Denture Base Materials

Question 1

State the ideal properties of a denture base (broad overview)

Answer 1

-Dimensionally accurate and stable in use -High softening temperature (Tg) -Unaffected by Oral fluids -Thermal expansion -Low density -High thermal conductivity -Radiopaque -non toxic, non irritant -Good colour/translucency -Easy and inexpensive to manufacture -Easy to repair -Good mechanical properties

Question 2

What are the ideal mechanical properties of a denture base?

Answer 2

• High Young’s (Elastic) Modulus (stiffer)
• High proportional limit (maximum stress that a dental material sustains without any deviation)
• High transverse strength
• High fatigue strength
• High impact strength
• High hardness/abrasion resistance

Question 3

What is transverse strength of a denture?

Answer 3

It is to do with flexure The strength/stress is measured at the point before the material fractures/yields There are 3 points of stress on the material

Question 4

Why does a denture base need a high impact strength?

Answer 4

Because if the denture is dropped and has a low impact strength then it may:

• fracture
• create micro-cracks causing the denture to fail later on

Question 5

What kind of polymerisation does acrylic resin undergo?

Answer 5

Free radical addition polymerisation

Question 6

What is free radical addition polymerisation?

Answer 6

The chemical union of two molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule.

It involves molecules with C=C double bonds.

Question 8

What are the stages in acrylic polymerisation?

Answer 8

• activation
• initiation
• propagation
• termination

Question 9

What are the 2 types of acrylic resin based on how they are cured?

Answer 9

Heat cured and self-cured

Question 10

In heat cured acrylic resin, describe the contraction and expansion that takes place during wear and manufacture.

Answer 10

PMMA contracts by 0.5% during manufacture

It expands by approx 0.4% during usage (water absorption)

This just about equates

Question 11

How does the composition/polymerisation of self-cured acrylics differ from heat cured acrylics?

Answer 11

The same as heat cured except benzoyl peroxide is activated by a promoter (tertiary amide) in the liquid instead of heat

Question 12

In order to get a polymer with good mechanical propertes, what does it molecular weight need to be? How is this achieved?

Answer 12

high molecular weight

Efficient polymerisation must take place for this to happen which is why a high temp is used for heat cured acrylic resins

Question 13

What is the problem with the high temps used to set acrylic resins? (heat cured)

Answer 13

There are gaseous porosity limits

i.e. air bubbles may form which weakens the material

Question 14

Is PMMA a non-toxic and non-irritant material?

Answer 14

• Non-toxic = yes
• Non-irritant = yes if no monomer is released (i.e. it is fully polymerised so no monomer can leach out). Few patients are allergic

Question 15

Is acrylic resin affected by oral fluids?

Answer 15

• Unaffected by oral fluids – it doesn’t absorb oral fluids and it is insoluble in fluids taken orally (if it did this could affect its fit)

Question 16

What are the mechanical properties like of acrylic resin? What is done to overcome this?

Answer 16

• Poor mechanical properties – need to increase the bulk of the material to compensate

Question 17

What is fatigue strength?

Answer 17

• Fatigue strength = ability to resist repeated application of low level stresses during things like mastication

Question 18

What is the fatigue strength like for acrylic resin?

Answer 18

• Reasonable fatigue strength/impact strenght but it is far from indestructable

Question 19

What is the ahrdness/abrasion resistance of acrylic resin like?

Answer 19

• Hardness/abrasion resistance – resists abrasion quite well but over prolonged use the surface layers may be removed, affecting its appearance and causing it to feel rough when contacted by patient’s tongue.

Question 20

Does the thermal expansion of acrylic resin match that of the artificial teeth?

Answer 20

• Good if acrylic teeth are used
• Expansion of acrylic is substantially higher than porcelain teeth

Question 21

What is the thermal conductivity of acrylic resin like?

Answer 21

Low which is poor

Question 22

What is the density of acrylic resin like?

Answer 22

• Density = low (good) but the increase in bulk to overcome poor mechanical properties offsets advantage
  
  • Want low dentisty so its lightweight and comfortable for patients

Question 23

What is the softening temperature of acryli resin like?

Answer 23

• Is 75 degrees celcius which is okay for injested hot fluids but can’t cope with boiling water which is something patients overlook when cleaning
• Need to advise to NOT use boiling water for cleaning

Question 24

Is acrylic resin dimensionally accurate and stable in use?

Answer 24

Just about

• Has a 0.5% linear contraction during the heat-curing manufacturing stage
• Expands approx 0.4% due to water absorption during use
• These just about balance each other out

Question 25

Compare the thermal contraction and dimensionally accuracy between self-cured and heat cured acrylics.

Answer 25

In self-cured acrylics there is no heat-cured stage so little thermal contraction will be experienced compared to heat cured acrylics. Self cured acrylics fit the original cast better than heat cured as less thermal contraction BUT water absorption will give expansion leading to an over-sized acrylic that will fall out Heat cured acrylcis have thermal contraction of 0.5% and expansion of 0.4% due to water absorption. This leads to a slightly under-sized fit which tends to be tolerated better by patients

Question 26

Compare heat-cured and self-cured PMMA

Answer 26

Heat-cured: * Has a higher molecular weight making it stonger meaning that it will survive for longer * But the curing process may cause porosity * You should examine each denture you get to check for flaws e.g. bubble below the surface Self-cured: * Higher unreacted monomer levels which can act as an irritant to the patient’s soft tissues * You need to inform the patient of this risk and instruct them to notify you ASAP if there are any signs * Fits cast better but water absorption makes it oversized in mouth * Poorer colour stability * The other properties are the same heat cured acrylics The end point is that neither is ideal!

Question 27

What improved forms of acrylics have been designed to improve the strenght of the material?

Answer 27

* Incorporate high impact resistant materials * Rubber toughening agent (butadien styrene) to stop crack propagation and long term fatigue problems * Incorporate fibres * However, have been proven difficult to manufacture

Question 28

A high impact heat cured acrylic resin is used in Glasgow Dental Hospital (called Ultra-Hi). What are its benefits?

Answer 28

* Greater degree of ductility * Increases fracture toughness by helping to mitigate the effects of any micro-cracks that may be present. * Increased flextural strength These 2 key features together gives the material a slight bending aspect which keeps the material from being brittle and subject to cracking and/or breaking.

Question 29

What are the components of light-activated denture resins?

Answer 29

- UDMA plus acrylic co-polymers - microfine silica (control flow of material) - photoinitiator

Question 30

How is a denture base made from the light cured acrylic? What is its downfall?

Answer 30

* The material is adapted to the patient’s cast and light-cured in a light chamber * Limited depth of cure which limits the thickness of the resin (thickness needed to imprive mechanical properties)

Question 31

Light-cured acrylic resin is mostly used for what?

Answer 31

used mostly as customised impression trays and for the repair of fractured dentures

Question 32

Why would it be ideal for a denture base to be radiopaque?

Answer 32

Incase it fractures and missing piece swallowed etc

Question 33

Has there been a way found to make acrylic radiopaque?

Answer 33

no (all weakened the material)

Question 34

What are some alternative polymers that were developed incase your pateint had an allergy?

Answer 34

* Nylons * Vinyl polymers * Polycarbonates.

Question 35

What are the flaws with nylons?

Answer 35

* Swelling due to water absorption * Softening

Question 36

What are the flaws with vinyl polymers?

Answer 36

* Have a softening of just 60 degrees Celsius meanign they’ll soften in use * Need to be produced using an injection mouling process which is very expensive

Question 37

What are the flaws with polycarbonates?

Question 38

Summary: * Heat-cured acrylic resin is the most commonly used denture base material * Most properties (mechanical, thermal) are satisfactory * Alternative materials all have deficiencies (no serious alternative has been developed)