L09: Denture Base Materials

Question 1

What was PMMA historically used for?

Answer 1

Direct restoratives

Question 2

Why is PMMA no longer used as a direct restorative?

Answer 2

Optically clear, shrinkage, high TEC, poor mechanical properties, water sorption (affecting mechanical and cosmetic properties eg staining)

Question 3

What is PMMA commonly used for?

Answer 3

Denture bases

Question 4

How are PMMA like materials used as direct restoratives?

Answer 4

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.

Question 5

How is PMMA used as bone cement?

Answer 5

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.

Question 6

What polymers can be used for denture based materials?

Answer 6

Heat cured polymethylmethacrylate (PMMA)
Auto-polymerised (cold cure) PMMA
Injection moulded - modern processes

Question 7

What other materials can be added to denture bases?

Answer 7

Permanent soft liners
Temporary soft liners

Question 8

What is the composition of the denture base?

Answer 8

Powder: PMMA, BPO, BMA
Liquid: MMA, EGDMA, HQ

Question 9

What reaction occurs to turn MMA into PMMA?

Answer 9

Free radical addition reaction
Exothermic

Question 10

What happens in the free-radical initiation of MMA to PMMA?

Answer 10

Benzoyl peroxide initiator, readily dissociates at a certain temperature. Heat fast cure and slow cure.

Question 11

What is the problem with pressure of the PMMA reaction?

Answer 11

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.

Question 12

How is heat and pressure control achieved in heat cured PMMA

Answer 12

Water bath, flask denture and clamp flask

Question 13

What is the reaction for auto-polymerised PMMA (cold cure acrylic)

Answer 13

Benzoyl peroxide initiator with tertiary amine activator (by product is toxic!). Radical sufficient at room temperature.

Question 14

Properties of cold cure acrylic compared to heat cured

Answer 14

Cold cured has
Decreased degree of conversion
Decreased mechanical properties (less polymer and lower strength)
Increased creep (time dependent response to load)

Question 15

What is creep?

Answer 15

Time dependent response to load

Question 16

What are the setting stages of PMMA?

Answer 16

Sandy stage: immediately after mixing
Stick or stringy stage
Dough stage
Rubbery stage

Question 17

Describe the sticky or stringy stage of PMMA reaction

Answer 17

Due to dilatant material in its monomer form, it gets thicker as you stir. Quite operator induced.

Question 18

Describe the dough stage of PMMA reaction

Answer 18

Mix is cohesive and less tacky easy to manipulate. Dough stage period is the working time of the material.

Question 19

Describe the rubbery stage of the PMMA reaction.

Answer 19

Would fail brittly but has some flexibility.

Question 20

What allows MMA to turn into PMMA?

Answer 20

MMA is hugely volatile, allowing beads to bond mechanically, making it solid PMMA

Question 21

What are the components of an RPD?

Answer 21

Denture base - acrylic saddles and metal alloy framework
Denture teeth - acrylic and porcelain

Question 22

Potential problems with denture materials

Answer 22

Denture toothwear
Porosity of the denture base
Acrylic discolouration

Question 23

What is an indication of having lots of different materials in a denture base?

Answer 23

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.

Question 24

What is bondage?

Answer 24

Attachment mechanisms

Question 25

What are the attachment mechanisms of denture teeth to denture base?

Answer 25

Gross mechanical retention
Micro mechanical
Pseudo-chemical

Question 26

How is gross mechanical retention used to attach denture teeth to the denture base?

Answer 26

Diatoric holes to help retain the teeth
Undercuts, mechanical retention

Question 27

How is micromechanical retention used to attach denture teeth to the denture base?

Answer 27

Teeth and base may be grit-blasted to increase surface roughness.

Question 28

How is pseudo-chemical retention used to attach denture teeth to the denture base?

Answer 28

Between acrylic teeth and denture base, DB monomer/DT. If done well, it improves stress transfer/interfacial leakage.

Question 29

By what mechanisms is the denture base held to the framework?

Answer 29

Gross mechanical retention
Alloy etching
Grit blast
No P-C (chemical bond): metal-coupling (using primers) or tribochemical

Question 30

What are light-cured acrylics made from?

Answer 30

Photoactive resin

Question 31

How do light cured acrylics work?

Answer 31

Cured by photopolymerisarion in a light box

Question 32

Uses of light-cured acrylic

Answer 32

Hard reline, repair, special trays

Question 33

What are the properties of light-cured acrylic compared with heat-cured?

Answer 33

No MMA
Decreased shrinkage (higher Mw polymer)
Decreased elastic modulus
Approximately equal impact strength