denture base materials

Question 1

What are the ideal properties?

Answer 1

Biocompatible- hygienic*

Physical properties- aesthetic, stable, high thermal conductivity*, low density, radiopaque

Mechanical properties- high strength/stiffness, hard, tough

Ease of processing, repair, adjustment, reproduction of surface detail

Reasonable cost

*PMMA doesn’t fit these criteria

Question 2

What are current materials we use?

Answer 2

PMMA- polymethyl methacrylate
Nylon polyamide- flexible denture (Valplast)
Polyetheretherketone (PEEK)
Nylon
Cobalt chrome alloy
Ti alloy

Question 3

What is PMMA?

Answer 3

Plastic
Organic polymer
Acrylic- really acrylate (acrylite and Perspex- trade names)
Thermoplastic- Tg ranges from 85-165C
Long chain molecule of repeated units of methyl methacrylate
To start process-
~initiator (benzoyl peroxide)- splits into 2 w one unpaired electron each and these attach the C=C bond
~cross linking agent- links polymer chains together for better properties (diethylene glycol dimethacrylate)

Question 4

What is the composition of PMMA?

Answer 4

Powder-
PMMA granules
Initiator
Pigments, dyes, opacifiers
Plasticisers
Synthetic fibres (nylon)

Liquid-
Methyl methacrylate monomer
Inhibitor (hydroquinone)
Cross linking agent

Question 5

What are problems with processing?

Answer 5

Porosity
Polymerisation shrinkage
Processing strains

Question 6

Why might you get porosity?

Answer 6

Volatisation of monomer during curing if temp raised too quickly
As reaction is exothermic and monomer has low boiling point

So, use slow heating cycle and keep under pressure

Question 7

Why might you get polymerisation shrinkage?

Answer 7

Denture doesn’t contact hard palate/contraction porosity

Use polymer beads in correct proportion to reduce extra polymerisation
Keep denture under pressure and use slow cool when curing
Incorporate post dam to compensate

Question 8

Why might you get processing strains?

Answer 8

Due to dissimilar materials being used (eg. Ceraminc teeth or CoCr components
Or if polymer cooled too quickly

Question 9

What are alternative processing routes?

Answer 9

‘Injection moulding’ of acrylic dough
Injecting moulding of acrylic above Tg
Milling from block
3D printing

Question 10

What are variants of PMMA?

Answer 10

Cold cure (auto cure)
~mainly used for repairs/attaching teeth to CoCr RPD, chairside relines
~lower molecular weight so more porosity
~less dense, softer, weaker and prone to discolouration

High impact
~bouncy due to additives
~contain copolymer of butadiene and styrene
~so dispersion of rubber inclusions
~carbon/glass fibre addition
~nanoparticles addition
~most of reinforcement methods have processing difficulties or high cost
~fatigue over time

Question 11

What are denture teeth made of?

Answer 11

Acrylic
Highly crossed link acrylic
Composite
Ceramic

Question 12

What are some adverse reactions to PMMA?

Answer 12

Traces of monomer remain after processing
Diffuse into oral tissues
~irritation/allergic reactions/burning mouth
Technicians in contact w monomer and polymer
~asthma, resp tract and eye problems, irritant contact dermatitis

Question 13

What is the microbiological impact on dentures?

Answer 13

Denture stomatitis
~growth of Candida albicans which adhere via London/electrostatic forces
~fungus acts as reservoir for bacteria
~factors contributing- other microorganisms, diet rich in sucrose, roughness of material

Formation of biofilms in cracks and imperfections (regular recall for repair)

Require good polish or glaze

Question 14

What are alternatives to PMMA?

Answer 14

CoCr- not complete alternative
Nylon
Thermoplastic PMMA

Question 15

Why might we use flexible denture materials?

Answer 15

If repeated denture fracture
Presence of large tori/deep undercut
Severe trismus eg. scleroderma and following surgery in oncology

Question 16

What are advantages of flexible materials?

Answer 16

Toughness- nearly unbreakabld
Coloured like mucosa (translucent)
Thin
Non-allergenic
No metal clasps
Can be used in deep undercut

Question 17

What are disadvantages of flexible materials?

Answer 17

Poor for gingival health

V difficult to process and adjust

Question 18

What is CAD and CAM?

Answer 18

Computer aided design
Computer aid med manufacture
Led to use of previously unworkable materials

Question 19

What is advanced manufacturing?

Answer 19

Opens up a range of materials
Industrial processing of materials
Hopefully efficient and repeatable

Question 20

What are mechanical properties?

Answer 20

Strength-
~static- tensile, compressive, flexural, impact
~dynamic- fatigue strength, creep strength

Stiffness

Resilience

Toughness

Bond strength

Question 21

What are relining materials?

Answer 21

When denture becomes ill fitting due to bone resorption

Use PMMA
Remove undercuts and periphery chair side
Take impression (ZOE/silicone)
Send to lab, model is cast
Vertical dimension recorded (articulated/reline jig)
Impression material replaced w wax
Processing replaces wax w acrylic

Question 22

What are variants to relining PMMA?

Answer 22

Cold cure resin- for temporary measure/on RPD

Chairside reline (Tokuso rebase)- poor control over vertical dimension, not pleasant for patient
~if removed too soon- distortion
~if removed too late- exothermic reaction (hot)
~residual monomer
~colour retention

Soft liner- should have low elastic modulus, high resilience, good adhesion, high tear strength, biocompatible, antibacterial, dimensionally stable, good surface wettability

Question 23

What are soft liner materials?

Answer 23

Plasticised heat cured

Silicone rubber- cold/heat cured

Silicone/acrylic co-polymer

Question 24

What are tissue conditioners?

Answer 24

Viscogel
Temp linings that allow traumatised tissue to recover whilst producing new denture

Contains-
~polyethylmethacrylate (low Tg)
~butyl phthalyl glycolate (plasticiser)
~ethyl alcohol (solvent)