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BDS2 CLINICAL Dental Materials > Composite Properties > Flashcards

Composite Properties Flashcards

1
Q

what is the fracture stress of composite

A

350 MPa

It is strong

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2
Q

what is the Young’s modulus of composite

A

15 GPa

it is rigid

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3
Q

what does a large posterior cavity require from composite

A

composite that is high strength, high rigidity and high abrasion resistance as it needs to survive its function and withstand biting forces

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4
Q

what do deciduous teeth require from composite

A

a composite that is strong in thin sections and it’s wear is the same of that of the tooth (i.e not amalgam) as well as other important properties such as bonding, microleakage etc

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5
Q

describe conventional composite

A

strong but problems with finishing and staining due to soft resin and hard particles. The softer the resin then the softer the resin on the surface and the more likely it is to uptake stain from food and drink etc.

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6
Q

describe microfine composite

A

smaller particles, smoother surface for better aesthetics for longer period but inferior mechanical properties (elastic limit and young’s modulus)

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7
Q

describe hybrid composite

A

originally compromise between conventional and microfine. Most modern composites are hybrids - improved filler loading (higher percentage of filler) and coupling agents have led to improvement in mechanical properties

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8
Q

what is hardness

A

refers to the materials surface – we want to know whether a material is resistant to scratching, whether it is able to resist indentation.

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9
Q

what does abrasion result in

A

removal of surface layers when two surfaces make frictional contact as these are the forces that happen when we bite and grind surfaces against the tooth/restoration

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10
Q

what is the hardness test

A

test uses an indentor made of stainless steel that has a specific shape at the end. It weighs around 100g and we leave it for a period of time, 30 seconds usually and this allows an indentation to appear.
If the material is hard then the indentation should be relatively small.

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11
Q

what is abrasion

A

when the tooth grinds/slides along the opposing tooth surface (or the restorative material at its surface.

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12
Q

what does the tooth surface being abraded result in

A

a loss of material surface layers and a roughened surface.

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13
Q

what does the surface roughness of composite affect

A

o Appearance
o Plaque retention
o Sensation when in contact with the tongue

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14
Q

what happens to composite in wear

A

As the surface is removed, the resin is removed, and the glass filler particles are exposed and it is only when they are exposed do we lose them.

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15
Q

how does the size of the filler particle affect surface roughness

A

The size of the filler particles on the surface will determine the degree of surface roughness.

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16
Q

what are the material factors affecting wear

A 
o	Filler material 
o	Particle size distribution
o	Filler loading
o	Resin formulation
o	Coupling agent
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17
Q

what are the clinical factors affecting wear

A 
o	Cavity size and design 
o	Tooth position 
o	Occlusion
o	Placement technique
o	Cure efficiency 
o	Finishing methods
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18
Q

what are the bonding properties of composite

A

Generally okay but some mismatches have been reported

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19
Q

how does composite bond to enamel

A

acid etch technique

20
Q

what is acid etch technique

A

uses 30% phosphoric acid, put on enamel for 20 seconds. It takes the original enamel surface and leave ‘microtags’ within the enamel structure which you can fill with an unfilled resin

21
Q

how does composite bond to dentine

A

through dentine/universal bonding systems

22
Q

what is the typical bond strength of composite to enamel/dentine

A

40 MPa

23
Q

why is bonding crucial

A

a good bond will reduce micro leakage and counteract polymerization’s stress
stress transfer

24
Q

what happens in polymerization shrinkage

A

material is trying to pull away from tooth surface it is bonded to and it tries to pull away with sufficient force and break away from tooth surface then this can create gaps

25
Q

what does composite bonding to tooth mean for cavity design

A

no need for retention undercut.

26
Q

why is bonding crucial for stress transfer

A

Good bonding means that when a force is applied the forces acting on the base of the composite are spread uniformly and evenly and we have a good bond then that the restoration is likely to survive a long time. If you have a poor bond then there is lots of gaps between the restoration and the tooth tissue and the stress that you put will not be spread evenly and there will be greater stress on certain regions and if there is greater stress then the material is more vulnerable to fracture.

27
Q

why are hybrid composites better than microfilled composites

A

o elastic limit is almost double that of microfilled composite
o hybrid is almost 3 x as hard making it more resistant to abrasion

28
Q

how does amalgam compare in terms of elastic modulus

A

double the elastic modulus meaning it is twice as rigid meaning it is twice as able to resist any movement when you subject it to stress

29
Q

what are the thermal properties of composite

A

low thermal conductivity (good) but has a high thermal expansion coefficient which is poor

30
Q

what is the thermal conductivity of composite

A

The thermal conductivity is low meaning that heat is transferred quite poorly as composite is an insulator not a conductor. The thermal conductivity should be low to avoid pulpal damage from hot and cold foods/fluids

31
Q

what is thermal expansion

A

The thermal expansion of composite resin should be equal to the tooth to reduce microleakage otherwise thermal contraction can occur if there is a cold stimulus and saliva and bacteria can get in. We want resin and enamel to contract at the same rate so that the microleakage is reduced. Bonding influences the gap formation

32
Q

what is the thermal expansion coefficient for enamel

A

11

33
Q

what is the thermal expansion coefficient for dentine

A

8

34
Q

what material has closest expansion coefficient to enamel/dentine

A

GIC

10-11

35
Q

what is the thermal expansion coefficient for composite

A

25-68 meaning that it tries to expand at twice the rate of the tooth meaning that there is more stress on that bond and the greater the stress then the more likely there is going to be a fracture

36
Q

what are the aesthetic properties of composite

A 
o	A good range of shade
o	Translucency
o	Maintenance of properties over lifetime
o	Resistance to staining
o	Surface finish
37
Q

is composite radiopaque

A

Some composites are radiopaque which allows them to show up in x rays which is useful when diagnosing secondary caries

38
Q

what is the handling/viscosity properties of composite

A

Light curing gives an ‘on demand’ setting but there are potential problems.
The mixing/working times depends on the specific material
different viscosities available

39
Q

is composite anticariogenic

A

Generally composite isn’t anticariogenic but there are a few products that claim to release fluoride

40
Q

Does composite have a smooth surface finish/polishable

A

It can be good but it is product/technique sensitive

The aesthetics of a composite resin restoration is one of the most important things for patients

41
Q

why is QUIXFIL more likely to experience debonding than FILTEK

A

QUIXFIL has some grey areas on the stress concentration map meaning there is stress exceeding 50MPa which is greater than the typical bond strength of 40

42
Q

how is the biocompatibility of composite

A

Generally thought to be okay but increasing concern about resins in general as not all monomer is polymerized and overtime as the material weakens the monomers could be released resulting in irritation around the surrounding tissue

43
Q

what are the properties that affect the choice of material

A 
o	Mechanical
o	Bonding
o	Thermal
o	Aesthetic
o	Handling
o	Surface finish
o	Polymerization shrinkage
o	Anticariogenic
o	Biocompatible
o	Radiopacity
44
Q

what is resin modified GI used for

A

high caries risk and frequent attenders

45
Q

what is composer used for

A

medium caries risk, keeps caries under control and for regular attenders

46
Q

what is composite resin used for

A

low caries risk patients

BDS2 CLINICAL Dental Materials (15 decks)

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