I. INTRODUCTION The finishing and polishing of restorative dental materials are important steps in the fabrication of clinically successful restorations. The techniques employed for these procedures are meant not only for removal of excess material but also to smoothen rough surfaces. The finishing of dental restorations prior to their placement in the oral cavity has therefore three benefits. 1. To promote oral hygiene (by resisting the accumulation of food debris and pathogenic bacteria). 2. Enhance oral function (so that food glides through occlusal and embrasure surfaces). 3. To improve esthetics. The materials employed for finishing and polishing of dental restorations are generally termed as abrasives. Thus an understanding of the characteristic features with their properties of these materials and the process of abrasion would aid in improved clinical usage of these materials and techniques.
II. INDIVIDUAL CONSIDERATIONS Abrasion Craig has defined abrasion as : “A process of wear whereby a hard rough surface (like a sand paper disk) or hard irregularly shaped particles grooves in a softer material and cause materials from these grooves to be removed from the surface. It has been stated by Skinner that abrasive wear could be a “two body” or a “three body” process. To understand this more specifically the terms Wear, Abrasive Wear and Erosive Wear need more understanding. a. Wear – is a material removal process that can occur whenever surfaces slide against each other. In dentistry the outer most particle or the surface material of an abrading instrument is referred to as an ABRASIVE. The material being finished is termed as a SUBSTRATE. b. Abrasive wear : this could be of two types : i. Two body wear – occurs when abrasive particles are firmly bonded to the surface of the abrasive instrument and no other abrasive particles are used. Eg. a diamond bur abrading teeth. ii. Three body wear – is when the abrasive particles are free to translate and rotate between two surfaces. Eg. dental prophylaxis pastes (between tooth and rubber cup. 2
c. Erosive wear â€“ This is caused by hard particles impacting a substrate surface, carried by either a stream of air or a stream of liquid. Erosive Wear should be differentiated from chemical erosion more commonly known as acid etching which involves acid and alkalis instead of the hard particles to remove the substrate material. Chemical erosion is employed to prepare teeth surfaces for bonding purposes and not for finishing of materials.
III. DESIRABLE CHARACTERISTICS OF AN ABRASIVE 1. It should be irregular in shape so that it presents a sharp edge. (that is why jagged particles are more effective. Round sand particles and cubicle particles of sand paper are poor abrasives). 2. It should be harder than the work it abrades. If it cannot indent the surface to be abraded then it cannot cut it and therefore wears out. 3. Abrasives should possess high impact strength or body strength. Abrasive points should always fracture than dull out so that always a sharp point or edge is available. The cuts also helps in shredding debris accumulated from work. For eg. a grinding wheel against a metal. 4. Abrasives should possess attrition resistance so that it does not wear.
IV. DESIGN OF ABRASIVE INSTRUMENTS The abrasives could be one of the three types : A. Abrasive Grits Abrasive grits are derived from (abrasive) materials that have been crushed and passed through a series of mesh screens to obtain different particle size ranges. The grits are classified as â€“ coarse, medium coarse, medium fine and super fine according to the particle size ranges.
B. Bonded Abrasives These consists of abrasive particles incorporated with a binder to form grinding tools. The abrasive particles are bonded by 4 methods 1. Sintering. 2. Vitreous bonding (Glass/Ceramic). 3. Resin bonding (usually phenolic resin). 4. Rubber bonding (usually silicone rubber). 1. Sintering – Sintered abrasives are the strongest variety since the abrasive particles are fused together. 2. Vitreous Bonded – In these the particles are mixed with a glassy or ceramic matrix material, cold pressed to the instrument shape and fired to fuse with the binder. 3. Resin Bonded – (Consists of phenolic resin) which are either cold or hot pressed and then heated to cure the resin. 4. Rubber Bonded – Consists of silicon rubber and is made in a manner similar to resin bonded abrasives. As far as bonded abrasives are concerned, the type of bonding affects the grinding behaviour of the tool on the substrate. Bonded abrasives that tend to disintegrate rapidly on the substrate are weak and result in increased abrasive costs because of reduced instrument life. 5
Bonded abrasives that disintegrate very slowly tend to clog with the grinding debris which results in loss of abrasive efficiency, increased heat generation and increased finishing time. Thus an ideal binder is one which holds the abrasive particles in the tool sufficiently long enough to cut, grind or polish the substrate, yet release the particle, either before its cutting efficiency is lost or before heat build up cause thermal damage to the substrate. A bonded abrasive should always be TRUED and DRESSED before its use. Truing – is a procedure through which the abrasive instrument is run against a harder abrasive block until the abrasive instrument rotates in the handpiece without eccentricity when placed on the substrate. Dressing – like truing is used to shape the instrument but accomplishes two different purposes. First – the dressing procedure reduces the instrument to its correct working size and shape. Second – it is used to remove clogged debris (also called abrasive blinding) from the abrasive instrument to restore grinding efficiency during the finishing operation. Why diamond is used as a dressing tool ? Because diamond is the hardest substance known and therefore strong binders are required such as resin 6
and additional retention is necessary for which nickel plating is done (additional advantage of nickel plating is to absorb generated heat). C. Coated Abrasive Disks and Strips These abrasives are supplied as disks and finishing strips. They are fabricated by securing abrasive particles to a flexible backing material (heavy weight paper or mylar). These disks are available in different diameters with thin and very thin backings. V. ABRASIVE ACTION The mode of action of the abrasives is similar to that of the dental burs that is it is merely a cutting action. Each fine abrasive particle thus presents as a sharp edge which cuts through the surface similar to a pointed chisel. During this cutting process the shavings thus formed is powdered and usually clogs the abrasive thus making periodic cleaning of the abrasive necessary. A. Abrasive Action on Metals On abrading metallic restorations, the metallic grain structure usually gets disoriented. The more the abrasion, the greater is the associated disorientation. Strain hardening of the abraded metal usually accompanies the grain disorientation that is the metal becomes stronger, harder, and less ductile when greater stresses are induced.
In the abrading of metals, the crystalline structure of the surface is disturbed, sometimes to a depth of 10µm. However this surface effect varies with different metals, for eg. in ductile metals (like gold) less of the surface metal may be removed by the abrasive than in a brittle metal. B. Abrasive Action on Resins The surface disturbance produced by abrasives on resins however leads to the creation of surface stresses that may cause distortion if the abrasion is too rigorous. The heat generated during this abrasion partially relieves the stresses.
VI. FACTORS AFFECTING RATE OF ABRASION Rate of abrasion is determined primarily by three factors: 1. Size of the abrasive particles – Larger the size – greater the abrasion. 2. Pressure of the abrasive against the substrate – When an abrasive is pressed against the substrate, scratches are deeper and abrasion is more rapid – so greater chances of the abrasives to fracture. 3. Speed at which the abrasive particles travels across the work – Greater the speed, greater would be the rate of abrasion. Speed employed is of two types: -
Rotational speed and
Linear speed i.e. speed with which particles pass over the substrate.
Linear speed is related to rotational speed as follows: V = Îł d n where
V = Linear speed d = diameter of the abrasive n = revolutions per minute
ABRASIVE These factors are as follows: 1. Hardness of the abrasive particles (diamond is hardest, pumice and garnet are relatively mild). 2. Shape of the abrasive particle (particles with sharp edges are more effective). 3. Particle size of the abrasive (longer particle size will cut deeper grooves). 4. Mechanical properties of the abrasive (if the material breaks, it should form a new cutting edge. Therefore brittleness can be an advantage). 5. Rate of movement of the abrasive particles (slower abrasion â€“ deeper scratches. 6. Pressure applied to the abrasive (too much pressure can fracture the abrasive instrument and increase heat of friction that has evolved).
7. Property of material being, abraded (brittle material can be rapidly abraded whereas ductile / malleable metal like gold will flow instead of being removed).
VIII. TYPES OF ABRASIVES I.
According to Craig : The abrasives used can be classified as :
A. Finishing Abrasives These are hard, coarse abrasives used initially to develop desired contours and remove off gross irregularities. B. Polishing Abrasives These are smaller particle size and are less hard than abrasives used for finishing. They are used for smoothening surfaces that are roughened by finishing abrasives. C. Cleaning Abrasives These are soft abrasives with small particle size and are intended to remove softer materials that adhere to enamel or a restoration. These dental abrasives could be employed in the form of three basic designs as mentioned previously.
Skinners has grouped the abrasives employed in dentistry as : A. Natural Abrasives B. Manufactured Abrasives Manufactured Abrasives are generally preferred because of their more predictable physical properties. A. Natural Abrasives : 1. Arkansas stone -
Semi translucent, light gray, siliceous sedimentary rock, mined in Arkansas .
It contains microcrystalline quartz.
Small pieces of this mineral is attached to metal and trued to various shapes for fine grinding of tooth enamel and metal alloys.
2. Chalk -
It is the mineral form of calcite.
Composed of calcium carbonate.
It is used as mild abrasive paste to polish teeth enamel, gold foil, amalgam and plastic materials.
3. Corundum (discovered in 1825) -
It is the mineral form of aluminum oxide and is usually white. 11
It is primarily used for grinding metal alloys.
Available as bonded abrasives in several shapes.
Commonly used in an instrument known as white stone.
4. Diamond - It is a transparent colourless mineral composed of carbon called super abrasive because of its ability to abrade any other known substance. -
It is the hardest substance known.
Used on ceramic and resin composite materials.
Supplied as a. Bonded abrasives rotary instrument. b. Flexible metal backed abrasive strips. c. Diamond polishing pastes. 5. Emery -
This abrasive is a grayish black corundum that is prepared in a fine grain form.
Supplied predominantly as coated abrasive disks.
Used for finishing metal alloys or plastic materials.
6. Garnet (glass like gem stone) 12
These minerals are silicates of aluminum, cobalt, iron, magnesium and manganese.
The garnet abrasive used in dentistry is usually dark red.
If garnet defines during grinding it forms sharp chisel shaped plates making it a highly effective abrasive.
Used in grinding metal alloys and plastic materials.
7. Pumice -
It is produced from volcanic activity.
Used in polishing teeth enamel, gold foil, dental amalgam and acrylic resins.
Flour of pumice â€“ extremely fine grounded volcanic rock derivative from Italy.
It is the most abundant.
8. Quartz -
Hard colourless and transparent.
Crystalline particles are pulverized to form sharp angular particles that are useful in making coated abrasive disks.
They are used mostly to finish metal alloys and may be used to grind dental enamel. 13
9. Sand (Oldest abrasive 25000 B.C.) -
It is a mixture of small mineral particles predominantly silica.
Particles have rounded to angular shape. a. Applied under air pressure to remove refractory investment materials (sand blasting). b. It is coated on paper disks for grinding of metal alloys and plastic materials.
10. Tripoli Derived from lightweight, friable siliceous sedimentary rock present in
Tripoli in North Africa. -
Can be white, gray, pink, red or yellow.
Gray and red types most frequently used in dentistry.
Rock ground to fine particles sizes and is used for polishing metal alloys and plastic materials.
11. Zirconium silicate -
It is an off white mineral ground to various sizes used to make coated disks and strips.
Also used as a component of dental prophylaxis paste.
12. Cuttle – Made from the pulverized internal shell of a Mediterranean white mollusk from the genus sepia, this abrasive is obtained as a white calcareous powder. – Relative synonyms of this abrasive are cuttle fish and cuttle bone. – Obtained as a coated abrasive and used for delicate operations like polishing of metal margins and dental amalgam restorations. 13. Kieselguhr -
Abrasive is obtained from siliceous remains of minute aquatic plants known as diatoms.
It is an excellent mild abrasive.
Coarser variety is known as Diatomaceous earth used as a filler for dental materials such as hydrocolloid impression materials.
Precautions must be taken while handling this abrasive as there is risk of respiratory silicosis due to long term exposure of this material.
B. Manufactured Abrasives 1. Silicon Carbide – (developed by Acheson 1891) -
This is the first of the synthetic abrasives to be developed.
Two types were manufactured i.e. i) green form and ii) blue black form both having similar physical properties.
The green variety is preferred because substrates are more visible against the green colour.
The cutting efficiency of silicon carbide abrasives is higher as the particles are sharp and break to form new sharp particles.
It is supplied as an abrasive in coated disks and in vitreous and rubber bonded instruments.
Used in cutting metal alloys, ceramics and plastic mateials.
2. Aluminium Oxide -
This is the second synthetic abrasive to be manufactured after silicon carbide.
This form of alumina can be much harder than its normal counter part that is corundum because of its purity.
The forms usually prepared are: i.
White stones â€“ made of sintered aluminium oxide are used for contouring enamel and finishing metal and ceramic materials.
Variations of aluminium oxide include those with chromium compound additions. Those obtained in pink and ruby colours are present as vitreous bonded non-contaminating mounted stones â€“ 16
used for preparations of metal ceramic alloys to receive porcelain. (It is important to note that remnants of these materials should not interfere with porcelain bonding to metal. Studies by Yamamoto in 1983 therefore show that carbides are better for this purpose). 3. Synthetic diamond â€“ (developed in 1955). -
Synthetic or manufactured form of diamond is produced at 5 times the level of natural diamond abrasive.
This synthetic diamond is used for the manufacture of diamond saws, wheels and burs.
Blocks with embedded diamond particles are used to true other types of bonded abrasives.
Synthetic diamonds are used primarily on tooth structure, ceramic materials and resin based composite materials.
Iron oxide is the fine red, abrasive component of rouge.
It is blended like tripoli, with various soft binders into a cake form.
It is used to polish high noble metal alloys.
May be impregnated in paper or fabric known as Crocus cloth. Tin Oxide 17
Extremely fine abrasive is used extensively as a polishing agent for polishing teeth and metallic restorations in the mouth.
It is mixed with water, alcohol or glycerin to form a mildly abrasive paste.
IX. POLISHING â€œPolishing is the production of a smooth mirror like surface without the use of any external filmâ€?. A number of reasons are elucidated for the importance of polishing dental restorations and teeth. These are: i.
The smooth polished restorative and teeth (enamel) surfaces are less receptive to bacterial colonization and dental plaque formation.
ii. The polished layer on metallic restoration aids in the prevention of tarnish and corrosion. iii. Finally, from the clinical point of view, polished restoration with rough surfaces are uncomfortable for the patient. The polishing procedure is one which is eventually initiated once the abrasive mechanisms eliminates or obliterates most of the finer scratches leaving a smooth finish. The smooth layer or smooth finish on the surface of the restoration is referred to by Skinner as a polish.
However it is difficult to draw out an exact line of differentiation between an abrasive and a polishing agent. For example, an agent which has a large particle size and that which produces deep or large scratches on the restoration surface could be termed as an abrasive. On the other hand a similar agent with a fine particle size which produces a smooth finish could be termed as a polishing agent. Surface structure not more than 0.005 mm is removed during polishing. It is therefore observed that the process of finishing, cutting / abrading and polishing have not been differentiated well in dentistry. The term ‘finish’ would actually relate to the final surface achieved or the final character of the surface of the material after the finishing procedure. If this explanation for the term finish is accepted then others i.e. cutting / abrading, grinding and polishing would be considered as a series of steps encompassed within the process of finishing. However minor differences exist in the cutting, grinding and polishing procedures. 1. A cutting operation as cited by Skinners “refers to the use of a bladed instrument or any instrument in a blade like fashion”. The substrate may sustain deep notches and grooves by the cutting operation. 2. A grinding operation removes small particles of a substrate through the action of bonded or coated abrasive instruments. The grinding 19
instruments may contain randomly arranged abrasive particles eg: a diamond coated rotary instrument. It is important to note that cutting and grinding are both Unidirectional in their course of action. 3. The polishing operation is one of the most refined finishing process. It produces scratches so fine that they are visible only when greatly magnified. The ideally polished surface is one which would be smooth with no surface imperfections. Polishing is considered to be Multidirectional in its course of action. Eg. of polishing instruments are : a. Rubber abrasive points. b. Fine particle disks and strips. c. Fine particle polishing pastes â€“ applied with soft felt points, muslin wheels, prophy cups or buffing wheels. d. Electrolytic polyshing (Cobalt â€“ Chromium Alloys).
X. HAZARDS ENCOUNTERED IN THE FINISHING AND POLISHING PROCEDURES. Aerosols, the dispersion of solid and liquid particles in air are generated whenever finishing and polishing operations are performed.
The dental aerosols contain tooth structures dental materials and microorganisms. These aerosols are potential sources of infections and cause chronic diseases of the eyes and lungs. Silicosis or grinders disease is a major aerosol hazard in dentistry because, a number of silica based materials are used in processing and finishing of dental restorations. These aerosols can remain for more than 24 hours before settling and are therefore capable of contaminating other zones of the treatment facility. Aerosols produced during finishing procedures can be controlled by three ways: 1. Controlled at the source through a. Adequate infection control procedures. b. Water spray. c. High volume suction. 2. Personal protection a. Safety glasses. b. Disposable face masks. 3. Adequate ventilation
XI. FINISHING AND POLISHING OF COMMONLY EMPLOYED RESTORATIONS. 1. Amalgam -
Burnishing of newly condensed amalgam is carried out with a metal having broad surface contact to make the surface ready susceptible to finishing and polishing procedures.
Burnishing can produce a tenfold reduction in surface roughness.
After initial carving, restoration is left undisturbed for an appropriate period (usually 24 hrs to obtain maximum strength) before finishing and polishing is initiated.
Polishing is done through the sequential use of finishing fine burs, stones and disks or strips.
Final polishing is done by the application of extrafine silex followed by a slurry of tin oxide with a rotating soft brush.
During final polishing, restorations should be kept moist to avoid overheating.
In case of high copper amalgam with high early strengths, finishing can be done at the first appointment.
Polishing is started 8-10 minutes after the start of trituration but care must be taken to use light pressure (of no more than 30 seconds per surface) 22
with triple x silex creamy paste and water and unwebbed polishing cup using slow handpiece. A three year study has showed no difference between the 8 min. and 24 hour polishing technique. Main advantage of 8min. polishing technique is the elimination of a second appointment. (Buffing refers to the process that is controlled by the use of abrasives carried via bristle brushes, treated leather and cloth materials). 2. For Acrylic Jacket Crowns 1. Initial trimming is done with tungsten carbide burs or alpine stones followed by Arkansas stone. 2. Finishing of the restoration is done using sand paper disks to remove irregularities on the surface. 3. Buffing is done with wet pumice and buffing wheel coated with tripoli. Pumice is made wet to prevent distortion of the resin by heat generation. 4. Dry buffing is done using a buffing wheel made of cloth material and a mild abrasive such as chalk is used. 3. For Cast Metal Restorations 1. The internal margin is inspected under magnification and adjusted as necessary with small diamond stones and carbide burs. Adjustments are restricted to areas where binding contact occurs.
2. The sprue is removed by abrasive (carborundum) discs. Discs and stones are used for gross recontouring at the attachment area. 3. Initially a wire brush is used on the occlusal surfaces. 4. A fine grit sand paper disc is applied for removing pits and irregularities from the axial walls. 5. Rubber points and small carbides are used for selective finishing of occlusal morphology. 6. Final polishing of the surface are done using rouge on a brush. 7. The polished restoration is then cleaned using an ultrasonic cleaner.
REVIEW OF LITERATURE 4. For Composite Restorations : A. Rotary instrument finishing of micro filled and small particle composite resins. JADA. Aug 1987. -
This study suggests that rotary instrument for finishing composites must be selected in accordance with the type of composites used.
Tungsten Carbide Burs at high speeds for trimming and finishing microfilled composites are contraindicated because they disrupt, the composite resin surface therefore for microfilled and small particle resins diamond burs at slow speeds are used.
Carbide burs at high speeds on small particle composites produces a surface free from the characteristic striations and grooves produced by diamond burs.
B. Effect of three finishing systems on four aesthetic restorative materialsâ€?. Operative Dentistry â€“ 1998. Two varieties of composites (Hybrid and Microfilled) and two types of GIC viz. Traditional / conventional GIC and a Resin modified GIC were employed for this study. -
Impregnated disks and diamond and carbide burs were used.
No difference was seen in the surface of any of the restorations. When the mylar matrix were used.
However the study concluded with the result that abrasive impregnated disks and aluminium oxide disks provided smoother finished surfaces on the GIC and composite than did the diamond and carbide finishing burs.
C. A quantitative study of finishing and polishing techniques for a composite”. Journal of Prosthetic Dent. 1988. As we know, finishing and polishing of composite has always been a problem because the resin matrix and inorganic fillers differ in hardness and do not abrade uniformly. This study evaluated and compared six finishing and polishing techniques to identify the most effective one for micro filled composite restorations. These six techniques were as follows : TECHNIQUE I
Use of disks of medium, fine and superfine grits in descending order.
Use of polishing points.
Use of polishing paste with rubber cup.
Finishing (40 m) and polishing (15 m) diamond burs were used. 26
TECHNIQUE V-IV Followed by polishing points. TECHNIQUE VI-IV Followed by polishing pastes. Conclusion of the Study Technique I produced the smoothest polished surface. Technique V was second best. For Glass Ionomer Cements “Finishing glass polyalkenoate cements (GIC)”
M.J. Woodfords – BDJ
(1988). This article reports on the SEM surface examination of GIC after finishing procedures using rotary and hand instrumentation. Anhydrous water hardening type and encapsulated type of GIC’s were employed. -
Finishing procedures employed were : 1. White stones and Vaseline in the slow hand piece at 5000 rpm. 2. Soflex disks (Al2O3 disks) and Vaseline in the slow hand piece at 5000 rpm. 3. Tungsten carbide burs in air turbine hand piece with a water spray as coolant.
Results : Ideal surface is produced by the matrix [any finishing inevitably disrupts this surface]. -
Soflex discs produce a relatively smooth surface.
Tungsten Carbide disrupts the surface of even mature GIC.
Hand instruments cause marginal breakdown.
XII. DENTIFRICES The functions of a dentifrice may extend beyond mere cosmetic objectives and include the incorporation of additives/ ingredients which would provide therapeutic benefits specially flourides. Dentifrices are made in three forms i.e. pastes, gels, and powders. They provide three important functions i.e. i. Efficient removal of plaque, debris and stained pellicle. ii. Polish teeth to increase the light reflectance and thus improve aesthetics. The high polish also aids in preventing the accumulation of microorganisms and stains. iii. They act as vehicles for delivery of therapeutic agents with known benefits.
Removal of Plaque / stain, polish tooth surface
Dibasic calcium phosphate dihydrate Hydrated alumina Hydrated silica Sodium bicarbonate Mixtures of listed abrasives Detergent
Sodium lauryl sulfate
Aids debris removal
Oils of spearmint, peppermint, Flavor wintergreen or cinnamon
Maintains moisture content
Thickener, prevents liquidsolid separation
Sodium Dental caries prevention monofluorophoshpate, sodium fluoride, stannous fluoride
Tartar control agents
Disodium pyrophosphate, tetrasodium pyrophosphate, tetrapotassium pyrophosphate
Inhibits the formation of calculus above the gingival margin.
Potassium nitrate, strontium chloride
Promotes occlusion of dentinal tubules
XIII SUMMARY & CONCLUSION Though a varied range of abrasive and polishing agents have been described with relation to individual dental materials, an ideal abrasive or a polishing agent which would satisfy all polishing needs of the dental materials, one would say is yet to be developed.
REFERENCES 1. Kenneth J. Anusavice “Philips Science of Dental Materials”. 10 th edition, 1998 ; W.B. Saunders Publications. 2. Ralph W. Phiips “Skinner’s Science of Dental Materials”. 9 th edition, 1992 ; W.B. Saunders Publications. 3. Combe E.C. “Notes on Dental Materials”. 6th edition, Churchill, Livingstone Publications. 4. Subbarao V. “Notes on Dental Materials”. 3 rd edition, 1997 ; V.K.S. Publications. 5. Robert G. Craig, William O’Brien and John M. Powers “Dental Materials – Properties and Manipulation”. 5th edition, 1992; Mosby Publications. 6. Stephen F. Rosenstiel “Contemporary Fixed Prosthodontics”. 2nd edition, 1997 ; Mosby Publications. 7. Marzouk M.A. “ Operative Dentistry – Modern Theory and Practice”. 1 st edition, 1997 ; Ishiyaku Euro. America. Inc.
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