ABRASION AND POLISHING AGENTS 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 viz. A) To promote oral hygiene – by resisting the accumulation of food debris and pathogenic bacteria. B) Enhance oral function – a well polished surface aids in gliding movement of the feed over occlusal surfaces and between embrassure spaces (smooth restoration contacts minimise wear rates on opposing and adjacent teeth. E.g. materials like ceramics which contain phases harder than the teeth enamel and dentine). C) To improve esthetics. The materials employed for finishing and polishing of dental restorations are generally termed as Abrasives. Thus an understanding of the characteristics / 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: 1. Abrasion Craig has defined abrasion as : “A process of wear whereby a hard rough surface (like a sand paper disk) or hard irregular shaped particles (like those in a abrasive slurry) plough grooves in a softer material and cause materials from these grooves to be removed from the surface”. It has been stated by Skinner’s that abrasive wear could be a “two body” or a “three body” process.
To understand this more specifically the terms viz. Wear, Abrasive wear and Erosive wear need more understanding. I) Wear – is a material removal process that can occur whenever surfaces slide against each other. In dentistry the outermost particle or the surface material of an abrading instrument is referred to as Abrasive. The material being finished is termed as a SUBSTRATE. II)
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 and abrasive particles are used eg, a diamond bur abrading a teeth. ii. Three body wear – when the abrasive particles are free to translate and rotate between two surfaces Eg, dental prophylaxis pastes (between tooth and rubber cup). iii. Erosive wear – 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 hand particles to remove the substrate material.
Chemical erosion is employed to prepare teeth surfaces for bonding purposes and not for finishing of materials. DESIRABLE CHARACTERISTICS OF AN ABRASIVE 1. It should be irregular in shape so that it presents a sharp edge. (- 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. Abrasive should posses a high impact strength or body strength. Abrasive point 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. Abrasive should posses attrition resistance so that it does not wear. DESIGN OF ABRASIVE INSTRUMENTS The abrasives employed could be one of the three types, viz: A. Abrasive Grits. B. Bonded Abrasives. C. Coated Abrasive Disks and Strips. A. Abrasive Grits Abrasive grits are derived from (abrasive) materials that have been crushed and passed through series of mesh screens to obtain different particle size ranges. The grits are classified as – COARSE, MEDIUM COARSE, MEDIUM FINE and SUPER FINE according to the particle size ranges. B. Bonded Abrasives These consists of abrasive particles incorporated through a binder to form grinding tools. The abrasive particles are bonded by 4 general 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 – 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 – are cold or hot pressed and then heated to cure the resin.
4. Rubber Bonded – made in a manner which is similar to resin bonded. As far as bonded abrasives are concerned the type of bonding employed affects greatly 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. Bonded abrasives that disintegrate very slowly tend to clog with the grinding debris and result in loss of abrasive efficiency.Thus an ideal binder is one which would hold the abrasive to the instrument sufficiently long as to cut, grind or polish the substrate, yet release the particle before its cutting efficiency is lost or before heat build up causes thermal damage to the substrate.The type of binder is also intimately related to the life of the tool in use. In most cases the binder is impregnated throughout within an abrasive so that as an abrasive particle is wrenched during use another abrasive particle takes its place as the binder wears e.g. for diamond rotary instrument electroplated with a nickel base matrix is used as a binder.The heat generated during abrasive activity is dependent on the efficiency of the abrasives. Cooling however may be required for e.g. in abrading polymeric materials, excessive heat should be avoided as it can cause stress relief and warpage (E.C. Combe).It is important that a bonded abrasive should always be TRUED and DRESSED before its use. Truing –procedure where bonded abrasive is first run against a harder abrasive block until it rotates in the handpieces without eccentricity or run out when placed on the substrate. Therefore TRUING is done for proper shaping of the bonded abrasive. Dressing – like Truing is done for proper shape of the abrasive and also for two different purposes. 1. Dressing procedure reduces the instrument to its correct working size and shape. 2. Used to remove clogged debris (ABRASIVE BLINDING) from the abrasive instrument to restore working efficiency. 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). The disks are available in different diameters with thin and very thin backings. Moisture – resistant backings are advantageous as the abrasive stiffness is not reduced by water degradation. 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 shaving thus formed is powdered and usually clogs the abrasive which thus makes periodic cleaning of the abrasive necessary. 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. (Strain hardening is a process wherein the metal becomes stronger, harder and less ductile when greater stresses are induced at the grain boundaries to provide slipping of the lattice network).In the abrading of metals, the crystalline structure of the surface is disturbed, sometimes to a depth of 10m. However this surface affect 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. Abrasive Action on ResinsThe surface disturbance produced by abrasion 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.If this heat generated is excessive it may even relieve stresses created during processing which leads to warpage of the resin along with melting of the surface of the resin. The heat generated is directly proportional to the efficiency of the abrasives. FACTORS AFFECTING RATE OF ABRASION Rate of abrasion of a given material by a given abrasive is determined primarily by three factors: 1. Size of the abrasive particle – larger the size – greater the abrasion. 2. Pressure of the work against the abrasive when work is pressed against the abrasive 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: -
Linear speed speed with which particles pass over the work.
Linear speed required is 5000ft/min.
Linear speed is related to rotational speed as follows: V = ď d n where V = Linear speed d = diameter of the abrasive n = revolutions per minute FACTORS INFLUENCING EFFICIENCY OF THE ABRASIVESThese factors are as follows: 1. The hardness of the abrasive particle (diamond is hardest, pumice and garnet etc., are relatively mild). 2. The Shape of the abrasive particle (particles with sharp edge 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. Properties of material that is being abraded. (A brittle material can be rapidly abraded whereas ductile / malleable metal like pure gold will flow instead of being removed by the abrasive). TYPES OF ABRASIVES I.
According to Craig : The abrasives used can be classified grouped as and : A. Finishing Abrasives. B. Polishing Abrasives. C. Cleaning Abrasives.
A. Finishing Abrasives These are hard, coarse abrasives used initially to develop desired contours and remove off gross irregularities.
B. Polishing Abrasives These have a smaller particle size and are less hard than abrasives used for finishing. They are used for smoothening surfaces that are typically roughened by finishing abrasives. A. Cleansing 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 3 basic designs as mentioned previously. Secondly Skinnerâ€™s has grouped the abrasives employed in dentistry as follows: A. Natural Abrasives. B. Manufactured Abrasives. A. Natural Abrasives
B. Manufactured Abrasives
1. Arkansas Stone 2. Chalk
Are generally preferred because of their more predictable physical properties.
1. Silicon carbide
2. Aluminium oxide
3. Synthetic diamond
5. Tin oxide
8. Quartz 9. Sand 10. Tripoli 11. Zirconium silicate 12. Cuttle 13. Kleselguhr
Under Natural Abrasives we have: 1. Arkansas stone -
Semitranslucent, light gray, siliceous sedimentary rock, mined in Arkansas.
It contains microcrystalline quartz.
Small pieces of this mineral is attached to metal shanks and trued to various shapes for fine grinding of tooth enamel and metal alloys.
2. Chalk -
Mineral form of calcite.
Contains calcium carbonate.
Used as mild abrasive paste to polish teeth enamel, gold foil, amalgam and plastic materials.
3. Corundum â€“ is largely replaced by ď â€“ Aluminium oxide due to its superior physical properties. -
However corundum is primarily used for grinding metal alloys and is available as a bonded abrasive.
4. Diamond 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 based composite materials.
Supplied as: a. Bonded abrasive 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 â€“ the term garnet includes several minerals which possess similar physical properties like Silicates of Al, Co, Fe, Mg and Mn. -
Garnet is dark red, extremely hard and when fractured during abrasive activity forms sharp chisel shaped plates â€“ therefore making Garnet an effective abrasive.
Garnet is available on coated disks and Arbor box.
Used in grinding metal alloys and plastic materials.
7. Pumice -
Is produced from volcanic activity.
Flour of pumice is an extremely fine grinded volcanic rock derivative from Italy.
Used in polishing teeth enamel, gold foil, dental amalgam and acrylic resins.
8. Quartz - the particles are pulverized to form sharp angular particles which are useful in making coated disks. -
Used to finish metal alloys and may be used to grind dental enamel.
9. Sand -
Is a mixture of small mineral particles predominantly silica.
Particles have rounded to angular shape.
Used to remove refractory investment material from base metal castings.
It is coated on paper disks for grinding of metal alloys and plastic materials.
10. Tripoli -
Derived from a lightweight, siliceous sedimentary rock
Could be white, gray, pink, red or yellow.
Gray and red are most frequently used.
Used for polishing metal alloys and some plastic materials.
11. Zirconium silicate Off white mineral, ground to various sizes used to make coated disks and strips.
Also used as a component of dental prophylaxis pastes.
12. Cuttle â€“ Made from the pulverized internal shell of a Mediterranean marine mollusk, this abrasive is obtained as a white calcareous powder. â€“ The related synonyms of this abrasive are cuttle fish and cuttle bone. â€“ It is obtained as a coated abrasive and used for delicate operations like polishing of metal margins and dental amalgam restorations. 13. Kleselguhr -
This abrasive is obtained from the silaceous remains of minute aquatic plants known as diatoms.
It is an excellent mild abrasive.
The coarser variety is known as DIATOMACEOUS EARTH used as a filler for dental materials such as hydrocolloid impression materials.
Appropriate precautions must be taken while handling this abrasive as there is risk of respiratory silicosis due to long term exposure of this material is significant.
Manufactured Abrasives 1. Silicon Carbide -
This is the first of the synthetic abrasive to be developed.
Two types were manufactured i.e. 1. green form and 2. Blue black form. Both are having similar physical properties.
However, 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.
Supplied as air abrasive in coated disks and vitreous and rubber bonded instruments. The silicon carbide is
Used in cutting metal alloys, ceramics and plastic materials.
2. Aluminium Oxide -
This is the second synthetic abrasive to be manufactured.
This form of alumina is much harder than its natural counterpart (CORUNDUM) because of its purity.
The forms usually prepared are: i.
White stones â€“ made of sintered aluminium oxide are used for contouring of enamel and finishing metal and ceramic materials.
Variations of aluminium oxide include those with chromium compound additions these obtained in pink and ruby colours are obtained as vitreous bonded non-contaminating mounted stones â€“ used for preparation of metal ceramic alloys to receive porcelain.
(Important to note that remnants of these abrasives should not interfere with porcelain bonding to the metal studies by Yamamoto 1987. 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 the natural diamond abrasive.
This synthetic diamond is used for the manufacture of diamond saws, wheels and burs and also diamond locks employed for truing of bonded abrasives.
Synthetic diamond abrasives are used primarily on tooth structure, ceramic materials and resin based components.
Principle component is iron oxide blended with various binders.
Used to polish high noble metal alloys.
May be impregnated in paper or fabric known as CROCUS CLOTH.
Tin Oxide -
Is composed of very fine abrasive particles.
May be employed in an abrasive paste form along with water, alcohol or glycerine.
Used as a polishing agent for teeth and metallic restorations.
POLISHING â€œPolishing is the production of a smooth mirror like surface without use of any external formâ€?.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.
Secondary, the polished layer on metallic restoration aids in the prevention of tarnish and corrosion.
iii. Finally, from the clinical view point unpolished restoration with rough surfaces are uncomfortable for the patient. The polishing procedure is one which is eventually initiated once the abrasive mechanism eliminates or obliterates most of the fine scratches leaving a smooth finish. The smooth layer or smooth finish on the surface of the restoration is referred to by Skinnerâ€™s as a polish. However it is difficult to draw or markout exact line of differentiation between an abrasive and a polishing agent. For example, given 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. The polishing materials also differ in the amount of material they remove from the surface. They remove molecule by molecule and thus provided smooth surface. In the process fine scratches and irregularities are filled in by powdered particulate being removed from the surface. The polished layer is therefore thought of as one made up minute crystals, thus known as the micro-crystalline layer of Beilby layer (named after a scientist who first noted such a surface layer after polishing which is easily kept clean and has a high corrosion resistance. 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 / applied to a material or the final character of the surface of the material.If this explanation for the term finish is accepted then others viz. 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 Skinnerâ€™s â€œrefers to the use of a bladed instrument or any instrument in a blade like fashionâ€?. The substrate of the cutting operation be divided into large separate pieces or 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 instruments may contain randomly arranged abrasive particles eg: a diamond coated rotary instrument. It is important to note here the 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 automatically smooth with no surface imperfections. Polishing is considered to be Multi Directional 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 – applied with soft felt points, muslin wheels, prophy cups or buffing wheels. d. Electrolytic polymers (Co – Cr – Alloys). -
is a reverse of electroplating.
Alloy to be polished is made the anode of the electrolytic cell.
As current passes ionic material from anode is dissolved leaving a bright surface.
Excellent method for Co-Cr alloys used in denture bases.
Polishing Procedure (for resins) The steps involved are: i.
Smoothen the work with a coarse abrasive to produce large scratches.
Remove large scratches with a finer abrasive but difference in fineness should not be too great as it causes streaking in the final surface. Keep changing direction of the abrasion so that new scratches appears at right angles to the coarser, scratches, to ensure uniform abrasion.
iii. Continue using the fine abrasive until scratches are no longer visible to the eye then accomplish: •
Primary polishing with pumice flour with a canvas buff wheel (mainly for resins).
Then the work is cleaned to remove all abrasive particles and then pumice and water paste of a muddy consistency is applied and buffing done till a bright and well polished surface is obtained. Glycerine is preferred instead of H 2O because water could evaporate, glycerine maintains consistency.
Clean the work with soap water and change to a lannel (rotate at high speed) buff wheel. Polishing cake with grease is held against buff wheel to impregnate with the agent.
Apply light pressure to avoid excessive heat generation. Care should be taken to avoid use of abrasive tool or slurry in a dry condition which would reduce efficiency of abrasive and also increased heat.
Speed employed for Polishing: -
is more than that for abrasion.
Linear speed of range 7500-10000 ft/min.
HAZARDS ENCOUNTERED IN THE FINISHING AND POLISHING PROCEDURES Aerosol Hazard: Aerosols, the dispersion of solid and liquid particles in air are generated whenever finishing and polishing operations are performed. The dental microorganisms.
These aerosols are potential sources of infections and chronic diseases of the eyes and lungs. Silicosis or grinderâ€™s disease is a major aerosol hazard in dentistry because of a number of silica based materials are used in processing and finishing dental restorations. These aerosols can remain for more than 24 hours before settling and are therefore capable of cross contamination other zones of the treatment facility. Aerosols produced during the finishing procedures can be controlled by 3 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 of the entire unit which efficiently removes any residual particulates from the air.
FINISHING AND POLISHING OF COMMONLY EMPLOYED RESTORATIVES 1. Amalgam -
After initial carving, restoration is left undistributed for an appropriate period (usually one day to obtain maximum strength) before finishing and polishing is initiated.
Polishing is done through the sequential use of finishing fine stone and disks or strips.
Final polish is done by the application of extra fine silen; followed by a thin slurry of tin oxide with a rotating soft brush.
During final polishing restoration should be kept moist to avoid overheating.
2. Composites: -
The smoothest surface on a freshly inserted composite can be obtained by allowing polymerization to occur against an inserted Mylar matrix.
Use of green/carbide stones 12 - blade carbide burs is also accepted for removal of excess near enamel margins of macro filled resins.
This is followed by use of: -
Aluminium oxide disks – for accessible areas finishing.
White source stones of suitable shapes – inaccessible areas.
Fine and micro fine diamonds – finishing of micro filled resins.
Detergent Coolants Flavoring Humecant Water Binder Fluoride
Tartar control agents Desensitization agents
Calcium carbonate Dibasic calcium phosphate dihydrate Hydrated alumina Hydrated silica Sodium bicarbonate Mixtures of listed abrasives Sodium lauryl sulfate Food colorants Oils of spearmint, peppermint, wintergreen or cinnamon Sorbitol, glycerine Deionized water Carrageenan Sodium monofluorophoshpate, sodium fluoride, stannous fluoride Disodium pyrophosphate, tetrasodium pyrophosphate, tetrapotassium pyrophosphate Potassium nitrate, strontium chloride
Removal of Plaque / stain, polish tooth surface
Aids debris removal Appearance Flavor Maintains moisture content Suspension agent Thickener, prevents liquidsolid separation Dental caries prevention
Inhibits the formation of calculus above the gingival margin. Promotes occlusion of dentinal tubules
REVIEW OF LITERATURE Rotary instrument finishing of micro filled and small particle composite resinsâ€?. 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.
“Effect of three finishing systems on four aesthetic restorative materials”. Operative Dentistry – 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.
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 cap.
Finishing (40 m) and polishing (15 m) diamond burs were used.
Followed by polishing points.
Followed by polishing pastes.
Conclusion of the Study Technique I produced the smoothest polished surface technique V was second best. “Finishing glass polyalkenoate cements (GIC)” M.J. Woodfords – BDJ (1988) This article reports on the SEM surface examination of GIC after furnishing procedures using rotary and hard instrumentation. Anhydrous water hardening type and an encapsulated type of GIC’s were employed. Finishing procedures employed were: 1. White stones and Vaseline in the slow hand piece at 5000rpm. 2. Soflex disks (Al2O3 disks) and Vaseline in the slow hand piece at 5000rpm. 3. Tungsten carbide blank 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 surface of even mature GIC.
Hand instrument cause marginal breakdown.
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