phonetics

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phonetics

  1. 1. ABRASION DEFINITION: The wearing away of a substance or structure through a mechanical process, such as grinding, rubbing or scraping.
  2. 2. TYPES OF ABRASION ABRASION MAY BE:  A two body process e.g. action of a diamond bur on enamel.  A three body process e.g. Pumice applied with a bristle brush.
  3. 3. SUPPLIED AS : In dentistry the abrasive is applied to the work by a variety of tools. ► paper / plastic coated: The abrasive particles may be glued on to a paper or plastic disc that can be attached to a handpiece .sand paper belongs to this category. ► Stainless steel coated strips: The abrasive (e.g.diamond ) may be attached to stainless steel or plastic strips. This category is similar to the above.
  4. 4.  Electroplating bonding : In case of diamond rotary instruments the diamond chips are attached to steel wheels,discs and cylinders by electro-plating with nickel based matrix.  Bonded stones : In grinding wheels and dental stones, the abrasive particles are mixed with a bonding agent that holds the particles together. before hardening, the matrix material with the abrasive is moulded to form tools of desired shapes.  Powder form : An abrasive may also be mixed with water or glycerine and to form a paste or slurry. It is applied with felt cone, rubber cup or brush and used for smoothening irregularities, e.g. pumise powder.
  5. 5.  CAKE FORM : They are also available in the form of cakes.  RUBBER IMPREGNATED : Abrasives can be incorporated into rubber or shellac discs or cups for “soft grade” abrasion.  PASTE FORM : The abrasive is made into a paste and supplied in a tube, e.g.Ivoclar polishing paste, tooth paste, etc. Abrasion is affected by properties of the abrasive as well as the material being abraded. The important properties are hardness, strength, ductility and thermal conductivity.
  6. 6. MECHANISM OF ABRASIVE ACTION The action of an abrasive is essentially a cutting action. Each tiny particle presents a sharp edge that cuts through the surface similar to a chisel. A shaving is formed which crushes to a fine powder. This powder clogs the abrasive tool and frequent cleaning is required.
  7. 7. STRESS, STRAIN AND HEAT PRODUCTION DURING ABRASION      In denture resins,too rigorous abrasion introduces stresses. The generation of heat during abrasion partially relieves such stresses, but if it is too great, it may relieve processing stresses and a warpage may result. The resin surface may even melt. Similarly ,high speed cutting of tooth structure generates excessive heat which can lead to pulpal damage. Therefore, it is very important to control the heat by air/water spray and intermittent cutting.
  8. 8. RATE OF ABRASION The rate of abrasion of a given material by a given abrasive is determined primarily by three factors. SIZE OF THE PARTICLES :  Larger particles cause deeper scratches in the material and wear away the surface at a faster rate.  The scratches caused by the coarse abrasive must then be removed by finer ones. PRESSURE :  Heavy pressure applied by the abrasive will cause deeper scratches and more rapid removal of material.  Heavy pressure is not advisable as it can fracture or dislodge the abrasive from the grinding wheel.
  9. 9. SPEED   The higher the speed, the greater the frequency per unit of time the particles contacts the surface. Increasing the speed increases the rate of abrasion.
  10. 10. CLASSIFICATION    Finishing abrasive : finishing abrasives are hard, coarse abrasives which are used initialy to develop contour and remove gross irregularities. E.g. coarse stones. Polishing abrasives : polishing abrasives have finer particle size and are less hard than abrasives used for finishing. They are used for smoothening surfaces that have been roughened by finishing abrasives. e.g. polishing cakes, pumice, etc. Cleansing abrasives : cleansing abrasives are soft materials with small particle sizes and are intended to remove soft deposits that adhere to enamel or a restorative material.
  11. 11. TYPES OF ABRASIVES :  EMERY : 1)Emery consists of a natural oxide of aluminium called corundum. 2)There are various impurities present in it such as iron oxide, which may also act as an abrasive. 3)The greater the content of alumina, the finer the grade of emery. 4)Pure alumina is also used as a polishing agent.
  12. 12.  ALUMINIUM OXIDE : 1) pure aluminium is manufactured from bauxite. 2) it can be produced in fine grain sizes and has partially replaced emery for abrasive purpose.  GARNET : 1) The mineral comprises of silicates of aluminium, cobalt, magnesium, iron, and manganese. 2) garnet is coated on paper or cloth with glue. 3) it is used on discs which are operated on handpieces.
  13. 13. PUMICE : It is a highly siliceous material of volcanic origin and is used either as an abrasive or polishing agent. KIESELGURH : 1) consists of siliceous remains of minute aquatic plants known as diatoms. 2) it is excellent as a mild abrasive and polishing agent. TRIPOLI : This mild abrasive and polishing agent.
  14. 14.  ROUGE : 1) Rouge is a fine red powder composed of iron oxide. 2) It may be impregnated on paper or cloth known as “crocus cloth” 3) It is an excellent polishing agent for gold and noble metal alloys.  TIN OXIDE : 1) Putty powder used as polishing agent for teeth and metallic restorations in the mouth. 2) It is mixed with water, alcohol or glycerin and used as paste.  CHALK : 1) It is calcium carbonate prepared by precipitation method. 2) There are various grads and physical forms available for different polishing techniques. 3) It is sometimes used in dentifrices.
  15. 15.  CHROMIC OXIDE : 1)A relativity hard abrasive capable of polishing a variety of metals. 2) used as a polishing agent for stainless steel.  SAND : Sand as well as other forms of quartz is used as sand paper or as powder in sandblasting equipment.  CARBIDES : 1) The silicon carbide is sintered, or pressed with a blinder, into grinding wheels or discs. 2) Most of the stone burs used for cutting tooth structure are made of silicon carbide.  DIAMOND : 1) It is the hardest and most effective abrasive for tooth enamel. 2) The chips are impregnated in a blinder or plated on to a metal shank to form the diamond ‘stones’ and disks so popular with the dental profession.
  16. 16. • ZIRCONIUM SILICATE : 1) Occurs in nature as zircon. 2) This mineral is ground to various particle sizes and used as polishing agent. • ZINC OXIDE : 1) Zinc oxide in alcohol can be used for polishing amalgam restorations. • DESIRABLE CHARACTERISTICS OF AN ABRASIVE : 1) Abrasive should be harder than the work it abrades. 2) If it cannot indent the surface to be abraded, it cannot cut it and the abrasive dulls or wears out.
  17. 17. POLISHING • It is production of a smooth mirror like surface without much loss of any external form. • If the particle size of an abrasive is reduced sufficiently, the scratches become extremely fine and may disappear entirely. • The surface then acquires a smooth shiny layer known as a polish. • The polishing agents remove material form the surface molecule by molecule. • The microcrystalline layer is referred to as polish layer or Beilby layer. • A polishing agent is employed only after an abrasive obliterates or eliminates most of the fine scratches, leaving a smooth finish.
  18. 18. NON –ABRASIVE POLISHING Polishing is usually achieved by an abrasive process. However , a smooth shiny surface can also be achieved through non-abrasive means. These include :  COMPOSITE GLAZING : A layer of glaze or gloss is applied over the restoration and cured. This results in a smooth highly glossy surface.  GLAZING CERAMICS : Ceramics are difficult to polish conventionally. The finished restoration is subjected to high temperatures. At this temperature the surface layer and flows to produce a smooth glass-like surface.  ELECTROLYTIC POLISHING : Electrolytic polishing is not truly an abrasive process. Although material is removed, it is removed through an electrochemical process rather an abrasive process.
  19. 19. BURNISHING  It is related to polishing in that the surface is drawn or moved. Instead of using many tiny particles, only one large point is used.  If a round steel point is rubbed over the margins of a gold inlay, the metal is moved so that any gap between the inlay and the tooth can be closed. A special blunt bur revolving at high speed can also be used.
  20. 20. DENTRIFICES • These are agents used with a toothbrush to cleanse and polish natural teeth. • They should have maximum cleansing efficiency with minimum tooth abrasion. • Highly abrasive dentrifices should not be used especially when dentin or cementum is exposed.
  21. 21. FUNCTION • Assists the toothbrush to mechanically remove stains, debris and soft deposits from the teeth. • To impart a polished surface to the tooth. • Thus, they help to reduce caries, maintain healthy gingiva, improve esthetics and reduce mouth odors.
  22. 22. PROPHYLACTIC PASTE • Oral prophylaxis is a widely used procedure in the dental office. • Prophylactic paste may be available commercially or can be made in the dental office. • They are usually employed in paste form by mixing with a suitable vehicle.
  23. 23. FUNCTION : o Remove extrinsic stains, pellicle, materiaalba and oral debris o Impart a highly polished and esthetic appearance. Different types of abrasives may be employed, e.g. zirconium silicate, magnesium silicate, silica, pumice, etc. In addition, some may contain fluoride in order to reduce caries.
  24. 24. PROPHYJET : The prophyjet is a relatively new dental prophylaxis system of removing intraoral stains. An abrasive blasting process is used to mechanically remove extrinsic stains as well as light supragingival adherent plaque the calculas. COMPOSITION : Consists of sodium carbonate, hydrophobic modified silica and some flavoring agent. The abrasive is available as powder in a small sachet. CLINICAL CONSIDERATIONS : The prophyjet is directed at 45 angles to the tooth surface. For obvious reasons it is less effective in proximal areas. The chances of soft tissue injury exist especially if the tissue is inflamed and friable.
  25. 25. DENTURE CLEANSERS BRUSHING : a) The dentures may be brushed using a soft bristle brush and gentle abrasive or cream. b) Hard abrasive and stiff bristles should be avoided because they may produce scratches on the denture surface. SOAKING : Chemical cleaners are an alternative to brushing especially among very old or handicapped persons. a) Alkaline perborates b) Alkaline peroxides c) Alkaline hypochlorites d) Dilute acids.
  26. 26. DR .K.JESUDOSS M.D.S DEPT OF PROSTHODONTICS

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