Materials used in restorations/ orthodontic course by indian dental academy


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Materials used in restorations/ orthodontic course by indian dental academy

  1. 1. MATERIALS USED IN FIXED RESTORATIONS Materials used in fixed restorations can be classified as: Metals Porcelain Resins and solders METALS Taggart in 1907 introduced the “lost wax” technique for casting dental restorations. Veneering of metal substructure with porcelain became successful in the late 1950’s Classification of dental casting alloys A. According to function 1. Gold casting alloys (Bureau of standards,1927). Type I (soft). Small inlays-easily burnished, subject to slight stress. Type II (medium) Inlays subject to moderate stress, thick three-quarter crowns, abutments, Pontics and full crowns. Type III (hard). Inlays subject to high stress, thin three quarter crowns, thin cast backings, abutments pontics, full crowns, denture bases and short span fixed partial dentures. These alloys can be age hardened. 1
  2. 2. Type IV (extra hard) Inlays subject to very high stress, denture base bars, clasps, long span FPDS, full crowns can be age hardened. Types III and IV are generally called “Crown and bridge alloys”. 2. Metal ceramic (hard and extra hard) Suitable for veneering with dental porcelain, copings, thin walled crowns, short span FPDS (hard type) and long span FPDS (extra hard type) 3. Removable partial denture alloys-Base metal alloys and type IV gold alloys. B. According to description (composition). 1. Crown and bridge alloys. a) Gold based (noble) i) type III& IV gold (high gold) ii) alternative crown and bridge alloys (low gold)containing less than 60% but more than 40%Au. b) Non gold-based i) Sliver palladium alloys 70 –72 %Ag, 25%Pd. Pd resists the tarnishing of Ag. Some Ag-pd alloys contain small amounts (15%) of Cu and have properties similar to type IV gold alloys. ii) Base metal alloys Ni-or co- based, cheaper 2
  3. 3. 2. Metal ceramic alloys a) Noble metal alloys i) Gold –platinum – palladium ii) Gold –palladium –silver iii) Gold –palladium iv) Palladium –silver v) High palladium b) Base metal alloys i) Nickel – chromium ii) Cobalt – chromium iii) Other systems The silver in pd-Ag alloys can cause discoloration (yellow, green or brown) of some porcelains. Non-greening porcelain systems have partially over come this. 3
  4. 4. Typical compositions of some modern noble metal dental alloys Au% Cu% Ag% Pd% In, Sn, Fe Ga, Zn Type I Gold 83 6 10 0.5 Balance Type II Gold 77 7 14 1 Balance Type III Gold 75 9 11 3.5 Balance Type III Low gold 46 8 39 6 Balance Type III Ag-Pd - - 70 25 Balance Type IV Gold 69 10 12.5 3.5 (+30.opt) Balance Type IV Low gold 56 14 25 4 Balance Type IV Ag-pd 15 14 45 25 Balance Metal ceramic (White) Gold 52 - - 38 Balance Metal ceramic Pd-Ag - - 30 60 Balance Metal ceramic (yellow) Gold 88 - 1 6.5(+4.0pt) Balance Metal ceramic High pd 0-6 0-15 or 0- 8Co 0-6.5 74-88 Balance The physical properties and handling characteristics of Ni-Cr alloys are improved by addition of 2%by weight of beryllium. One particular brand of Ni- Cr-Be alloy has a low enough casting temperature to be successfully cast into a gypsum bonded investment. Ni gives strength and Cr the passivating effect which makes the alloy corrosion resistant. Be reduces vision temperature, 4
  5. 5. improves casting characteristics, refines grain structure and participates in porcelain bonding. Metal ceramic alloys have 3 common features: a) The potential to bond to dental porcelain b) Coefficient of thermal expansion compatible with porcelain c) Sufficiently high solidus temperature permitting the application of low fusing porcelains. Properties of modern crown and bridge alloys Among the minor additives, zinc is added primarily as an oxygen scavenger. In the absence of Zn, silver causes absorption of O2 during melting, the O2 rejected during solidification causes gas porosity. Indium, tin and iron harden the alloy. The elimination of Ag from these alloys markedly decreases the propensity for the green stain at the margins of the metal porcelain interface. All modern noble metal crown and bridge alloys are fine grain. Copper is the principal hardener; in excessive amounts it reddens the yellow alloys and reduces resistance to tarnish and corrosion. Silver minimizes this reddening effect. Pd hardens and whitens the alloy and reduces its cost. 5
  6. 6. Lower gold content alloys A 42%gold alloy containing 9% palladium was clinically found to tarnish less than a75%gold alloy containing no palladium. This knowledge led to the introduction of Ag-pd type III and IV alloys containing little, of any, gold. In type IV Ag-Pd alloy, gold is added not for its nobility and colour, but for its age hardening effect. When Cu is added to the Ag-pd alloy, the melting range is reduced to permit the use of gypsum bonded investment and gas air torch. Physical properties The upper limit of the melting range is the liquidus. When 75to 1500 c is added to the liquidus, we arrive at the casting temperature. The lower limit or solidus can similarly be used to obtain the maximum soldering temperature. The metal ceramic alloys should have high melting range so that the metal is solid well above the porcelain baking temperature to minimize distortion (sag) of the casting. Heat treatment of noble metal alloys Gold alloys can be subjected to hardening heat treatment or age hardening, if the alloy contains sufficient amount of Cu. Type I, and II alloys do not harden like type III and IV alloys. The alloys can also be softened by softening heat treatment or solution heat treatment. 6
  7. 7. Softening heat treatment The casting is placed in a furnace at 700°C for 10 minutes and then quenched in water. All intermediate phases are changed to a disordered solid solution, and the rapid quenching prevents ordering during cooling. Tensile strength, proportional limit and hardness are reduced by such a treatment, but the ductility is increased. This enables the metal to be ground, shaped or otherwise cold worked, either in or out of the mouth. Hardening heat treatment The dental casting is soaked or aged for 15 to 30 minutes at 200°C to 450°C. The casting is subjected to a softening heat treatment to relieve all strain hardening before a hardening heat treatment. The proportional limit (or yield strength) and modulus of resilience and hardness are increased which makes the prosthesis withstand mechanical stresses without permanent deformation. Some ductility is essential if margin and adjustment and burnishing are to be done. But a cast prosthesis that has undergone plastic deformation has failed in service. Ductility is decreased by age hardening. Casting shrinkage This occurs in three stages 1) Thermal contraction of the liquid metal between the temperature to which it is heated and the liquidus temperature. 2) Contraction of metal from liquid to solid state; and 3) Thermal contraction of solid melt down to room temperature. 7
  8. 8. Linear casting shrinkage of inlay casting gold alloys Metal Casting shrinkage (%) Gold (100%) 22-karat alloy Type-I Type-II Type-III Base metal 1.67 1.50 1.56 1.37 1.42 2.4% Platinum, palladium and copper are effective in reducing casting shrinkage. As thermal contraction of the alloy as it cools to room temperature dominates casting shrinkage the higher melting alloys tend to exhibit greater shrinkage. General features of metal ceramic alloys Porcelain has low tensile and shear strength but can resist compressive stresses with reasonable success. To facilitate compressive loading, and porcelain is fused to a cast alloy substructure which fits over the prepared tooth, this can avoid or minimize brittle fracture. Earlier, mechanical retention and undercuts were used to prevent detachment of the ceramic veneer. By adding less than 1% oxide forming elements such as iron, indium and tin to the high gold content alloy, the porcelain-metal bond strength was improved 3 times. 8
  9. 9. Mechanical properties The prosthesis should be rigid to avoid brittle fracture of porcelain. Doubling the thickness of the metal substructure increases the rigidity by a factor of 8. But occlusion and esthetics limit the extent to which the metal thickness can be increased. Base metal alloys have a modulus of elasticity approximately thrice that of previously used gold alloys and hence are more suitable for long span bridges and thinner castings. Base metals are harder, reducing occlusal wear significantly. Density of base metal alloys is 8.0 gm/cm3 compared to 18.39 gm/cm3 for comparable noble metal alloys, thus making centrifugal casting of base metal alloys easier and precise. Sag resistance is the ability of an alloy to resist permanent deformation or wear induced by thermal stresses. It is particularly important in long span bridges during porcelain firing. Base metal alloys will deform less than 0.001 inch, while a noble metal alloy will deform 0.009 inch. The higher fusion temperature of base metal alloys also contributes to their superior sag resistance. To be compatible, the alloy must not interact with the ceramic so as to visibly discolour the porcelain, and their bond should be strong. The use of base metal alloys has increased rapidly at the expense of the high noble metal ceramic alloys. Working characteristics a) Ease of casting : Alloy must be easy to melt and must rapidly fill the mold. 9
  10. 10. b) Ease of soldering : the liquid solder must wet the alloy surface readily, noble metal alloys render themselves well to both pre-ceramic and post- ceramic soldering. c) Ease of burnishing : noble metals with high-gold or high palladium contents are burnishable. Ni-Cr alloys have lower casting accuracy and greater surface roughening than cold alloys, but higher strength and sag resistance. Casting investments a) Gypsum bonded investments – for gold based crown and bridge alloys. b) Carbon containing phosphate bonded investments – for gold based metal ceramic alloys. c) Non-carbon phosphate bonded investments – for non gold based alloys like Ni-Cr or Co-Cr alloys. Biological considerations Inhalation of dust and fumes of Beryllium is toxic and hence exhaust ventilation is necessary. Aspiration of Ni containing dust can be carcinogenic. Ni can also cause contact dermatitis and hence is contraindicated in Ni- sensitive patients. Etching base metal alloys Maryland bridge utilizes micromechanical retention of etched-metal resin-bonded retainers. The etching of metal surface can be done either electrolytically or using chemical etchants. 10
  11. 11. Recycling of noble metal casting alloys Noble metal alloys are significantly stable to react two or three times. The non-volatile base metals like Zn, In, Sn and Fe may be lost during remelting and this loss can be compensated by adding equal amounts of fresh alloy to the scrap before melting. Dental Ceramics Dental porcelains are used to make denture teeth, single unit crowns, fixed partial dentures and labial veneers. Single unit crown may be porcelain jacket crown (PJC), a metal ceramic crown or porcelain-fused to metal restoration (PFM), or the newer glass-ceramic crown. General Considerations Composition i. Silica (SiO2) the crystalline form or quartz is used. ii. Sodium, potassium or calcium carbonate increases the fluidity and decreases the softening temperature. These glass modifiers are added in varying amounts to produce three types of porcelains based on their firing temperature. High fusing : 2350 to 2500°F (1290 to 1370°C) Medium fusing : 2000 to 2300°F (1095 to 1260°C) Low fusing : 1600 to 1950°F (870 to 1065°C) 11
  12. 12. iii. Feldspar – a natural mineral containing potash (K2O), alumina (Al2O3), and silica (SiO2). Fledspar when fired at high temperatures can form a glass phase that softens and flows slightly at porcelain firing temperatures. This softened phase allows the porcelain particles to coalesce togather at high temperature without complete melting – a process referred to as sintering. Feldspar when heated between 1150°C and 1530°C undergoes incongruent melting to form the crystalling mineral leucite, which is potassium-aluminium-silicate mineral with a large coefficient of thermal expansion. This property is utilized in the manufacturer of porcelains for fusing to metal. iv. Other additions : Boric oxide (B2O3) is added in small amounts to act as a glass modifier to decrease the viscosity and lower the softening temperature. It also forms its own glass network. Pigmenting oxides are added to obtain various shades to simulate natural teeth. These pigments are produced by fusing metallic oxides together with fine glass and feldspar and then regrinding to a powder. These powders are blended with unpigmented powdered frit to provide proper hue and shade. Brown – Fe or Ni oxides Green – Cu oxide Yellowish brown – Ti oxide Lavender – Mn oxide Blue – Co oxide Opacity is achieved by adding Zirconium, Titanium, Tin oxides. 12
  13. 13. Mechanical behaviour and physical properties Materials fail to exhibit the strengths that we expect from interatomic bonds. This is because of the minute scratches and other defects present on their surface. The defects have sharp notches whose tips are as narrow as the spacing between the atoms. Due to stress concentration at the tips of the notches the bonds at the notch tip break leading to crack propagation. As the brittle ceramic have no mechanism for yielding to stress without fracture as do metals, cracks propagate at low stress levels. So their tensile strengths are much lower than their compressive strengths. Methods of strengthening porcelain Smoothen and reduction of surface flow is one of the reasons for glazing dental porcelain, which produces a very large increase in their strength. Strengthening of brittle materials can be done either by the introduction of residual compressive stresses into the surface of the material or by the interruption of crack propagation through the material. 1) Introduction of residual compressive stresses Strengthening is gained by virtue of the fact that there residual stresses must first be negated by the applied force before any tensile stresses can be created in the object. Residual compressive stresses can be introduced by the following techniques. 13
  14. 14. a) Ion exchange (chemical tempering) A sodium containing glass article is placed in a bath of molten potassium nitrate, when some K ion in the bath exchange places with Na ions on the glass surface. The larger K ions squeeze into the same space occupied by the Na ions leading to a very large increase in residual compressive stress in the glass surface. b) Thermal tempering Here the object is rapidly cooled (quenched) while it is in the soft (molten) state. As the solidifying molten cone tries to shrink or pull the rigid solidified outer skin, residual compressive stresses are created in the outer skin. c) Thermal expansion coefficient mismatch Here a metal housing slightly larger coefficient of thermal expansion is used. During cooling from the firing temperature, the metal contracts slightly more than the porcelain. This mismatch leaves the porcelain in residual compression. 2) Interruption of crack propagation a) Dispersion of a crystalline phase. A tough crystalline material such as alumina (Al2O3) is added to glass in a particulate form. The glass is toughened and strengthened because the crack cannot penetrate the alumina particles easily. This technique has been utilized 14
  15. 15. in the development of “alumina particles easily. This technique has been utilized in the development of aluminous porcelains”. This technique is also used in the cast glass crown Dicor where the glass crown is subjected to a heat treatment that causes microscopic mica crystals to grow in the glass, these crystals interrupt crack propagation. For maximum reinforcing effect, the dispersed phase should have a minimum difference in thermal expansion with the glass. b) Transformation toughening This involves incorporation of a crystalline material that is capable of undergoing a change in crystal structure when placed under stress, absorbing the energy from the crack. Partially stabilized zirconia (PSZ) is the usually used crystalline material. The disadvantage is that it can produce an opacifying effect. Design of ceramic restorations The design should avoid subjecting the porcelain to high tensile stresses. So PJCs are contraindicted for restoring posterior teeth. Even on anterior teeth with deep vertical overlap and moderate horizontal overlap PFM restoration is to be preferred over PJC. To prevent stress concentration sharp line angles on the preparation and sudden changes in porcelain thickness should be avoided. The coping surface in PFM should also not have sharp line angles. 15
  16. 16. Colour of porcelain Porcelain is an esthetic restorative material capable of matching the adjacent tooth in translucence, colour and intensity. Complete colour matching is difficult. The same object may show slight variation in colour when viewed under different types of light sources this is the phenomenon of metamerism. A shade guide is used to match the colour. Ideally colour matching is done under the illumination of northern light from a blue sky as this light contains the most even balance of light wavelengths. If this light source cannot be obtained, colour matching should be done under two or more different light sources. The opacity, of the cementing medium also affects the esthetic qualities of a PJC. Zinc phosphate cement is opaque whereas silicophosphate and glass ionomer cements are more translucent. Many cements are specifically tinted for colour matching. Fabrication of a ceramic restorations Condensation : Porcelain is supplied as a fine powder that is mixed with distilled water or another vehicle and condensed into the desired form. Particles of different sizes allow dense packing. Dense packing has the benefits of lower firing and less porosity in the fired porcelain. Condensation is achieved by vibration, spatulation and brush techniques. When mild vibrations used for packing, the excess water is blotted away with a clean tissue. In the second method, a small spatula is used to apply and smooth the wet porcelain. The smoothen action brings excess water to the surface, where it is removed. In the brush technique, dry powder is placed with a brush to the side opposite from an increment of wet porcelain. As the water is 16
  17. 17. drawn toward the dry powder, the wet particles are pulled together (by capillary action). The porcelain must never be allowed to dry out before condensation is complete. Firing procedures After condensation, the restoration is placed on a fire-clay slab or tray and inserted in the muffle of a porcelain furnace. Porcelain should not contact the muffle walls or floor. Porcelain can embrittle the heating element if the latter is contracted. During firing the powder particles fuse together. The condensed porcelain is first placed in front of the muffle of a preheated furnace (approximately 650°C) for 5 minutes to permit the water vapour to dissipate, before the firing. During firing, the porcelain particles unite at their points of contact and then the fused glass gradually flows to fill up the air spaces. However, the mass is too viscous to allow the escape of air. Porosity can be reduced by vacuum offset firing. Glazing Stains and glazes provide a more life-like appearance. External staining is subject to chemical durability, problems. Internal staining is permanent and life-like, particularly when simulated craze lines are built into it. Internal staining and characterization have the disadvantage that the porcelain must be completely stripped if staining is unsuitable. Glazed porcelain is much stronger and prevents crack propagation then adjust the occlusion, the transverse strength is halved. 17
  18. 18. Cooling : sudden cooling from the firing temperature can fracture the glass. Cooling a metal ceramic restoration too slowly can cause the coefficient of thermal expansion of porcelain to increase and can actually make it more likely to crack or craze. Metal ceramic crown When porcelain is bonded to an inner skin of metal, cracks can develop only when the metal is deformed or broken. Esthetically, the PFM restorations are slightly inferior to the PJC. PFM utilizes cast or non cast metal copings. Cast coping To be fused to metal, the porcelains have to be low fusing and have a coefficient of thermal expansion considerably higher than ordinary porcelains. The alloys used should have higher melting ranges to prevent sag, creep or melting during firing. Gold alloys used for cast coping contain about 1% of base metals such as Fe, In and Sn which form a surface oxide layer during “degassing” and this layer is responsible for development of a bond with porcelain. The porcelain – metal bond is primarily chemical in nature and is capable of forming even when the metal surface is smooth i.e. when there is no opportunity for mechanical interlocking. Both metal and ceramic must have closely matched coefficients of thermal expansion, to minimize residual thermal stresses in the latter. 18
  19. 19. In PFM fabrication, the ceramic should contain greater amounts of soda and potash to increase the thermal expansion to a level compatible with the metal. Opaque porcelains contain large amounts of metallic oxide opacifiers to conceal the underlying metal and to minimize the thickness of the opaque layer. The metal and porcelain should preferably have compatible thermal conductivity to resist thermal shock. Because of the high melting temperature of the alloys, gypsum investments can not be used, a phosphate bonded or silica bonded investment is used. Thermal expansion is utilized to compensate casting shrinkage. The casting should be carefully cleaned to ensure a strong bond to porcelain. Degassing also burns off surface impurities. Oil from fingers can be a contaminant. Ceramic bonded stones may be used for cleaning the surface. Final texturing with an 25 alumina air abrasive makes porcelain bond to mechanically receptive surface. Opaque porcelain is condensed to a thickness of 0.2mm and fired to its maturing temperature. This is followed by translucent porcelain and finally the glaze. Unlike acrylic resin veneered structures there is almost no wear by abrasion or change in colour because of microleakage between porcelain veneer and metal PFM requires removal of more tooth structure than for PJC. Bonded platinum foil coping Here tin oxide coating on platinum foil is utilized for porcelain bonding. Here the thicker metal coping is replaced by a thin platinum foil, giving more room for porcelain. Aluminous porcelain is used. 19
  20. 20. Swaged gold alloy foil coping Renaissance is a laminated gold alloy foil having a fluted shaped which is swaged on to the die and flame –sintered to form a coping. An “interfacial alloy”, powder is applied and fired, and the coping is then veneered with porcelain. Porcelain-metal bond Chemical and mechanical bonds exist. Alloys that form adherent oxides during degassing form good chemical bond with porcelain, whereas those alloys with poorly adherent oxides form poor bonds. Minor elements like Sn or In are believed to migrate to the interface where they oxidize and form covalent or ionic bonds across the interface. Some Pd-Ag alloys form no external oxide at all, but rather oxidize internally, these alloys need mechanical bonding. Shear tests show that bond failure can be cohesive through the porcelain, metal-oxide or metal, or adhesive at the metal-porcelain, metal oxide-porcelain or metal oxide-metal interfaces, or a mixture of cohesive and adhesive shear strength and tensile strength of porcelain are when fired in oxygen than when vacuum fired. Bonding using electrode position Electrode-position of a layer pure gold onto the cast metal, followed by a short “flashing deposition of tin, has been shown to improve the wetting of porcelain onto the metal and to reduce porosity at the porcelain metal interface. The electrodeposited layer also inhibits ion penetration from the metal, and acts as a buffer zone to absorb stresses caused by differentials in the coefficients of 20
  21. 21. thermal expansion between the metal casting and the porcelain during cooling. The gold colour of the oxide film enhances the vitality of the porcelain when compared with the normal dark oxides that require heavy opaque layers. Castable Glass ceramic crown The castable glass ceramic, or Dicor was introduced to dentistry in 1984. Glass ceramics are composite materials of a glassy matrix phase and a crystal phase. Dicor is comprised of SiO2, K2O, MgO, MgF2, small amounts of Al2O3 and ZrO2 and a fluoresing agent. It is technically described as Tetrasilicic fluoromica glass-ceramic. It is formed into full crown restorations by a lost wax casting process. After the transparent glass casting is recovered, it is subjected to a heat treatment to induce partial devitrification (i.e. loss of glass structure by crystallization), a process called ceramming. Ceramming causes microscopic plate- like particles of crystalline material (mica) to grow within the glass matrix. After ceramming, it is coated with a thin layer of porcelain to provide esthetics. The final colour of the restoration is due, in part from the colour picked up from the adjacent teeth (“chemeleon” effect) and in part from the tinted cements used in luting. Flaws (Griffith’s flaws) developing on the surface of glass are prevented from propagating, by the mica crystals. The marginal adaptation or fit of Dicor is better than gold crowns. The biocompatibility of glass ceramics is excellent. The soft tissue response of glass ceramic restoration is similar to that of unrestored control teeth because: a. The marginal adaptation is exceptional 21
  22. 22. b. The fluoride content of the material inhibits bacterial colonization and c. The surface of the restoration is smooth and non porous. Dicor has a low wear potential and low thermal conductivity that insulates the underlying tooth from changes in temperature. The fabrication of Dicor is simple, as the lost wax technique is used. Castable ceramics provide life like vitality. Dicor can be used for single restoration like full veneer restorations on anterior and posterior teeth, inlays, onlays, three-quarter crowns, partial veneers and recently laminate veneers. It is contraindicated on teeth with short clinical crowns. Another castable glass ceramic developed in Japan produces hydroxyapatite crystals in the glass matrix instead of mica crystals, on ceramming. Injection molded glass ceramic crown This is a shrink free ceramic crown, marketed originally under the name Cerestore. Conventional ceramics shrink 10 to 20% during firing. The primary constituents in Cerestore are MgO, Al2O3, glass frit, silicone resin and kaolin. These non shrink ceramics have good flexural strength. The technique involves construction of a special non shrinking epoxy die of the prepared tooth. A wax pattern of the coping is found on this die. The die and pattern are invested in a gypsum bonded investment and the wax removed with boiling water. The investment mold is then heated to 180°C. the ceramic material supplied as dense pellets is heated to until the silicone retim carrier in the ceramic is flowable and then injection molded by pressure into 22
  23. 23. the heated mold. The green state coping is retrieved, sprue removed and any adjustments made. It is then subjected to a very high temperature, firing cycle to form a true glass ceramic core or coping. Over this coping low fusing dentin and enamel porcelains are applied to develop the external shape and esthetics. The equipment required is specialized and expensive. The technique is time consuming and calls for extra attention to detail. Porcelain veneers, inlays and onlays Here the tooth enamel or metal is etched and resin cement is used as the cementing agent for the porcelain laminates. Ceramic veneers can be used on stained hypoplastic teeth, and provide excellent esthetics. Cost and wear of opposing natural teeth are the drawbacks. Chemical stability Topical fluorides such as APF and stannous fluoride, used for caries control, produce hydrofluoric acid which etches glass and leads to surface roughness of ceramic restorations. Hence, APF gels should not be used when glazed porcelain restorations are present. If such a gel is used, the surface of the restoration should be protected with Vaseline, cocoa butter or wax. Resins Resins may be indicated for an individual restoration or as a veneer over a casting. 23
  24. 24. Advantages Esthetics Low cost Convenient repair, even intra orally Ease of fabrication No abrasion of opposing teeth Disadvantages Low proportional limit and pronounced plastic deformation distortion on occlusal loading, hence resin should be protected with metal occlusal surface. Microleakage and staining under veneers Dimensional change during thermal cycling and water sorption Surface staining and intrinsic discolouration. Tooth brush wear Resin veneered metal restoration unsuitable for RPD clasping. Types of synthetic resins Type I (acrylic) Type II (dimethacrylate) Type III (composite) Acrylic resins are powder liquid systems based on methyl methacrylate and similar to self cured acrylic resins. Dimethacrylate resins are cured at higher temperatures and yield cross linked wear resistant resins. Microfilled 24
  25. 25. composite resins use BIS-GMA, Urethane dimethacrylate, or 4, 8 di(methacryloxy methylene)-tricyclo-( decane resin matrixes. These new resins are polymerized using light, or heat and pressure. Microfilled resins have superior physical properties including better wear resistance than the original unfilled resin. Earlier resins had low strength and hardness, and high water sorption. The accelerated loss of material exposed the metal framework, which required repair with a direct filling resin. The disparity in thermal expansion and lack of adhesion between resin and metal lead to percolation of fluids at the resin-metal interface contributing to discolouration of the resin and corrosion of non-noble alloys. Rigidity of metal frame work is needed to prevent plastic deformation. Processing porosity also leads to weakness of resin, opaque appearance, potential for incubating micro-organisms and tissue irritation due to roughness. Porcelains have largely replaced resins. Resins are indicated where porcelains cause undue wear of opposing teeth and restorations. The development of wear resistant, esthetic resin material is warranted to meet clinical demands. An acrylic resin called Pyroplast is still used for esthetic veneering of castings. The polymer is mixed with monomer and applied in small increments to the casting. It is cured in a special curing over at 275°F for 8 minutes. Then the gingival and incisal colours are applied and blended, curing follows each lamination. After processing the veneer is finished and polished. 25
  26. 26. Composite Resins Isosit, was the first chemically activated composite resin used for FPD work. It is cured using pressure and temperature. The majority of composite resin material, use visible light for polymerization. A single paste is used. One system utilizes a diketone, camphoroquinone, and a reducing agent N,N-dimethyl aminoethyl- methacrylate. Resin Retention Mechanical retention or an intermediary coupling agent is used in bonding, resin to metals framework. Retentive beads, loops or ladders have been suggested. Opaque layer does not obstruct the retentive patterns completely, the resin is also locked in. Adhesive coupling agents are a recent introduction one system utilizes flaming silica onto the metal. Another system of resin retention involves electrolytically etching a microretentive surface, high bond strengths are accomplished. These new techniques allow a more conservative preparation, reduced cost and improved esthetics. A disadvantage is the difficulty in clinical repairs of fractured veneers. Clinical applications In resin veneer areas, tooth structure is to be reduced by 1.5 to 2mm depth. A beveled solder is prepared on the labial surfaces into the interproximal 26
  27. 27. surfaces. It blends into a Chamfer finish line in veneer areas. The occlusal surface of the restoration should be in metal. Complete crowns in resin are only interim restorations. In mandibular central and lateral incisors extensive tooth reduction can be avoided using resin over minimum metallic framework. 1 to 1.5mm reduction is enough. The pontic of an acid etched, resin bonded retainer Maryland bridge is usually fabricated with dental porcelain and render itself to electrolytic etching. A heat cured composite material for pontic is a reasonable solution, which is costless. The tissue surface of pontic can be alloy, which produces a favourable tissue response. Custom laminate veneers Here the teeth are minimally prepared to receive resin veneers 0.5 to 1mm reduction gives attractive results. The fabricated heat cured laminates are bonded to the etched enamel surface using a composite resin. Additional applications Recently composite resin veneering materials have been considered for use as implant materials and also for custom occlusal splint therapy. Solders Soldering is the joining together of metal parts by melting a filler between them at a temperature below the solidus temperature of the metal being joined and below 450°C. 27
  28. 28. Jelenko classifies solders as: Group I – traditional gold containing solders Group II – others (special solders) Pre ceramic soldering refers to soldering before porcelain application and post ceramic soldering after porcelain application. Pre-ceramic solders are high fusing, fusing only slightly beneath the softening point of the parent alloy. They should flow well above the fusion temperature of the subsequently applied porcelain. Post ceramic solders must flow well below the pyroplastic range of porcelain. Dental gold solders are given a fineness number to indicate the proportion of pure gold contained in 1000 parts of alloy. A 585 fine solder contains 58.5% Au, 14% Ag, 19% Cu, 3.50% Sn and 4.5% Zn, and has a flow temperature of 780°C. The main requirement of solder is that it fuses safely below the sag or creep temperature of the casting to be soldered. Pre-ceramic soldering is relatively difficult and structurally hazardous due to volatilization of base metal solder constituents due to overheating. Volatilization causes pitting or microporosity. Porcelain does not chemically bond equally well to all solders. Solders should also resist tarnish and corrosion, should flow easily, match the colour of the units being joined and be strong. 28
  29. 29. The noble metal content and Ag: Cu ratio determine the solder’s tarnish resistance. If the composition of solder and the parent metal differ galvanic corrosion results. The surfaces to be soldered should be smoothed with abrasive disks and not with rubber wheels or polishing compounds. The solder must wet or flow freely over the metal surface. Ag increases and Cu decreases the flow Low fineness gold solders are often more fluid. Proximal contacts are added, if needed with a higher fineness solder since it flows less. The strength of most solders is greater than the parent metal. Brittleness is often seen with gold based Cu containing solders, on cooling to room temperature. FPDs fabricated from type III gold alloys are joined with gold based solders and usually water quenched 4 to 5 minutes after soldering. Quenching immediately after soldering causes warping of the FPD; not quenching leaves a joint with little or no ductility. A brittle joint may easily fracture. Thus a disadvantage of post-ceramic soldering is the loss of joint ductility. Since the components are partially porcelain, quenching is not possible because porcelain fracture will occur. Conclusion The development of newer alloys and porcelains with better working properties are progressing in an encouraging manner. Researchers are hopeful in their endeavour to minimize or totally eliminate the drawbacks that are associated with these materials. 29
  30. 30. References 1. Skinner’s Science of Dental Materials, 9th Ed – Ralph W. Phillips 2. Contemporary fixed Prosthodontics, 1st Ed – Stephen F. Rosenstiel, et al 3. Tylman’s Theory and Practice of fixed Prosthodontics, 8th Ed – W.F.P. Malone et al. 30