finishing and polishing materials in dentistry /orthodontic courses by Indian dental academy
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Benefits of Finishing and Polishing Restorative MaterialsPrinciples of Cutting, Grinding, Finishing, and PolishingAbrasion and ErosionAbrasive Instrument DesignTypes of AbrasivesFinishing and Polishing ProceduresDentifricesConclusion
Abrasive :A hard substance used (or grinding, finishing, or polishing a less-hard surface.
Buffing: Process of producing a lustrous surface through the abrading action of fine abrasives bound to a nonabrasive binder medium.
Bulk reduction: Process of removing excess material by cutting or grinding a material with rotary instruments to provide a desired anatomic form.
Contouring: Process of producing a desired anatomical form by cutting or grinding away excess material.
Cutting: Process of removing material from the substrate by use of a bladed bur or an abrasive embedded in a binding matrix on a bur or disk.
Finished and polished restoration: A prosthesis or direct restoration whose outer surface has been progressively refined to a desired state of surface finish.
Finishing: Process of removing surface defects or scratches created during the contouring process through the use of cutting or grinding instruments or both.
Glaze ceramic: A specially formulated ceramic powder that, when mixed with a liquid,applied to a ceramic surface, and heated to an appropriate temperature for a sufficient time,forms a smooth glassy layer on a dental ceramic surface.
Grinding: Process of removing material from a substrate by abrasion with relatively coarseparticles.
Natural glaze :A vitrified layer that forms on the surface of a dental ceramic containing a glass phase when the ceramic is heated to a glazing temperature for a specified time.
Over glaze: Thin surface coating of glass formed by fusing a thin layer of glass powder the matures at a lower temperature than that associated with the ceramic substrate.
Polish: Luster or gloss produced on a finished surface.
Polishing: Process of providing luster or gloss on a material surface.
Benefits of Finishing and Polishing
Finished and polished restorations provide three benefits of dental care :Oral health, Function, and Aesthetics.
A well-contoured and polished restoration oral health by resisting the accumulation of food debris and pathogenic bacteria.
This is accomplished through a reduction in total surface area and reduced roughness of the restoration surface.
Smoother surfaces have less retention areas and are easier to maintain in a hygienic state when preventive oral home care is practiced because dental floss and the toothbrush bristles can gain more access to all surfaces and marginal areas.
Tarnish and corrosion activity of some dental materials can be significantly reduced.
Oral function is enhanced with a well-polished restoration.
Rough material surfaces lead to the development of high-contact stresses that cause the loss of functional aid stabilizing contacts between teeth.
Roughness on ceramics also act as stress concentration points.
Finishing and polishing surfaces can improve the strength of the restoration, especially in areas under tension.
Finally, aesthetic demands may require the dentist to handle highly visible surfaces of restorations differently than those that are not accessible.
PRINCIPLES OF CUTTING, GRINDING, FINISIHING, AND POLISHING
Even though there are distinct differences in function of cutting, grinding and polishing, at times they overlap, depending on the hardness, shape the abrasive particle used and the speed of the handpiece.
Grinding, finishing, and polishing systems vary considerably. They consist of abrasive-coated paper or plastic disks, abrasive impregnated tipped mandrels, diamond-bonded burs, and abrasive pastes.
The goals of finishing and polishing procedures are to obtain the desired anatomy, proper occlusion, and the reduction of roughness, gouges, and scratches that were produced by the contouring and finishing instruments.
The instruments available for finishing and polishing restorations include fluted carbide burs, diamond burs, stones, coated abrasive disks and strips, polishing pastes, and soft and hard polymeric cups, points, and wheels impregnated with specific types and sizes of abrasive particles.
Subtle differences distinguish the cutting, grinding, and polishing processes.
A cutting operation usually refers to the use of a bladed instrument or the use of any instrument in a bladelike fashion. Substrates may be divided into large separate segments, or they may sustain deep notches and grooves by the cutting operation.
High-speed tungsten carbide burs have numerous regularly arranged blades that remove small shavings of the substrate as the bur rotates.
The unidirectional cutting pattern reflects the action of the regularly arranged blades
The pattern produced by a diamond bur
A separating wheel is an example of an instrument that can be used in a bladelike fashion.
A separating wheel does not contain individual blades, but its thin design allows it to be used in a rotating fashion to slice through cast metal sprues and die stone materials.
A grinding operation removes small particles of a substrate through the action of bonded or coated abrasive instruments.Grinding instruments contain randomly arranged abrasive particles.Each particle may contain several sharp points that run along the substrate surface and remove particles of material.
For example a diamond-coated rotary instrument may contain many sharp diamond particles that pass over a tooth during each revolution of the instrument.
Because particles are randomly arranged, innumerable unidirectional scratches are produced within the material surface.
It shows a tooth surface ground by a diamond bur. Cutting and grinding are both considered to be predominantly unidirectional in their course of action. This means that a cut or surface exhibits cuts and scratches oriented in one predominant direction.
Different types of burs have unique effects on surfaces. The 16-flute carbide produces a smoother finish than the 8-flute carbide bur, but the latter removes material more rapidly. Similarly, the coarsest diamond bur removes material quickly but leaves a rougher surface.
Polishing procedures, the most refined of the finishing processes, remove surface particles. Each type of polishing abrasive acts on an extremely thin region of the substrate surface. Polishing progresses from the finest abrasive that can remove scratches from the previous grinding procedure and is completed when the level of surface smoothness is achieved.
Each step is followed by the use of progressively finer polishing media until no further improvement in surface finish is observed. The final stage produces scratches so fine that they are not visible unless greatly magnified.
Examples of polishing instruments are rubber abrasive points, fine-particle disks and strips, and fine-particle polishing pastes. Polishing pastes are applied with soft felt points, muslin (woven cotton fabric) wheels, prophylaxis rubber cups, or buffing wheels.
A nonabrasive material should be used as an applicator while using polishing pastes. Felt, leather, rubber, and synthetic foam are popular applicator materials for buffing.
A common feature of some of these materials is their porous texture that allows fine abrasive particles to be retained during the buffing procedure.
Polishing is considered to be ,multidirectional in its course of action. This means that the final surface scratches are oriented in many directions.
Some examples of ground and polished surfaces are shown
Bulk Reduction ProcessBulk Reduction Process
Bulk reduction can be achieved through the use of instruments such as diamond, carbide, and steel burs, abrasive-coated disks, or separating disks. Whereas diamond burs and abrasive-coated disks provide this action by grinding, steel and carbide burs remove materials through a cutting action of the hard blades.
Abrasive-coated disks are popular instruments for bulk reduction of resin-based composite restorations. For bulk reduction, the clinician should choose 8 to 12-fluted carbide burs or abrasives with a particle size of 100μm or larger with sufficient hardness (9 to 10 Mohs hardness).
Even though contouring can be achieved during bulk reduction, in some cases it requires finer cutting instruments or abrasives to provide better control of contouring and surface details. At the end of this process, the desired anatomy and margins should be established.
The smoothness of the surface at this stage depends on the instrument used and may require extra steps to establish a smoother surface Usually, 12 to 16 fluted carbide burs, or abrasives ranging in size from 30 to 100μm, provide the fine contouring action.
In general, finishing and polishing processes require a stepwise approach, introducing finer scratches to the surface of the substrate in order to methodically remove the deeper scratches. This process may require several steps to reach the desired surface smoothness.
Surface imperfections can be an integral part of the internal structure , or they can be created by the instruments that are used for gross removal because of the size of the abrasives or the flute geometry. Finishing provides a relatively blemish-free, smooth surface.
The finishing action is usually accomplished using 18- to 30-flute carbide burs, fine and superfine diamond burs or abrasive between 8 and 20μm in size.
The purpose of polishing is to provide an enamel-like luster to the restoration. Smaller particles provide smoother and shinier surfaces. The speed of achieving a luster, however, depends on the hardness and size of the abrasive particles and the method of abrasion (e.g., two-body abrasion or three-body abrasion).
Ideally, abrasive particles ranging up to 20 μm provide luster at a low magnification. At the end of this process, there should be no visible scratches. However there will always be scratches that are detectable at higher magnification.
The surface must be cleaned between steps, because an abrasive particle left on the surface from the previous step can cause deep scratches. The quality of the surface finish and polish can be characterized by the measurement of the surface roughness using a profilometer, an optical microscope, or a scanning electron microscope (SEM).
In clinical practice, the surface luster is usually judged without magnification. Even though, most of the time surface smoothness is correlated with the luster, as in cases such as resin-based composite restorations, the smoothest surface does not necessarily provide the most lustrous surface.
For industrial applications, reflectometers are used to measure the luster. However, it is difficult to use them successfully for dental applications because of the irregular contour and small size of dental restorations.
Heat generation during cutting, contouring finishing, and polishing processes of direct restorations is a major concern. To avoid adverse effects to the pulp, the clinician must cool the surface with a lubricant, such as an air-water spray, and avoid continuous contact of high-speed rotary instruments with the substrate.
Intermittent contact during operation is necessary, not only to cool the surface but also to remove debris that was formed between the substrate and the instrument. The effectiveness and the speed of the contouring, finishing and polishing procedures will be greatly improved by removal of debris.
Dispersions of solid particles are generated and released into the breathing space of laboratories and dental clinics whenever finishing operations are performed. These airborne particles may contain tooth structure, dental materials and microorganisms.
Such aerosols have been identified as potential sources of infectious and chronic diseases of the eyes and lungs and present a hazard to dental personnel and their patients. Silicosis, also called grinder’s disease, is a major aerosol hazard in dentistry because a number of silica-based materials are used in the processing and finishing of dental restorations.
Silicosis is a fibrotic pulmonary disease that severely debilitates the lungs and doubles the risk for lung cancer. The risk of silicosis is substantial because 95% of generated aerosol particles are smaller than 5μm in diameter and can readily reach the pulmonary alveoli during normal respiration.
Additionally, 75% of airborne particles are potentially contaminated with infectious microorganisms. Furthermore, aerosols can remain airborne 24 hrs before settling and are therefore capable of cross-contaminating other areas of the treatment facility.
Aerosol sources, in both the dental operatory and laboratory environments, must be controlled whenever finishing procedures are performed. A concise and informative source of information on aerosol hazards has been written by Cooley (1984).
Aerosols produced during finishing procedures maybe controlled in three ways:
First, they may be controlled at the source through the use of adequate infection control procedures, water spray, and high-volume suction.
Second, personal protection, such as safety glasses and disposable facemasks, can protect the eyes and respiratory tract from aerosols. Masks should be chosen to provide the best filtration along with ease of breathing for the wearer.
Third, the entire facility should have an adequate ventilation system that efficiently removes any residual particulates from the air.
Wear is a material-removal process that can occur whenever surfaces slide against each other. The process of finishing a restoration involves abrasive wear through the use of hard particles. In dentistry, the outermost particles or surface material of an abrading instrument is referred to as the abrasive.
The material being finished is called the substrate. In the case of a diamond bur abrading a tooth surface, such as that illustrated
the diamond particles bonded to the bur represent The abrasive, and the tooth is the substrate. Also notice that the bur in the high speed handpiece rotates in a clockwise direction as observed from the head of the handpiece.
Abrasion is further divided into the processes of two-body and three-body wear. Two-body abrasion occurs when abrasive particles are firmly bonded to the surface of the abrasive instrument and no other abrasive particles are used. A diamond bur abrading a tooth represents an example of two-body wear.
Three-body abrasion occurs when abrasive particles are free to translate and rotate between two surfaces. An example of three-body abrasion involves the use of nonbonded abrasives, such as exist in dental prophylaxis pastes. These nonbonded abrasives are placed in rubber cup, which is rotated against a tooth or material surface.
Diamond particles may debond from a diamond bur And cause three-body wear. Likewise, some abrasive particles in the abrasive paste trapped in the surface of a rubber cup and cause two-body wear. Lubricants are often used to minimize the risk for these unintentional shifts from two-body to three-body wear and vice versa.
Thus the efficiency of cutting and grinding will be improved with lubricants. Water glycerin, or silicone can be used as lubricants. lntraorally, a water-soluble lubricant is preferred. Excessive amounts of lubricants may reduce the cutting efficiency by reducing the contact between the substrate and the abrasive.
Erosive wear is caused by hard particles impacting a substrate surface, carried by either a stream of liquid or a stream of air, such as occurs when sandblasting a surface. Following figure illustrates schematically two-body abrasion, three-body abrasion, and hard-particle erosion.
Most dental laboratories have air-driven grit-blasting units that employ hard-particle erosion to remove surface material.A distinction must be made between this type of erosion and chemical erosion, which involves chemicals such as acids and alkalis instead of hard particles to remove substrate material.
Chemical erosion, more commonly called acid etching, is not used as a method of finishing dental materials. It is used primarily to prepare surfaces to enhance bonding or coating.
The inherent strength of cutting blades or abrasive particles of a dental instrument must be great enough to remove particles of substrate material without becoming dull or fracturing too rapidly. The durability of an abrasive is related to the hardness of its particles or surface material.
Hardness is a surface measurement of the resistance of one material to be plastically deformed by indenting or scratching another material. The first ranking of hardness was published in 1820 by Friedrich Mohs, a German mineralogist. He ranked 10 minerals to one another by their relative scratch resistance.
The least scratch-resistant mineral, talc, received a score of one and the most scratch-resistant mineral, diamond, received a score of ten. Mohs’ scale was later expanded in the 1930s to accommodate several new abrasive materials that received scores in the 9 to 10 range.
Abrasive instrument design Abrasive grits Abrasive grits are derived from materials that have been crushed and passed through a series of mesh screens to obtain different particle size ranges. Following Table lists grit and particle sizes for commonly used dental abrasives. Dental abrasive grits are classified as coarse, medium coarse, medium, fine, and superfine according to particle size ranges.
Experience generally indicates which grades of an abrasive give the best results in finishing a given material. Keep in mind that the rate of material removal is not the only important factor.
The surface finish obtained with each abrasive is just as important. If too hard an abrasive is used, or if the grain size is too coarse for use on a given material, deep scratches result in the substrate that cannot be removed easily in subsequent finishing operations.
Additionally, if an abrasive does not have the proper particle shape or does not break down in a manner that creates or exposes new sharp-edged particles, it will tend to gouge the substrate.
Bonded AbrasivesBonded abrasives consist of abrasive particles that are incorporated through a binder to form grinding tools such as points, wheels, separating disks, coated thin disks, and a wide variety of other abrasive shapes.
Particles are bonded by four general methods:
Vitreous bonding (glass or ceramic),
Resinoid bonding (usually phenolic resin), and
Rubber bonding (latex-based or silicone-based rubber).
Because most of the rubber wheels, cups, and points contain latex, a known allergen, all residues must be removed from polished surfaces.
Abrasive disks are used for gross reduction, contouring, finishing, and polishing of restoration surfaces. Most types of disks are coated with aluminum oxide abrasive. Abrasive strips with either a plastic or metal backing are also available to smooth and polish the proximal surfaces of all direct and indirect bonded restorations.
Metal strips are usually limited to situations in which very tight proximal contacts are involved. They are particularly useful for ceramic restorations, but are also used for composites and amalgams. However, care must be taken to avoid lacerating the gingival tissues.
The metal-backed strips are more costly, but they can be autoclaved and used several times if they are not damaged. Plastic-backed strips are used primarily for composites, compomers, hybrid ionomers, and resin cements.
Sintered abrasives are the strongest type because the abrasive particles are fused together. Vitreous-bonded abrasives are mixed with a glassy or ceramic matrix material, cold-pressed to the instrument shape, and fired to fuse the binder. Resin-bonded abrasives are cold-pressed or hot-pressed and then heated to cure the resin.
Several examples of bonded abrasives are illustrated in Figure.
A bonded abrasive instrument 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 or runout when placed on the substrate.
The dressing procedure, like truing, is used to shape the instrument, but it accomplishes two different purposes as well. First, the dressing procedure reduces the instrument to its correct working size and shape. Second, it is used to remove clogged debris from the abrasive instrument to restore grinding efficiency during the finishing operation.
The clogging of the abrasive instrument with debris is called abrasive blinding. Abrasive blinding occurs when the debris generated from grinding or polishing occludes the small spaces between the abrasive particles on the tool and reduces the depth that particles can penetrate into the substrate. As a result, abrasive efficiency is lost and greater heat is generated.
A blinded abrasive appears to have a coating of the substrate material on its surface. Frequent dressing of the abrasive instrument during the finishing operation on a truing instrument, such as that illustrated in Figure below maintains efficiency of abrasive in removing the substrate material.
Binders for diamond abrasives are manufactured specially to resist abrasive particle loss rather than to degrade at a certain point and relase particles. One reason For this is that diamond is the hardest material known - so hard that diamond abrasive particles do not lose their cutting efficiency against substrates.
Diamond particles are bonded to metal wheels and bur blanks with special heat-resistant resins such as polyimides. The super-coarse through fine grades are then plated with a refractory metal film such as nickel. The nickel plating Provides improved particle retention and acts as a heat sink during grinding.
Titanium nitride coatings are used as an additional layer on some of the recent diamond abrasive instruments to further extend their longevity.Finishing diamonds used for resin-based composites contain diamond particles 40μm or less in diameter, and many are not nickel-plated.
Therefore they are highly susceptible to debonding and should always be used with light force and copious water spray to ensure retention of the very-fine diamond particles. Diamond but should always be used with water spray and at rotational speeds of less than 50,000 rpm.
Disposable diamond burs recently gained popularity from maintenance and OSHA viewpoints because of three factors:
(1) Optimal instrument efficiency, (2) Concerns over the reuse of disinfected abrasive devices and (3) Minimal heat build-up.
Diamond instruments are preshaped and trued; they are not treated as other bonded abrasives. Diamond cleaning stones are used on the super-coarse through fine grades to remove debris build-up and to maintain grinding efficiency.
An example of a diamond cleaning stone is shown in Figure following. Cleaning stones should not be used on finishing diamonds because their bonded particles are quickly removed. Manufacturers provide special operating and cleaning instructions for these instruments.
Coated abrasives are fabricated by securing abrasive particles to a flexible backing material (heavyweight paper, metal, or Mylar) with a suitable adhesive material. These abrasives typically are supplied as disks and finishing strips. Disks are available in different diameters and within and very thin backing thicknesses.
A further designation is made in regard to whether or not the disk or strip is moisture-resistant. It is advantageous to use abrasive disks or strips with moisture resistant backings because their stiffness is not reduced by water degradation.
Furthermore, moisture acts as a lubricant to improve cutting efficiency. Examples of coated abrasives are shown here.
Polishing pastes are considered as nonbonded abrasives and are primarily used for final polishing. They need to be applied to the substrate with a nonabrasive device such as synthetic foam, rubber, felt, or chamois cloth. The abrasive particles are a persed in a water-soluble medium such as glycerin for dental application. Aluminum oxide and diamond are the most popular nonbonded abrasives.
Abrasive motionThe motion of abrasive instruments is classified as rotary, planar, or reciprocal. In general, burs are considered rotary, disks are planar, and reciprocatng handpiece provide a cyclic motion and are reciprocal in relationship to their direction of motion. Different sizes of abrasives can be incorporated with each motion
Reciprocating hand pieces especially provide the benefit of accessing interproximal and subgingival areas to remove overhangs, to finish subgingival margins without creating ditches, and to create embrasures.
Many types of abrasive materials are available, but only those commonly used in dentistry are discussed in this section. Natural abrasives include Arkansas stone, chalk, corundum, diamond, emery, garnet, pumice, quartz, sand, tripoli, and zirconium silicate.
Cuttle and kieselguhr are derived from the remnants of living organisms.
Manufactured abrasives are synthesized materials that are generally preferred because of their more predictable physical properties. Silicon carbide, aluminum oxide, synthetic diamond, rouge, and tin oxide are examples of manufactured abrasives.
Arkansas stone is a semitranslucent, light gray, siliceous sedimentary rock mined in Arkansas. It contains microcrystalline quartz and is dense, hard, and uniformly textured. Small pieces of this mineral are attached to metal shanks and trued to various shapes for fine grinding of tooth enamel and metal alloys.
One of the mineral forms of calcite is chalk, a white abrasive composed of calcium carbonate. Chalk is used as a mild abrasive paste to polish tooth enamel, gold foil, amalgam, and plastic materials.
This mineral form of aluminum oxide is usually white. Its physical properties are inferior to those of manufactured alpha (α) aluminum oxide, which has largely replaced corundum in dental applications. Corundum is used primarily for grinding metal alloys and is available as a bonded abrasive in several shapes. It is most commonly used in an instrument known as a white stone.
Diamond is a transparent, colorless mineral composed of carbon. It is the hardest know substance. Diamond is called a superabrasive because of its ability to abrade any other known substance.
Diamond abrasives are supplied in several forms, including bonded abrasive rotary instruments, flexible metal-backed abrasive Strips, and diamond polishing pastes. They are mostly used on ceramic and resin-based composite materials.
The advantages of synthetic diamonds over natural diamonds include their controllable, consistent size and shape, as well as their lower cost compared with natural diamonds. The shape of the diamonds determines the binder needed for its use. The binders can be either resin or metal. Resin-bonded diamonds have sharp edges.
During use, the sharp edges break down and expose new sharp edges and corners. On the other hand, metal-bonded diamonds are regular and more consistent in size. They function as cutting points or edges primarily through the benefit of their hardness rather than their shape.
Larger synthetic diamond particles appear greenish because of the chemical reaction with nickel during the manufacturing process. Manufactured diamond is used almost exclusively as an abrasive and is produced at five times the quantity of natural diamond abrasive. This abrasive is used in the manufacture of diamond saws, wheels, and burs.
Blocks with embedded diamond particles are used to true other types of bonded abrasives. Diamond polishing pastes are also produced from particles smaller than 5µm in diameter. Synthetic diamond abrasives are used primarily on tooth structure, ceramic materials, and resin-based composite materials.
This abrasive is a grayish-black corundum that is prepared in a fine-grain form. Emery is used predominantly in the form of coated abrasive disks and is available in a variety of grit sizes. It may be used for finishing metal alloys or acrylic resin materials.
The term garnet includes a number of different minerals that possess similar physical properties and crystalline forms. These minerals are the silicates of aluminum, cobalt, iron, magnesium, and manganese, The garnet abrasive used in dentistry is usually dark red.
Garnet is extremely hard and, when fractured during the grinding operation, forms sharp, chisel-shaped plates, making it a highly effective abrasive. Garnet is available on coated disks and arbor bands. It is used in grinding metal alloys and acrylic resin materials.
Volcanic activity produces this light-gray, highly siliceous material. It is used mainly in grit form but can be found in some rubber-bonded abrasives. Both pumice forms are used on acrylic resin materials. Flour of pumice is an extremely fine-grained volcanic rock derivative from Italy that is used in polishing tooth enamel, gold foil, dental amalgam, and acrylic resins.
The most commonly used form of quartz is very hard, colorless, and transparent. It is the most abundant and widespread of minerals. Quartz crystalline particles are pulverized to form sharp, angular particles that are useful in making coated abrasive disks. Quartz abrasives are used primarily to finish metal alloys, and they may also be used to grind dental enamel.
Sand is a mixture of mineral particles predominantly composed of silica. The particles represent a mixture of colors, making sand abrasives distinct in appearance. Sand particles have a rounded to angular shape. They are applied under air pressure to remove refractory investment materials from base metal alloy castings. They are also coated onto paper disks for grinding of metal alloys and acrylic resin materials.
This abrasive is derived from a lightweight, friable siliceous sedimentary rock. Tripoli can be white, gray, pink, red, or yellow. The gray and red types are most frequently used in dentistry. The rock is ground into very fine particles and formed with soft binders into bars of polishing compound. Tripoli is used for polishing metal alloys and some acrylic resin materials.
Zircon or zirconium silicate is supplied as an offwhite mineral. This material is ground to various particle sizes and is used to make coated abrasive disks and strips. It is frequently used as a component of dental prophylaxis pastes.
Commonly referred to as cuttlefish, cut bone, or Cuttle, this abrasive is a white calcareous powder made from the pulverized internal shell of a Mediterranean marine mollusk of the genus Sepia. Cuttle is available as a coated abrasive and is useful for delicate abrasion operations such as polishing of metal margins and dental amalgam restorations.
This material is composed of the siliceous remains of minute aquatic plants known as diatoms. The coarser form of kieselguhr is called diatomaceous earth and is used as a filler in many dental materials, such as the hydrocolloid impression materials.
This extremely hard abrasive was the first of the synthetic abrasives to be produced. Green and blue-black types of silicon carbide are produced ; both types have equivalent physical properties. The green form is often preferred because substrates are visible against the green color. Silicon carbide is extremely hard and brittle. Particles are sharp, and they break to form new sharp particles.
This results in highly efficient cutting of wide variety of materials, including metal alloys, ceramics, and acrylic resin materials. Silicon carbide is available as an abrasive in coated disks and as vitreous-bonded and rubber-bonded instruments.
Fused aluminum oxide was the second synthetic abrasive to be developed. Synthetic aluminum oxide (alumina) is made as a white powder and can be much harder corundum (natural alumina) because of its purity. Alumina can be processed with different properties by slight alteration of the reactants in the manufacturing process. Several grain sizes of alumina are available, and it has largely replaced emery for several abrasive uses.
Aluminum oxide is widely used in dentistry to make bonded abrasives, coated abrasives, and air-propelled grit abrasives. Sintered aluminum oxide is used to make white stones, which are popular for adjusting dental enamel and for finishing metal alloys, resin-based composites and ceramic materials.
Pink and ruby variations of aluminum oxide abrasives are made by adding chromium compounds to the original melt. These variations are sold in a vitreous-bonded form as non contaminating mounted stones for the preparation of metal-ceramic alloys to receive porcelain.
Remnants of these abrasives and other debris should be removed from the surface of metals used for metal-ceramic bonding so as not to prevent optimal bonding of porcelain to the metal alloy. A review by Yamamoto (1985) suggests that carbide burs are the most effective instruments for finishing this type of alloy because they do not contaminate the metal surface with entrapped abrasive particles.
Iron oxide is the fine, red abrasive component of rouge. Like tripoli, rouge is blended with various soft binders into a cake form. It is used to polish high the metal alloys.
Tin oxide is an extremely fine abrasive 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.
The most commonly used abrasive pastes contain either aluminum oxide (alumina) or diamond particles. Alumina pastes should be used with a rotary instrument and increasing amounts of water as polishing proceeds from coarser to finer abrasives. Diamond abrasive pastes are used in a dry condition.
The instruments that apply the paste to the material surface are equally important, These include ribbed prophy cups (the ribbed type or the more flexible, nonribbed type), brushes, and felt wheels.
Abrasive pastes have several disadvantages. First they are relatively thick and cannot gain access into embrasures. Second, the pastes tend to splatter off of the instruments. Third, heat is generated when insufficient coolant is used or when an intermittent polishing technique is not used.
The ideal surface for ceramic restorations is a polished and glazed surface. The production of a glaze layer through a natural glaze or overglaze process will not necessarily yield a smooth surface if the initial ceramic surface has significant roughness.
The smoothest surfaces can he achieved extraorally before a prosthesis is cemented. In the mouth however, minor roughness can be successfully polished without compromising the surface quality. In addition, polishing can improve the strength within the surface region of a ceramic prosthesis because it removes pores and microcracks.
Adequate cooling is important in vivo when finishing and polishing ceramic restorations. Using an air-water spray and maintaining intermittent contact between the restoration and the rotary instrument are critical during the operation.
Continuous contact between the restoration and the rotary instrument should be avoided. Heatless stones (silicone carbide) provide heat reduction and can be used as an alternative. Several kits are available for finishing and polishing ceramic restorations.
Manufacturer’s instructions should be followed when using different systems. Depending on the preference of the dentist, a general technique is as follows:
(1) Contour with flexible diamond disks, diamond burs, heatless or polymer stones or green stones (silicone carbide).
(2) Finish with white or abrasive-impregnated rubber disks, cups, and points. This process may require two or three steps, depending on the system used.
(3) Polished with fine abrasive impregnated rubber disks, cups, and points, or, if necessary, use a diamond paste applied with a brush or felt wheel.
(4) Apply an overglaze layer, or natural glaze the ceramic if necessary. For intraoral polishing, use intermittent application of rotating instruments with a copious amount of water as a coolant.
Acrylic resins are relatively soft materials. To avoid overheating, apply a large amount of pumice slurry to the surface. Intermittent contact with the substrate also helps to avoid overheating. The following technique steps are recommended:
(1) Contour with tungsten carbide bur and sandpaper.
(2) Use a rubber point to remove the scratches.
(3) Apply pumice with a rag wheel, felt wheel, bristle brush, or prophy cup, depending on the size of the area that needs to be polished.
(4) Apply tripoli or a mixture of chalk and alcohol with a rag wheel.
As an alternative to the use of rotary instrument cutting, air-abrasive systems can deliver a fine, precisely controlled high-pressure stream of 25-to 30µm aluminum oxide particles to remove enamel, dentin, and restorative materials. Because air abrasion generates minimal heat or vibration, the potential for tooth chipping or microfracturing is minimized.
These systems have been used for the following applications: cavity preparation, removal of defective composite fillings, endodontic access through porcelain crowns, minimal preparation to repair crown margins, tunnel preparations, superficial removal of stains, cleaning of tooth surfaces before adhesive bonding and roughening of internal surfaces of indirect porcelains or composite restorations before adhesive bonding.
Often referred to as air polishing, air-abrasive polishing is based on the controlled delivery of an air, water, and sodium bicarbonate slurry to remove plaque and stains from tooth surfaces. Compared with rubber cup and prophylaxis paste techniques, it is more time-effective, and it is possible to access many tooth surfaces with this technology.
However, it is reported that surfaces of softer restorations, such as glass ionomers, can be damaged. Therefore it should be used with caution around cosmetic restorations.
Dentifrices, available as toothpastes, gels, and powders, provide three important functions. First, their abrasive and detergent actions provide more efficient removal of debris, plaque, and stained pellicle compared with use of a toothbrush alone. Second, they polish teeth to provide increased light reflectance and superior aesthetic appearance.
The high polish, as an added benefit, enables heat to resist the accumulation of microorganism and stains better rougher surfaces. Finally, dentifrices act as vehicles for the delivery of therapeutics agents with known benefits; for example, fluorides, tartar control agents, desensitizing agents, and remineralizing agents.
Fluorides improve resistance to caries and may, under a proper oral hygiene regimen, enhance the remineralization of incipient noncavitated enamel lesions. Tartar control agents, such as potassium and sodium pyrophosphates, can reduce the rate at which new calculus deposits from supragingivally.
Desensitizing agents with proven clinical efficacy are strontium chloride and potassium nitrate. The therapeutic benefits of other additive such as peroxides and bicarbonates are under investigation. The products advertised as “whitening tooth paste” may contain a abrasive agent alone or a chemical agent and a abrasive agent. The former type of additive acts through a surface stain removal mechanism whereas latter additives act through a combined mechanism of abrasion and bleaching.
CompositionThe abrasive concentration in paste and gel dentifrices are 50% to 75% lower than those of powder dentifrices Therefore powders should be used more sparingly and with greater caution by patients (especially where cementum and dentin are exposed) to avoid excessive dentinal abrasion and pulpal sensitivity.
The ideal dentifrice should provide the greatest possible cleaning action on tooth surfaces with the lowest possible abrasion rates.
Dentifrices do not need to be highly abrasive to clean teeth effectively. This is fortunate because exposed root surface cementum and dentin are abraded at rates of 35 and 25 times that of enamel, respectively.
Currently the preferred means of evaluating dentifrice abrasivity is to employ irradiated dentin specimens and brush them for several minutes with test and reference dentifrices.
An abrasivity ratio is then calculated by comparing the amounts of radioactive phosphorus ( 32P) released by each dentifrice, and this value is multiplied by 1000. A dentifrice must obtain an abrasivity score of 200 to 250 or less to satisfy the abrasivity test requirements proposed by the American Dental Association (ADA) and the International Organization for Standardization (ISO).
This means that a test dentifrice must abrade dentin at 20% to 25% of the rate of the reference standard to be considered safe for normal usage. A problem with this laboratory test is that it does not account for all variables that would affect abrasivity under in vivo conditions.
Some of the factors affecting dentifrice abrasivity are :
ADA Acceptance Program
The ADA designates a dentifrice a “Accepted” only if the dentifrice meets specific requirements.
First, the abrasivity of the dentifrice must not exceed the maximum acceptable abrasivity value of 250. (also a limit for the ISO standard).
Second, the manufacturer must produce scientific data, usually from clinical trials, that verify any claims the manufacturer wishes to make on the product package or in commercial advertisements, which are also periodically reviewed by the appropriate ADA Council.
Toothbrush bristle stiffness alone has been shown to have no effect on abrasion of hard dental tissues. However, when a dentifrice is used, there is evidence that more flexible toothbrush bristles bend more readily and bring more abrasive particles into contact with tooth structure albeit with relatively light forces.
This interaction should produce more effective abrasion and cleaning action on areas that the bristles can reach. Battery-powered toothbrushing devices provide a variety of cleaning actions that are claimed to improve tooth-cleaning actions even further than those achieved by manual toothbrushes.
William J. 0’Brien: Dental materials and their selectionRobert G. Craig: Restorative dental materials.John F McCabe: Applied dental materials.E.C.Coombe: Notes on dental materials.Kenneth J Anusavice: Science of dental materials. E.H. Greener: Material science in dentistry.Bernard G. N. Smith: The clinical handling of dental material.