composite resins

Question 1

What are the advantages of composite resins?

Answer 1

Aesthetic
Tough, durable and insoluble in oral environment
Low failure rates
Good insulators + coefficient thermal expansion close to tooth tissue
Minimal tooth prep
Safe if handled and used correctly

Question 2

What challenges remain for composite resins?

Answer 2

Hydrophobic so requires adhesion to tooth tissue
Technique sensitivity
Polymerisation shrinkage

Question 3

What is an ideal restorative material?

Answer 3

Safe
Aesthetic
Durable
Chemically stable
Tasteless
Poor conductor of heat
Minimal tooth prep
Easy to use
Cost effective

Question 4

What is a composite material generally?

Answer 4

Two or more materials combined to produce improved properties
E.g. fibreglass
Component materials largely unchanged (not a solution)
Some form of interfacial bonding (not a mixture)

Question 5

What is the composition of a composite resin?

Answer 5

Matrix component
~mixture of methacrylate resin monomers - mainly bis glycidyl methacrylate (bisGMA) formed by reaction between glycidyl methacrylate w bis-phenol A
Filler component
~silica (SiO2) powder
~silane coupling (interfacial bonding between filler and matrix)
Photoinitiator component
~e.g. camphorquinone
Inhibitor component 
~increases shelf life

Question 6

What is the methacrylate polymer?

Answer 6

Methacrylate
~methyl group on the alpha carbon of the vinyl monomer
Acrylate
~large family of vinyl polymers
Vinyl monomer
~small organic molecule w electron rich C=C, e.g. ethylene

Question 7

What is the filler?

Answer 7

Amorphous silica glass OR quartz powders
~irregular shapes
~ranges from few micrometers to nanoscale
~may include other glasses n ceramics, e.g. zirconia
~hard, insoluble and light scattering (for tooth colour)

Question 8

How are composite resins set?

Answer 8

Chemical cure
~2 pastes mixed together (activator w free radical intitiator)
Light activation***
~photoinitiator exposed to appropriate lamp
e.g. camphorquinone - 470nm - blue light used

Question 9

What are some other methacrylate resin monomers?

Answer 9

Triethylene glycol dimethacrylate, TEGDMA
Urethane dimethacrylate, UDMA
~e.g. reduce viscosity and improve mixing

Question 10

What is free radical polymerisation?

Answer 10

In a methacrylate based monomer, the C=C bonds are electron rich.
Photoinitiator creates reactive chemical species to unlock electrons.
The released internal energy is used to polymerise other methacrylate groups.
Leads to polymerisation shrinkage.

Question 11

What are coupling agents?

Answer 11

E.g. silanes
Improves interfacial bonding of composite to increase strength
Chemically coats filler particle surfaces
However, ages during storage and loses potency and are sensitive to water
During restoration, water absorbed may result in hydrolysis of silane bond and gradual loss of properties
Common silane agent e.g. vinyl triethoxysilane

Question 12

What are the structure property relationships in a composite resin?

Answer 12

Organic phase (resin matrix)
~monomer, initiator, inhibitors, pigments
~polymer backbone provides tensile strength
Dispersed phase (inorganic filler)
~glass, quartz, colloidal silica
~improves mechanical properties (wear, compressive strength)
~decreases shrinkage
Interfacial phase (coupling agent)
~adhesive bond between organic + inorganic

Question 13

Why do the filler particles vary in size?

Answer 13

Macrofilled
~larger particles (10-50micrometers) scatter light dramatically so v opaque, high wear n shrinkage
Microfilled
~(0.04micrometers), better wear resistance, more polishable but particles aggregate, high shrinkage and lacks strength
Hybrid*
~variously sized particles so better packing efficiency so less shrinkage, better mechanical p. and retains polish-ability
Nanofilled*
~less technique sensitivity and improved clinical success

Question 14

Why are nanofilled composites claimed to be so good?

Answer 14

• Superior mechanical properties
• High polish-ability
• Very low p. shrinkage
• Improves continuity between natural tooth and nanofiller so better interface
• High SA to vol ratio so high filler loading so won’t shrink as much
• Unable to scatter or absorb visible light- increases translucency
• However, remains controversial

Question 15

Why are nano particles so difficult to manufacture?

Answer 15

Even if you manage to work at such a tiny scale, the particles tend to aggregate due to forces e.g. London forces

Question 16

Why are nanocomposites better than microcomposites in terms of nanoindentation?

Answer 16

The forces required to measure indentation on the materials are used to plot data
Nanocomposites have a smaller range of variation in hardness so the more homogenous the surface, the less prone to wear or friction for example
However, clinically, bonding is probably more important

Question 17

What are the advantages of smaller filler sizes?

Answer 17

Increased resistance to wear
Potentially greater strength
Better polish-ability
Improved translucency
Potentially decreased polymerisation shrinkage

Question 18

How is polymerisation shrinkage reduced clinically?

Answer 18

Don’t do bulk placement
~inadequate cure at depth so poor interface w tooth and therefore risk of micro leakage and secondary caries
Should do incremental layering of about 2mm
~if layers are too thin, oxygen diffuses in and inhibits polymerisation