Denture base materials

Question 1

What are the ideal properties of a denture base material (or any dental material)

Answer 1

Biocompatible
Aesthetic
Hygienic (Resistant to bacterial contamination)
Dimensionally stable
High Strength, stiffness, hardness and toughness
High thermal conductivity
Low density
Cost
Ease of:
Processing
Repair/adjust
Reproduction of surface detail
Radiopacity

Question 2

What is the most common denture base material?

Answer 2

polymethyl methacrylate

Question 3

What molecule starts the process of polymerisation?

Answer 3

benzoyl peroxide (initiator) -  This molecule readily splits into two, each fragment having one unpaired electron. 
These fragments attack the C=C bond starting the polymerisation

Question 4

What does the cross linking agent do?

Answer 4

This forms addition bonds between polymer chains resulting in improved mechanical and physical properties.
-Diethylene glycol dimethacrylate

Question 5

What processing problems can occur?

Answer 5

Porosity
Polymerisation shrinkage
Processing strains

Question 6

How does porosity in the acrylic occur?

Answer 6

Volatisation of monomer during curing if the temperature is raised too quickly.
This is because the reaction is exothermic and the monomer has a relatively low boiling point.

Question 7

How would you solve porosity?

Answer 7

Use a slow heating cycle when curing and keep under pressure.

Question 8

What can polymerisation shrinkage do to the denture?

Answer 8

Can result in the denture not contacting the hard palate or porosity
- instability/poor fit

Question 9

How would you solve polymerisation shrinkage?

Answer 9

• Use polymer beads to reduce the necessary polymerisation to a minimum
• Keep denture under pressure and use a slow cool when curing
• Incorporate a post dam on to the denture to compensate

Question 10

Why may processing strains arise?

Answer 10

These may arise due to dissimilar materials being used (e.g.ceramic teeth or CoCr components)
Also if the polymer is cooled too quickly

Question 11

What alternative processing routes exist?

Answer 11

‘Injection molding’ of acrylic dough

• Injection molding of acrylic above Tg (glass transition temperature)
• Milling from block

Question 12

What happens in true injection moulding?

Answer 12

Heat material to above to above Tg
Squeeze into mould using high pressure
- can use PMMA, polyamide, PEEK etc

Question 13

What is the composition of PMMA powder?

Answer 13

Polymethyl methacrylate granules
Initiator
Pigments, Dyes & Opacifiers
Plasticisers
Synthetic fibres (nylon)

Question 14

What is the composition of PMMA liquid?

Answer 14

Methyl methacrylate monomer
Inhibitor (hydroquinone)
Cross linking agent

Question 15

What variants of PMMA exist?

Answer 15

Cold cure - auto cure - mainly used for repairs or attaching teeth to a CoCr RPD

• high impact Contains copolymer of butadiene and styrene . Results in a dispersion of rubber inclusions
• colours
• soft-liners

Question 16

What are the negatives of cold cure?

Answer 16

Has a lower molecular weight more porosity

Therefore is less dense, softer, weaker and is prone to discolouration

Question 17

Why may adverse reactions to denture materials occur?

Answer 17

Traces of monomer remain in denture base after processing - diffuse out and cause problems - delayed hypersensitivity
- irritant contact dermatitis

Question 18

Why may bacteria grow on the fitting surfaces?

Answer 18

cant polish surface of fitting surface - lose retention - can encourage bacterial growth on rough surfaces

Question 19

What are the indications for flexible material?

Answer 19

repeated denture fractures
large undercuts occur
presence of large tori makes use of rigid matrerials challenging
patients cant open wide enough 
single maxillary denture

Question 20

What are the advantages of flexible materials?

Answer 20

-

Question 21

Why might you reline a denture?

Answer 21

carried out when denture becomes ill fitting due to bone resorption using PMMA
- can also use cold cure resin if this is a temporary measure or procedure carried out on an RPD

Question 22

How would you reline a denture?

Answer 22

- remove undercuts and periphery
take impression 
send lab, model is cast
vertical dimension recorded
impression material replaced with wax
usual processing to replace wax with acrylic

Question 23

What are tissue conditioners?

Answer 23

Temporary linings that allow traumatised tissue to recover before carrying out definitive treatments