Denture Base Materials

Question 1

Ideal properties

Answer 1

Biocompatible
Aesthetic
Hygienic (resistant to bacteria contamination)
Dimensionally stable
High strength, stiffness, hardness and toughness
High thermal toughness
High thermal conductivity
Low density
Cost
Ease of processing, repair/ adjust, reproduction of surface detail
Radiopacity

Question 2

Materials

Answer 2

Polymethyl methacrylate (PMMA) - main
Polyetheretherketone (PEEK)
Nylon: flexible dentures (Valplast)
Polyamide (Bredent)
Cobalt Chrome alloy
Titanium

Question 3

PMMA properties

Answer 3

Plastic
Organic polymer
Acrylic: really acrylate (Acrylite and Perspex trade names for PMMA)
Thermoplastic Tg ranges from 85-165C

Question 4

PMMA structure

Answer 4

Long chain molecules of repeated units of methyl methacrylate
-produced by free radical polymerisation

Question 5

Polymerisation

Answer 5

To start the process we use an initiator called benzoyl peroxide

• this molecule readily splits into 2, each fragment having one unpaired e-
• these fragments attack C=C bond starting the polymerisation

Question 6

Cross linking agent forms

Answer 6

Addition bonds between polymer chains resulting in improved mechanical and physical properties
Deithylene glycol dimethacrylate

Question 7

Processing

Answer 7

Two part mould and acrylic dough

Question 8

Processing problems

Answer 8

Porosity
Polymerisation shrinkage
Processing strains

Question 9

Porosity problems

Answer 9

• volatisation of monomer during curing if T is > too quickly
• because reaction is exothermic and monomer has relativiely low B.Pt
• use slow heating cycle when curing and keep under P

Question 10

Polymerisation shrinkage problems

Answer 10

• can result in denture not contacting hard palate or porosity
• pain may result
• use polymer beads to reduce necessary polymerisation to minimum
• keep denture under p and use slow cool when curing
• incorporate post dam on to denture to compensate

Question 11

Processing strains problems

Answer 11

These may arise due to dissimilar material being used

• e.g. ceramic teeth or CoCr components
• also if the polymer is cooled too quickly

Question 12

Alternative processing routes

Answer 12

‘Injection molding’ of acrylic dough
Injection molding of acrylic above Tg
Milling from block

Question 13

Injection moulding

Answer 13

Heat material to above Tg
Squeeze into mould using high pressure
-PMMA
-polyamide
-PEEK     and more

Question 14

Composition of PMMA

Answer 14

Powder
-PMMA granules
-initiator
-pigments, dyes and opacifiers
-plasticisers
-synthetics fibres (nylon)
Liquid
-PMMA monomer
-inhibitor (hydroquinone)
-cross linking agent

Question 15

Variants

Answer 15

Cold cure (auto cure)
-mainly used for repairs or attaching teeth to CoCr RPD
-lower molecular weight, more porosity
-therefore less dense, softer, weaker and prone to discolouration
High impact
-contains copolymer of butadiene and styrene
-results in dispersion of rubber inclusions
-glass fibres
-nanoparticles
-however most of reinforcement methods exhibit processing difficulties or high cost
Colours

Question 16

Denture teeth

Answer 16

Acrylic
Highly crossed linked acrylic
Composite
Ceramic

Question 17

Adverse reactions

Answer 17

Irritant contact dermatitis
-dental technicians handling acrylic dough
-in denture wearers usually associated with release of residual monomer
-delayed hypersensitivity (type IV)
Traces of monomer remain in denture base after processing
-diffuse out of dental tissues
-allergies including burning mouth sensations

Question 18

Microbiological resistance

Answer 18

Denture stomatitis

-growth of candida albicans on fitting surface of acrylic denture materials

Question 19

Alternatives to PMMA

Answer 19

Flexible materials (nylon)

• where repeated denture fracture occurs
• presence of large tori
• engagement of deeper undercut
• cases where pxs reject metal appearance of clasps

Question 20

Advantages of flexible materials

Answer 20

Toughness 
-Colored like mucosa
-Thinner than conventional materials
-Non-allergenic
-No metal clasps
-Can be used with deep undercuts
Not allowed to use in hospital

Question 21

What next with denture base materials?

Answer 21

CAD design

CAM materials

Question 22

CAM

Answer 22

Opens up range of materials
Industrials processing of materials
Repeatable
Efficient

Question 23

Mechanical properties

Answer 23

Strength (tensile, compressive, flexural, impact; fatigue strength, creep strength)
Stiffness
Resilience
Toughness
Bond strength (to teeth or repair)