L09: Denture Base Materials Flashcards
What was PMMA historically used for?
Direct restoratives
Why is PMMA no longer used as a direct restorative?
Optically clear, shrinkage, high TEC, poor mechanical properties, water sorption (affecting mechanical and cosmetic properties eg staining)
What is PMMA commonly used for?
Denture bases
How are PMMA like materials used as direct restoratives?
Thermoplastic with longer polymer chains that don’t tend to cross-link together, giving it less rigidity. BisGMA are difunctional which cross-links together, making it a higher modulus material, which is required for a direct restorative on an occlusal surface. Methacrylate group. Similar chemistry but different properties.
How is PMMA used as bone cement?
Controversially used in situ. Can cause tissue necrosis and has a high exotherm. Thermal curing process means you can never achieve 100% conversion, with residual monomer that can leak out.
What polymers can be used for denture based materials?
Heat cured polymethylmethacrylate (PMMA)
Auto-polymerised (cold cure) PMMA
Injection moulded - modern processes
What other materials can be added to denture bases?
Permanent soft liners
Temporary soft liners
What is the composition of the denture base?
Powder: PMMA, BPO, BMA
Liquid: MMA, EGDMA, HQ
What reaction occurs to turn MMA into PMMA?
Free radical addition reaction
Exothermic
What happens in the free-radical initiation of MMA to PMMA?
Benzoyl peroxide initiator, readily dissociates at a certain temperature. Heat fast cure and slow cure.
What is the problem with pressure of the PMMA reaction?
Processing PMMA without pressure means when temp is reached, it all heats up and goes mad with higher rates of polymerisation and the exotherm. The exotherm is so high that it exceeds boiling point of the monomer, so will evaporate the monomer, forming porosity and a crap denture base.
How is heat and pressure control achieved in heat cured PMMA
Water bath, flask denture and clamp flask
What is the reaction for auto-polymerised PMMA (cold cure acrylic)
Benzoyl peroxide initiator with tertiary amine activator (by product is toxic!). Radical sufficient at room temperature.
Properties of cold cure acrylic compared to heat cured
Cold cured has
Decreased degree of conversion
Decreased mechanical properties (less polymer and lower strength)
Increased creep (time dependent response to load)
What is creep?
Time dependent response to load
What are the setting stages of PMMA?
Sandy stage: immediately after mixing
Stick or stringy stage
Dough stage
Rubbery stage
Describe the sticky or stringy stage of PMMA reaction
Due to dilatant material in its monomer form, it gets thicker as you stir. Quite operator induced.
Describe the dough stage of PMMA reaction
Mix is cohesive and less tacky easy to manipulate. Dough stage period is the working time of the material.
Describe the rubbery stage of the PMMA reaction.
Would fail brittly but has some flexibility.
What allows MMA to turn into PMMA?
MMA is hugely volatile, allowing beads to bond mechanically, making it solid PMMA
What are the components of an RPD?
Denture base - acrylic saddles and metal alloy framework
Denture teeth - acrylic and porcelain
Potential problems with denture materials
Denture toothwear
Porosity of the denture base
Acrylic discolouration
What is an indication of having lots of different materials in a denture base?
Different materials means varied stiffness of each material. Mismatches in flexibility and modulus means it could fracture easily at the interface due to stress fracture, and early failure could occur.
What is bondage?
Attachment mechanisms
What are the attachment mechanisms of denture teeth to denture base?
Gross mechanical retention
Micro mechanical
Pseudo-chemical
How is gross mechanical retention used to attach denture teeth to the denture base?
Diatoric holes to help retain the teeth
Undercuts, mechanical retention
How is micromechanical retention used to attach denture teeth to the denture base?
Teeth and base may be grit-blasted to increase surface roughness.
How is pseudo-chemical retention used to attach denture teeth to the denture base?
Between acrylic teeth and denture base, DB monomer/DT. If done well, it improves stress transfer/interfacial leakage.
By what mechanisms is the denture base held to the framework?
Gross mechanical retention
Alloy etching
Grit blast
No P-C (chemical bond): metal-coupling (using primers) or tribochemical
What are light-cured acrylics made from?
Photoactive resin
How do light cured acrylics work?
Cured by photopolymerisarion in a light box
Uses of light-cured acrylic
Hard reline, repair, special trays
What are the properties of light-cured acrylic compared with heat-cured?
No MMA
Decreased shrinkage (higher Mw polymer)
Decreased elastic modulus
Approximately equal impact strength
