Dental Materials for Denture Bases

Prosthetic Dentistry - Study Notes

• Prosthodontics: The branch of dentistry dealing with the restoration and replacement of missing teeth and oral structures.
• Topics Covered: Flasking, denture base materials, packing procedures, and curing cycles.
• Clinical and Laboratory Steps: A process involving primary impressions, record bases, jaw relations, and teeth setting. Secondary impressions, mounting, wax-up, and try-in procedures follow
• Flasking: The process of investing a cast and a waxed denture in a flask to create a sectional mold for the acrylic resin denture base. The last step in denture construction.
• Flasking Materials: Metal cases or sectional metal flasks used in investing procedures, using artificial stone or plaster of Paris.
• Flask Components: Lids, upper and lower components (shaped depending on top or bottom), and a knockout disc. The lid and components have ID numbers.
• Flasking Techniques:
  • Compression moulding (most common)
  • Injection moulding
  • Polymerization via microwave energy
  • Fluid resin technique
  • Light-activated denture base resins
• Flasking Process:
  • Placing the cast in flasks (maxillary parallel to base, mandibular inclined from back to front). Maintaining a 3-6 mm space between teeth and flask. Reducing cast base if needed.
  • Lubricating casts with petroleum jelly. Preparing the flask with petroleum jelly.
  • Investing the lower half of the flask with plaster. Positioning the cast to allow excess plaster to surround it.
  • Trimming off excess plaster to leave the cast clear of the flask lip.
  • Applying separating medium (like petroleum jelly or tinfoil substitute) to the plastered surfaces.
• Wax Elimination: Dentures are placed in boiling water for 4-6 minutes to soften the wax, and then residual wax is removed with a stream of boiling water.
• Mold Formation: After trimming, a sectional mold is manufactured.
• Denture Base Materials: Must meet specific mechanical and chemical requirements. These materials usually have minimum requirements since they are definitive restorations, not temporary, and need to be rigid, resistant to fracture, and maintain their structure over time.
• Heat-Activated Denture Base Materials: Composition is important as it impacts various properties during the process. Examples of components include PMMA beads, initiator (peroxide), monomers, and pigments.
• Heat-Cured PMMA: Features favorable working characteristics, acceptable aesthetics, and physical/mechanical properties. However, it undergoes water absorption, has low thermal conductivity, may deform under load, and doesn't always bond well with non-acrylic teeth. Possible water absorption during and after manufacturing can create colonization by candida.
• Other Types of Base Materials: Injection molding, chemically activated resins, microwave-activated polymers, and modified acrylic materials.
• Polymerization Cycles: Steps for the final stage of denture processing involving controlled heating. Two cycles exist: long cycle with a constant temperature of 74°C for 8 hours or more followed by a brief boil, and a short cycle 74°C for 2 hours followed by 1 hour of boiling.
• Bench Cooling: Procedure to cool down the flasks after packing.
• Deflasking the Dentures: Process to remove the prosthetic from the flasks after the polymerization process. Using tools and sometimes a spring clamp.
• Internal Porosity: Internal air bubbles that can occur due to high temperatures, unreacted monomer, and poor thermal control during the polymerization process.
• Important Considerations: Safety and precautions related to monomers like allergic reactions are important.