Finishing and polishing


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Finishing and polishing

  1. 1. CONTENTS  Introduction  Objectives of finishing and polishing  Finishing, cutting, grinding and polishing procedures  Finishing and polishing instruments • Hardness of abrasives • Abrasive instrument design A) Abrasive grits • Type of abrasives B) Bonded abrasives • Finishing burs • Diamond instruments C) Coated abrasive disks and strips D) Cloth and felt  Abrasive and erosive wear,  Other finishing procedures • Microabrasion and macroabrasion • Burnishing,  Finishing of different restorative materials • Amalgam • Composite • Glass ionomer cement • Direct gold restorations • Cast gold restorations • Porcelain  Health hazards of finishing and polishing
  2. 2.  References  Conclusion. Introduction : The main objective of an operative dentist is to restore the individual tooth to its form and functions along with imparting pleasing esthetics and maintaining periodontal tissue in good esteem. It has been established that rough or uneven surfaces initiate microbial flora to flourish and also light reflected by these surfaces may not be even and uniform. Therefore a restored tooth should be evenly smooth and reflect light uniformly. The process of making the surface smooth is known as finishing and polishing. Finishing is defined as the transformation of an object from a rough to a refined form. The procedure involves removal of surface irregularities and shaping the restoration according to functional occlusan polishing is defined as the production of a shiny mirror like surface, which reflects light similar to enamel. Objective of Finishing and Polishing : The objective of finishing and polishing of any restoration are obtaining adequate adaptation and continuity of the restoration. Margins, with the tooth, optimum restoration contour, proper occlusal contacts and a surface free of any scratches or irregularities to make it biologically acceptable. All these objectives lay the foundation for optimum oral health function and esthetics. i) Oral health: a well contoured and polished restoration will have reduction in total surface area and reduced roughness of the restoration surface which promotes the oral health by resting the accumulation of food debris and pathologic bacteria. Smoother surfaces are easier to maintain in a hygiene state when preventive oral home care is practiced.
  3. 3. With some restorations tarnish and corrosion activity can be significantly reduced if the entire restoration is highly polished, which is very important for the biocompatibilite property of a material. ii) Oral function: Oral function is enhanced with a well polished restoration because food glides more freely over occlusal and embrasure surfaces during mastication. Smooth restoration contacts, minimize wear rates on opposing and adjacent teeth, which is particularly true for restorative materials such as ceramics that contain phases that are harder than enamel and dentin. Friction is the resistance to motion of one material body over another. If an attempt is made to more one body over the other a restraining force to resist motion is produced. The co-efficient of friction is reduced if the surfaces are smooth and in single plane. Reduction in friction results in reduction in wear which interns helps in longevity of restoration. Rough material surfaces lead to the development of high-contact stresses that can causes the loss of functional and stabilizing contacts between teeth. iii) Esthetics : Aesthetic demands may require the dentist to handle highly visible surfaces of restorations differently than those that are not accessible, Because. When white light shines on a solid, some of the light is directly reflected from the surfaces and remains white. This light is mixed with the light reflected from the body of the material and dilutes the color. As a result an extremely rough surface appears lighter than the smooth surface of the same material. This problem is associated with unpolished or worn glass ionomers and composite restorations. The opacity, translucency and transparency of a dental material is affected by the surface characteristic of the material.
  4. 4. Opacity is a property of materials that prevents the passage of light. An opaque material may absorb some of the light and reflect the remainder. For example the core material of porcelain is opaque it reflects yellow light so it appears yellow. Translucency is a property of substrate that prevents the passage of light but disperses the light, so objects can’t be seen through the material. Some translucent materials used in dentistry are ceramics, resins, composites etc. Transparent material allow the passage of light in such a manner that little distortion takes place and objects may be clearly seen through them. Finishing, Cutting, Grinding and Polishing Procedures : Finishing can also be described as a process where by substrate particles are removed by the action of cutting and or grinding. The surface of the substrate in made to come in to frictional contact with a comparatively harder material. This contact generates enough tensile and shear stresses to over come the forces of atomic bonds and there by release particles from the substrate. In a cutting operation the substrate particles are, removed by the use of a bladed instrument or any instrument in a blade like fashion. A grinding operation on the other hand removes small particles of the substrate through the action of bonded or coated abrasive instruments. These instruments contain randomly arranged abrasive particles. Both cutting and grinding procedures produce unidirectional scratches. Polishing is the most refined process and acts on an extremely thin region of the substrate surface. It procedures very fine scratches that can be visible only under very high magnification. Basically polishing is the process in which the polishing material dos’t cut or grind, but fills fine scratches and procedures a perfectly smooth surface. During the polishing of metals a highly stressed microcrystalline
  5. 5. layer is formed on the surface called the “Beilby layer”. It is believed that because of the rapid movement of the polishing agent, top layer of the material gets heated up causing it to flow and fill the scratches. During these procedures : i) Surface blemished and imperfections are removed ii) The material is shaped to an ideal form and iii) The outermost surface of the material is developed to a desired state. Finishing and Polishing Instruments : Hardness of abrasives: One of the most important characteristics of abrasive particles on dental instruments is, it must be great enough to remove particles of substrate material without becoming dull or fracturing too rapidly. The hardness of a material can be measured by using MOHS, KNOOP or VICKERS hardness tests. Abrasive Instrument Design : A) 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. Dental abrasive grits are classified as coarse, medium fine and super fine, according to particle size. Along with this the shape of particle also plays a major role in its selection. Many types of abrasive materials are available they can be classified mainly in to two types. i) Natural abrasives : Arkansas stone, Chalk, Sand Corundum stone, Diamond, Tripoli Emery stone, garnet, zirconium silicate Pumice stone, quartz ii) Manufactured or synthesized materials.
  6. 6. They are generally preferred because of their more predictable physical properties. Silicon Carbide, Aluminium Oxide Synthetic Diamond, Rough, Tin Oxide B) Bonded abrasives : Bonded abrasives consists of abrasive particles which are incorporated through a binder to form grinding tools such as points, wheels, separating disks and a wide variety of other abrasive shaper. Particles are bonded by four general methods. a) Sintering b) Vitreous bonding (Glass or ceramic) c) Resinoid bonding d) Rubber bonding Sintered abrasives are strongest type because the abrasive particles are fused together Vitreous bonded abrasive are mixed with a glassy or ceramic matrix material, then cold presses to the instrument shape and finally fired to fuse the binder. Resin bonded abrasives are cold pressed or hot pressed and then are heated to cure the resin. Rubber bonded abrasives are made similarly. The type of bonding method employed for the abrasive greatly affects the grinding behaviour of the tool on the substrate. An ideal binder holds the abrasive particle is the tool sufficiently long enough to cut, grind or polish the substrate, yet release the particles either before its cutting efficiency is lost or before heat built-up causes thermal damage to the substrate. A bonded abrasive instrument should always be trued and dressed before its use.
  7. 7. Truing is a procedure through which the abrasive instrument is run against the harder abrasive block until the abrasive instrument rotates in the hand piece without eccentrically or runout when placed on a substrate. The dressing procedure 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 from the abrasive instrument to restore grinding efficiency during the finishing operation. The clogging of abrasive instrument with debris is called abrasive blinding, which results in reduction in abrasive efficiency and more heat generation. So frequent dressing of the abrasive instrument during the finishing operation on a truing instrument, maintains the efficiency of the abrasive. Diamond instruments are pre-shaped and trued so are not treated like the bonded abrasives. Diamond clearing stones are used on the super coarse through fine grades to remove debris build up and to maintain grinding efficiency. C) Coated abrasive disks and strips : Coated abrasives are fabricated by securing abrasive particles to a flexible backing material (heavy weight paper or Mylar) with a suitable abrasive material. These abrasives typically are supplied as disks and finishing strips. D) Cloth : Cloth carried on a metal wheel may be used for final polishing with or without a polishing medium. E) Rotary rubber instruments : Rubber ended rotary tools are commonly used for polishing procedures. They can be obtained in various shapes of cups, wheels, cones etc. and are commonly used with other abrasives or polishing pastes.
  8. 8. ABRASIVE AND EROSIVE WEAR : Wear is a material removal process that can occur whenever surfaces slide against each other. In dentistry the process of finishing a restoration involves abrasive wear through the use of hard particles. In case of a diamond bur abrading a tooth surface, the diamond particles bonded to the bur represent the abrasive and the tooth is the substrate. It is important to observe the rotational direction of a rotary abrasive instruments to control its action on the substrate surface when a hand piece and bur are translated in a direction opposite to the rotational direction of the bur at the surface being abraded, a smoother grinding action is achieved. However when the hand piece and bur are translated in the same direction as the rotational direction of the bur at the surface, the bur tends to “run away” from the substrate, these by producing a more in controlled grinding action and rougher surface. Abrasive wear is further divided in to the processes of two body and three body wear. a) Two body wear: This occurs when abrasive particles are free to the surface of the abrasive instrument and no other abrasive particles are used. Ex: A diamond bur abrading a tooth. b) Three body wear: This occur when abrasive particles are bonded to translate and rotate between two surfaces. Dental prophylaxis which involves the use of a rotating rubber cup and an abrasive paste on a tooth or a material surface is an example of a three body wear. Erosive wear : Erosive wear is caused by hard particles impacting a substrate surface, carried either by a steam of air or a steam of liquid. It is mainly
  9. 9. used in laboratories for 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 alkalies instead of hard particles to remove substrate material, chemical erosion is commonly called as acid etching. Other Finishing Procedures : i) Micro and macro abrasion a) Micro abrasion: Earlier in this technique a paste made of 18% hydrochloric acid, pumice and water was used. The paste was then applied to the desired surface with a hand device like tongue blade or a rotary driven rubber cup, which has a fluted edges, the rubber cup should be rotated at a very slow speed to avoid removing any excess tooth structure and prevent spatter. Croll further modified the technique by reducing the concentration of the acid to approximately 11% and increasing the abrasiveness by using silicon carbide particles instead of pumice, is a water soluble gel paste, this is more safer due to low acid content. Recently EMS has introduced AIR-flow HANDY systems which uses sodium bicarbonate as an abrasive agent. The powder is passed through the narrow nozzle of specially designed hand piece with the help of compressed air. This technique is used to remove extrinsic stains on the tooth surface. b) Macro abrasion: This is a also a technique for the treatment of surface stains and defects but utilizes a 12-fluted carbide bur or a micro diamond point revolving at high speed to remove the defect. Adequate air/water spray should be used not only as a coolant, but also to facilitate proper visibility of the defect.
  10. 10. ii) Burnishing : Burnishing is defined as the process of rubbing a metal over the restorative material to make it shiny or lusterous. Further it acids in adaptation and compaction of material at margins. The metal of the burnishing instrument should not combine easily with the metal or alloy being burnished; otherwise some metal may be transported either on to the restoration or restoration on to the instrument. Stainless still or chromium plated instruments are commonly employed either by hand or in the form of smooth rotating engine burnisher. This technique is employed in, • Finishing amalgam • Finishing gold restoration • Finishing cast gold restorations FINISHING OF DIFFERENT RESTORATIVE MATERIALS : 1) Dental amalgam restoration : Although few authors say that polishing of high copper amalgam is less important as it is resistant to tarnish and corrosion it is often necessary to i) Complete carving ii) Refine the anatomy contours and marginal integrity iii) Enhance surface texture of the restoration. Additional finishing and polishing procedures of amalgam restoration is not initiated until first 24 hrs as the crystallization process is not complete. • Burnishing the restoration before and after carving reduces the surface roughness by ten times. • After 24 hrs, the surface of the restoration is usually rough due to the heterogenous structure of amalgam on setting.
  11. 11. Finishing is then begun with the use of steel finishing burs or stones. This includes trimming any overextended and excess margins, creating adequate contour and correcting any occlusal disharmonies that may have been overlooked during carving process. A high point in an amalgam restoration appears as a shiny area, which is reduced with carborundum stones or sharp finishing burs. For the proximo-occlusal restorations, finishing begins with a careful appraisal of the cervical margins, buccal and lingual proximal and occlusal margins. Despite the fact that matrix bands and wedges are used during restorations, gingival overhangs of varying degree may be present. These overhangs are reduced by using rhein trimmers, bard parker knives, periodontal files or gold foil knives. Finishing of cervical areas is done by inserting fine water resistant strips cervical to the contact area through the interdental space and moving them to and fro. Removal of superficial scratches and irregularities is accomplished simultaneously. It should be remembered that abrasives always should be applied through a descending grade, i.e. coarse, medium, fine and ultrafine to achieve the best, finish. Suitable polishing agent like tinoxide, zinc oxide, chalk, pumice, extra fine silex etc. carried with a soft rotating brush or rubber cup. For polishing in this contact areas and gingival embrasures, the abrasive is introduced through polishing strips and dental tape. During polishing, the restoration should be kept moist and only low rotational speeds with light intermittent pressure should be used, to avoid any over heating. Excessive heat production (600 C) will permanently damage the pulp tissue and the mercury present in the restoration reaches the surface which leads to loss of material property of amalgam.
  12. 12. With the advent of high copper amalgams, it is now possible to finish the restorations much earlier, i.e. 15-30 minutes after condensation. Finishing and Polishing of Composite Restorations : Composite reins have been widely used as an anterior restorative material and are now being used in posterior teeth also. The surface smoothness varies with the type of composite resin, owing to the nature of the filler particles. i) Conventional composites: These pose greatest difference in hardness of organic and inorganic phases. The resin matrix is soft and filler particles are hard if fine grit polishing methods are used, the foster rein matrix abrades away easily leaving behind the harder filler particles, which gives it a rough surface. If coarse abrasions are used on the other hand, the organic and inorganic phases are removed equally but the abrasives have behind rough marks. ii) Hybrid composites can be polished to a semi-gloss but the surface is somewhat hydrophobic which makes it quite unpleasant for the patient. iii) Microfilled composite materials can undoubtedly be polished to the highest gloss and are considered to be esthetically best amongst all composites. The surface of these restorations is highly smooth and the chances of extrinsic staining are minimal. Finish obtained on micorfilled composites is a glossy one where that obtained on conventional composites is a satin finish. A surface finish attained with the use of a plastic matrix bond is the most desirable finish for resin restorations, but this is rarely obtainable because of the need for contouring and removal of excess material, which is often required clinically. Hence it is advisable to contour the unpolymerized composite with hand instruments, so that the need for removal of large amounts of set resin leading to surface damage are minimized.
  13. 13. a) Finishing of direct composite restorations : Excess composite at the cavosurface margins is scraped away using a scalpal or a sharp gold knife. The use of stainless steel instrument should be avoided, as these tend to leave grey marks on the restoration. For gross contouring the finishing of the concave and comparatively non-accessible areas on the occlusal surface, the alpine stone, 12-30 fluted carbide burs and diamond points are recommended. Rotary instruments should always be used in slow speed and with a stream of water and little pressure. High pressure tends to loosen and dislodge the filler particles. In the accessible and convex areas of the occlusal surface agents such as aluminium oxide, cuttle fish or silicon dioxide coated disks and strips are used in a descending grade of their abrasiveness. Vaseline or petroleum jelly should be used by used as lubricant with these disks and strips. These disks should be used in to and fro motion. Strips are used to polish the proximal surface with short strokes rather than rapid lengthy strokes in the embrasure. In class-V restorations care must be taken to avoid gingival damage in class-V restorations. The final luster is obtained with polishing pastes that may contain pumice, silica, alumina, tin oxide etc. After the final polishing of the composite is completed a thin layer of glaze may be applied to improve surface smoothness. Glaze is a film of unfilled polymers with a composition similar to the resin matrix. Studies have shown that the soflex disks which has aluminium oxide polishing are superior to finish and polish composite restorations. Indirect composite restorations : The cured inlays / onlays is primarily trimmed and finished on the die, using the armamentarium as in a direct composite restoration. Chair side procedure : at the stage of the restoration, excessive pressure should not be applied while seating, because of high fragility of composite
  14. 14. material. If the restoration dos’t fit in to the cavity, it should be reduced using disks. Once the marginal integrity has been established, the occlusion is then checked and adjusted, occlusal adjustments are made using fine grit diamond instruments followed by 30 fluted carbide burs. Final adjustments are made with same instruments and materials as used for direct composite restorations. Finishing and polishing of glass ionomer restorations : The finishing of glass ionomer cement has been described as an exercise in damage. The ideal surface finish for glass ionomer cements is produced by the matrix strip and any interference with hand or rotary instruments inevitably disrupts the surface. However, clinically it is almost impossible to place a restoration that adequately fills the cavity without requiring any excess to be trimmed. It is therefore essential to have satisfactory procedure for finishing. The surface of glass ionomer cement is sensitive to both dehydration and water contamination during initial setting phase. If the restoration is exposed to air during this period without any protective covering like varnish, it loses water and develops crazing and cracks. Water contamination on the other hand dissolves the matrix forming ions, resulting in weak and more soluble restoration. Damage should be avoided by checking against both these conditions. It is therefore preferable to delay finishing and polishing of the restoration for at least 24 hrs after insertion so that setting is complete in case of fast cements and light cure glass ionomer cement, finishing can be started as early as 10 mins after insertion. Rotary cutting instruments are preferred than hand cutting instruments to prevent the tear of the material at the margins. 12 fluted carbide-finishing burs or fine diamond burs are preferred. Woolford observed that softex disks with Vaseline produced the smoothest surface. A
  15. 15. lubricant such as coca butter or petroleum jelly should be used along with finishing and polishing instruments. Finishing and polishing of direct gold restorations : Burnishing is the first step in finishing a gold restoration. For occlusal surfaces, a spratley burnisher or ball burnisher is moved with condensable pressure from the metal to the tooth surface. A spratley blade carver is used in the embrasure for proximal restorations. The next step is giving an optimum contour to the restoration. Instruments used for this purpose include Morse scaler, Jones knife or cleoid descoid carver. On the occlusal surface, sued from the centre to margins. In the proximal portions there is a need to remove any excess, if it exists which is facilitated by tooth separation. Abrasive disks and strips are used, the disks should be small in diameter to prevent gingival damage. After the surface has been smoothed, burnishing is repeated once again on all the margins. Polishing with tin oxide or extra fine silex applied with a rubber cup gives the restoration a high metallic lustore and satin finish. Shofer brown and green cups have also been shown to be very effective. These rubber cups have an abrasive impregnated in the rubber. However they should be used only at slow speeds. Finishing and polishing of cast gold restorations : An obvious extension of the casting beyond the margins removed upto the finish line with a small cylindrical point rotating away from the operator. The remaining thick margin is carefully thinned with fine disks. Occlusal surface is then finished using abrasive stones and disks in an order of descending abrasiveness. The proximal surface is contoured lightly with a ½" to 5/6" carborundum disk followed by smoothening with medium and fine cattle paper disks.
  16. 16. The occlusion is checked with an articulating paper and premature contacts, if any are refined with a selective grinding. The roughness created is then polished with rubber polishing disks or points. Finally after the insertion of the restoration it is burnished again. Finishing and polishing of porcelain restorations : The high esthetic quality of porcelain is attributed to surface texture, which is characterized by reflection and absorption of light rays. These are three methods of fabrication currently available for ceramic inlay and onlays. i) Conventional filling on a refracting die using firing porcelain, ii) Milling from a preformed ceramic block using machenable ceramic and iii) Casting by the lost wax technique using castable and pressable ceramic. The second and third methods do not use the application of a glaze layer to attain the final surface finish. i) Restoration formed by firing porcelain : The desired finish on the fired ceramic restoration is obtained preferably by glazing and to a smaller extent by polishing. Any adjustment of the fabricated restoration needs to be done at the expense of the glaze layer. It is proven that 40-46% of the strength of the porcelain restoration will be reduced if the glaze surface is removed. But the strength of a glazed porcelain and one highly polished are comparable. Hence with the use of polishing systems like, softex disks, shofu porcelain laminate polishing kit etc. it is now possible to minimize the damage caused by intra oral adjustment. Glazing is of two types autoglazing and overglazing.
  17. 17. Autoglazing is a process in which a smooth surface is obtained without the use of an additional glaze. By careful control of the furnace temperature (9550 C – 10650 C), a glaze is created. At this temperature the surface porcelain melts slightly, flows and glazes with a little roundening of the corners. Overglazing on the other hand is the application of an external glaze layer over the surface of the completed body of porcelain. Such a glaze contains silica and low fusing fluxes and increases translucency. Chair side finishing procedures i) Use fine grit diamonds. ii) 30 fluted carbide finishing burs iii) Rubber abrasive points and cups at slow speed with air water spray. iv) Diamond polishing pastes applied with bristle brush. ii) Restoration formed by milling ceramic blocks : These ceramic inlays and onlays are fabricated using CAD-CAM technology (computer assisted designing and computer assisted manufacturing). Diamond finishing burs are used followed by soflex discs to polish the restoration. iii) Restorations formed by casting ceramic : The fabricated restoration is finished and polished on the master die similarly as described earlier for ceramic restorations. Hazards of finishing and polishing : 1) High temperature may damage pulp during finishing and polishing. This raise in temperature may also damage the properties of restorative material, like bringing mercury to the surface in case of amalgam restorations.
  18. 18. 2) Microstructural defects or cracks can extend below the surface during finishing and polishing. 3) As finishing proceeds, voids incorporated during the filling process may be exposed, lodging unsightly stains. 4) The edges of the inflexible disks tend to scratch the surfaces and the central metallic mounting hub of certain disks also have a tendency to cut the surface. 5) Restorative materials may be lodged in the oral soft tissues during finishing and shaping procedures, which can lead to persistent chronic inflammation of those tissues. 6) Loss of external stains may occur during finishing of castable ceramic restorations. 7) If attention is not given an excess of glazing could lead to complete loss of anatomy. 8) And one of the most important complication is the aerosol production. i.e. dispersions of solid particles are generated and released into the breathing space of laboratories and dental clinics whenever finishing and polishing operations are performed. These airborne particles may contain tooth structure, dental material and microorganisms. Such aerosols have been identified as potential sources of infections and chronic diseases of the eye and lungs and present a hazard to dental personnel and their patients silicosis also called grinders disease is a major aerosol hazard is dentistry because a number of silica based materials are used in the processing and finishing of dental restorations. Aerosol produced during finishing and polishing can be reduced by three ways.
  19. 19. i) They can be controlled at the source through the use of adequate infection control procedures, water spray, and high volume suction. ii) Personal protection like safety glasses, disposable face masks can protect the eye and respiratory tract from aerosols. iii) The entire facility should have a adequate ventilation system. REFERENCES 1. Finishing techniques for high copper amalgam restoration. A laboratory study. Journal of dental research vol – 67, page – 633, 1988. CONCLUSION A beautiful statue, even if it is Beautifully sculptured, if left unfinished will attract no spectators. Similarly a dental restoration without finishing and polishing will be unesthetic, but this is not the only problem it will face. The effects are cumulative which will finally lead to its failure. So why not spend a few more minutes of out precious time finishing and polishing the restoration rather than see all our hard work go in vain.