DENTURE BASES

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 base materials 1. Strength and durability 2. Satisfactory thermal properties 3. Processing accuracy and dimensional stability 4. Chemical stability (unprocessed as well as processed material) 5. Insolubility in and low sorption of oral fluids

6. Absence of taste and odor 7. Biocompatible 8. Natural appearance 9. Color stability 10. Adhesion to plastics, metals, and porcelain teeth 11. Ease of fabrication and repair 12. Moderate cost

Different denture base materials

Character Material Advantages 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 Moderate mechanical properties Could be scratched Do not transmit heat May cause allergy to the patient Crazing & bleaching may occur Bacterial & fungal colonization may occur I. Non-Metallic 1. Heat-cured Polymethyl methacrylate (PMMA) 2. Self-Cure (PMMA) 3. Light cure (PMMA) 4. Pour-type denture base resin (fluid resin), chemical cure 5. High-impact strength material (rubber-reinforced), heat cure 6. Rapid-heat polymerized acrylic, 7. Hydrophilic Polyacrylate. 8. Soft acrylic.  

Character Material Advantages High mechanical properties Could not be scratched Transmit heat Minimal allergy to the patient Crazing not occur Minimal bacterial & fungal colonization Disadvantages 1. Color not matches gingival tissues 2. Not easy constructed & repaired 3. Not easy to be finished & polished 4. Not easy to reline 5. Heavy in weight 6. Relatively expensive 7. Corrosion may occur II. Metallic materials Cast gold alloys Cast cobalt-chromium Cast titanium & titanium alloy Swaged stainless-steel

Heat-cured acrylic denture base Physical form and composition Denture base plastics are commonly supplied in a powder-liquid form

Principle ingredient of acrylic denture base

1. Heat-cured polymethyl methacrylate a. Powder Pre polymerized polymethyl methacrylate in the form of small beads. Initiator: benzoyl peroxide. Plasticizers: dibutyl phthalate to soften the material and make it more flexible.

Pigments: various tissue like shades. Acrylic fibers: simulate the minute blood vessels. Glass fibers: to increase the stiffness. Barium salts: improve the radiopacity of the polymer.

b. Liquid (present in dark glass bottle) Monomer: methyl methacrylate. Inhibitor: to prevent premature polymerization which is hydroquinone. Cross-linkage agent: glycol dimethacrylate to decrease the solubility and water sorption and increase the resistance to minute surface cracking or crazing.

Denture base construction using compression molding technique 1. Flasking procedure 2. Wax elimination 3. Proportioning and mixing

Polymer / Monomer ratio The proportion of polymer to monomer is 3:1 by volume and 2:1 by weight. 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. A plastic mass is produced after mixing.

On standing this workable mass pass through five stage: 1. Sandy stage The polymer gradually settles into the monomer and a some what fluid incoherent mass is formed which is grainy. 2. Stringy or sticky stage The monomer attaches the surface of the polymer beads. This stage is characterized by a stringiness and adhesiveness if the mixture is touched or pulled apart.

3. Dough stage The monomer diffuses into the polymer gradually and the mass becomes more saturated with polymer in solution, it becomes smooth and dough like. It is no longer tacky and does not adhere to the wall of the mixing jar. 4. Rubbery stages The monomer disappears by evaporation and by further penetration into the polymer. The mass becomes more cohesive and rubber like. 5. Stiff stage Evaporation of free monomer.

During the various stages, the reaction is physical in nature and include solution of polymer in monomer and some absorption of monomer by the polymer as well as wetting of the polymer particles. 4. Packing 5. Trial closure 6. Curing

Curing Curing cycles are divided into 2 main groups: The curing cycle described the heating processes employed to control polymerization reaction of the dough, to transform methyl methacrylate to polymethyl methacrylate. Curing cycles are divided into 2 main groups: 1. Short curing cycle (about 2.5 hours) Put the flask in a water bath and increase the temperature gradually from room temperature to 65 ᵒC within 30 minutes and keep at 75 ᵒC for one and half hour, then keep at boiling for 30 minutes.

2. Long curing cycle (about 9 hours) Put the flask in a water bath, then the temperature is raised gradually to 65 ᵒC within 30 minutes, then the temperature is raised to 80 ᵒC within 8 hours and kept at boiling for 30 minutes. Usually long curing cycle is used for curing bulky prosthetic appliance.

Importance of the boiling stage It is important to reduce the level of residual monomer in the finished denture, to complete polymerization to reduce the allergic effect of residual monomer. Excessive boiling is contraindicated as the resin becomes brittle and easily broken. 7. Deflasking and finishing 8. Polishing

Steps of the polymerization reaction: 1. Initiation 2. Propagation 3. Termination Direct coupling or by chain transfer

Defects which may occur during processing of heat-cured polymethyl methacrylate: A. Porosity B. Crazing C. Warpage

A. Porosity Small voids which may be present internally or externally on the surface of the denture. 1. Internal porosity Causes -If large mass of the resin is heated within the flask, the temperature in the center of the resin will rise. The resin and plaster or stone are both bad conductors of heat, so they can not dissipate all the elevated heat away from the center of the bulky resin. The temperature at the center of the resin rises above the boiling point of the monomer (100.30 0C) leading to formation of bubbles or pores. Site -In the upper denture, at thick portions as tuberosity. -In the lower denture, in the bulky portions of the lingual flanges.

2. Gaseous porosity Cause Any excessive or elevated curing temperature which lead to boiling of the monomer as in internal porosity. Site It appears on the superficial surface of the denture. N.B: Gaseous porosity is usually accompanied by internal porosity. 3. Contraction porosity Causes -Decrease the powder/ monomer ratio -Lack of pressure during packing and curing -Under packing (small amount of dough is used in the mold). At the edges of the flanges of the denture.

4. Granular porosity Causes -Increase powder/monomer ratio -Packing in sandy stage Site It appears on the surface of thin areas of the denture.

How can porosity be avoided ? -Proper proportion of polymer and monomer -Proper mixing leading to homogeneity of the mass -The glass jar must be cool all the time mixing is done -Never pack at any stage other than the dough stage -Under packing may lead to tooth contraction and granular porosity -Too rapid rise of heating temperature may lead to both types of porosity internal and gaseous.

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. Causes -Opening of the flask before cooling -Too rapid cooling of the flask in tap water -Excessive heating release during polishing, repair by heat cure. -Excessive pressure during processing of heat cured PMMA resin when the temperature is about 70 0C, the polymer monomer mix, expands thermally. If the flask is clamped rigidly, such expansion of the resin must be compensated for by enlargement of the mold space.

C. Crazing Crazing is small linear cracks that appear to originate at denture’s surface, may result from stress application or partial dissolution of a resin. It begins at the surface of a resin and oriented at right angles to tensile forces due to separation of individual polymer chains. Causes 1. Solvent-induced crazing result from prolonged contact with liquids as ethyl alcohol. 2. Alternative drying and wetting of the denture. 3. Thermal stresses as a result of differences in coefficient of thermal expansion between acrylic denture base and porcelain teeth.

Chemical-cured acrylic denture plastics

Called self-curing, cold-curing or autopolymerizing resins. It has the same composition as that of heat-cured acrylic denture base, but some differences exist The powder contains polymers beads of low molecular weight. The liquid contains a chemical activator which is responsible for starting the polymerization reaction at room temperature as tertiary amine that reacts with the peroxide initiator and sufficient free radicals are produced to initiate the polymerization reaction. Called self-curing, cold-curin

Differences between heat and chemical cured acrylics

Difference Heat-cured Chemical-cured Polymerization reaction Addition polymerization activated by heating Addition polymerization starts at room temp. in presence of amine system Degree of polymerization Higher due to heating Lower Polymerization shrinkage Higher Residual monomer content 0.2-0.5% 3-5% Mechanical properties Water sorption Color stability Good Discoloration due to oxidation of tertiary amine

Dimensional changes after several months of service *Under size (0.3-0.4%). *Polymerization shrinkage *Thermal shrinkage *Relief of internal stresses *Over size (0.1%). Practicability -Long processing time (2-8 hours). -Compression or injection molding tech. could be used -Short processing time (10-15 min.) -Compression or fluid resin tech could be used Applications Making permanent denture bases and removable orthodontic appliances Denture bases, repair of broken dentures and special trays

Advantages Disadvantages Denture base fabrication in short time Dimensional accuracy Disadvantages Lower mechanical properties as stiffness Higher porosity Color instability which can be reduced by the addition of stabilizing agent Higher residual monomer that could irritate the patient’s soft tissues and act as plasticizer that result in decreased transverse strength

Light-activated acrylic resin

Applications Denture bases, repair of broken dentures and special trays.

A sheet of the material is adapted to the working cast and cured for 5 min. in a light-curing unit (Triad). 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.

Advantages Disadvantages No MMA monomer found → reducing the possibility of allergic reaction. Lower rate of polymerization reaction→ better fitness of denture base. Short processing time with no need for flasking. Disadvantages Lower mechanical and physical properties

When patients are known to have suffered from an allergic reaction Processing the denture for extended periods (such as 24 versus 8 hours) may be helpful. Processing heat polymerized poly (methyl methacrylate) in a water bath for 7 hours at 70" C, followed by boiling for 1 hour. Boiling has only a slight effect on the dimensional accuracy of the processed dentures. Vinyl acrylic or light-activated denture base materials are an alternative for those patients who are sensitive to methyl methacrylate monomer.

Polymerization shrinkage   The density of methyl methacrylate monomer is only 0.945 g/cm3 at 20º C, compared with 1.16 to 1.18 g/cm3 for poly (methyl methacrylate). This increase in density is mainly accounted for by an approximate 21% decrease in volume of monomer during polymerization. Because the ratio of polymer to monomer used in the preparation of dental poly(methyl methacrylates) and polyvinyl acrylics is usually 3: 1, the free volumetric shrinkage amounts to approximately 6%