DML1: Structure of composites

Question 1

What is a compostie

Answer 1

two or more materials put together with each contributing to the overall properties (thus if one was missing the material would fail)

Question 2

Why are silicate cements not used any more

Answer 2

• they are soluble in oral fluids and so the properties are weakened over time
• it means that the material is swallowed
• it also discolours

Question 3

Why were silicate cements originally used

Answer 3

• they used to be the only tooth coloured restorative

- they released fluoride which reminineralises

Question 4

What are the advantages of direct filling acrylics such as polymethyl methacrylate (PMMA)

Answer 4

• tooth like appearance
• insoluble in oral fluids
• easy to manipulate
• low cost

Question 5

What are the disadvantages of direct filling acrylics such as polymethyl methacrylate (PMMA)

Answer 5

1. There is high setting shrinkage and so secondary caries occurs at the margins
2. Poor wear resistance (especially in posterior cavities)
3. The coefficient of thermal expansion is different to the tooth and so there are stresses at cavity margins

Question 6

How can the disadvantageous properties of PMMA direct filling acrylics be reduced

Answer 6

Inclusion of quartz power as a filler - this makes the filling a composite

Question 7

What are resin based direct composite filling materials such as Bowen’s resin made from

Answer 7

Bis-GMA (dimethacrylate made from Bis-phenol A and glycidyl methacrylate)

The filling material consists of three phases

Question 8

Why are resin based direct composite filling materials used

Answer 8

They are tooth coloured and so a choice for direct aesthetic anterior restorations

Question 9

What are the three phases in dental composites

Answer 9

1. Organic matrix = plastic monomer/resin viscous fluid material that polymerises to form a continuous phase, binding the filler particles via a coupling agent (this is hydrophobic)
2. Inorganic filler = reinforcing particles/fibres dispersed in the matrix which enhances the materials mechanical properties (this is hydrophilic)
3. Coupling agent = bonding agent promotes adhesion between the filler and resin matrix

Question 10

How does the organic matrix set

Answer 10

It cure-polymerises into a rigid polymeric material via a free radical, addition polymerisation reaction

Question 11

What does the organic matrix in the monomer phase consist of

Answer 11

1. Aromatic dimethacrylate-Bis-GMA
2. Bis-GMA

As this is not pure, it will not polymerise to 100% and so the two components will leach into the patients mouth

Question 12

What are the constituents of Bis-GMA

Answer 12

1. Bisphenol A

2. Glycidyl methacrylate

Question 13

Why is UDMA (urethame dimethacrylate) used instead of Bis-GMA

Answer 13

Because it is a monomer with a lower viscosity because it doensn’t have aromatic rings

Question 14

Why are diluent monomers added to the organic matrix

Answer 14

Because both Bis-GMA and UDMA become unworkable due to their viscosity which is further increased by the addition of fillers

Diluent monomers act as viscosity controllers

Question 15

Give an example of a diluent monomer

Answer 15

Glycol dimerthacrylates

Question 16

Why are dimethacrylates used instead of methyl methacrylates

Answer 16

because methyl methacrylate has a lower molar volume and molar weight than dimethacrylates like bis-GMA have larger monomers and this reduces the linear shrinkage that will occur (this is because the C=C double bonds will be broken)

Question 17

What additives does the organic matrix contain and why

Answer 17

• hydroquinone
• DHPT activator/initiator system for room temperature cure
• camphorquinone activator/initiator system for light cure
• fillers to reduce polymerisation shrinkage
• pigments to give correct shade
• UV stabilisers
• optical brighteners to give flouresence

Question 18

What are the benefits of including a filler phase within composites

Answer 18

1. reduces setting shrinkage as less resin is required
2. reduces coefficient of thermal expansion (this has to be close to that of the tooth tissue)
3. Enhances modulus, compressive strength and hardness
4. Provide radiopacity
5. Control of aesthetics

Mechanical properties of composite improve with higher amounts of filler

Question 19

Why must the refractive index of filler and resin match

Answer 19

To avoid scattering of light otherwise the full depth of cure will not be achieved when light curing the resin

Question 20

What do smaller filler particles result in

Answer 20

A smoother finish of the restoration and they can be polished

Question 21

Why are macrofillers no longer used

Answer 21

Because they give a poor finish with a dull appearance as the filler particles protrude through the resin surface

Question 22

What are the positives of macrofillers

Answer 22

They have a high maximum filler loading

Question 23

What is the issue with microfillers

Answer 23

A very large surface area of filler is in contact with the resin and so there is difficulty obtaining a high filler loading - this gives worse mechanical properties even though it is more aesthetic

Question 24

How is adequate microfiller loading achieved

Answer 24

2 STAGE PROCESS (has greater cost implications)

1. A microfilled composite is polymerised and ground to particles 10-40um in size
2. These particles are then used as a filler

Thus these are composite containing composite filler particles

Question 25

What are the benefits of hybrid fillers

Answer 25

The packing density is increased while the size of the filler has been reduced

• improved wear resistance
• radiopacity

This is why they are used for posterior restorations because they are stronger

Question 26

What are the diadvantages of hybrid fillers

Answer 26

They have an increased surface roughness with time as the resin wears away and the filler particles protrude through the matrix

Question 27

What are the benefits of nanofilled composites

Answer 27

They have the strength of a hybrid and polishing finish of a microfilled material

Question 28

What are microfilled composites used for and why

Answer 28

For anterior restorations due to lower filler loading and this decreased mechanical properties

Question 29

Why are silane coupling agents needed

Answer 29

Because there is phase separation due to hydrophobic resin and hydrophilic filler and these need to be bonded to eachother for the composite to have the desired mechanical properties - coupling agents thus give improved wear resistence

Question 30

How does the silane coupling agent react to bond to the filler and resin

Answer 30

Filler: hydrophilic end of coupling agent reacts with hydroxy groups of filler via condensation reactions

Resin: bonds form to the organic matrix via free radical addition polymerisation

Question 31

What happens if bonding is unsuccessful between the filler and resin via the silane coupling agent

Answer 31

1. Unstable bonds cause fracture and disintegration of material
2. If no bond is made there will be stress transfer due to phase separation
3. Crack initiation sites will occur which will cause fatigue failure of composite

Question 32

What are the three initiator systems for polymerisation and curing of composites? Outline their strengths and weaknesses

Answer 32

1. Heat cured - impractical for direct filling materials and cannot be used within patients mouth; it is used for indirect composites (inlays/onlays)
2. Room temperature - dispensed as two pastes; on mixing air bubbles are incorporated and this makes mixing difficult due to viscosity of pastes and porosity will occur
3. Light cured - react in blue light to form free radicals which initiate addition polymerisation

Question 33

What are the two pastes used in room temperature cured composites

Answer 33

• benzoyl peroxide

- dihydroxyethyl-p-toulidine (DHPT) = tertiary amine