Non-Metallic Denture Base Processing PDF

Non Metallic Denture Base Definition The denture base is that part of the denture which rests on the tissues and carries the artificial teeth. Functions Support & retain denture teeth. Stress distribution. Improve esthetics. Requirements of denture bases 1. Strength and durability. 2. Satisfactor...

Non Metallic Denture Base Definition The denture base is that part of the denture which rests on the tissues and carries the artificial teeth. Functions Support & retain denture teeth. Stress distribution. Improve esthetics. Requirements of denture bases 1. Strength and durability. 2. Satisfactory thermal properties. 3. Processing accuracy and dimensional stability 4. Chemical stability. 5. Insolubility in oral fluids. 6. Absence of taste and odor. 7. Biocompatible. Advantages of non metallic denture base 1. Color matches gingival tissues. 2. Easy constructed & repaired. 3. Easy to be finished & polished. 4. Easy to reline. 5. Light in weight. 6. Not expensive Disadvantages of non metallic denture base. 1. Moderate mechanical properties. 2. Be scratched. 3. Do not transmit heat. 4. May cause allergy to the patient. 5. Crazing& bleaching may occur. 6. Bacterial & fungal colonization may occur.  Types of Acrylic Resin 1. Heat cured acrylic resin 2. Chemical cured acrylic resin 3. Light cured acrylic resin Steps of the polymerization reaction 1. Initiation: activation of the initiator by heat, light or chemical producing 2 free radicals. 2. Propagation: each free radical react with monomer converting it to new free radical, with repetition. 3. Termination: reaction with 2 growing chains to form the polymer.  1. HEAT CURED ACRYLIC DENTURE BASE Powder 1. Pre-polymerized poly-methyl methacrylate. 2. Initiator: benzoyl peroxide. 3. Plasticizers: make the material to be resilient. 4. Pigments: give various tissues like shades. 5. Acrylic fibers: simulate the minute blood vessels. 6. Glass fibers: increase stiffness.  Liquid (present in dark glass bottle) 1. Monomer: methyl methacrylate. 2. Inhibitor: to prevent premature polymerization.  2. Chemical-Cured Acrylic Denture Plastics (Self-curing, cold-curing or autopolymerizing resins) The liquid contains a chemical activator which is responsible for starting the polymerization reaction at room temperature is called tertiary amine that reacts with the peroxide initiator and sufficient free radicals are produced to initiate the polymerization reaction. Uses 1. Denture bases. 2. Repair of broken dentures. 3. Special trays. Advantage Denture base fabrication in short time. Disadvantages 1. Lower mechanical properties as stiffness. 2. Higher porosity. 3. Color instability due to tertiary amine.. 4.The unreacted monomer cause 2 major difficulties: - It acts as plasticizer –decreased strength - Residual monomer irritate the patient’s soft tissues.  3. Light-Activated Acrylic Resin Composition UDMA (Urethane dimethacrylat) and PMMA. A photo initiator as Comphro-quinon is also added. When the material is subjected to the blue visible light, free radicals will be released. Properties A sheet of the material is adapted to the working cast and cured for 5 min in a light-curing unit. The rope material is adapted as horse shoe over the cured base and the artificial teeth are arranged over it. Curing for another 5 min occur. Applications Denture bases, repair of broken dentures and special trays. Advantages 1. No MMA monomer found → reducing the possibility of allergic reaction. 2. Lower rate of polymerization reaction→ better fitness of denture base. 3. Short processing time with no need for flasking. Disadvantages It has lower mechanical and physical properties  Denture processing use two techniques 1. Compression molding, most common one. 2. Injection molding. Denture base construction using compression molding technique 1. Flasking procedure. 2. Wax elimination. 3. Proportioning and mixing. 4. Packing. 5. Curing. 6. Cooling &De-flasking. finishing. 7. Finishing&Polishing.  1. Flasking Is the process of investing the cast with the waxed denture in a flask to make a mold that is used to form the acrylic resin denture base. The Inner surface of the flask is coated with Vaseline, while the base of the cast is painted with separating medium, to prevent the investment material (plaster of Paris) from attaching to the cast.  The first layer of gypsum investment is poured in the lower half and the cast is placed on top of the investment. After set of the first gypsum investment layer, a separating medium is painted on it, to prevent the sticking of the second layer of gypsum investment to the first layer.  The body of the flask is put in place, then the second mix of gypsum investment is placed on the first layer and covers the wax, denture base and teeth, to fill the body. Then the lid is placed on the body.  2. Wax Elimination Dewaxing is done to remove the wax in the wax pattern, so that a mold space is created for acrylic to fill in. The flask is immersed in boiling water (100 C ) for 5 min.  Separating medium is applied to dewaxed mold space prior to packing the acrylic resin. Roles of separation medium to prevent the passage of water from gypsum to resin, to minimize the passage of monomer to the plaster and to facilitate separation of the flasks.  3. Proportioning and mixing The proportion of polymer to monomer is 3:1. The powder and liquid are mixed with stainless steel spatula and kept in a sealed glass jar during the initial stages of reaction to avoid the loss of the monomer by evaporation. Plastic mass is produced after mixing. Workable mass pass through five stages; 1. Sandy stage: The polymer gradually settles into the monomer and grainy incoherent mass is formed. 2. Stringy or sticky stage: This stage is characterized by a stringiness and adhesiveness if the mixture is touched or pulled apart. 3. Dough stag. The mix becomes smooth and dough like. It is no longer tacky and does not adhere to the wall of the mixing jar. stage of mix 4. Rubbery stage. The monomer disappears by evaporation and the mass becomes more cohesive and rubber like. Non workable mass 5. Stiff stage. Evaporation of free monomer occurs.  During the various stages, the reaction is physical in nature and includes solution of polymer in monomer and some absorption of monomer by the polymer as well as wetting of the polymer particles, so no polymerization reaction takes place.  4. PACKING Packing is introduction of denture base resin into the mold cavity at the dough stage. Over-packing (placement of too much material) give excessive thickness & mal-positioning of teeth Under-packing (use of too little material)- leads to denture base porosity. To Minimize Over-paking & Under-paking: 1. Performed in dough-like stage. 2. Shaped and adapted over the teeth. 3. Polyethylene sheet placed over the resin. 4. Designed pressed, until flask is closed. 5. Open the flask and remove the excess, then closed again. 6. Repeat this procedure untill there is no excess and obtaining metal-metal contact.  5. Curing The curing cycle described the heating processes employed to control polymerization reaction of the dough, to transform methyl methacrylate to Poly- methyl methacrylate.  6. Cooling & Deflasking The flask should be cooled slowly ( Bench cooled ), then placing under tap water for 15 min, then remove the denture from the flask with care.  7. Finshing and Polishing Polishing wheel and suspension of fine pumice in water is used. Water cooling is important to avoid excessive heat production, so avoid warpage of the denture  Defects during processing A. Porosity Small voids which may be present internally or externally on the surface of the denture. Causes. 1. Excessive or elevated curing temperature which lead to boiling of the monomer. 2. Decrease the powder/ monomer ratio. 3. Lack of pressure during packing and curing. 4. Under packing (small amount of dough is used in the mold). 5. Increase powder/monomer ratio. 6. Packing in sandy stage. b. Warpage Warpage or distortion is the change of the final shape of the upper or lower denture due to dimensional changes and release of internal stresses. 1. Too rapid cooling of the flask in tap water. 2. Excessive heating release during polishing. c. Crazing Crazing is small linear cracks that appear to originate at dentureꞌs surface. Causes 1. Alternative drying and wetting of the denture. 2. Thermal stresses as a result of differences in coefficient of thermal expansion between acrylic denture base and porcelain teeth. Properties of acrylic denture bases 1. Mechanical properties Conventional heat-accelerated acrylic resins are still the predominant denture base materials in use, as its mechanical properties are higher than chemical cured resins.  But in general, acrylic resins are low in strength, brittle on impact if dropped on hard surface. And fairly resistant to fatigue during repeated bending in service. Strengthening of acrylic denture base could be done by incorporation of fibers as carbon and glass fibers. 2. Thermal properties Dental plastics are poor thermal and electrical conductors. Low thermal conductivity allows plastic denture bases to serve as an insulator between the oral tissues and hot or cold materials placed in the mouth.  Plastics have relatively high thermal coefficients of expansion. This leads to cracking of denture base attached to porcelain teeth, due to large differences of coefficients of expansion of acrylic and porcelain. The result is loosening of artificial teeth. 3. Water Sorption The sorption of water increases weight of the resin causing expansion. This expansion compensates for polymerization shrinkage of the resin. Drying out of the resin is associated with shrinkage, so denture should be kept wet when not in service 4. Tissue Compatibility Completely polymerized poly methy-meth- acrylate rarely cause allergic reactions but that methyl methacrylate monomer or other trace components in the monomer may produce an allergic reaction. The allergic reaction tends to be immediate and is more likely to occur with chemically -cure resin. 5. Residual monomer Even in properly cured acrylic, 0.2 to 0.5 % of the monomer remains unreacted. Processing at too low temperature or for too short time gives higher residual monomer values, this should be avoided because: 1. Free monomer irritates the oral tissues. 2. Residual monomer act as a plasticizer making the resin weaker and more flexible.  4-Microwave  Polymerization shrinkage Heat cured acrylic resin has higher polymerization shrinkage than chemical cured one. The lower shrinkage of chemically cured type results because less stress is induced in the denture during processing.