Denture base resins/ cosmetic dentistry training


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Denture base resins/ cosmetic dentistry training

  1. 1. SEMINAR ON DENTURE BASE RESINS INDIAN DENTAL ACADEMY Leader in continuing dental education
  2. 2. Contents Introduction History Requirements Classification Composition - heat cure - autopolymerising resin -other resin materials Aspects of manipulation Recent advances(modified acrylics) Summary and conclusion References
  3. 3. Introduction Denture base is that part of a denture that rests on the foundation tissues and to which teeth are attached.Although individual denture bases may be formed from metals or metal alloys, the majority of denture bases are fabricated using polymers.
  4. 4. History 1851 – Vulcanite was developed. 1853-1st denture base from vulcanite was constructed by Nelson Charles Goodyear, 1891- use of vulcanite as a denture base material was considered “universal”. 1869- Celluloid was introduced, developed from plant framework. It had natural gingival colour but short shelf life.therefore, NEW MODE CONTINOUS combination product, of rubber denture base material, a celluloid gum work area and porcelain teeth.
  5. 5. 1910-1940 Other polymers:vinyl acrylics,Polystyrene, Epoxy,Nylon, Vinyl styrene, Polycarbonates, Polysulfonates-unsaturated Polyester, Polyurethane,Polyvinyl acetate- ethylene, Hydrophilic polyacrylate,Silicones,Light activated urethane dimethacrylate,Rubber reinforced acrylics, and Butadiene- reinforced acrylics. 1930-1940 the Phenol formaldehyde products were 1st to compete with vulcanite. Self cure were introduced after world war II.
  6. 6. 1936- Vernonite or poly methylmethacrylate(PMMA). 1st practical replacement for Vulcanite. 1946- 95% of dentures were made with Vernonite
  7. 7. Plastic other than acrylic resins Nylon: found unsatisfactory owing to its poor ability to resist oral conditions-swelled badly. -if left outside water will shrink. The epoxy resins:drawbacks -high rate of water sorption with dimensional change,fracture,loss of posterior teeth,heavy deposition of various stains and calculus,colour change and some fouling odour. -Phenol formaldehyde(Bakelite):proved too difficult to process. •Color instability. Vinyl resins:low resistance to fracture. •Failure were common.
  8. 8. Polycarbonates:Contains 10% glass fibers of 150um length and 10 um diameter. •Good impact strength- nine times more than PMMA. •More difficult to mould dentally, since injection moulding technique is required. •Greater water sorption,higher flexibility,lower hardness,lower adhesion to acrylic teeth. Acrylics: Better aesthetic quality •Easy to process using inexpensive techniques. •Shrink at the rate of 1um in eight hours. if left outside
  9. 9. Requirements 1. Biological: - non toxic and non-irritant. 2. Chemical: - inert , in-soluble and non-absorbent. 3. Physical: - Aesthetically satisfactory. - The softening temperature should be higher than the temperature of liquids and food ingested. - Dimensionally stable. - Low value of specific gravity or density.
  10. 10. - High value of thermal conductivity: to maintain the healthy mucosa,and to retain normal reaction to hot and cold stimuli. - It should be radio-opaque. 4. Mechanical properties: - A high value of modulus of elasticity- For greater rigidity. - A high value of elastic limit to prevent permanent deformation. - A combination of both above would allow the base to be fabricated in thin sections.
  11. 11. - Sufficient flexural strength to resist fracture. - An adequate fatigue life and a high fatigue limit. - Good impact strength. - Sufficient abrasion resistance to prevent excessive wear. 5. Miscellaneous - Relatively inexpensive. - Easy to manipulate and fabricate. - Easy to clean. - Easy to repair.
  12. 12. CLASSIFICATION I. According to ISO 1567 Type Class Description 1 1 Heat processing (powder and liquid) 1 2 Heat processing (plastic cake) 2 1 Self cure (powder and liquid) 2 1 Self cure (powder and liquid pour type resins) 3 - Thermoplastic blank or powder 4 - Light-activated materials 5 - Microwave
  13. 13. II. According to material system 1. Metallic – stainless steel denture base 2. Non-metallic – acrylic resins, etc. III. According to mode of polymerization 1. Addition polymer – e.g. acrylic resins, polyvinyl chloride, polymethyl methacrylate 2. Condensation polymer – e.g. bakelite, nylon IV.According to method of activation 1. Thermal – heat cure acrylics 2. Chemical- self cure acrylics 3. Light activated 4. Microwave
  14. 14. Acrylic resin materials Polymerisation reaction: The polymerisation of PMMA involves the conversion of low molecular weight monomer to high molecular weight polymer. Free radical addition polymerisation which involves activation, initiation ,propagation and termination.
  15. 15. Initiation Free radical Monomer
  16. 16. Propagation
  17. 17. Termination
  18. 18. Chain transfer
  19. 19. Composition of heat cure denture base resin Powder a. Prepolymerised polymethyl methacrylate in the form of beads or granules. b. Initiators – Benzoyl peroxide (0.5-1.5%), to initiate the polymerization of the monomer c. Plasticizer – Dibutylpthalate (8-10%), increases the rate of dissolution of polymer in monomer. d. Pigments – Cadmium selenide (pink), Mercuric sulfide (red),Rouge/ Ferric oxide(brown). e. Synthetic fibers:Acrylic or nylon to give veined appearance.
  20. 20. f. Inorganic particles-Glass fibers zirconium silicate, silicone carbide etc. to increase the stiffness and decrease the coefficient of thermal expansion. g. Opacifiers-Heavy metal compounds, e.g. salts of Barium and Strontium. Liquid a. Methyl methacrylate monomer- It has a limited shelf life b. Inhibitor- Hydroquinone, added to improve the shelf life of monomer c. Cross linking agent- Ethylene glycoldimethacrylate, it improves the mechanical properties, more resistant to surface crazing.
  21. 21. d. Plasticizer- Dibutylpthalate, to produce a softer and more resilient polymer. Gel types – e.g. Vinyl acrylics •Liquid and powder have been mixed to form a gel and shaped into a thick sheet. •Chemical accelerators cannot be used.
  22. 22. Self cure resins: Dispensed as powder and monomer liquid. Composition: Polymer: PMMA in the form of beads. Copolymer-PEMA Initiator (< 2%) Benzoyl peroxide Plasticizer dibutyl phthalate Colour pigments: salts of Mercury and Iron Dyed synthetic fibers Inorganic particles glass fibers, Zirconium Silicate Radio-Opacifiers-salts of Barium .
  23. 23. Monomer: Methylmethacrylate monomer. Inhibitor- hydroquinone Cross linking agent- glycol dimethacrylate Plasticizer- dibutylpthalate Activator- dimethyl para toludine Setting reaction: PMMA +initiator+MMA+inhibitor+activator (powder) (liquid) free radicals(R°)
  24. 24. R° + M RM° Liberation of heat. The rate of polymerisation is influenced by particle size. Properties: Free of residual monomer- 3-5% Polymerisation of self cure is never completed. The effects of free residual monomer:released from the denture thus irritate the mucosa. it will act as plasticizer and make the resin weaker and more flexible. The degree of polymerisation is
  25. 25. Shorter working time due to gradual increase in viscosity. Curing is done at room temperature. So better fit and more stable. The color stability is not good. Easy to manipulate. Manipulation: Compression moulding technique -Allowed to polymerize at room temperature in a pressure chamber. -Dough forming time is 5 mins. -Trial closure only 1 or 2 . -For complete polymerisation kept for 2 ½ hrs or overnight.
  26. 26. Benefits: - Eliminates the processing stresses. - Less thermal contraction. Disadvantages: More residual monomer Low mechanical properties. Colour instability. Uses: Occasionally used for denture bases. -Preparation of special trays. -Denture repairs, relining and rebasing. -For removable orthodontic appliances.
  27. 27. property Heat cure Self cure Method of activation heat Chemical Composition No DMPT Present Polymerisation reaction Heated to 60 to 70°C On mixing Degree of polymerisation high low Mechanical Superior inferior
  28. 28. Compressive strength(Mpa) 75 65 Impact strength(MPa) 1.5x106 at 17.2MPa Less Tensile strength(MPa) 65 lesser Residual monomer .2-.4% 3-5% Thermal shrinkage higher Lesser crazing greater Better dimensional
  29. 29. Water solubility 0.02 mg/cm 2 0.05mg/cm 2 Water sorption(mg/cm 2 ) 0.6 0.7 Aesthetic quality Good color stability No good color stability Processing shrinkage 0.53% 0.26%
  30. 30. Aspects of manipulation 1. Preparation of the mould 2. Selection of separating medium 3. Powder liquid ratio 4. Polymer monomer interaction 5. Dough forming time 6. Packing 7. Bench curing 8. Polymerization
  31. 31. 1. Preparation of the mould CAP COPE DRAG
  32. 32. 2. Selection of separating medium Used to separate the denture base material from the mould surface. Formerly, Tin foil was used, but it was a time consuming and difficult process. Causes: a. Dimensional inaccuracy b. Poor reproduction of details Tin foil substitutes: -Cellulose lacquers -Evaporated milk -Solutions of alginate
  33. 33. - Soap - Sodium silicate - Starch The most commonly used separating medium are water soluble alginates. Composition: 1. Sodium/potassium alginate- 2-3% 2. Di/tri sodium phosphate- 0.7% 3. Preservatives- 0.3% 4. Glycerin- 4% 5. Alcohol- 7% 6. Balanced with water- 85%
  34. 34. Setting reaction: Potassium alginate + calcium sulphate calcium alginate + potassium sulphate Functions: 1. To prevent the diffusion of water from the mold into the un polymerized packed dough. 2. To prevent the diffusion of the monomer from the unpolymerised packed dough into the mould material. Precautions: The coating should be uniform. -De waxing must be done thoroughly. -The resin teeth should not be
  35. 35. : Powder liquid ratio It controls the workability of the mix and the dimensional change on setting. •Too much powder: Under wetting of the polymer beads •Too much of monomer-Excessive Polymerisation shrinkage and loss of quality of fit. •To reduce the volumetric Polymerisation shrinkage from 21% to 7% and linear shrinkage to 0.5% - Use of prepolymerised resin is recommended - 3:1 polymer and monomer ratio.
  36. 36. Polymer monomer interaction The powder is slowly added to the liquid to get a workable mass which passes thro five distinct stages: (1) Wet, sand like stage (2) Stringy stage/ a tacky fibrous stage. (3) A smooth Dough like stage:On molecular level:Increase in no. of polymers chains, a large amount of undissolved polymer remains. clinically, no longer tacky and does not stick to mixing spatula.The latter phases of this stage is ideal for compression moulding.
  37. 37. (4) Rubbery or elastic stage:dissipation of monomer. clinically, rebound and stretches. (5) Stiff stage:evaporation of free monomer. clinically, the mixture appears dry and is resistant to mechanical deformation. DOUGH FORMING TIME: The time required for resin mixture to reach a dough like stage.According to ADA-12 it is less than 40 mins & clinically it reaches in 10 mins. WORKING TIME:According to ADA-12,the dough to remain moldable for atleast 5 mins.
  38. 38. Packing The dough is packed into the mould cavity. •Packed at sandy or stringy stage, too much monomer will be present between the powder particles. •If too early : porosity. •If done at rubbery or stiff stage, material too viscous to flow. •Over packing:Excessive thickness and mal positioning of prosthetic teeth. •Under packing: Noticeable porosity. •The acrylic dough is packed into the flask by use of pneumatic, hydraulic or mechanical press.
  39. 39. Bench curing After the final closure of the flask , it is allowed to stand for 1 hour. Objective: For more uniform dispersion of monomer throughout the mass.
  40. 40. Polymerisation cycle or Curing cycle: The heating process used to control polymerisation is termed polymerisation cycle.This process should be well controlled to avoid the effects of uncontrolled temperature rise, such as boiling of the monomer, or denture base porosity.The following techniques are there: 1.Involves processing the denture base resin in a constant temperature water bath at 74°C(165°F) for 8 hours or longer, with no terminal boiling treatment.(slow curing) 2.Processing in a 74°C water bath for 8 hour and then increasing the temperature to 100°C for 1 hour.
  41. 41. 3.Processing the resin at 74°C for approximately 2 hour and increasing the temperature of the water bath to 100°C and processing for 1hr.(fast curing) BENCH COOLING: It is cooled slowly as rapid cooling may result in warpage because of differences in thermal contraction of resin and investing stone. Bench cooled for 30mins and subsequently immersed in cool tap water for 15mins.
  42. 42. Processing defects of dentures Rough or irregular surface. Porosity Dimensional changes Processing stresses Crazing Fractures
  43. 43. Rough or irregular surface Improper wetting of wax denture by plaster mix. Entrapment of air bubbles in investing plaster. Low liquid powder ratio. Incomplete de waxing. Insufficient or improper coating of separating agent. delayed packing. Insufficient material in the mold.
  44. 44. porosity Undesirable effect on the strength and optical, hygienic properties.The causes are: Polymerisation shrinkage termed contraction porosity-appear as irregular voids throughout, and on the surface of denture. causes:insufficient material, insufficient pressure. Gaseous porosity shows fine uniform bubbles. Particularly in thicker regions. Localised porosity-improper mixing of components or early
  45. 45. Granular porosity-granular effect on the denture surface, due to low powder liquid ratio. Blotchy, opaque surface – when mixing vessel’s lid is not covered. Rapid heating Insuffiecint mixing Insuffient pressure
  46. 46. Processing stresses Polymerisation shrinkage. Mechanical stresses on repeated drying and wetting of the denture, causing alternate contraction and expansion. During deflasking internal stresses are developed. Difference in the coefficient of thermal expansion of acrylic resin and investing material.
  47. 47. Crazing May appear on the surface and has weakening effect. Delayed packing. Constant cycle of drying and wetting done by patient. it is around the tooth neck Difference in the coefficient of thermal expansion of porcelain teeth and acrylic resin.(1:10) Solvent action-Use of MMA monomer during denture repair. Sudden cooling. During polishing .
  48. 48. fractures Dentures may break: On impact(extra orally) Due to fatigue, form repeated bending of the denture in service.(if parallel cracks).(intra orally) Presence of notches. Inadequate curing time. Excessive grinding and polishing
  49. 49. INJECTION MOULDING TECHNIQUE In this technique, for injection of resin, a hollow sprue connects the mould cavity created by wax boil out to an external opening on the flask and a high pressure injection cylinder is connected to the opening. The pressure is maintained during the polymerisation cycle.
  50. 50.
  51. 51. Advantage: -increased dimensional accuracy. -Elimination of trial closures. -It can be used for microwavable and pour type resins. Drawbacks: -inadequate spr uing will lead to under filled moulds. -Expensive equipment. -Injector is difficult to clean.
  52. 52. Pour type denture resins/ pour and cure resins/ fluid resins These are cold cure resins. Difference in the size of polymer. Slurry mixed which is poured into agar- hydrocolloid mold and allowed to polymerize under pressure at 0.14MPa for 30 –45 mins.. Centrifugal casting and injection molding techniques are used.
  53. 53. Fluid resin technique Employs a pour able resins, chemically activated resin. Dispensed as a powder liquid system.(2.5:1by wt.)
  54. 54.
  55. 55. Uses: Three methods of duplicating dentures with pour type autopolymerising resin.The method differ in the type of flask and the investing media. 1. Modified denture flask method (Brewer and morrow, 1975) 2. The pour –resin flask method(Boos and carpenter 1974) 3. The cup and flask method(Wagner, 1970 Singer, 1975) -in the preparation of the saddles for partial dentures, in which trial packing is difficult.
  56. 56. Advantages: - Improved adaptation to underlying soft tissues - Decreased probability of damage to prosthetic teeth and denture bases during deflasking - Reduce material cost - Simple procedure Disadvantages: - Noticeable shifting of teeth - Air entrapment within the denture base material - Poor bonding between the denture base material and teeth - Technique sensitive
  57. 57. Modified acrylics Objective is to improve the impact strength, fatigue resistance, or radiopacity. The impact strength or high impact strength materials -improved by adding -Elastomers: able to absorb energy. -Use of acrylic-elastomer copolymer e.g. methylmethacrylate-butadiene or methylmethacrylate- butadiene-styrene copolymers. Increased by 10 fold. Drawback is greater
  58. 58. THE FATIGUE RESISTANCE:or fibre reinforced acrylics 1. adding carbon fibers: Positioning of fibers is critical,placed in that part of denture which is under tensile stress. bonding between the fibres and acrylics is difficult to achieve, further weaken the denture therefore the technique is a complicating factor. Unaesthetic. 2.other reinforcing fibres: Kevlar (aramid),nylon, glass fibres Ultra high modulus polyethylene polymers(UHMPE) 3. Metal inserts(wires,plates,fillers)
  59. 59. Advantages: -Increase in impact and flexural strength -Improvement in fatigue resistance. -Minimizing denture fractures. Drawbacks: -Tissue irritation from glass fibres -Straw colored Kevlar fibres. -Increase in production time. -difficulties in handling. -Metal inserts failure due to stress concentration.
  60. 60. RADIOPACITY:Incorporating atoms of high atomic numbers than the C,H, and O atoms of which acrylic resin is made. Radiopaque additives comments Metal inserts or powdered metals May weaken the base & is unaesthetic. Inorganic salts(barium sulphate), Bismuth(10 to 15%) At 8% insufficient radiopacity At 20% weaken the
  61. 61. Co-monomer containing heavy metals e.g barium acrylate Polymer has poor mechanical properties Halogen - containing Co- monomer or additives e.g. Tribromophenylmethacrylate Bromine containing Co- monomers. Additives may act as plasticizer Co-monomers are expensive. Most promising ones.
  62. 62. ALTERNATIVE POLYMERS: Polycarbonates and certain vinyl polymers. Indications: allergic patient. when greater impact strength is required. Processing: Injection molding. So special equipment is required. RAPID HEAT POLYMERISED RESINS OR HYBRID ACRYLICS:polymerized in boiling water immediately after being packed into a denture flask.
  63. 63. procedure: after placing the denture in boiling water , boiled for 20 mins and then bench cooling . The initiator is formulated form both chemical and heat activated initiators to allow rapid polymerisation. Polymerisation shrinkage-0.97-0.43% double the conventional ones.
  64. 64. Hydrophilic acrylics consist of co-polymer of methylmethacrylate and hydroxyethyl methacrylate. This material is easily wetted so enhanced denture retention. Drawback: Poorer mechanical properties than PMMA when material is saturated with water.
  65. 65. Microwave activated resins Electromagnetic waves. A special glass fibre reinforced plastic flask. Processing technique: compression moulding. microwave energy in an oven at 500-600W curing time as short as 3 mins. Advantages: -short processing time.,easy and clean . -Dimensional accuracy. - comparable physical
  66. 66. Disadvantages: - high capital cost. - Flasks easily fracture. - Porosity if condition are not controlled.
  67. 67. Light activated denture base resins Single component denture base resin supplied in pre mixed sheet or rope forms It is a composite having a matrix of urethane dimethacrylate, microfine silica and high molecular weight acrylic resin monomer; no methyl methacrylate is present a) Organic filler- acrylic resin beads b) Activator- visible light c) Initiator- camphoroquinone
  68. 68.
  69. 69. The acrylic is polymerized in a light chamber with blue light of 400 to 500nm. The denture base rotates in the chamber. Setting reaction is free radical addition polymerisation. A bonding agent – mixture of acrylic monomers, including methyl methacrylate;cured by visible light. Advantages: No flask required. Short processing time. Improved fit, comparable impact strength & hardness
  70. 70. Disadvantages: High capital cost. Inferior bond to resin denture. Lower elastic modulus and slightly lower flexural strength-increase deformation. Recently, they have been trying the use of lasers for polymerisation of resins.
  71. 71. Summary and conclusion Denture bases are responsible for artificial tooth fixation,stability and distribution of masticatory forces over a large tissue bearing far acrylic resins are most commonly used and still the research is going on the resins which will fill all the requirements of dentist and patient.
  72. 72. References Anusavice Kenneth J.: Phillips Science of Dental Materials. 10th edition, W.B. Saunders, 1999. Combe E.C.: Notes on dental materials: 6th ed. Churchill Livingstone, 1992. Craig Robert G. and Powers J.M.: Restorative dental materials. 11th ed. Mosby Inc. 2002. Gladwin Marcia, Bagby Michael: Clinical aspects of dental materials, Lippincott, 2000. Vermilyea SG, Powers JM and Koran A. the rheological properties of fluid denture base resins. J
  73. 73. Austin AT, Basker RM. Residual monomer levels in denture bases. Br Dent J 1982;153:424 McCabe JF and Walls AWG: Applied dental materials, 8th ed. Blackwell Science Limited, 1998. O’Brien WJ: Dental material and their selection, 2nd ed. Quintessence, 1997. Van Noort Richard: Introduction to dental materials, Mosby 1994. Devlin H, Watts DC. Acrylic “allergy”. Br Dent J 1984;157:272 Elahi JM, Abdullah MA. Effects of different polymerisation techniques on dimensional stability of record bases. J Prosthet Dent 1994;71:150-3
  74. 74. •Consani RLX, Dometti SS and Consani S. effect of new tension system, used in acrylic resin flasking, on the dimensional stability of denture bases. J Prosthet Dent 2002;88:285-9 •Morata H, Toki K, Hong G and Hamda T.effect of tissue conditioners on the dynamic viscoelastic properties of a heat polymerised denture base. J Prosthet Dent 2002;88:409-14 •Yannikakis S,Zissis A,Dent,Polyzios G & Andreopuolos A.Evaluation of porosity in microwave-processed acrylic resin using a photographic method.J Prosthet Dent 2002;87:613-9 •Rueggeberg FA.From vulcanite to vinyl, history
  75. 75. Keenan PL,Radford DR & Clark RKF.dimensional change in complete dentures fabricated by injection moulding and microvave processing. J Prosthet Dent 2003;89:37-44 Zappini G,Kammann A & Wachter W.comparison of fracture tests of denture base materials. J Prosthet Dent 2003;90:578-85
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