. Enamel rods appear wavy in section of enamel
Fig 9. Gnarled enamel
Fig 11. Hunter – Shregar Bands when enamel is viewed under polarized light
Fig 10. Photomicrograph of enamel illustrating phenomenon of light & dark bands (Hunter Shregar Bands) 23
Enamel Tufts Enamel tufts are hypo-mineralized structures of enamel rods and inter-rod substance that project between adjacent groups of enamel rods from the dentino-enamel junction (FIG. 12 and 13) these projections arise in the dentin, extend into the enamel in the direction of the long axis of the crown, and may play a role in the spread of dental caries. These regions are of high porosity, as they cut across the prism structure, in which crystals are small and dispersed and protein abundant (Orams et al 1976). Enamel Lamellae They are thin leaf faults between enamel rod groups that extend from the enamel surface towards the dentino-enamel junction, sometimes extending into the dentin (FIG. 12). They contain mostly organic material, which is a weak area predisposing a tooth to the entry of bacteria and dental caries (FIG. 14). Enamel Spindles Odontoblastic processes sometimes cross the dentino-enamel junction into the enamel; these are termed enamel spindles when their ends are thickened (FIG. 12). They may serve as pain receptors, there by explaining the enamel sensitivity experienced by some patients during tooth preparation (FIG. 15). Incremental Lines of Enamel - Striae of Retzius Enamel rods are formed linearly by successive opposition of enamel in discrete increments (FIG. 16). The resulting variations in structure and mineralization are called the Incremental Striae of retzius and can be considered growth rings (FIG. 12). 24
Fig 12. Photomicrograph exhibiting enamel tuft, enamel lamellae, enamel spindle, striae of retzius, Dentino enamel junction
Fig 13. Transmitted light micrograph of DE junction showing enamel tufts
Fig 15. Enamel spindles
Fig 14. Enamel lamellae 25
In horizontal sections if the tooth, the Striae of Retzius appear as concentric circles. In vertical sections, the lines transverse the cuspal and incisal areas in a symmetric arc pattern descending obliquely to the cervical region and terminating at the dentino-enamel junction. When these circles are incomplete at the enamel surface, a series of alternating grooves, called the imbrication lines of Pickerill, are formed. The elevations between the groves are called Perikymata; these are continuous around the tooth and usually lie parallel to the cemento-enamel junction and each other. The enamel of deciduous teeth develops partly before and partly after birth. The boundary between the two portions of enamel in the deciduous tooth is marked by an accentuated incremental line of retzius, the neonatal line or neonatal ring (FIG. 17). It appears to be the result of abrupt change in the environment and nutrition of the newborn infant. The prenatal line is usually well developed than the postnatal enamel.
This is explained by the fact that the foetus
develops in a well-protected environment with an adequate supply of all the essential materials, even at the expense of the mother. In addition, it has been reported that there is locally increased porosity at the incremental growth lines (Newman and Poole 1974). As a result, enamel structure is altered along these lines and electron microscopy has reveled a possible decrease in the number of crystals in the striae. There is also increased porosity on the cross striations (Boyde 1989), which are a pattern of periodic banding noted at 2-6 micron meter intervals along the length of the prisms, and which represent
the
circadian
variation
in
secretory
activity
of
the
ameloblast. Shellis (1996) produced methacrylate replicas of some cross striations in inner enamel, but was unable to do so in outer enamel, suggesting that the pores at most striations are very small or inaccessible. 26
In cuspal enamel the prism curvature gives rise to a related but often apparently more complicated appearance of gnarled enamel. Bands in which the prisms run parallel with the section plane reflect the light to a different degree compared with those in which the prisms are perpendicular to the section plane (Silverstone 1982). Because of the deviations in prism orientation, inner enamel is relatively porous. It is thought that the relatively complicated prism arrangement within the Hunter-Schreger bands to reduce the propagation of fractures (Osborn 1968, Boyde 1989). In the outer enamel, the prisms are straight and parallel in the cuspal and lateral regions; so do not show Hunter-Shreger banding. The angle at which prisms reach the surface varies with the anatomical location on the tooth. At the cervical margin, the prisms follow an undulating course and approach the surface at very variable times acute angles (Boyde 1989). Occlusally different orientation is noted, with prisms on the lateral surface of the crown being angled at approximately 70째, whilst on the cuspal surface the angle returns to approximately 90째. Prism Shape and Crystal Orientation The cross sectional appearance of prisms is by the interrelationship of prismatic and inters prismatic enamel (FIG. 18). Three classical prism patterns have been defined, termed (1-3) (Boyde 1989). Pattern 1 Is characterized by prisms with complete boundaries, separated by well defined inter prismatic regions.
Pattern 2 27
The prisms have incomplete outlines and are arranged in rows. Within each rows narrow bridges of inter prismatic enamel separate the rows. Pattern 3 Is the structure observed in human enamel, containing alternating prisms with horseshoe shaped boundaries. Although pattern 3 is predominant in human enamel (Boyde 1989), the other patterns can be found in restricted areas. In particular, pattern 1 enamel, occurs close to the dentinoenamel junction and also near the outer surface i.e., in the enamel formed at the beginning and end of the ameloblast life cycle. Comparative studies show that there is no correlation between prism pattern and incremental rate. In all the three patterns, crystals in
the
inter
prismatic
regions
are
oriented
approximately
perpendicular to the general forming surface (i.e., perpendicular to the plane of the retzius lines), while the crystals within the prisms form perpendicular to the floor of the Tome’s process pit. In human enamel, this results in a gradual divergence of the crystals
in
the
tail
region
from
the
parallel
intra
prismatic
arrangement by angles of about 15°-45° in the cervical direction (Poole and Brookes 1961). In pattern 2 enamel it results in a large angle between the interprismatic crystals and those in the prism sheets. This distinction between pattern 2 and pattern 3 is important because of the widespread use of rodent and bovine enamel (pattern 2) in dental research.
Crystal Size and Morphology 28
The crystals of mature enamel appear to grow and fill the bulk of the space available within the prism. The apatite crystals characteristically exhibit considerable irregularity of outline, but are roughly hexagonal in cross-section, with a mean width of 68.3 nm and mean thickness of 26.3nm. Many of the crystals in mature enamel show evidence of crystallographic defects (Ichijo et al. 1993). Aprismatic Enamel Aprismatic enamel, up to 100-micron meter thick, has been reported to be present at the surface of both permanent and deciduous human enamel (Boyde 1989, Kodaka et al. 1989) (FIG. 19). The thickness of aprismatic enamel varies both within and between tooth types. Within aprismatic surface enamel, the crystals are arranged parallel to each other and perpendicular to the surface, although some deviation in crystal orientation, due to the presence of remnants of prism boundaries, may be detectable in some areas (Kodaka et al. 1989). Because of the parallel alignment of crystals and the absence of prism boundaries, the surface layer is generally more highly mineralized than the subsurface enamel (Robinson et al 1971). This relatively featureless layer is thought to be result from the loss of the tomes ‘process by the ameloblast; thus the structural feature which directs the deposition of crystal into prisms and interprismatic material is lost, altering enamel structure as a consequence.
Dentino Enamel Junction 29
The
interface
of
the
enamel
and
dentin
is
called
the
dentinoenamel junction (FIG 12). It is scalloped or wavy in outline, with the crest of waves penetrating toward the enamel. The rounded projections of the enamel fit into the shallow depressions of the dentin. This inter digitations seems to contribute to affirm attachment between dentin and enamel. The dentino-enamel junction is also a hypo-mineralized zone about 30 micrometer thick (FIG. 20). Enamel is incapable of repairing itself once destroyed because the ameloblast cell degenerates following formation of the enamel rod. The final act of the ameloblast cell is secretion of a membrane covering the end of the enamel rod. This layer is referred to as Nasmyth membrane, or the primary enamel cuticle. This membrane covers the newly erupted tooth and is worn away by mastication and cleaning. The membrane is replaced by an organic deposit called a pellicle, which is a precipitate of salivary proteins. Microorganisms may invade the pellicle to form bacterial plaque, a potential precursor to dental disease.
30
Fig 16. Ground section of enamel viewed under transmitted light showing striae of retzius
Fig 17. Photomicrograph showing prenatal and post natal enamel in primary teeth
Fig 18. Different prism patterns in transverse section
DENTIN
Fig 19. Aprismatic enamel
Fig 20. The Scalloped appearance of dentino enamel junction 31
Dentin provides the bulk and general form of the tooth and is characterized as a hard tissue with tubules throughout the thickness. It forms slightly before the enamel; it determines the shape of the crown, including the cusps and ridges and the number and size of the roots (FIG. 21).
Along the crown, the dentin is covered by enamel,
along the root by cementum. It encloses the dental pulp, with which it shares a common origin from the dental papilla. The dentin and pulp can be considered as a single development and functional unit, often described as pulpodentinal complex. Dentin can be defined as porous biological composite composed of apatite crystal filler particles in a collagen matrix (Pashley 1996). The apatite crystallites are thought to provide strength, where as the collagen matrix provides toughness. Dentin mineralized
contains (95%
dentinal
volume
tubules
mineral
surrounded
phase)
by
highly
intratubular
dentin
embedded within a partially mineralized (30% volume mineral phase) collagen matrix (inter tubular dentin) (Marshall et al. 1997). The majority of tooth structure is composed of dentin, which is the vital component of the tooth. When compared with the enamel (Knoop hardness number KHN 343), dentin is much softer (KHN 68) (Craig 1993), a characteristic explains why dentin exhibits much faster wear. In addition the modulus of elasticity of enamel is approximately 84 Gpa (Craig 1993) compared with a value of 13-17 Gpa reported for dentin.
Physical Properties 32
It is light yellow in colour and becomes darker with age and less translucent. It is harder than bone and cementum but softer and less brittle than enamel. Dentin has greater compressive strength and tensile strength than enamel because it is traversed by tubules. The dentin is readily permeable. Specific gravity – 2 .1g/ml. Dentin is elastic and subject to slight deformation and acts as a shock absorber to overlying enamel. The lower mineral salt content in dentin renders it more radiolucent than enamel. Compressive strength of dentin - 40 – 50,000 PSI. Modules of resilience vital dentin – 100-140 LBS/Inch. Modules of vital dentin – 1,90,000 psi.
Composition of Dentin 70% - In organic material 20% - Organic Materials 10% - Water The inorganic substance consists of hydroxyapatite crystals and small amount of phosphate, carbonates and sulfates (FIG. 23). The organic substance consists of type-1, collagen containing 20% of matrix with proteoglycans between the fibres.
Structure of Dentin Dentinal Tubules The dentinal matrix contains tubules, each or which ranges from about 1 to 2micro meter in diameter at its outer end and 3 to 4micrometer at is pulpal side. The number of tubules are about 15,000 /mm2 near the dentinoenamel junction and it is 65,000mm 2 near the pulpal surface. The dentinal tubules are fine canals that extend across entire width
of the dentin. They contain odontoblastic process. The course
of the dentinal tubules follows a gentle curve, which is “S” Shaped. 33
They show two curvatures - primary curvature and secondary curvatures (FIG. 22). Primary curvature start at right angle from the pulpal surface, the convexity of this curved course is directed towards the apex of the root and the curvature in the outer half is directed towards the occlusal or incisal surface. These tubules end perpendicular to the dentino-enamel junction and cemento-dentinal junction. It is almost straight at the root apex, incisal edges and cusps. Over their entire, length, the tubules exhibit minute relatively regular secondary curvatures (FIG. 24). The fore most morphological characteristic of dentin is it’s tubular branched structure the pulp to the dentino-enamel junction. Under normal conditions the tubules are filled with fluid, may be important in hydraulically transferring and relieving stresses imparted to dentin through the supporting structures of the periodontium and Fig 21. Structures seen in dentin Fig 22. S-shaped the enamel. Indeed this may explain why endodontically treated teeth dentinal tubules are more brittle than vital teeth. When isolated from the dentin, each individual dentinal tubule would have the appearance of an inverted cone; with the smallest dimension being recorded at the dentinoenamel junction end the largest dimension adjacent to the cell body in the pulp. Canaliculi or Microtubules The dentinal tubules have lateral branches throughout the Fig 23. Composition of dentin by volume dentin termed as canaliculi.percentage These canaliculi are1micrometer or less in diameter and originate more or less at right angle to the main tubule.
34
Fig 24. Dentinal tubules seen in longitudinal ground section showing primary and secondary curvatures
Enamel Spindles
35
Near the dentino-enamel junction, the dentinal tubules divide into
several
terminal
and
form
an
inter
communicating
and
anastomosing network. Some dentinal tubules extend into the enamel for several millimeters. These are formed as enamel spindles (FIG. 15). Peritubular Dentin The dentin that immediately surrounds the dentinal tubules is called peritubular dentin. This dentin forms the walls of the tubules. It is more highly mineralized about 9% than the intertubular dentin. It is completely broken down and disappears on being subjected to routine decalcification methods. Intertubular dentin The main body of the dentin is composed of intertubular dentin. It is located between the dentinal tubules or between the zones of peritubular dentin. Although it is highly mineralized this matrix, like bone and cementum is retained after decalcification. About one half of its volume is organic matrix, specially collagen fibres which are randomly oriented around the dentinal tubules. The fibres have a lattice like arrangement coursing in gentle curves between the tubules and their peri-tubular zones. The fibres also exhibit cross-bonding. Hydroxyapatite crystals are formed along the fibres. Within each tubule is a collagen-deficient, hyper mineralized layer of dentin, which has been termed as peritubular dentin, and which may be more accurately termed periluminal (Pashley 1996) or intratubular dentin, which is calcium deficient carbonate rich hydroxyapatite. The small crystals present have a higher crystallinity and are five times harder than the intertubular dentin, with KHN of 250 36
compared with a KHN of 52 for intertubular dentin. The presence of this intertubular dentin narrows the lumen of the tubule from its original 3-µm to as little as 0.6- 0.8 µm in superficial dentin near the dentino-enamel junction. The width of intratubular dentin decreases in a pulpward direction, where there is a zone in which there is no intratubular dentin present and the tubule (luminal) diameter is approximately 3µm (Garberoglio and Brannstrom 1976). There is little published information on the biological control of intra tubular apposition, but it is known to be a slow process, slower than the incremental formation of secondary dentin in the pulp chamber. Pre dentin Predentin is located adjacent to the pulpal tissue and is 2µm to 6µm wide. It is the first formed dentin and is not mineralized. As the collagen fibres undergo mineralization at the pre-dentin front, the predentin then becomes dentin and a new layer of predentin forms circumpulpally (FIG. 25). Odontoblasts The cells, which are related to the deposition of dentin, are the odontoblasts. The odontoblasts are a layer of specialized cells, which lie on the surface of the pulp against the internal surface of the dentin. In a fully formed tooth, the odontoblasts are arranged at a single layer of closely packed cells, which are pyriform, in shape. As the cells are the cells are at different levels in the layer, on erroneous impression of stratification results. Each odontoblast possesses a long process (Tome’s Fibres), which passes from the distal end of the cell into the substance of the dentin where it is housed in a fine canal, the dentinal tubules. The 37
odontoblastic processes are largest in diameter near the pulp (3 to 4Âľm) and taper upto 1mm further into dentin (FIG. 26, 27 and 28). Primary Dentin The dentin that forms the initial shape of the tooth is called primary dentin. It is usually completed three years after tooth eruption. It consists of mantle and circumpulpal dentin (FIG. 29). Mantle Dentin Fig 25. Predentine Mantle dentin is the name of the first formed dentin in the crown underlying the dentino-enamel junction. It is thus the outer (or) most peripheral part of the primary dentin and it is about 20Âľm thick. The fibrils formed in this zone are perpendicular to the dentinoenamel junction and the organic matrix is composed of the collagen fibrils (FIG. 30). Circumpulpal Dentin Fig 26. Odontogenic zone comprising Fig 28. Extension of odontoblast odontoblasts, cell rich zone, cell free process in dentinal tubule zone Circumpulpal dentin forms the remaining primary dentin or bulk of the tooth. It is circumpulpal dentin that represents all of the dentin formed prior to root completion. The fibrils in circumpulpal dentin are much smaller in diameter and are more closely packed together. The circumpulpal dentin may contain slightly more mineral than mantle dentin.
Fig 29. Primary dentin and secondary dentin
Fig 27. SEM of deep dentin showing odontoblastic process
38
Fig 30. Histology of mantle dentin
Secondary Dentin Secondary dentin is a continuation of primary dentin that forms at a slower rate as the tooth ages physiologically. It is a narrow band 39
of dentin bordering the pulp and represents the dentin formed after the root completion. Secondary dentin formation takes place without any external stimuli. In secondary dentin, the tubules take a different directional pattern in contrast to primary dentin (FIG. 29). Incremental Lines The incremental lines von ebner or imbrication lines appear as fine lines (or) striations in dentin (FIG. 31). They run at right angles to the dentinal tubules and correspond to the incremental lines in enamel (or) bone. These lines reflect the daily rhythmic, recurrent deposition of dentin matrix as well as hesitation in the daily formative process. The distance between lines varies form 4 to 8Âľm. In the crown to much less in the root. The course of the lines indicates the growth pattern of the dentin. Contour lines of Owen Occasionally some of the incremental lines are accentuated because of the disturbances in the matrix and mineralization process. Such lines are readily demonstrated in ground sections and are known as contour lines. The most consistently seen contour lines is at the junction of the primary and secondary dentin (FIG. 32). Neonatal Lines In the deciduous teeth and in the first permanent molars, where dentin is formed partly before and partly after birth, the prenatal and postnatal are separated by an accentuated contour line. This is termed as neonatal line and is seen in enamel and as well as dentin. This line reflects the abrupt change in the environment that occurs at birth. The dentin matrix formed prior to birth is usually of
40
better quality than that formed after birth and neonatal line may be a zone of hypo-calcification (FIG. 33). Inter Globular Dentin Some times mineralization of dentin begins in small globular areas that fail to fuse into a homogenous mass. This results in zone of hypo-mineralization between the globules. These zones are known as interglobular dentin. Inter globular dentin forms in the crown of teeth in the circumpulpal dentin just below the mantle dentin, and it follows the incremental pattern (FIG. 34). Fig 31. Von Ebner’s lines The dentinal tubules
Fig 32. Contour lines of un-interruptedly through Owen interglobular dentin, thus demonstrating defects of mineralization and pass
not of matrix formation. In dry ground sections some of the interglobular dentin black in transmitted light. However, spaces in interglobular dentin are not believed to occur naturally. Granular Layer When dry ground section of the root dentin is visualized in transmitted light, there is a zone adjacent to the cementum that appears granular. This is known (or) granular layer (FIG. Fig 33. Neonatal as linetomes in dentin in amount from the Cementoenamel 35). This zone increases slightly junction to the root apex and is believed to be caused by a coalescing and looping of the terminal portions of the dentinal tubules. The cause of development of this zone is probably similar to the branching and beveling of the tubules at the dentinoenamel junctions.
Fig 34. Ground section of dentin viewed under transmitted light showing interglobular dentin
41
Fig 35. Ground section of dentin, viewed under polarized light showing granular layer
AGE AND FUNCTIONAL CHANGES 42
Reparative Dentin Reparative dentin is formed by the replacement (or) secondary odontoblast in response to irritation caused by attrition, abrasion, erosion, trauma, dental caries, some operative procedures and other irritants (FIG. 36 and 37). Reparative dentin is formed when Tomes Process are cut within 1.5 mm from the pulp. The cut fibres die along with the corresponding odontoblasts leaving dead tracts. New odontoblasts are differentiated from mesenchymal cells of the pulp in about 15 days and these replacement odontoblasts lay down the reparative dentin. Dead Tracts This is a type of reaction dentin, which appears to result from irritation of greater severity. The odontoblast process in the whole length of the injured tubule degenerates and at the same time is sealed off at the pulpal end by a deposit of reactionary dentin (FIG. 38). In dried ground section of normal dentin the odontoblast processes disintegrate and the empty tubules are filled with air. They appear black in transmitted and white in reflected light. Loss of odontoblast process may also occur in teeth containing vital pulp as a result of caries, attrition, abrasion, cavity preparation (or) erosion, (When the tomes process are cut more than 1.5mm).
These areas
demonstrate decreased sensitivity and appear to a greater extent in older teeth.
Sclerotic Dentin 43
Sclerotic dentin results from aging or mild irritation (such as slowly advancing caries) and causes a change in the composition of the primary dentin. The peritubular dentin becomes wider, gradually filling the tubules with calcified material, progressing from the D.E. Junction pulpally. These areas are harder, denser, less sensitive, and more protective of the pulp against subsequent irritations (FIG. 39). The deposition of intratubular dentin, as a result of ageing or in response to attrition, results in a progressive reduction in the tubule lumen, and if continued, obliterates the tubule. If this occurs in several tubules in adjacent areas, theReparative dentin dentin assumes a glassy Fig 36. appearance. The term used to describe this progressive deposition and obliteration of the tubule is SCLEROSIS, resulting in sclerotic dentin. This process begins in root dentin of 18 – year old premolars without any external influence. It can therefore be assumed that this is a physiological response and the occlusion of the tubules is achieved by continued intratubular deposition. The mechanism by which intratubular dentin is formed are poorly understood and three possible mechanisms have been suggested (Torneck 1994). Firstly, it has been suggested that there may be a passive redistribution of mineral from inter tubular dentin into the tubules around the pre-existing components of the tubule.
Secondly there
may be an active response on the part of the odontoblast process, resulting in an organic matrix that is actively mineralized as a result of odontoblast activity. Finally, it has been suggested that the Fig 37. Types of reparative dentin odontoblast may produce an organic matrix that becomes mineralized by redistribution of mineral from intertubular dentin, as in the first case. In which ever way it is formed, the net result is that intratubular dentin is deposited at the expense of the odontoblast process, which is either retracted or shortened by the loss of it’s distal extremity.
44
Fig 38. Dead tracts - ground section of dentin viewed under transmitted light
Fig 39. Sclerotic dentin
The amount of sclerosed dentin increases with age and is most frequently encountered in the apical third of the root. Sclerosis 45
reduces the permeability of dentin and thus may help prolong pulp vitality. Processes, which contribute to sclerotic dentin in the crown in response to attrition and caries, may differ from the physiological deposition of sclerotic (translucent) dentin in the root, which is agedependent and whose rate of deposition is not altered by attrition. Although there is a little evidence in the literature, it is thought sclerosis resulting from aging is physiological dentin sclerosis and that resulting from mild irritation is reactive dentin sclerosis. Eburnating dentin is a term referring to the outward portion of reactive sclerotic dentin where slow caries has destroyed formerly overlying tooth structure, leaving, a hard, darkened, cleanable surface. The refractive indices of dentin in which the tubules are occluded
are
equalized
and
such
areas
become
transparent.
Transparent (or) sclerotic dentin can be observed in the teeth of the elderly people, especially in the roots. Sclerotic dentin may also be found under slowly progressing caries. Mineral density is greater in this area of dentin as shown both by radiography and permeability studies. It appears transparent or light in transmitted light and dark in reflected light.
INNERVATIONS OF DENTIN 46
Intertubular nerves Dentinal tubules contain numerous nerve endings in the predentin and inner dentin no further than 100 to 150Âľm from the pulp. Most of these small vesiculates endings are located in the tubules in the coronal zone, specifically in the pulp horns. The nerves and their terminals are found in close association with the odontoblast process within the tubule (FIG. 28). Nerve grows into the papilla in the bell stage of tooth development
(Byers
1980)
both
afferent
neurons
and
efferent
automatic nerves that innervate pulpal blood vessels are present. The number of myelinated axons in permanent teeth increases with age and /or tooth development, reaching a plateau value of about 500 myelinated axons per human premolar at age 15, which remains constant upto 60 years. There may be single terminals or several dilated and constricted portion. In either case, the nerve endings are packed with small vesicles, either electron dense or lucent, which probably depends on whether there as been discharge of their neuro transmitter substance. In any case, they interdigitate with the odontoblast process, indicating an intimate relationship to this cell. It is believed that most of these are terminal processes of the myelinated nerve fibres of the dental pulp. The primary afferent somato sensory nerves of the dentin and pulp project to the main sensory nucleolus of the midbrain. Extent of Odontoblastic Process
During tooth development, at the bell stage, odontoblast processes extend from the odontoblast cell body through predentin to the dentino-enamel junction.
As the thickness of dentin increases,
the cellular processes must elongate.
47
However, the true length of the processes in mature dentin, in the absence of blood vessels or supporting cells, is an issue that is open to debate (FIG. 28) In human teeth, the thickness of dentin is about 3-3.5mm Such that if an odontoblastic process were to pass the entire distance from the pulpal border to the DEJ, then the volume of the cellular process would be four fold larger than that of the cell body (Pashley 1996). This difference in volume between the cell body and the process is even greater if the situation with cuboidal or flattened odontoblasts is considered, as seen in the root towards the apex. It is generally agreed that the process of most odontoblasts is between 0.1 and 1.0mm (Byers 1996). The question of how far the odontoblast process penetrates dentin is of vital importance when considering dentin sensitivity. If odontoblasts were to participate directly in the sensitivity of dentin to surface stimuli, then the stimuli must interact directly with the process, which is unlikely to be the case. Normally dentin is covered coronally with and on the root surface by cementum. When these surface coverings are lost, dentin is subjected to a variety of stimuli, including mechanical, chemical, thermal and smaller mechanical stimuli to which intact teeth are responsive. When exposed, it is proposed that the fluid filled tubules allow minute fluid shifts across the dentin when exposed to thermal, tactile, evaporative or osmotic stimuli. The effect of this is that mechanoreceptors in the pulp are stimulated (Pashley 1996). These fluid shifts can directly irritate odontoblasts, pulpal nerves and sub odontoblastic blood vessels by applying large sheer forces on their surface as the fluid streams through narrow spaces. The effect of fluid shift on the release of neuro peptides has been 48
assessed (Kimberly and Byers 1988, Byers et al 1990, Byers 1996), and results in the release of calcitonin gene related peptide (CGRP) or substance p (SP) from the pulpal nerves to generate a local neurogenic inflammatory condition. Dentin Characteristics Change With Depth Both primary and secondary dentin contains tubules. The circumference of the dentin at the most peripheral part of the crown or root is much greater than that of the final circumference of the pulp chamber or root canal space this results in the odontoblasts being much more crowded as they approach their final position, thus leading to the appearance of a columnar layer of odontoblasts, especially over the pulp horns. The convergence of odontoblasts towards the pulp creates a unique structural organization, with functional consequences. The convergence has been estimated to be 4:1. The number of tubules per unit area and the radius of the tubules increases in the direction from the dentino-enamel junction to the pulp, thus the area occupied by tubule lumina also increases. Pashley (1984) calculated the area occupied by tubule lumina at the dentino-enamel junction to be approximately 1% of the total surface area of the dentino-enamel junction and 22% of the pulp. As this area is occupied by dentinal fluid, which is 95% water. (Pashley 1996), the surface area figures are also approximately equal to the tubule water content of these regions. Therefore, the water content or wetness of dentin increases 20 fold from superficial to deep dentin. This factor has clinical implications; in terms of dentin bonding of restorative materials to deep dentin the water competes with resin monomers for surface collagen fibrils (Pashley and Carvalho 1997). 49
Fluid Flow In clinical conditions there is an outward fluid flow across exposed dentin in response to the low but positive pulpal tissue pressure. The composition of this fluid is uncertain, but must have an ion product of calcium and phosphate, which is above or near the solubility product constants for a number of forms of calcium phosphate (Pashley 1996). This would in turn lead to the formation of mineral deposits in dentinal tubules which have many forms (Mjor 1985), as the dentinal fluid moves outwards, larger amounts of mineral ions are presented to the walls of tubules than would occur in sealed tubules. Indeed, Shellis
(1994)
used
this
principle
to
reduce
the
depth
of
demineralization in vitro under stimulated caries forming conditions, by using a supersaturated surrogate dentinal fluid, which was perfused through the pulp chamber. When examined microscopically, translucent bands resembling sclerotic dentin were sometimes observed. Clinically, patients who complain of dentin sensitivity report that a cold stimulus elicits a greater response than evaporative, tactile or osmotic stimulation (Orchardson and Collins 1987). Outward direct fluid movement (in response to cold) is far more effective at activation pulpal mechanoreceptors than is the inward movement of fluid (seen following a hot stimulus). Dentin Permeability The structure of dentin is tubular, as previously stated, and it is this characteristic that provides the channels for the permeation of solutes and channels for the permeation of solutes and solvents across dentin. 50
The density of tubules per mm square varies from 15,000 at the dentino-enamel junction to 65,000 at the pulp boundary be predicted from tubule density and diameters, due to the presence of intra tubular material such as collagen fibrils and mineralized constrictions of the tubules (Pashley 1996). Dentin permeability can be subdivided into two broad categories (Pashley 1996): Transdentinal movements of substances through the entire thickness of dentin via dentinal tubules (such as fluid shifts in response to hydro dynamic stimuli). Intradentinal movement of exogenous substances into the infiltration of hydrophilic adhesive resins into demineralized dentin surfaces during resin bonding or demineralization of inter tubular dentin by bacterially derived acids (Kinney et al 1995), where the material enters the tubules but does not travel across the tubules. The presence of the smear plugs and / or intra tubular deposits (i.e. sclerotic dentin) is thought to lower intratubular permeability to minimal values (Pashley et al 1991). Dentin permeability (Transdentinal or intratubular) is not uniform across the tooth. Coronal dentin permeability is much higher than that of the root. This can be attributed to the convergence of tubules towards the pulp chamber, the tubule density increases about four fold in coronal dentin, but only two fold in root dentin.
51
Thus, within any location on the tooth peripheral dentin has a lower
permeability
than
deeper
dentin.
The
permeability
of
intertubular dentin has never been quantified, but it must be very low and limited to patent lateral canals that branch off from tubules (Chappell et al 1994, Mjor and Nordahl 1996). Numerous
methods
have
been
used
to
assess
dentin
permeability (Pashley 1990). The easiest method of measuring trans dentinal permeability is to quantify its hydraulic conductance. This measures the ease with which fluid can filter across a unit surface area of dentin in a unit time under a unit pressure gradient (Pashley 1990). It has been reported, in unobstructed dentin, that the hydraulic conductance increases as dentin thickness decreases. However, the presence of intratubular dentin and hence lowers its permeability (Pashley 1996). The structure of dentin makes it act both as a barrier and a permeable structure, depending on its thickness, age and other variables (Pashley and Pashley 1991). Dentin is very porous because of its tubular structure and the minimum porosity of normal peripheral coronal dentin is about 15000 tubules per square. If the dentin is uncovered, then the tubules provide a diffusion channel from the surface to the pulp. The rate at which diffusional flux of exogenous material crosses dentin to the pulp is highly dependent on dentin thickness and upon the hydraulic conductance of dentin (Pashley 1985, 1990). The Pulpo-Dentinal Complex Dentin and pulp are
embroyologically,
histologically and
functionally united and there is much evidence to support the concept of viewing the dentin and pulp as a functionally coupled unit, which 52
act as an integrated system. As soon as the tissues, which normally cover dentin, are lost, then normal compartmentalization between the tissues
is
lost
(Pashley
1996)
and
they
become
functionally
continuous. The pulp responds to the stimuli generated by the loss of dentinal covering, in the short term, by mounting an outward movement
of
fluid
(Vongsavan
1994,
Mathews
1996)
and
macromolecules (Byers 1996). The long-term response to the stimulus is the production of tertiary dentin, which is a biological response to reduce the permeability of the dentin of the dentin –pulp complex.
53
ACID ETCHING ON ENAMEL The developed materials that adhere or bond to tooth structure would minimize removal of healthy tissue, thus allowing a more conservative preparation and providing for an impenetrable seal at the margin between the tissue and restoration. Criteria for Bonding Three basic criteria necessary for bonding. The surface with which the bonding is to occur should be: 1: Similar to the surface. 2: Free of contamination. 3: Smooth and uniform.
Steps In Acid Etch Technique Enamel Prophylaxis The mechanical cleaning of the enamel is an important first step clinically in the bonding procedure. Maximum bond strength was developed only when an oral prophylaxis was done before etching. An examination of etched enamel surfaces not receiving an oral prophylaxis
shows
pellicular
remnants
and
microorganisms
contaminating the enamel. Clearly, acid alone cannot remove all contaminants. This especially true of calculus and a careful inspection should be made for the presence of this accretion which should be removed by scaling.
54
Because there is concern for interference of flavoring oils, glycerin and fluorides with the etching process, the use of watery slurry of flow pumice has been recommended. There is no significant difference in the retention rate of sealants with or with out pre-etch pumice prophylaxis (Donnan and Ball 1988). However, no clinical or laboratory evidence has been presented to preclude the use of commercial pastes, even those containing fluoride. Studies in 1980’s have showed no difference in the clinical performance of a sealant whether fluoridated or nonfluoridated toothpaste was used for the prophylaxis. Further research is indicated. Pellicle Removal An oral prophylaxis should remove all gross deposits and accretions from enamel, but it may not remove all integuments such as subsurface pellicle. In addition, some protein may become smeared over the surface during the prophylaxis. Some of this pertinacious constituent may go into the solution in the acid while the remainder may be floated away mechanically as the phase of enamel is solubilized. Application of Etchant (FIG. 40) In the next step, with the teeth dried and properly isolated from saliva, the acid is applied by one of several means including a cotton pellet, brush or minisponge. The object is to gently agitate the acid for a minute for maximal effect. This can be achieved using a gentle swabbing motion. Clinical reports have suggested extending the etching time upto 2 minutes in relatively high fluoride areas and highly calcified mature enamel as for an adult. 55
Pre operative - silver amalgam restoration
After cavity preparation
Acid Etching
Rinsing with water
Blot excess water using mini sponge or cotton
Application of bonding agent
Placement of composite restoration
Fig 40 56
Finished composite restoration
It is important not to rub the enamel during acid application, since burnishing the friable rods and their crystallites will reduce the surface area available for bonding. This has been shown to reduce bond strength. Scrubbing or rubbing may push the decalcified material back into the pores that are being formed. No apparent difference exists in the degree of irregularity after etching acid solution compared with an acid gel. Gels provide better control for restricting the etch area but may require more through rinsing afterward.
The most popular enamel / dentin etchant in
general dentistry is phosphoric acid blue gel.
This gel is syringe
dispensed, as adequate colour contrast, smooth consistency and almost ideal viscosity for application and rinsing off cleanly, and provides and even, nicely demarcated white frosted appearance. This etchant is recommended whenever extra good etching of enamel is desired, such as deciduous teeth. Studies and clinical experience indicate the 15 seconds is probably adequate for etching most young permanent teeth. However, individual variation exists in enamel solubility between patients, between teeth, and with in the same tooth, and 30 to 60 seconds may recommended for molars and adult teeth. Longer periods provide no more, but actually less, retention because of loss of surface structure. Caution should be exercised when etching over acquired and developmental demineralization.
It is best to avoid it.
If this is
impossible a short etching time the applicant of the sealant, and the use of direct bonding with extra attention to not having areas of adhesive deficiency are important. The presence of avoids, together the poor oral hygiene, can lead to indelible staining of underlying developmental white spots.
57
Washing There is a significant increase in bond strength values when enamel is washed for 60 secs compared to 15 secs. These observations were made using phosphoric acid in concentration of 30% and lower. The chemical composition of the rinsing solution did not affect the bond strength. 1% potassium Chloride solution was found to improve bond strength. The presence of contaminants in the post etch rinsing solutions could adversely affect the composite bond strength. Given the size of dentinal tubules any contaminant that is small enough to penetrate or obstruct the flow of monomers into the dentinal tubules may influence the process of polymerization and ultimately affect the development of hybrid layer and potential bond strength. Significant reduction in the bond strength was demonstrated when saline was used as rinsing solution, due to the presence of ions, which interfered with the formation of hybrid layer (Eric C. Sung et al 2002). In clinical procedure involving the etching of dentin with phosphoric acid, it requires complete removal of etchant and reaction products that are formed on the etched dentin surface, as in complete removal of reaction products will intervene with bond strength (Bates et al 1982). At the end of etching period the etchant is rinsed off the teeth with abatement water spray. A high-speed evacuator is strongly recommended for increased efficiency in collecting the etchant - water rinse and to reduce moisture contamination on teeth and Dri-Angles. Salivary contamination of the etch must not be allowed (If it occur, rinse with the water spray or re-etch for few seconds; the patient must not rinse).
58
Drying Next, the teeth are thoroughly dried with a moisture-and-oil-free air source to obtain the well-known dull, frosty appearance.
Teeth
that do not appear dull and frosty white should be re-etched. Cervical enamel, because of its different morphology, usually looks somewhat different from the centre and incisal portions of a sufficiently etched tooth. It should not be re-etched in attempts to produce a uniform appearance over the entire enamel surface.
Effects Of Etching On Enamel A routine etching removes from 3 to 10µm of surface enamel. Another 25µm reveals subtle histologic alterations, creating the necessary mechanical interlocks. Deeper localized dissolution will Fig 41. Etching pattern of enamel after acid etching generally cause penetration to a depth of 100µm or more. Although laboratory studies indicate that enamel alterations are largely (though not completely) reversible, it can be stated that the overall effect of applying etchant to healthy enamel is not detrimental.
This is
augmented by the fact that normally enamel is from 1000 to 2000µm thick. (except as it tapers toward the cervical margin), abrasive wear of facial enamel is normal and proceeds at the rate of upto 2µm per year, and facial surfaces are self-cleaning and not prone to caries. On the other beII exercised when etching Type Ihand, caution should Type Type IIIdamaged teeth with exposed dentin, deep enamel cracks or external or internal Fig 42a. Different types of etching pattern demineralization. Pattern of Etching Silverstone et al (1975) studied the morphological changes produced on the acid etched enamel surface scanning electron microscope. Exposure of human enamel to conditioning solutions produces three basic etching patterns (FIG. 42a). 59
Fig 42b. Acid etched enamel rod core dissolved to greater extent than rod sheath
Type 1 60
Prism core material is preferentially removed, leaving the prism peripheries relatively intact, resulting in a honeycomb appearance (FIG. 42b). The average diameter of the hollowed prism cores measures about 3Âľm. This pattern is most common of the three types observed. Type 2 The peripheral regions of the prism are dissolved preferentially, leaving the prism cores relatively intact, resulting in a cobblestone appearance. Type 3 Etching pattern contains areas, which resembles both type 1 and type 2 along with some distinct areas where the pattern of etching appears to be unrelated to the enamel prism morphology. Studies with polarized light microscope showed that sound enamel etched with phosphoric acid to be affected at 3 distinct levels and may be described in terms of three specific zones (Silverstone 1974). A superficial etched zone, which is a narrow zone of enamel of about 10 microns in depth that is removed by etching.
A Qualitative
porous zone of about 20 microns in depth. It is rendered porous by the acid attack and may be identified qualitatively using polarized light. A Quantitative porous zone of about 20 microns depth that qualitatively indistinguishable form adjacent enamel. Enhancement of Enamel Porosity Enamel is a porous tissue that contains approximately 0.1% to 0.2% by volume of space. Many of the pores communicate to allow for transport of tissue fluid and ions in solution. Poole and his coworkers (1961) showed that enamel behave like a molecular sieve, allowing passage of only the smallest molecules comparable in size to that of 61
water. Acid etching enhances not only the size of the pores to permit access of relatively large resin molecules, but does so far distances approximately 20 to 30 micrometer in from the tissue surface. Decreased concentration of phosphoric acid enhances porosity to greater depths in the enamel. This observation holds significance for the depth to which resin may penetrate into the tissue. Antimicrobial Property of Etchants Lsettembrine et al (1997) at the university college of dentistry New York concluded that all phosphoric acid etchant materials tested demonstrated antimicrobial activity against several bacteria commonly found in the oral cavity.
They also reiterated that addition of
antimicrobial agents to etchant or cavity preparation may not be necessary given the antimicrobial activity of the etchant, if the current bonding systems can provide and sustain sealed tooth restorative interface.
Role Of Flourides In Etching Of Enamel Enamel is soluble when exposed to an acid medium, but the dissolution is not uniform. Solubility of enamel increases from, the enamel surface to the dentino-enamel junction. When fluorides are present during enamel formation or are topically applied to the enamel surface, the solubility of the surface enamel is decreased. Flouride concentration
decrease
towards
the
dentino-enamel
junction.
Flouride additions can affect the chemical and physical properties of the apatite mineral and influence the hardness, chemical reactivity, and stability of enamel while preserving the apatite structures. Trace amount of fluorides stabilize enamel by lowering acid solubility, decreasing the rate of demineralization and enhancing the rate of 62
remineralization. Evidence also shows that topical fluorides alter the oral bacterial flora, there by increasing resistance to dental caries. It has been accepted to etch apparently normal enamel for 15 secs and enamel that shows signs of fluoridation for double that time or more. The use of prophylaxis pastes containing fluorides and topical fluoride treatments prior to etching is slowly diminishing.
There is
virtually no evidence that the fluoride incorporated in enamel prior to etching will significantly interfere with etching or will significantly affect bond strengths. It is well known that even the fluoride-acquired from acidulated topical fluoride solutions is poorly retained and easily removed under oral conditions in a short time. There is no contraindication to the use prior to etching because the fluoride from these agents including acidulated or nonacidulated sodium fluoride and stannous fluoride, will most likely find its way into the deep recesses of the pits and fissures and benefit in sealing them. This solid fluoride may conceivably be retained in the pits and fissures even after etching. Once sealed in the fissure by a sealant, the fluoride may gradually react with enamel (and perhaps with dentin, which in accessible at the base of some fissures) to produce a resistant tooth structure that can afford protection against caries even when sealant application is no longer provided. Fluorides should be avoided as part of the etching solution or immediately prior to regular bonding.
Studies have shown that
fluorides react with etched surface to produce reaction products that may interfere with bonding. These reaction products appear to interfere with optimal adhesive penetration resulting in weaker bond and/or bonds that will 63
not survive as long under conditions of oral moisture. It should be noted that washing away the acid conditioning solution with water containing 1or 2 parts per million of fluoride is not expected to interface with achieving high bonding strengths. Uptake of Fluorides in Etched Enamel Most important use of fluoride is after bonding procedures of all types. During etching more enamel surface usually is intentionally or unintentionally etched than is subsequently covered by adhesive, such as inter proximal areas etched by acid spillover. Etched enamel is highly reactive and readily combines with and better retains many times more fluoride than a natural unetched enamel surface.
The
large amount of fluoride thus acquired by etched but uncovered enamel from a topical fluoride application, may confer on the etched enamel surface a greater resistance to cavities, normally have this capacity.
Infact, it has been suggested that a mild acid etch
(independent of bonding procedures) be employed prior to application of acidulated sodium fluoride in order to enhance the acquisitions and retention of fluoride from this source. Retief et al (1986) has stated that fluoride is not evenly distributed through out enamel, the fluoride concentration follows a negative exponential distribution being highest in the surface enamel. The loss of fluoride rich surface enamel during the etching procedure may
make
the
enamel
more
caries
susceptible
environment.
Factors Affecting Etching On Enamel 64
in
the
oral
TIME Increased Time Application High fluoride content and primary teeth require longer etching time. The increased etching time is needed to enhance the etching pattern on enamel that is more a prismatic than that of permanent enamel. Currently 15 sec, a sufficient time to produce a bond equivalent to that produce by a 60 sec etching time is used routinely. Shorter Etching Time C.J. Guba et al (1994) highlighted that etching times and etchant consistency were not critical to enamel bond strengths.
It
yields acceptable bond strength. It conserves enamel and saves time. They also found that on microscopic examination of a 10 secs etch versus a 60 sec etch showed that etching the enamel for 10 sec produced a very superficial etch compared to a very deep etch with a 60 sec etch. However this did not have significant impact on the tensile strength. Though some researchers suggested that the etching effect is reduced when the etching viscosity of the acid is high, this study showed no significant difference as related to their viscosities. ACID CONCENTRATION An interesting and important phenomenon is the existence of an inverse relationship between the etching effect of phosphoric acid and it’s concentration. The phenomenon was first observed and reported in 1965 and subsequently confirmed by others. The same etch time lower concentrations of acid tend to be more destructive of the enamel than higher concentrations (FIG 43a to f).
65
Fig 43b. 32% H3PO4
Fig 43a. 10% H3PO4
Fig 43d. 35% H3PO4 for 2 minutes
Fig 43c. 35% H3PO4
Fig 43f. 37% H3PO4
Fig 43e. 36% H3PO4 for 30 secs
Etching pattern with various concentration of phosphoric acid (SEM)
Concentrations of phosphoric acid over 65% tend to show minimal changes. The concentrations of acid, producing consistent, 66
more or less evenly distributed relatively deep etch pattern, appear to be in the range of 30 to 50%. Bond strengths are greater with 30 to 50% acid concentration, the difference between their values and those obtained on surfaces etched with 10 to 70% acid were not as great. The higher concentration of acid may not produce a sufficient in depth etch to provide adequate resin penetration (tag formation) and / or sufficient bonding area to resist repeated long-term masticatory and other dislodging stresses encountered in the oral environment. In an in vitro study carried out on bovine substrate by M.J.Shingi et al (2000) it was concluded that milder concentrations of phosphoric acid or less aggressive acids could be used to pretreated enamel for orthodontics adhesive systems and sealants if the diffusion potential of applied monomers is high enough. According to Unos (1996) depths of demineralization increased by both acid concentration and conditioning times following a logarithmic relationship. Chow and Brown (1973) demonstrated that the application of phosphoric acid solutions greater than 27% Phosphoric acid resulted in the formation of monocalcium phosphate monohydrate while dicalcium phosphate dehydrate was formed with phosphoric acid concentrations less than 27%. The former product is readily soluble and would be completely washed away in the clinical situations.
VARIATIONS IN ACID ETCHING METHODOLOGIES 67
Currently phosphoric acid is the acid of choice, but it is possible that other acidic etching agents such as pyruvic acid may be used in the
future.
A
concentration
of
controversial phosphoric
issue, acid.
however, The
is
most
the
optimal
widely
used
concentrations of phosphoric acid used in clinical practice exceed 30% phosphoric acid.
This is partly based on the findings the phase
diagram of the phosphoric acid Âą calcium hydroxide Âą water ternary system. They demonstrated that the application of phosphoric acid solutions greater than 27% phosphoric acid resulted in the formation of monocalcium phosphate monohydrate. While dicalcium phosphate dihydrate was formed with phosphoric acid concentrations less than 27% phosphoric acid.
The former product is readily soluble and
would be completely washed away in the clinical situation, while the latter product is less soluble. The reaction products, if not completely removed after the etching procedure, may interfere with the bonding of composite resins to etched enamel surfaces. The effect of phosphoric acid concentration on the tensile bond strength of a conventional composite resin to enamel surfaces etched with 10, 20, 30, 40, 50, 60, and 70% phosphoric acid was determined. The tensile bond strength to enamel surfaces etched with 70% phosphoric acid was significantly lower than the bond strengths recorded to enamel surfaces etched with other phosphoric acid concentrations. The application of a phosphoric acid etching solution to freshly cut dentin may elicit a pulpal response.
To prevent the flow of
phosphoric acid applied to the enamel walls of preparations to the freshly exposed dentin at the floors of the preparations phosphoric acid gels were recently introduced. The objective was to confine the acid-etching agent to the intended site of application.
It is
recommended that the etching agent should be applied to the enamel surface using a dabbing action as opposed to rubbing. Another issue 68
that has not been resolved is the optimal duration of etching with phosphoric acid. It is surprising that some authors recommend that the etchant should remain on the tooth surface for at least 60seconds to develop an appropriate etched pattern. The etch duration is of
particular
importance in acid etching enamel prior to the direct bonding of orthodontic attachments, as it is practically impossible to confine the bonding site. Fluoride is not evenly distributed in enamel but allows a negative exponential distribution with fluoride concentration being in the surface enamel. The loss of fluoride rich enamel surface during prolonged etching may make the adjacent enamel more susceptible to enamel decalcification during orthodontic treatment.
The reaction products
that are formed on the enamel surface after phosphoric acid etching should be removed completely, as incomplete removal may interfere with bond strength. Etched surface should be washed for at least 15 secs to remove the reaction products. The tooth to be restored should be isolated with a rubber dam to prevent saliva contamination prior to acid etching and the placement of
composite
resin.
It
is
generally
recommended
that
saliva
contaminated etched enamel should be washed and retched. O’Brien and others showed, however that it was not necessary to re-etch an enamel surface contaminated briefly with saliva, as a thorough washing of such a surface did not have a detrimental effect on bond strength. Buonocore M (1955) introduced a simple conservative technique for bonding restorative resins to enamel. He placed a drop of selfcuring acrylic resin on the labial enamel surface of upper central incisor of ten subjects. One surface was treated prior to resin 69
placement with 85% phosphoric acid for 30 seconds. He noted that the acid conditioning of the enamel resulted in on uncontioned control surfaces lasted less than 12 hours. After three decades of laboratory and clinical research, Buonocore’s method is widely adopted and has added a new and exciting technical dimension to the practice of dentistry. Gwinnett,
Matsui
and
Buonocore
(1969)
suggested
that
formation of resin tags was the primary attachment mechanism of resin to phosphoric acid. Acid etching removes about 10 microns of the enamel surface and creates a porous layer ranging from 5-50 microns deep. When a low viscosity resin is applied, it flows into the micro porosities and channels to this layer and polymerizes to form a micro mechanical bond with the enamel. Fusayama et al (1979) introduced an etching technique for both the enamel and the dentin cavity wall using 37% phosphoric acid followed by a dentin-bonding agent containing methacryloxyethyl hydrogen phenyl phosphate (phenyl-P). This improved bond strength greater extent and dentinal etching has become fairly common practice in Japan. However, the concept of total etching only recently has gained acceptance in the United States.
ACID CONDITIONING OF DENTIN Any discussion of the effects of acid conditioning of dentin must begin with the acid etching of enamel. This was first proposed by 70
Buonocore (1955) as an attempt to clean enamel, increase the microscopic surface area for bonding, and infiltrate unfilled resins into enamel porosities. Many investigators were alarmed at what was then regarded as an unconventional and even reckless approach to the problem. Buonocore, Wilernan and Brudevold (1956) not only introduced the acid etching of enamel to dentistry, they also were among the first to attempt to bond resins to acid-etched (7% HCL, one minute) dentin. Their success with acid etching of enamel led them to try to acid etch dentin. Unlike enamel, when dentin is etched, its surface becomes mineral -poor protein rich; and it tends to become wetter (Brannstrom and Nordenvall 1977). Unfortunately, Buonocore and his colleagues' success with dentin was never realized, because the relatively crude resin materials that were available at that time would not wet dentin very well. Buonocore, however, was very much aware or the requirements for good bonding. For clinical success, the conditioned dentin must be sealed to prevent sensitivity and to prevent the pathology (Brannstrom, 1981) associated with the increased permeability of the dentinal tubules. Conditioning of dentin will be defined as any alteration of dentin done after the creation of dentin cutting debris, termed the smear layer. The objective of dentin conditioning is to create a surface capable of micro-mechanical and possible chemical bonding to a dentin-bonding agent. Goals of acid conditioning of dentin ♌
Remove the intrinsic weakness of the smear layer to permit bonding to underlying dentin. 71
♦
Demineralize the superficial dentin matrix to permit resin infiltration into surface.
♦
Uncover both intertubular and peritubular dentin.
♦
Clean the dentin surface free of any biofilms. It is important to define the purpose of the acid etching of
dentin so that once identified, these goals can be tested in a systematic scientific manner. As the smear layer is intrinsically weak, the first goal is to loosen it or remove it so that subsequently placed adhesive resins can interact with solid dentin adhesive resins can interact with solid dentin matrix. Most smear layers are 1-2 µm thick; they are composed of the cutting debris of the materialized tissue on which they lie. (Ruse and Smith 1991) The reason for acid etching is to demineralize the solid dentin matrix (both intertubular and peritubular dentin) to increase the porosity of the dentin. While this is analogous to why enamel is etched, the porosities that are produced are of the order of 0.05 – 1-3 µm in peritubular dentin rather than the 5-7 µm diameter of enamel prisms. Further, acid-etched enamel can be thoroughly dried, while that foal is much more difficult in vital, normal dentin. Enamel contains little protein that is at risk of being denatured by acid treatment. Dissolving away hydroxyapatite mineral crystallites from the collagen component of dentin matrix creates dentin porosities. The crystals tend to stabilize collagen and prevent its denaturation. There is a risk that the acid used to demineralize the dentin may denature or weaken the collagen. As denatured proteins generally change their dimensions, the pores may become smaller if the collagen is denatured. This may interfere with subsequent resin infiltration and prevent the formation of a hybrid layer (Nakabayashi, 72
Nakamura and Yasuda 1991). Another danger in the etching step is that the demineralized zone may extend, for instance, 5 µm into the dentin, while the resin infiltration may only extend 4 µm, leaving a 1 µm demineralized zone at the base of the hybrid layer that is unprotected by mineral or resin and that may be structurally weak. If the pulpodentin complex can re-mineralize this unprotected basal 1 µm of demineralized dentin (Tatsumi, 1989; Tatsumi and others 1992), then the layer may become as strong as normal dentin, rather than be a zone of debonding that has been seen in vitro (Nakabayashi and others 1991). Another purpose of acid etching dentin is to clean the dentin surface. Often dentin is inadvertently contaminated with blood during the cavity preparation. Acid etchant, by dissolving most of the smear layer, tend to float these biofilms on the dentin when it is rinsed. The low pH of the etchant may also denature the plasma proteins and hemoglobin. The purpose of acid etching may vary depending upon the material. If the intention is simply to remove the smear layer but leave the smear plugs in place, as when one uses glass-ionomer cements, then short etching times with dilute acids would seem to be indicated (Bowen 1978; Pashley and Others 1981; Hamlin and Others 1990a). However,
if
one
wants
to
create
a
resin
hybrid
layer
(Nakabayashi and Others 1991) in the dentin rather than on the dentin, then one must demineralized more deeply and in the process, removes smear plugs. This can still be accomplished using dilute acids, but the etching time may have to be extended.
Effects Of Conditioning Of Dentin The principal effects of conditioning of dentin may be classified as a)
Physical changes
b)
Chemical changes 73
Physical changes Increases or decreases in the thickness and morphology of smear layer changes in the shape of dentinal tubules. Chemical changes a) Modification of the fraction of organic matter b) Decalcification of the inorganic portion
Conditioning of dentin may be done by several means 1) Chemical a) Acids b) Calcium chelators 2) Thermal a) Lasers 3) Mechanical a) Abrasion When dentin is cut for cavity preparation, the wrenched cutting debris of the dentin forms a thin smear layer on the surface. It is also driven into the dentinal tubule apertures displacing the odontoblast process and forming a smear plug at a depth of less than 10-micron meter. Etching dissolves the smear layer and part of the peritubular dentin, leaving tapered cylindrical holes of that depth. In an experiment on monkey, dentin wall demineralized with a phosphoric acid jelly etchant for 60 sec was completely re-mineralized after 4 months. This results indicates that etching did not result in deleterious effect upon either the collagen fibers or the odontoblast processes, because the presence of collagen fibers maintaining their proper cross bonded structure as a base for apatite crystals to attach 74
to and of the vital odontoblast processes to supply the calcium phosphate from the pulp is essential for remineralization of dentin. A.J.Gwinnett and M.D.Jendresen (1978) have concluded from their
experiments
and
observations
that
the
surface
of
acid
conditioned eroded dentin is significantly different from that of acid conditioned normal dentin. They further observed the depth of penetration of resin is also less in acid treated eroded dentin where many tubules remain partially occluded by intratubular insoluble deposits. Ruse and Smith (1991) found when common conditioning agents were used, it has been found by X-ray photo electron microscopy that the outermost surface contains only 10% or less of the calcium and phosphorus initially present. They concluded that the treatment of dentin with acidic conditioners leaves the surface so depleted
of
calcium
and
enriched
by
organic
residues
that
subsequently placed bonding systems should be based upon agents able to interact with organic components of dentin. Bonding agents that rely on chelation to calcium are unlikely to be successful when applied to acid etched dentin unless they penetrate into the demineralized matrix to reach normal, mineralized dentin. Acid etching of dentin is not harmless but represents one more source of acute irritation to the pulpodentin complex in addition to the vibratory,
thermal,
mechanical
and
evaporative
stimuli
that
accompany cavity a preparation. However, it is not as irritating as has been previously thought. Nakabayashi (1982) introduced the concept of hybridization. The technique consists of applying an acid, ranging in concentration from 10% to 30% to the surface of dentin. Within 15 minutes the acid selectively dissolves away the inorganic component of the dentin to a depth of 5 to 10 microns. It then flows in the dentinal tubule for up to 75
100 microns at which point it diffuses laterally into the peri-tubular dentin for up to 10 microns. As in the previous case the calcium component is selectively eliminated. Then these spaces are replaced by an insoluble resin component that completely encapsulates all exposed collagenous fiber. He also reported that dentin conditioning by citric acid containing ferric chloride followed by a dentin bonding agent containing 4 META (methacryloxyethyl trimellitate anhydride) was effective method of dentinal bonding. Concerning the bonding mechanism, he proposed that diffusion and impregnation of monomers into the subsurface of pretreated dentinal substrate and their polymerization, creating a hybrid layer of resin reinforced dentin. This newly formed hybrid layer may be thought of as an admixture of polymer and dentinal components, creating a resin dentin composite. This technique not only enhances the shear bond strength of the resin to the dentin but also increase the potential against micro leakage and postoperative sensitivity. Nakabayashi
(1985)
suggested
that
the
acidic
treatment
partially demineralized a zone of the dentin near the surface, facilitating an infiltration process of compatible monomers. The polymerized resin forms a reinforced zone of dentin on which a resin based restorative material can be bonded. The bond strength is not dependent upon interlocking at the dentinal tubules. Kurosaki et al (1987) found that etching of dentin of the clinical cavity floor allows the chemically adhesive composite resin to produce resin tags of tapered, cylindrical or tubular form as well as 76
impregnated dentinal layers. These changes will considerably improve the bond strength as well as the tubule aperture seal. Surface Interactions Of Dentin Conditioners Smear layer removal One purpose of a dentin conditioner is removal of the smear layer to provide a surface that is more suitable for adhesion; however this does not necessarily apply to all systems. For example, the AllBond system can be used with the SA-HEMA conditioner, which is weakly acidic and probably modifies the smear layer without removing it, except where the smear layer is quite thin. Dentin permeability changes Removal of the smear plugs results in increased permeability of the dentin, and the rate of removal by conditioner can be examined by measurements of permeability increase for different application times. This is controlled by the strength of the acid, it’s concentration, and whether there are modifying components in the conditioning solution (FIG. 44). Because acid etching increases dentin permeability and dentin wetness, successful bonding at adhesive resins to acid etched dentin requires the use of hydrophilic resins that bond equally well to both peritubular and intratubular dentin. Future trends seem to be toward lowering both the concentration of acids and the time of etching dentin. Conditioner Application Time To Achieve Maximum Permeability CONDITIONER
Time (sec) 77
2.5%Nitric Acid
<10-
6%Citric Acid
~15
37%Phosphoric acid
<15
2.5%Maleic Acid
<60
55%HEMA
~12
Demineralization of dentin surface by conditioning agents Conditioning agents not only remove the smear layer, but also cause demineralization of the underlying dentin.
Complete loss of
peritubular dentin occurred 15 microns deep when 37% phosphoric acid was applied for 60 seconds. Demineralization depths of 10-15 microns may be greater than monomers can effectively penetrate. Demineralization of dentin to a depth greater than monomers can infiltrate and reinforce the collagen network can lead to decreased adhesion (Nakabayashi 1989; Kiyomura 1987).
When the Gluma
system was used with various acid conditioners, bond strength decreased with decreasing PH of the acid used (Asmussen and Bowen, 1987) (FIG. 45). Similarly, bond strength has been found to decrease as dentin surface hardness decreases when acid concentrations and self-etching primers of various strengths are used (Chiba, Itoh and Wakumoto 1989; Ingaki and Others). Surface
hardness
gives
an
indication
of
surface
demineralization, and a 60 second treatment with phosphoric acid of 30-45% concentration can reduce the surface hardness to 15-30% of its initial value and cause demineralization of greater than 10 micron deep. In another study, complete loss of peritubular dentin occurred with 15 micron deep when 37% phosphoric acid was applied for 60 seconds (Erickson and others 1991). Demineralization depths of 10-15 78
micron may be greater than monomers can effectively penetrate. Cadiaco Mc et al has revealed that both 10% maleic acid and 36% phosphoric acid completely the smear layer and demineralized the dentin, leaving a layer of collagenous network. Effect on collagen Conditioning agents may also affect collagen of the dentin surface. Nakabayashi (1985, 1989) has shown that the conditioning dentin with phosphoric acid and citric acid resulted in low bond strengths when used with the 4META/MMA/-TBB adhesive systems. However, addition of ferric chloride to the conditioner improved the bond strength drastically, and this was attributed to stabilization of the collagen by ferric ions during etching. It has been shown that phosphoric acid denatures collagen fibers are still surrounded by hydroxy
apatite
crystals
because
of
their
buffering
capacity
(FIG.46a,b). Alteration of surface wettability Because wettability of the surface is the first requirement of adhesion, it is of interest to examine the effect that dentin conditioners have an effect on surface wet ability. Dentin with a smear layer has been found to have a critical surface tension of wetting. (Benediktson1991).
Following
conditioning surface with EDTA
solution of Gluma was reduced to 29.48 dynes/cm and similarly, the conditioner of Tenure reduced to 27.27dynes/cm.
Erickson in 1989 studied the surface region of specimens where all bond systems has been applied with the technique using 10% Phosphoric acid as a conditioner. There were numerous resin 79
projections from the tubular tags that represent penetration into lateral canals and indicate the good wetting. Smear layer The
smear
layer,
an
incidental
consequence
of
cavity
preparation with rotary instruments, has been the subject of considerable investigation. The particles that remain on the surface of dentin after tooth preparation vary in size from 0.05 to 10Âľm. Although they tend to be irregular, the larger particles have plate like conjurations to stack into a layer that is seldom more than 1Âľm thick. The composition of these particles is similar to the dentin form, which they are formed.
Intentional removal of the smear layer during
restorative procedures is controversial (FIG. 47). One school of thought suggests that because the smear layer occludes the dentinal tubules, it precludes bacterial invasion. Others advocate removal of the smear layer to enhance the bonding of adhesive restorative materials and encourage the ingress of tags into the enlarged orifices of the dentinal tubules. A further argument in favour of removal is that the smear layer, although composed primarily of inorganic material, may have a significant organic component including viable bacteria and their byproducts that might provide a reservoir of irritants. Thus complete removal of the smear layer would be consistent with the elimination of irritants. When normal dentin is cut with a bur, grinding debris is forced into open tubules to form smear plugs of variable length. Further their grinding debris is burnished over the smear plugs to form a smear layer. The combination of smear layer and smear plugs forms a physiologic unit that is responsible for reducing the permeability of 80
prepared dentin. The length of the smear plugs may depend on the diameter of the tubules. Dentinal tubules are inverted cones with the bases at the pulpal surface of dentin. Thus, one would expect deep dentin to have longer smear plugs than superficial dentin with smaller diameter tubules (Van Meerbeek and others, 1992). If the smear plugs are so long that the hydrogen ions in the acidic primers cannot reach them in the typical 30- 60-second application time, then these agents could remove the smear layer but does not increase dentin permeability. If these primers remove both the smear layer and smear plugs then dentin
permeability
will
increase
to
near
maximum
values.
Additionally, resin tags of varying lengths with obviously penetrate further into tubules that are devoid of smear plugs than in tubules containing residual plugs. H. Koibuchi et al (2001) concluded that self etching prime can provide an effective alternative to conventional phosphoric acid etchant in conditioning the enamel surface to secure a durable bonding and marginal seal of composite resin restoration, tested on bovine enamel and demonstrated quiet clearly that hybridized dentin indeed form beneath resin impregnated smears which bonds well for bond longevity and prevention of noxious stimuli from reaching the pulp. Self-etching bonding systems that function with intact smear layers greatly simplify clinical adhesion procedures. They however recommended delaying final judgment until more clinical evidence is obtained.
The mineral component of the smear layer can be dissolved in a few seconds of etching, depending upon the pKa of the acid, its pH, chemical concentration and viscosity. The collagen phase of the smear 81
layer is acid, insoluble and presumably remains on the etched surface. Effect of
smear layer on bond strength
The retention of smear layer not only lowers dentin permeability (Pashley, Michelich and Kehl, 1981) but may prevent the decrease in bond strength seen with some bonding systems as deeper dentin is prepared
(Prati, Pashley and Montanari, 1991; Tao, Tagami and
Pashely, 1991). It also greatly lowers the effects of pulpal pressure on bond strength
(Tao and Pashely, 1989).
Thus, there are many
reasons to retain smear layer as bonding substrate Some investigators recommend that the smear layer be removed withconditioning various acids Fig 45. Acid stage to Fig 44. Intertubular porosity optimize the bonding of restorative materials to dentin, while others feel it can be left but modified, since its presence reduces the permeability of dentin (Stanley, 1989; Brannstrom, 1981; Schulein, 1988) (FIG. 47). Effect of chemical conditioners on smear layer Inokoshi and others (1989) have investigated the effects of a wide range of dentin conditioners and bonding agents. This study has demonstrated differences in the depth of dentin decalcification as well as the thickness of the resulting hybrid (dentin and resin) layers. The
A
B hybrid layer results when resins are applied to conditioned dentin. Fig 46. A. SEM of etched dentin showing exposed collagen fibers, B. conditioners A higher magnification showingare collagen banding All the investigated capable of removing the smear layer, but not all will remove the plugs from the tubules. The chemistry of the applied conditioner was subsequently shown (Suizaki, 1991) to affect the thickness of
the resulting hybrid layer.
This may be due to interaction with collagen in the outer decalcified zone.
82
Fig 47. Impregnation of smear layer
Fig 48. SEM of a smear plug blocking entrance of dentinal tubule
Moist versus dry dentin surface
83
Vital dentin is inherently wet, therefore, completely drying of dentin is difficult to achieve clinically. Water has been considered an obstacle for attaining an effective adhesion of resins to dentin. The “wet –bonding” technique prevents the spatial alterations (i.e., collagen collapse) that occur upon drying demineralized dentin (FIG. 49).
Such alterations may prevent the monomers from penetrating
the
labyrinth
of
nano
channels
formed
by
dissolution
of
hydroxyapatite crystals between collagen fibers. The wet bonding technique has been shown repeatedly to enhance bond strengths because water preserves the porosity of collagen network available for monomer interdiffusion. If the dentin surface is dried with air, the collagen undergoes immediate collapse and prevents the resin monomers from penetrating. When etched dentin is dried using an air syringe, bond strength decrease substantially, especially for acetone and ethanol based dentin
adhesive
system.
When
water is
removed,
the
elastic
characteristics of collagen may be lost. The collapse of the collagen fibers upon drying may therefore be a result of the change in the molecular arrangement. While in a wet state, wide gaps separate the collagen molecule from each other, in a dry state, the molecules are arranged more compactly (FIG. 50). This is because extra fibrillar spaces in hydrated Type 1 collagen are filled with water, while dried collagen has fewer extra fibrillar spaces open for the penetration of the monomers included in the adhesive systems. During air drying water that
occupies
the
interfibrillar
spaces
previously
filled
with
hydroxyapatite crystals is lost by evaporation, resulting in a decrease of the volume of the collagen network to approximately to one –third of it’s original volume. When air-dried demineralized dentin is rewet with water, the collagen matrix may reexpand and recover its
84
Fig 49. Moist dentin surface â&#x20AC;&#x201C; Dynamics
Fig 50. Collapse of etched dentin by air drying
Fig 51. Etching pattern of dentin, etched with maleic acid primary dimensions to the levels of original hydrated state. This spatial reexpansion occurs because the spaces between fibers are 85
refilled with water and because Type1 collagen itself is capable of undergoing expansion upon rehydration.
Factors Affecting Dentin Conditioning TIME Duration of application Brannstrom and his colleagues achieved good bonds of resin to dentin using a five-second application of 37% phosphoric acid (Brannstrom, Johnson and Nordenvall, 1979). Perhaps 30 seconds of 5% Phosphoric acid would have been equally effective. The same agent may remove the smear layer in five seconds can cause considerable decalcification if left in place for 30 seconds and produce pulpal damages began to appear using less concentrated acids with higher molecular weights and shorter time intervals, such as tannic acid 25%, poly acrylic acid 25%, deducing 0.09% (power & other, 1982; Van de Voorde, Gerdts and Muexhision 1988) buffered formic acid and 2 butyric acid (Mjor and Others 1982) which produced minimal pulpal responses. Tagarni and Others (1999) Compared a series of acids for their ability to clear tubules of smear layers and smear plugs by measuring the increase in the hydraulic conductance of smear layer-covered dentin as a function of etching time. Presumably, the same amount of demineralization was occurring in intertubular dentin Blosser and Others (1989) reported that etching normal dentin for 20-30 seconds with 2.6% nitric acid was as effective at was as effective at removing the smear layer and opening up dentinal tubules as 50 or 60 seconds of etching. Etching times may also need to be adjusted when attempting to etch sclerotic dentin such as excavated carious dentin or abraded 86
cervical root surfaces. Dluke and Lindemuth (1990-1991) have shown that this dentin is much less etchable and more resistant to acid attack than normal dentin. Presumably, this is because much of it is made up of whitlockite, a relatively acid-resistant form of calcium phosphate. Thus the etching conditions required to create a hybrid Layer in sclerotic de tin have not yet been demonstrated. In an in vitro study conducted at the university of Istanbul faculty of dentistry the following conclusions were obtained Benderli (1999) The application of strong acids (ferric chloride/citric acid or phosphoric acid to a dentin surface for 15 secs significantly gave higher bond strength values than 60 secs applications.
Applying
weak acids (maleic acid or Na-EDTA) to dentin for 15 secs significantly decreased the bond strength values compared to 60 secs applications. The highest value of the weakest acid and the lowest value of the strongest acid (phosphoric acid) for the same period (60 secs) were not statistically significant.
The highest bond strength values were
obtained with the application of maleic acid (60 secs) or Phosphoric acid15secs or ferric chloride/citric acid 15 secs. N. Brannstrom and K. J. Nordenvall (1977) in their study showed that etching prepared cavities using 37% phosphoric acid for 30 seconds and 2 minutes had no demonstrable difference in appearance in enamel and dentin. Though acid etching could open dentinal tubules, only incomplete tags were formed when enamel bond resin was forced into them. This produced a restoration of insufficient marginal adaptation. Shortening of application time of conditioning agents 87
In 1977, Brannstrom and Nordenvall noted demonstrable difference between dentinal surfaces etched for 15 seconds or two minutes and recommended shorter etching times. This appeared to be the beginning of a new phase in dentin conditioning. Concentration Y. Benderli (1999) studied the effect of various dentin acid treatment on tensile bond strength of composite to dentin.
He
concluded that the application of strong acids such as ferric chloride or phosphoric acid to dentin surface for 15 sec. gave significantly higher bond strength values than for 60 sec. application.
He also
stated that when weak acids such as maleic acid was applied for 15 sec. to dentin showed decreased bond strength values compare with 60 sec. application. This is because weak acids remove the smear layer after an increase in the etching time. On the other hand the strong acids remove the smear layer in 15 sec., when applied for a long time (60 sec.), they removed both smear layer and peritubular dentin. This causes the protein contents to rise to dentin surface resulting in bonding strength. Effect of additives Additives to the acid solution can affect the rate of removal of smear layer and plugs as shown in table. Addition of 55% HEMA to 2.5%maleic acid more than doubles the time required to reach maximum permeability even though the measured PH is the same.
This might be explained by absorption of HEMA on the Hydroxyapatitie crystals, thereby protecting them from dissolution 88
Permeability has also been shown to decrease when aluminium or ferric oxalate is added to nitric acid conditioning solution. Addition of 6.8% Oxalate to 2.5% nitric acid caused permeability to drop below the initial smear layer value when application times of 30-60seconds were used.
An initial increase in permeability was
followed by a marked decrease, presumably due to precipitation of aluminium or ferric phosphate and calcium oxalate in the tubules. The Tenure conditioner, having only 2.5% aluminium oxalate was not as effective in reducing permeability.
B.E. Causton et al.,
(1984) showed that a ten-minute application of ITS mineralizing solution over dentin, improved the performance of 37% phosphoric acid as adhesion promoters in combination with NPG-GMA (Nphenylglycine with glycidyl methacrylate) a chelating primer. Hosada et al and Sauk et al (1989) reported that when new generation clearfil liner bond system was used, in which they treat the dentin with 10% citric acid containing 20% calcium chloride. This high concentration of calcium present may stabilize collagen during surface
etching,
but
it
may
also
decrease
the
extent
of
demineralization of hydroxyapatite by a common ion effect. Viscosity The viscosity of the solutions used to etch has a significant effect on the amount of demineralization. Many etchants are purposely thickened to improve handling and limit their distribution on teeth. However, free diffusion slows down as viscosity is increased so that gets require more etching time than liquids or stated another way, gels etch dentin less than do liquids of The same acid concentration and etching time (Takahashi Suzuki and Naka, 1991; Erickson 1992.) According to study conducted by C.J. Guba (1994), when three enchants of different viscosities were compared under the scanning 89
electron microscope, the liquid and thin gel produced a more even etch pattern than a thick gel. In addition the thin gel appeared to produce the most well defined pattern of three conditioners. The highest mean tensile bond strength was recorded by the thin gel phosphoric acid (207.2<52.8kg /cm 2). The liquid phosphoric acid group etched for 10 secs had the lowest tensile bond strength (150.3,46.3 kg/cm2). The primary advantage of using a gel is that it gives better control of the acid solution when etching several teeth, and other difficult to reach places Brannstrom, Nordnwall (1977). Morphological studies have shown no significant differences based on the viscosity of the etchant. Molecular weight An other important variable is the molecular weight or size of the etchant, since diffusion varies inversely with the molecular weight. While this does not vary much for many acids (molecular weights: nitric acid, 63: phosphoric acid. 98; citric acid, 192). It is more significant with polyacrylic acids, which can have molecular weights that vary from 5000 to 25 000 or higher, which decreases their diffusion coefficients. If they also were viscous, then both effects would summate to limit their etching effects. Smear layer Smear tubules (Pashley and others, 1988a; Van Meerbeck and others, 1992). Their length (2-6 Âľm) is much longer than the thickness of the smear layer. It is the smear plugs that are responsible for most of the reduction in dentin permeability.
90
Brannstrom, Nordenvall and Glantz (1980) advocated treatment of dentin with dilute (0.2%) EDTA to remove the smear layer without removing the smear plugs. This can be accomplished, but the smear plugs will prevent adhesive
resin
penetration
into
the
tubules.
(Hamlin,
Samarawickrama and Lynch 1990b). Under some conditions this might be desirable, while in others it may lower bond strength below what might have been achieved had the resin both penetrated and bonded to peritubular dentin. However, Bergenholitz (1982) consideration on this view is questionable, since solitary microorganisms in particulars anaerobes, with high demands for proper growth conditions, would probably not survive and or multiply on the dentin floor where the nutritional supply is poor and where the microorganisms would be exposed to host defense factors in the fluids of the dentinal tubules. In general, most materials are toxic and bactericidal when they are prepared fresh but lose their antibacterial effects as they cure and age (Hensten â&#x20AC;&#x201C; Pettersen, 1987, 1986 Meryon, 1988). In the first few days, amalgam and silicate cement are as antimicrobial as zinc oxide eugenol, but the antibacterial effects of zinc oxide eugenol last much longer (Tobias, 1988). Originally, the following of the Brannstrom concept placed great emphasis on the presence or absence of microorganisms in the smear layer. Novrdenvall, Brannstrom and Torstenson (1979) predicted that if one microorganism was left in the smear layer, over 100 billion organisms cloud develop within 24 hours if the conditions were favorable.
91
However, Bergenholitz (1982) consideration on this view is questionable, since solitary microorganisms in particulars anaerobes, with high demands for proper growth conditions, would probably not survive and or multiply on the dentin floor where the nutritional supply is poor and where the microorganisms would be exposed to host defense factors in the fluids of the dentinal tubules. Consequences of removal of smear layer A major problem associated with etching dentin is the increased wetness that results when the smear layer is removed. Brannstrom and Nordenvall (1977) have presented evidence indicating that the dentin surface becomes decreases the ability of hydrophobic resins to wet such surface, leading too poor adhesion. Recently, this problem has been partially solved by using hydrophilic resins such as HEMA in concentrations ranging from 35 to 55% (Scotchbond 2, Gluma), giving higher dentin bond strengths. These higher bond strengths are possible, because these systems remove the limitations of the smear layer by acid etching (maleic acid in Scotch bond 2) or chelation etching (0.5M EDTA in Gluma) of the surface. If used properly, these agents remove the smear layer and increase bond strengths, but they also increase dentin permeability. This raises the question of the toxicity of etching agents to the pulp. Hamin, Lynch and Sarnarawickarama
(199O) in a vitro study
demonstrated that the application of a solution of aluminum nitrate and oxalic acid for five seconds was all that was needed to remove Smear plugs.
92
Ferric oxalate used to dissolve smear layer Bowen, Cobb and Rapson (1982) introduced the mordanting acidified ferric oxalate technique subsequently replaced by aluminum oxalate, which dissolved the original smear layer and smear layer. Stanley, Bowen and Cobb (1984) and Bowen and others (1989) found very little pulpal responses, due to the blocking of the dentinal tubules with the new artificial smear layer. Blosser R.L (1987) indicated that the use of purified ferric oxalate solution is ineffective in removing the smeared surface material from either dentin or enamel. The removal of this smeared surface layer, previously associated with ferric oxalate solution, was apparently due to the presence of residual nitric acid from the commercial synthesis of ferric oxalate powder.
Blosser and Bowen
(1988) and Blosser and others (1989) also found that solutions of aluminum nitrate (2.5%) and oxalic acid (1.5%) or just 2.4% nitric acid and 5.7% NPG (N-phenly glycine) applied for 60 seconds were equal effective conditioning agents without the ferric or aluminum oxalate. Formation of insoluble precipitates Another approach in avoiding the intrinsic weakness of the smear layer (Pashley 1991) is to remove it, but use some of the mineral in the smear layer to react with ions in the etchant to form insoluble precipitates that adhere to each other and to the underlying dentin matrix more firmly than did the original smear layer particles. An example of that approach was the use of ferric oxalate, in an acid environment, could form insoluble crystals of ferric phosphate (Eick 1992), calcium oxalate, and several forms of calcium phosphate. It is interesting to speculate that the ferric or aluminum ions may also stabilize
dentin
collagen
or
other
demineralization step. 93
macromolecules
during
the
Arguing against that notion is the observation that dentin collagen exposed during the conditioning
step in the
Mirage
(Chamelon Dental products) bonding system (2.5% nitric acid containing 4% N-phenylglycine) retains its cross-banding. This suggests that it is not denatured by acid treatment. However, some laboratories report relatively low bond strength with this system and claim that the bond fails at the dentin-resin interface. Smear plugs Brannstrom and Johnson (1974) noticed that they could remove most of the smear layer but leave the smear plugs in the tubules when they initially scrubbed the surface for five seconds with microbicidal fluoride solutions and then left it for a remaining 60 seconds (FIG. 48). Result of removal of smear plugs Removal of the smear plugs by acidic conditioners may result in dentinal fluid flowing from the tubules onto the dentin surface, and this can interfere with the adhesion process. In vitro stimulation of fluid flow to the surface has shown that adhesion is adversely affected by increased permeability. Effect of presence of surface moisture Also quantitative measurements of surface moisture have demonstrated that adhesion can be significantly reduced by very small amounts of water on the surface. This moisture does not necessarily have to come from the dentinal tubules but could be due to inadequate drying of the tooth surface after application of a waterbased primer. Whatever the source, physisorption of an adhesion promoter can be disrupted by small amounts of moisture and this would affect the ability of bonding agent to wet the surface. 94
Conditioners On The Dentin Surface The concentrations of acids should be lowered to levels that are isotonic (approximately 1%) with body fluids. The etching times should be limited to that required to produce optimum bonding. Ideally, the depth of etching should not exceed 1-2 µm, should not denature collagen, and should leave residual remnants of smear plugs in the tubules to maintain dentin permeability at a low level. The chemical effects of conditioners are generally limited to the top 5µm of dentin and, in some cases, to the top 0.1µm of the surface. The study of these chemical changes is best-done using surface analysis techniques, using x-ray photoelectron spectroscopy, recently reported that the elemental composition of fractured dentin and that of smear layer-covered dentin were very similar. The application of 3% H2O2 to smear layers produced no detectable effects.
When
Scotchbond Dual Cure adhesive was allowed to react with the smear layer for two minutes, it produced surface demineralization’s that were limited to the outer 0.05 mm of the smear layer. Treatment of dentin with acids severely depleted the surface of calcium and phosphate. All these conditioners reduced the surface concentration of calcium and phosphate.
Further, the surfaces became enriched with alcohol
groups when treated with Scotchprep (HEMA), carbonyl groups with Tenure Conditioner (oxalate), and carboxyl groups when treated with Gluma Cleanser (EDTA). Further, they found no aluminum remaining on the surface after treatment with Tenure Conditioner (aluminum oxalate).
Most importantly, since x-ray photoelectron spectroscopy
provides information about the binding energies of various chemical groups, they found no evidence of any shifts in binding energies of reactive groups on dentin that would signal the development of primary bonding.
The treatment of dentin with acidic conditioners
leaves the surface so depleted of calcium and enriched by organic residues that subsequently placed bonding systems should be based 95
upon agents able to interact with the organic components of dentin. Bonding agents that rely on chelation to calcium are unlikely to be successful when applied to acid-etched dentin unless they penetrate into the demineralized matrix to reach normal mineralized dentin X-ray
photoelectron
spectroscopy
(XPS)
combined
with
secondary ion mass spectrometry (SIMS) is very useful in the study of the effects of conditioner treatments on dentin surface. The use of ion beams to erode the surface under controlled conditions permits depthprofile analyses to be made.
Energy-dispersive x-ray spectroscopy
(EDS) lacks the ability to restrict the analysis to the surface, as if penetrates several micrometers into the surface. However, it has the advantage of high-resolution (spot size), and it can provide maps of the distribution of elements.
None of these surface analytical
techniques can determine whether; the tertiary structure of collagen is normal or how well resins have wetted dentin surfaces.
Thus,
transmission and scanning electron microscopy remain important research tools in this area. The concentration of acids used to etch dentin is evolving to lower concentrations than the initial 37% phosphoric acid concentration (Fusayama1984). Bisco now uses 10% phosphoric acid in their AllEtch technique. Tenure and Mirage Bond both use 2.5% nitric acid treatment. This procedure was shown to be without significant pulpal response in experimental animals. Just as investigators have recently recommended shorter etching times and lower concentrations of acids for enamel, and recently recommended shorter etching times for dentin conditioners. If reductions in acid concentration and etching time do not lead to a decrement in bond strength, then they should be employed. Bowen has long advocated the use of isotonic acid solutions to avoid osmotic insult to odontoblasts. 96
BIOCOMPATIBILITY Since the mid 1950s, a number of publication have reported that etching of vital dentin will cause pulp inflammation and eventual necrosis. (Kramer and McLean, 1952; Franquin and Broulliet 1988). Professor Fusayama (1987) reported that in vivo etching of dentin and proper sealing of the dentinal tubule and Fusayamau1982; Fujiniti 1986) have demonstrated that direct application of various etchants to vital dentin and subsequent treatment with their compatible adhesive systems provided a seal against microleakage. No pulpal response was recorded when these etchants and bonding materials were evaluated for international biocompatibility testing. A recent study of a vital acid etching system (White and Others 1992) employing 10% phosphoric acid treatment placement onto vital dentin of freshly prepared on non human primate teeth demonstrated no pulpal inflammation or bacterial leakage after 25 and 80 days as per ISO, FDI and ADA testing criteria for usage evaluation. Most of the recent generation of dentin bonding agents has recommended removal of the outer contiguous smear layer as well as well as all or portions of underlying smear plugs. With the advent of enamel窶電entin etch ants that may disinfect the remaining affected dentin, as well as the newer hydrophilic primer and bonding system that infiltrate into the dentin substrate, our clinical profession may expect to place adhesive and composite systems that are biologically compatible with the dentin and pulp. In addition, these newer systems provide a durable bonding and hybrid mechanism to the tooth structure
that
should
be
expected
to
continuously
prevent
microleakage of bacterial contaminants. It has been suggested that conditioning agents (etching agents used on dentin) should be: 97
♦
Isotonic to avoid osmotic pressure changes in dentinal tubules
♦
Of neutral pH or at least between pH 5.5 and pH 8.0;
♦
Nontoxic to dentin, pulp and gingival tissue
♦
Compatible with the chemistry of the materials it will contact
♦
Water soluble and easily removed
♦
Unable to deplete the enamel or dentin chemically
♦
Able to enhance the surface chemically in preparation for bonding. Important variables regulating the type of pulpal response to
etched dentin The type of acid, pKa and pH Applied concentration (which determines its chemical potential and osmotic pressure) The time of etching (acid challenge = time X concentration) Remaining dentin thickness (+<0.6mm>). The ability of subsequently placed restorative materials to seal dentin. pH Many dentin conditioning agents have pH values much lower than 5.5. Acids can challenge pulp vitality no matter what the source, and they can harm the pulp if they approach or contact it. The smear layer and tubular plugs (the smear unit) and per tubular dentin (sclerotic
dentin)
can
be
rapidly
dissolved
by
strong
acidic
conditioning agents when applied for excessive (extended) time intervals (Mount, 1990; Pashley and Depew, 1986; Pashley, 1988; Pashley and others, 1988; Stanley, 1990)
98
Pulpal reactions depending on ph Cytotoxic effects within the pulp tissues can be caused by low pH hypertonic and hypotonic concentrations (abnormal osmolality), or chemical interferences with vital biochemical reactions (Mjor, Hensten â&#x20AC;&#x201C; Pettersen and Bowen, Johnson and others 1970). Concentration of acid The concentration of acids used to etch dentin is evolving to lower
concentrations
than
the
initial
37%
phosphoric
acid
concentration (Hosoda and Fusayama, 1984). Bisco now uses 10% phosphoric acid in their All Etch technique. Tenure and Mirage Bond both use 2.5% nitric acid treatment. This procedure was shown to be without significant pulpal response in experimental animals (Blosser and others, 1989). Just as investigators have recently recommended shorter etching times and lower concentrations of acids for enamel times (Bastos and others, 1988; Legler and others, 1989), Blosser (1990) recently recommended shorter etching times for dentin conditioners. If reductions in acid concentration and etching time do not lead to a decrement in bond strength, then they should be employed. The concentration of an acid reaching the pulp tissue is determined by how much penetrates through dentinal tubules and reaches along the way with hydroxyapatite and proteins contained within the tubules. The absolute concentration of an acid is reduced over distance such that at relatively large thickness (>1.0 mm) the concentration of a substance is relatively low by the time it reaches the pulpal surface.
99
As the remaining dentin thickness decreases, there is less recognition of solute concentration, causing a meager concentration at the pulpal surface as the diffusing solutes reach the end of the tubules (Pashley, 1979; Pashley, 1985 and Others). Contributing to the immediate pulpal irritation of acid etching is the fact that in etchants are very hypertonic and tends to osmotically draw fluid from the pulp to etchants (Macko and Others, 1978). Acidic solutions can and do denature proteins including collagen, noncollagenous proteins, enzymes in odontoblastic processes. Thus, the acidity and hypertonicity of acidic conditions should cause an immediate pulpal reaction. As
protons
are
buffered
by
the
trivalent
phosphate
of
hydroxyapatite, the mono and divalent forms of phosphate no longer fit
the
crystalline
lattice
structure
of
hydroxyapatite
and
it
disintegrates. These changes in dentin are readily seen in Scanning Electron Micrograph (SEM) examinations of fractured surfaces of acid etched dentin- The funneling of the top of dentinal tubules due to the dissolution of peritubular dentin provides physical evidence of the depth of penetration of hydrogen ions during the brief (30-60 seconds) etching periods used clinically. The loss of peritubular dentin mineral is more dramatic than that of intertubular dentin, because it contains little collagen and thus disappears when it is demineralized. The same buffering of hydrogen occurs in intertubular dentin, but because of its high collagen content. There is little loss of substance that can be seen by Scanning Electron Micrograph. H. R. Stanley et al (1975) reported that 50% citric acid or phosphoric acid is detrimental to pulp when placed over dentin. The intensity of pulp response to composite was increased after acid pretreatment of the dentin. It was also suggested that when the 100
remaining dentin thickness was one mm or less than that, the area had to be protected, even when a composite of neutral ph containing no methacrylic acid was used. Perdiago et al (1996) studied the effect of six phosphoric acid etching agents and depth of demineralization human dentin under scanning electron microscope. The results obtained suggest that similar concentrations of phosphoric acid etchants containing distinct thickness result in different demineralization. Numerous investigators have reported that acid etching of dentin leads to pulpal inflammation. Several in vitro studies indicated that dentin restricts the penetration of hydrogen ions. Their results indicate that brief exposures to acid lead to little penetration of hydrogen ions across dentin that is 0.4 to 0.5 mm thick. This is due, in part, to the excellent buffer capacity of dentin. The calculated depth of penetration of a small acid such as nitric acid into dentin in 40 seconds is 89 Âľm. Blosser (1990) recently reported that 2.5% nitric acid funneled the orifices of dentin to a depth of 5Âľm in 40 seconds. Funneling of tubule orifices provides physical evidence of the depth of penetration of protons into dentin. Holzâ&#x20AC;&#x2122;s group (Cotting and Others 1980) also found the acid etching of human dentin in vivo with 37% phosphoric acid produced only a mild pulpal response if the etched dentin was subsequently covered with calcium hydroxide. If it was covered with calcium hydroxide first and then acid etched, there was more severe pulp response due, in part, to the greater loss of the bases during acid treatment.
101
Etching techniques that utilize a high concentration of acid with a low pH and a long interval of application (60 â&#x20AC;&#x201C; 120 seconds) when loosely approximating the pulp can intensify the pulpal response with neutrophilic infiltration. Local tissues injury that damages small vessels
causes
leakages
of
plasma
containing
fibrinogen
and
fibronectin. The extravasated fibrinogen becomes clotted and crosslinked to trap migrating neutrophils. Neutrophils arrive first at the site of injury not only due to their mobility, but because they are also chemo attracted by the fibronectin. Neutrophils then produce their own chemo-attractants that induce the migration of additional occur with zinc oxide eugenol. Therefore, it is unlikely that the influx of neutrophils following dentin conditioning techniques is due to increased permeability of the dentin and the increased ability for the eugenol to reach the pulp, it appears that certain conditioning agents can of themselves causes or intensify pulpal responses. Recovery of pulp Within 24 - 48 hours the pulp should recover from these sterile insults, as the tubules should be sealed by adhesive resin films. If one observes pulpal irritation beyond three days, then one should suspect that there is some continual source of irritation that has nothing to do directly with the acid used to etch the dentin or the subsequently placed resin, because most of the reachable products from such resins diminish rapidly over time (Hume and Mount. 1988; Hanks and others 1988). Likely candidates for such irritation are bacteria and their products, which gain access to the cavity walls because the restoration did not seal the dentin well. Germ-free studies indicate that pulps heal rapidly even when bonding agents are placed directly on the pulp (Inoue and Shimono, I992). 102
Duration of acid exposure into a smear layer It is useful to calculate how many seconds of acid exposure are required to allow the acid to penetrate 1mm into a smear layer surface (the thickness of an average smear layer). If one assumes that the acid has a diffusion coefficient of 1 X 10 -6 cm2/sec, the diffusion time is only five milliseconds. Assuming a 10-fold lower effective diffusion coefficient (which may be the case for higher molecular weight polyacrylic acids gels) extends the time to 0.5 seconds. Thus, it appears that the etching times that have been used are more than those required to diffuse through the smear layer. Chemical effects of conditioners The chemical effects of conditioners are generally limited to the top 5 Âľm of dentin surface (Ruse and Smith, 1991). Using x-ray photoelectron spectroscopy, recently reported that the elemental composition of fractured dentin and that of smear layer- covered dentin were very similar. They found that application of 3% hydrogen peroxide to smear layer produced no detectable effects. Difference in reaction of tooth according to age The pulp of younger virgin tooth with open dentinal tubules to begin with is more susceptible to the toxic components of dental materials and responds with more intense inflammatory reactions than does an older tooth, which over the years has produced a considerable amount of sclerotic dentin, and reparative dentin that blocks the tubules.
Sure attempts to protect the pulp with (a)
sclerosis of dentin, either as a natural process of aging or induced by the irritation from caries, attrition, abrasion and erosion; and b) reparative dentin formation, induced by the above restorative factors and also by tooth cutting and restorative procedures; Stanley and others, 1980). 103
VARIOUS ACID CONDITIONERS Phosphoric acid The Fist Dentin Conditioner phosphoric acid liquid removes some surface dentin and leaves a clean, well 窶電efined etching pattern. The tubule orifices are enlarged into a funnel shape. Phosphoric acid gels, thickened with fumed silica, similarly open the tubules but also leave a substantial covering of the thickening agent on the dentin. Regardless of extensive washing, the silica is not entirely removed. Fusyama and Others (1979) were the first to report the successful use of phosphoric acid to remove the smear layer, etch the dentin and restore with adhesive composite resin. For some years there was an unresolved discrepancy between clinical success and etched dentin (Shintani, Sataou and Satou, 1989) and research that seemed to contraindicate phosphoric acid etched dentin. Recently Kanca reevaluated the literature. His concluding hypothesis was that eugenol, not phosphoric acid, most likely led to pulpal irritation, as previously thought to be associated with phosphoric acid. It was the first dentin conditioner that was successfully used to remove the smear layer etches the dentin and restore with adhesive composite resin by Fusayama and Others (1979).
This helps in
removing the surface dentin, leaving a clean wee defined etching pattern
where
the
tubules
are
enlarged
into
funnel
shaped.
Phosphoric acid is the acid of choice currently for the etching purpose. However controversy remains about the optimal concentration of phosphoric acid. The most widely used concentrations in clinical practice exceed 30% phosphoric acid.
Nitric acid
104
It is stronger than phosphoric acid, it easily removes the smear layer. Used in concentration of 2.5% causes funneling of the orifices of dentin to a depth of 5Âľ in 40 seconds. Nitric acid conditioners are highly adhesive and provide good tubule seals. Citric acid 10% citric acid is used for the purpose of removing the smear layer .It has been reported by Nakabayashi (1989) that such treatment tends to lower the porosity or permeability of the demineralized surface, possibly by denaturing the collagen. 10% citric acid plus 3% ferric chloride and the combination was developed by Nakabayashi. The divalent caution seems to stabilize the dentin matrix during its demineralization by citric acid. This combination was found to be particularly effective for methacrylate based adhesives containing 4META. Several Japanese products remove the smear layer with 10%citric acid. Nakabayashi (1989) has reported that such treatment tends to lower the porosity or permeability of the demineralized surface, possibly by denaturing the collagen. solution
He developed 10-3
(10% citric acid plus 3% ferric chloride), in which the
divalent cation seems to stabilize the dentin matrix during its demineralization by citric acid. metharyloxyethyl tetrabutyl
borane
trimellitic oxidized
The higher bond strengths of 4-
anhydride complex
inflammation or necrosis.
Pyruvic acid 105
/
failed
methylmethacrylateto
produce
pulpal
Pyruvic acid and pyruvic acid buffered with glycine have been reported to satisfactorily acid etch both enamel and dentin (Asmussen and Munksgaard, 1988) when using the Gluma bonding system. Glycine was used to adjust the pH and perhaps to facilitate polymerization reactions. Lactic acid The study conducted by Ayad M F et al (1996) revealed that lactic acid dissolved the smear layer with various degrees of etching and demineralization .The degree of smear layer and matrix removal was proportional to the concentration of the acid and the length of application time. A 20% lactic acid concentration applied for 10secs produced a clearly etched surface with minimal demineralization. A 30% concentration not only removed the smear layer and enlarged the dentin tubule orifices but also appeared to affect the collagen matrix. Polyacrylic acid Polyacrylic acid is effective in removing the smear layer
and is
commercially available in products such as Clearfil Tooth cleanser (Dents ply Ltd) that contains 25% phosphoric acid and is marketed as a cavity conditioner for glass polyalkeonate restorative materials for 10 seconds. NaOcl as a dentin conditioner The manufacturers of Dentin Adhesit (Vivadent, Tonaeanda, NY 14150) included a solution of 5% NaOcl as a dentin conditioner. This was designed to solubilize the denatured collagen that had become gelatinized on the surface. 106
As this treatment did not change the permeability of smear layer covered dentin
(Pashley, unpublished observation), many thought
that such treatment was without effect. However, recently published transmission electron microscopy of NaOcl - treated smear layers revealed that the agent penetrated 510 mm below the smear layer. It also seemed to alter the subsurface dentin matrix in a subtle manner. The functional significance of this is unknown. The observation emphasizes the consequences of attempting to modify the surface of a smear layer 1-2mm thick.
These agents easily diffuse through the
smear layer in a few seconds and may produce undesirable subsurface effects. Chelators EDTA Contrary to the use of strong acid etchants, chelators are used to remove the smear layer without decalcification or significant physical changes to the underlying substrate. The best known chelating conditioner is ethylenediamine tetra acetic acid (EDTA) adjusted to PH of about 7.4. It was developed for use in the Gluma (Miles, INC, Sout11 1985). While the smear layer is removed, no significant surface concavity is formed and the funnel shape changes associated with phosphoric acid is not evident.
Brannstromâ&#x20AC;&#x2122;s
concerns that bacteria might be incorporated into smear layers and infect the dentin surfaces of cavities led him to develop a dentin conditioner containing 0.2% ethylene diamine tetraacetic acid, (EDTA, pH 7.4) and 0.1% benzalkonium chloride as a surface-active disinfectant.
(Braannstrom,
Nordenvall
Brannstrom and Others, 1982). 107
and
Glantz,
1980;
This agent, marketed under the name Tubulicid
(Dental
Therapeutics AB, Nacka, Sweden), is scrubbed on the surface of the smear layer for a few seconds, then left on passively for another 60 seconds, followed by additional scrubbing.
Such treatment indeed
does remove the smear layer and generally leaves smear plugs intact. The permeability of dentin remains low and unchanged by such treatment, although a few areas can be seen where the smear plugs are gone.
The lack of change in dentin permeability
suggests that
the latter effect is minor. Stagel, Ostro and Cesare (1987) reported that dentin conditioned with Tubulicid increased the bond strength of Scotchbond / Silux (3M Dental Products).
Tao and Pashley
(1988) reported a
significant decrease in the shear bond strength of Scotchbond /Silux when samples were treated with Tubulicid, although there was no change in dentin permeability (Tao and Pashley, 1989b) Presumably, the dilute solution of EDTA removed some surface calcium that is thought to be important in the mechanism of bonding of Scotchbond Dual Cure to dentin.
This was probably responsible for the fall in
bond strength. Causton (1984) reported that Scotchbond Dual Cure gave lower bond strengths to deep dentin compared to superficial dentin.
He
assumed that this result was due to a lower mineralization in deep dentin. Topical application of a mineralizing solution (Causton, 1984) increased the bond strength of Scotch bond to deep dentin but did not enhance bonds to superficial dentin.
The use of mineralizing
solutions as dentin conditioners has not received much further attention.
To be practical, they should produce a significant effect
within two minutes.
108
Most of the second-generation dentin bonding agents that did not use dentin primers or conditioners produces bond strength to smear layer-covered dentin of about 5-7 Mpa (Surmont and Others, 1989)
on
extracted
human
dentin.
Careful
scanning
electron
microscopy examination of both sides of the failed bonds revealed smear layer particles on each surface (Pashley, 1991). This indicated that the 5 Mpa really represented the cohesive strength of the smear layer. That is, the bond to the top of the smear layer remained intact and the â&#x20AC;&#x153;bondâ&#x20AC;? of the lower half of the smear layer to the underlying dentin matrix remained intact, but the smear layer had split. When the smear layer was treated with Gluma (Miles, Inc, Dental Products, South Bend, IN 46614) Primer (5% glutaraldehyde in 35% HEMA), we obtained a significant increase in shear bond strength. Similar results were obtained in vivo (Pashley and Others 1988). This is primarily due to improved wetting of the surface. However, it is possible that the smear layer is falling closer to the dentin surface, where the cohesive strength of the smear layer may be higher. Others have obtained relatively high bond strengths to smear layers. Surmont and others (1989), using the Gluma bonding system, compared the tensile bond strengths in three different treatment groups. The group in which the smear layer was treated with water gave a mean tensile and strength of 10.0 + 1.9 MPa. The
group
treated
with
Neo-Sabeny / Tubulicid (which
removed the superficial smear layer but not the smear plugs) had a mean bond strength of 10.1+ 2.0 MPa. The group treated with EDTA had a lower bond strength (7.4 + 1.4 MP a). The latter value is similar to what was obtained for Gluma / Scotchbond / Silux)
109
The smear plugs in the dentinal tubules are not fully removed by 30 seconds application of the conditioner. The system uses both glutaraldehyde and HEMA in a primer that is applied after the EDTA conditioner removes the smear layer. Maleic acid Maleic acid (e.g., Scotochbond 2, 3M dental products) also results in removal of the smear layer but not the smear plugs. Although it is acidic it does not appear to decalcify deeply, and the hybrid layer is comparatively thin. The thickness of the hybrid layer does not have much effect on the dentin bond strength (FIG.51). Brannstromâ&#x20AC;&#x2122;s concerns that bacteria might be incorporated into smear layers and infect the dentin surfaces of cavities led him to develop a dentin conditioner containing 0.2% ethylene diamine tetraacetic acid (EDTA, pH 7.4) and 0.1% benzalkonium chloride as a surface active disinfectant (Brannstrom and Nordenvail 1982). This agent, marketed under the name Tubulicid (Dental Therapeutics AB, Nacka, Sweden), is scrubbed on the surface of the smear layer for a few seconds, then left on passively for another 60 seconds, followed by additional scrubbing. Such treatment indeed does remove the smear layer and generally leaves smear plugs intact. The permeability of dentin remains low and unchanged by such treatment, although a few areas can be seen where the smear plugs are gone.
The lack of
change in dentin permeability suggests that the latter effect is minor. The dilute solution of EDTA removed some surface calcium that is thought to be important in the mechanism of bonding of Scotch bond Dual Cure to dentin.
110
Lasers Hard tissue LASERS in dentistry are an emerging technology. A pulsed Nd:YAG laser will not disturb the pulp, even when the approach is as close as 1mm (White and others 1990).
Heat is
dissipated between the 10 to 30 pulses per second. The mechanism of dentin removal is microscopic explosions caused by the thermal transients. While most research has been conducted on dry dentin, the laser will operate on dentin immersed in saliva and water. The carbonized, black soot that results is easily washed off with water. The lased surface results if desensitized dentin, presumably by occlusion
of
the
open
and
permeable
dentinal
tubules.
Microorganisms and organic debris are eliminated from the lased surfaces (White, Goodis, Cohen 1991). The laser decreases the organic fraction and increases the inorganic fraction of dentin surface (White and Others 1991). Lasing of the dentin has the potential to increase the bond strength of current dentin bonding agents. Its effect on the bond strength of Scot bond 2 was recently presented by White and others (1991). The bond strength increased about 60% compared to the smearlayered dentin presumably by increasing the bondable inorganic fraction of the dentin surface. The laser may create micro mechanical retention, which is an analogue to the effect seen on laser-etched surface. According to Arturo Martinez-Insua et al (2000), tensile bond strength s of bonded brackets obtained after laser etching were significantly lesser than those obtained with acid etching. 111
They also showed that enamel and dentin surfaces prepared with Er-YAG laser etching showed extensive subsurface fissuring that is unfavourable to adhesion. Microabbrasion Modification of dentin by micro abrasion is another emerging technology. Micro abrasion with aluminium oxide removes healthy as well as diseased dentin and results in a smear layer. The abrasion action of aluminium oxide depends on the particle size as well as on the velocity. Particles 0.5Âľm or less in diameter do not affect the enamel except to cleanse it. The 0.5-micron or larger particles create a smear layer on the dentin and increase the surface area. (Blake 1991). This smear layer might be used to enhance the bond strengths of smearmediated dentin bonding agents. Universal conditioners Several laboratories have attempted to produce Conditionersâ&#x20AC;?.
â&#x20AC;&#x153;Universal
That is, conditioners that remove the smear layers
from both enamel and dentin in a single treatment. Most dentin conditioners
(for example, 10-3 solution) do not
adequately etch enamel in 30 seconds. Thus it would save time and simplify the technique, if a single treatment could etch both surfaces.
112
SELF-ETCHING PRIMER Self-etching primers can provide an effective alternative to conventional phosphoric acid etchant in conditioning the enamel surface to secure a durable bonding and marginal seal of composite resin
restorations.
Self-etching
primer
adhesive
systems
are
characterized by the simultaneous demineralization of tooth surfaces and monomer diffusion into tooth substances. Contemporary self-etching primers and the recently introduced all in one adhesive are attractive additions to the clinicians bonding armamentarium. They are user friendly in that the number of steps required in the bonding protocol is reduced. As the smear plugs are not removed prior to the application of these adhesives, the potential for postâ&#x20AC;&#x201C;operative sensitivity that is caused by incomplete resin infiltration of patent dentinal tubules can be substantially reduced. More over, as water is an essential component of these systems to enable ionization of the acidic monomers for demineralization of hard dental tissues, the technique sensitivity associated with variations in the state of hybridization of a demineralized collagen matrix is also eliminated. Due to itâ&#x20AC;&#x2122;s intrinsic acidity, the self â&#x20AC;&#x201C;etching primer dissolves the enamel surface and thereby creates a three dimensional micro retentive surface pattern, while simultaneously promoting monomer infiltration. Depth off enamel demineralization and penetration depth of bonding agents are therefore identical, since both processes run parallel
to
each
other.
As
a
result,
light
curing
of
these
interpenetrated monomers and copolymerisation with the over lying resin bonding agent and composite resin form a continuous bond with the enamel surface capable of resisting the effect of micro leakage. 113
Since the primer is not rinsed after application, but air-dried only, the calcium and phosphate ions that were dissolved from the hydroxyapatite crystals must be suspended in the watery solution of the primer. When the water is evaporated during air-drying, the concentrations of solubilized calcium and phosphate salts within the primer may exceed the solubility product constants for the number of calcium phosphate salts. Presumably, minerals will then precipitate within the primer. These high concentrations of calcium phosphate will tend to limit further dissolution of the apatite due to common ion effects of calcium and phosphate and thereby limit the depth of enamel surface demineralization. On the other hand, it is very likely that the binding of calcium ions to the phosphate residues in the primer molecules contribute to the inactivation of molecules acidity. In addition, evaporation of water during air drying, as well as light curing of the primer and subsequently applied bonding agents, and restrict and inhibit the self etching effect of primer molecules. For adhesion to dentin, all self-etching primers dissolved the smear layer and caused better infiltration of hydrophilic monomer into dentin to create a hybrid layer. Another reason for better marginal sealing may be improvement in the mechanical properties of bonding resins. It has been suggested that improvement of the mechanical strength of the bonding resin leads to an enhancement of bond strength. M.Hanning et al (1999) in their article concluded that self etching primer can provide an effective alternative to conventional Phosphoric acid etchant in conditioning the enamel surface to secure a durable bonding and marginal seal of composite resin restoration, tested on bovine enamel. 114
According to Shahabi et al (1997), the shear bond strengths in the order of 10 Mpa can be obtained reliably on human teeth using laser conditioning with pulsed modes in the absence of any other preparation of the natural enamel surface. Miyasaka K. (1999) showed that high quality hybridization was possible by combining EDTA conditioner and phenyl-P/HEMA selfetching primer. This bonding system was promising for bonding resin to human dentin.
115
EFFECT OF ETCHING ON PRIMARY TEETH Silverstone (1974) found that a 60sec application of an unbuffered solution of 37% phosphoric acid produced the most favorable conditions for bonding. Fukes et al (1984) and Eidelman et al (1984) showed, respectively, that a 20 sec etch provided similar leakage resistance and retention rates when compared to a 60 sec etch. This has very obvious implications for the management of young children when scaling their teeth and therefore upon the success of the sealant itself. Acid etching has two distinct actions on human enamel. Firstly, it removes superficial plaque, debris and a very shallow layer of enamel, including chemically inert enamel crystallites. Secondly, it renders the enamel surface more porous. A honey-combed latticework is produced within the remaining superficial enamel, where enamel tags are left projecting in different planes and at different angles. There is differential demineralization of the prisms because the primary attack occurs on the cores of enamel rods to produce the microspaces. However, this depends on the incident angulation of the enamel rods to the tooth surface. On an average, the etching is about 25Âľm deep in permanent teeth. It was thought the outer prism less layer of primary enamel (Ripa 1966), prevented the penetration of resins in the surface of etched primary enamel (Sheykholeslam an Bunocore 1972). There is no universal agreement that prism less enamel occurs in all surfaces of all primary teeth (Mortimer1970; Silverstone 1970). To obtain a pattern of etching comparable to that found in permanent teeth, Silverstone and Dogan (1976) found it necessary to etch for 120 seconds. However, Redford et al (1986) looked at the effect of different etch times on the sealant bond strength, etch depth and pattern, on 116
primary teeth. They found that the bond strength was no greater at 60 or 120 seconds than at 15 or 30 seconds, but the standard deviation was greater at the two shorter times; the etch depth was not very different at the three shorter times but was five times greater at 120 seconds. They felt unable to recommend an etch time for primary enamel of less than 60 second because of the greater variability of bond strength at 15 and 20 seconds. Our clinical experience is of good results with a 30 second etch.
117
APPLICATIONS OF THE ACID CONDITIONERS Acid etch technique is now widely used for most composite restorations as a means of aiding retention and reducing or preventing micro leakage.
For class-4 cavities the acid etches technique has
replaced the gold inlay as the treatment of choice for restoring the tooth contours and function. Bonding of resins using the acid etch technique has also been used as a means of splinting teeth which have been weakened by cavity preparation. Fissure sealants are now widely used for preventing pit and fissure caries. Resin systems are now used for attaching orthodontic brackets. Composites are gaining in popularity for the attachment of bridges. The principle of the resin-bonded system is that the composite bonds mechanically to the etched enamel of the tooth and also to the surface of the cast alloy framework of the bridge. Another application of the acid etch technique is the attachment of acrylic or porcelain labial veneers in order to improve the appearance of stained, discolored or misshapened teeth. All in all, the accumulating knowledge indicates that since only the surface of the dentin, needs to be modified and not its depth, conditioning techniques that utilize weaker acids, shorter periods of applications,
and
the
elimination
of
rubbing
and
scrubbing
procedures perform quite satisfactory and produce minimal pulpal responses. Rupp (1981) 6% citric acid, 15 seconds; AL-Nahedh and Others (1990).
118
DISADVANTAGES OF ACID CONDITIONERS There are a number of disadvantages to acid etching of dentin. These include: ♦
Increasing dentin permeability
♦
Increasing dentin wetness
♦
Increasing the potential for pulpal irritation by leakage of microbial products
♦
Increased potential for denaturation of collagen and/or reducing the porosity of the demineralized matrix by precipitation of calcium and phosphate ions.
♦
There is also the danger that adhesive resins will not penetrate into the matrix as deeply as the acidic conditioners.
This may
leave an intrinsically weak zone that may give high bond strengths initially, but weaken over several years due to slow hydrolysis of exposed, unprotected collagen.
119
FACTORS TAKEN INTO CONSIDERATION When evaluating whether an etching technique is good or bad, one must remember that subtle changes in technique and methods can cause important changes in results, and that the following factors must be taken into consideration: ♦
Type of acid
♦
Concentration
♦
Time interval of application;
♦
Active (rubbing, scrubbing) or passive (soaking) application.
♦
Cavity preparation or just exposed superficial dentin
♦
Consideration of Remaining Dentin Thickness (RDT)
♦
Presence or absence of sclerotic dentin and reparative dentin
♦
Age of the patient and species and age of experimental dentin
♦
Pulpal responses to subsequent type of restoration .
120
o Condensation of amalgam o Self –cured composite resin, placed under pressure. o Visible – light cured composite resin placed incrementally; and ♦
Pulpal response to a fresh mix of restorative materials or to a cured disc of the material in question placed in a leaching solution to measure the release of H+.
ACID
CONDITIONERS
FOR
GLASS
IONOMER
CEMENTS Glass Ionomer Cements Like most etching procedures, acid etching of dentin before placement of glass - ionomer cements has evolved from relatively harsh treatment (40% polyacrylic acid for 30-60 seconds) to relatively mild treatment (10% Polyacrylic acid for 15 seconds, Powis and Others 1982; Berry, Von der Lehr and Herrin 1987). Few investigators have defined the purpose of what this etching was designed to accomplish other than to increase the bond strength of glass-ionomer cement to dentin. Presumably, they were attempting to remove the loose particles that make up the smear layer from the surface to permit interaction of the glass-ionomer cement
with the
underlying surface. However, as conventional glass-ionomer cement seems to interact with surface calcium (Wilson and Mclean 1988), it 121
would appear that any acid etching that is sufficient to remove the smear layer would deplete the surface of calcium and interfere with subsequent surface reactions. Indeed, careful examination of the literature revels as many reports of acid etching having no effect at increasing glass- ionomer cement bond strength as those that show an increase. SURFACE CONDITIONS Adhesion to dentin is enhanced by surface conditioning with 25% solution of polyacrylic acid for 10 seconds. Glass-ionomer cements, being polar substances, are capable of ion-exchange with the tooth structure, and they have similar thermal expansion to enamel and dentin and also very low shrinkage on setting.
By contrast dentin
bonding agents used with composite resins, although showing higher initial bond strengths, generally deteriorate under stress caused by flexure of the tooth, polymerization shrinkage or hydrolytic in stability (Wilson and McLean 1988).
122
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