1
Q
Historical development of tooth coloured filling materials
A
- silicate cements and acrylic resins
- silicate developed to glass ionomer cements and acrylic to composites
- 3 types used today - resin-modified glass ionomer cements, giomers and compomers
2
Q
Silicate cements are developed from …
A
construction material
3
Q
Composition of silicate cements
A
- white-ish powder hand mixed with liquid
- powder is fluoroaluminosilicate glass
- liquid is 50% phosphoric acid
4
Q
Setting reaction of silicate cements
A
- acid and base = salt and water
- fluoride released
5
Q
How does set silicate cement resemble amalgam?
A
- composite structure
- original powder surrounded by reaction products
6
Q
Advantages of silicate cement
A
- good initial appearance - shade and translucency similar to enamel
- coefficient of thermal expansion - same as tooth
- low thermal diffusivity - protects pulp but never used for deep cavities
- fluoride release - unexpected, wasn’t understood
7
Q
Disadvantages of silicate cements
A
- sensitive to moisture
- acidic due to phosphoric acid
- post-op sensitivity - acid may have irritated pulp
- no adhesion - needed undercut, marginal gaps formed
- very brittle
- solubility
8
Q
Silicate cement was brittle. How was this controlled?
A
- only used in small anterior cavities
9
Q
Why is solubility of silicate cement a disadvantage?
A
- poor stability and durability
- due to acids and dietary factors
10
Q
How were acrylic resins developed?
A
- polymers based on acrylic monomers
- 1930s-50s
- polymethyl methacrylate - maybe useful
- had very large shrinkage on polymerisation
- so used mix pre-polymerised PMMA into monomer (21 to 6% shrinkage)
11
Q
Acrylic resins contained what powders?
Role of them
A
- polymethylmethacrylate beads
- benzoyl peroxide to initiate
- pigments - tooth like appearance
12
Q
Liquids in acrylic resins
Roles
A
- methylmethacrylate monomer
- NN’ dimethyl-p-toluidine - activator (at low temps)
13
Q
Setting reaction of acrylic resins
A
free-radical addition polymerisation
14
Q
Stages of acrylic resin reactions
A
- activation
- initiation
- propagation
- termination
15
Q
Advantages of acrylic resins
A
- good initial appearance from pigments
- low solubility - more stable than silicates
- low acidity - less post-op irritation
- good insulator - protect pulp
- less brittle than silicates
16
Q
Disadvantages of acrylic resins
A
- exothermic reaction
- residual monomer
- flexible
- soft and easily damaged
- marginal leakages and staining
17
Q
Why is acrylic resin having an exothermic reaction a disadvantage?
A
- temp rise
- can cause pain and pulp damage
18
Q
Why is acrylic resin having residual monomer a disadvantage?
A
- low degree of polymerisation
- irritant to tissues
- plasticiser
19
Q
Why is acrylic resin being flexible a disadvantage?
A
- easily deformed in mastication
- plasticiser makes it worse
20
Q
Why is acrylic resin having marginal leakage and staining a disadvantage?
A
- high shrinkage (incremental technique to build up layers didnt help)
- high coefficient of thermal expansion - higher than enamel and dentine
- no adhesion
- water absorption
21
Q
What did we learn from silicate cement and acrylic resin to take into modern day materials?
A
- addition of pre-polymerised PMMA
- reduced polymerisation shrinkage, reduces setting temp rise
- since it’s already polymerised and takes up space in cavity
- but doesn’t improve water absorption, thermal expansion or mechanical proporties significantly
22
Q
Define ‘composite materials’
A
- blend of 2 or more materials
- distinct phases relating to original components
- has all properties of composite not achieved by using them independently
23
Q
Uses of composite
A
- automobile and aviation
- sports
- medicine
24
Q
Filler makes the … phase of composite and matrix has the …
A
- discrete
- continuous
25
Components of dental composites
- glass component or filler (simple quartz or more complex)
- resin component or matrix (one or many monomers)
- coupling agent
- initiator activator system
- pigments
- radio-opacifiers
- stabilisers
26
What does the coupling agent do?
- bonds the matrix to filler
- improves properties
27
What is the initiator-activator system?
- relates to how the composite polymerises
28
Why is glass filler added?
- glass is strong and stiff
- has low coefficient of thermal expansion
- insulating
29
First composites were called ... composites
Features
- conventional
- had quartz/silicon dioxide particles
- 1-50 micrometres in size
30
The first fillers needed a ... adding but this could ... the composite
- radio-opacifier
- weaken
30
As filler concentration increases, what properties reduce?
- polymerisation shrinkage
- coefficient of thermal expansion
- setting temp rise
30
As filler concentration increases, what properties increase too?
- hardness
- stiffness
- strength
- fracture toughness
31
Whats the maximum filler possible?
- up to 75% weight (50% by volume)
32
What was the first effort to increase filler concentration?
Problem
- reduce size of particles
- decrease in size leads to increase in energy so particles stuck together and couldn't be mixed
33
2 new product classes of filler
- microfilled - one size of small particle
- hybrid - two or more blends of particle size
- can have nanocomposites but not new
34
How to add more filler to microfilled composites?
- high temp to reduce monomer viscosity
- high pressure
35
Explain microfilled composites
- manufacturer mixes particles and monomer
- polymerise composite in factory
- break it up into particles
- add composite particles to monomer
- inorganic glass in polymer matrix
36
Clinical uses of microfilled composites
- anteriorly mainly
37
Advantages of microfilled composites
- good aesthetics
- small particles give smooth surface for better gloss
38
Disads of microfilled composites
- poor mechanical properties
- pre-polymerised polymer-glass components weakly bonded to matrix
- care needed in polishing
39
Explain structure of hybrid materials
- 2 or more filler particle sizes
- smaller ones can fit between bigger particles
- combine them to optimise content
- get increased mechanical properties
40
Ads of hybrid material
- better mechanical properties
- more suitable for posterior use and larger cavities
- higher filler conc and lower shrinkage
41
Nanozised particles have been used in dentistry for ... years. They're cheaper/dearer
- 40 +
- dearer
42
Advantages of nanoparticles
- wear resistance possibly
- not lab data to prove
43
Nanoparticles are made via different techniques termed ...
bottom up
44
Disads of nanoparticles
- more expensive but not much different
- small particles clump together and act like big particles so no advantage
- clumps are also weakly bound together so even worse
45
What form do current nanocomposites take?
- combination of larger particles and nanoparticles
- called nanohybrids
46
Which is the strongest of these materials?
- hybrid
- for compressive, yield, tensile and flexural strength
47
Alternatives to conventional composite filler
- amorphous silica
- silica-based glasses
- colloidal silica
- cermaics
48
Why is amorphous silica an alternative to conventional composite filler?
- same composition as quartz
- not as hard
49
Why is silica-based glasses an alternative to conventional composite filler?
- contain metals so radio-opaque
- softer than SiO2 - easier to polish
- not inert - degrades in water and acid (bad)
50
Why is colloidal silica an alternative to conventional composite filler?
- small particle size
- but hydrophilic/hydrolytic degradation and not radio-opaque
51
2 ceramics used as filler
- zirconia-silica
- zirconium oxide
Dental Materials
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