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Y3 Dental Materials > L15 Resin Composites 2 > Flashcards

L15 Resin Composites 2 Flashcards

1
Q

What is the role of filler?

A

Reduces polymerisation shrinkage
Reduces coefficient of thermal expansion
Improves mechanical properties
Increases elastic modulus
Provides radiopacity
Adjusts handling
Controls translucency and aesthetics

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2
Q

How does filler reduce polymerisation shrinkage?

A

More filler means less resin means less shrinkage
But can still be enough to generate sufficient stress at the interface to cause gaps

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3
Q

Compare the TEC of the resin monomer and the tooth

A

Expansion at different rates is unwanted. Resin thermal properties do not match the tooth. Monomer - 80ppm/0C, enamel - 11ppm;0C, glasses - 8ppm/0C

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4
Q

Which mechanical properties does filler improve?

A

Fracture toughness and overall strength

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5
Q

How does filler increase elastic modulus?

A

Increases stiffness and reduces flexibility of the polymer. Creates appropriate wear resistance.

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6
Q

What does filler making resin more radioopaque mean?

A

It is harder to spot on radiographs than amalgam
(Zirconia is added to help this)

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7
Q

What are the indications of using a relatively large filler?

A

You have a higher SA and a differential wear rate - filler does well at resisting wear but the resin matrix wears away quicker, reacting a rough surface where the fillers can fall out of.

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8
Q

What is the issue with acid-modified resin?

A

Self adhesive properties with acid modified resin with low pH becomes very hydrophilic. Will soak up a lot of moisture and degrade quicker, reserved for adhesive.

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9
Q

What are the issues with light curing?

A

Cannot place more than a certain amount to cure because you won’t get effective light transmission through the material.

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10
Q

What can be filled more overcoming the light cure issues?

A

Bulk fill

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11
Q

What is the order and development of light-curing units?

A

Self-cured, UV, visible light

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12
Q

Order in development of fillers

A

Microfilled, hydrids,munivsersal flowables and packables, microhybrids, nano filled and nanohybrid

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13
Q

Order in development of resins

A

Low-shrinkage, self-adhesive, bulk-fill

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14
Q

How has particle size of fill restorative resins changed over time?

A

Decrease in average particle size range since the introduction of RBCs
Filler particle sizes manipulated to achieve desired mechanical and physical properties

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15
Q

In hybrid resin composites, how are properties optimised?

A

Filler packing. To increase load, use a range of filler sizes so smaller ones will fill inside the gaps if the larger ones, so a distribution of sizes.

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16
Q

How would you increase viscosity of a resin composite?

A

Increase amount of filler or decrease amount of resin

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17
Q

When might a flowable composite be used?

A

Likely used to line a cavity not on an occlusal surface as it will likely have less filler and have less wear resistance than a higher viscosity type.

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18
Q

What are the key aspects of curing efficiency?

A

Degree of conversion, cross-linking
Depth of cure
Shrinkage and shrinkage stress
Mechanical properties - strength characteristics, modulus
Trapped radicals, residual monomer

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19
Q

What is degree of conversion and how is it optimal for curing efficiency?

A

The degree to which you’ve converted monomer to polymer units - want to maximise this and not have excess uncured material

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20
Q

How does depth of cure affect curing efficiency?

A

DC with depth? Not only amount of cross-linking creating, but how much is created with depth.

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21
Q

How does shrinkage affect curing efficiency?

A

Stress generation is more critical. Gaps can cause secondary infection.

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22
Q

How might trapped radicals and residual monomer be formed, from curing efficiency?

A

Consequences of monomer elation
Always be some residual monomer

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23
Q

Why is there a limited depth of cure?

A

Due to filler particles placed in the way, because of the pigments that are incorporated into the composite, photo-initiator has to absorb some of the light to create the polymerisation reaction. Surface reflection.

24
Q

What affects absorption of light?

A

Photoinitiator chemistry
Pigments

25
Q

How do pigments affect absorption?

A

Pigments are one of the biggest absorber of light, so to compromise and make something more translucent, but you won’t have as many shades to choose from.

26
Q

Why does light scatter?

A

Light scatters because of the filler particles, by their size shape or optical properties. If there is no match in density between the 2 phases, there will be scattering at the interface because of a refractive index mismatch.

27
Q

How can resin composites be optimised optically then?

A

Can optimise resin composites by trying to change the density of the filler to match the optical property of the resin (to increase translucency)

28
Q

What factors affect interfacial scattering?

A

Porosity, filler density, resin density, refractive index mismatch

29
Q

What can vary the depth of cure? (Graph pg78)

A

How well it is cured, light intensity, material itself etc

30
Q

Why do we place in increments of 2mm or less for conventional RBCs?

A

After about 2mm there is an exponential drop off in hardness of the material, so don’t place increment in more than 2mm.

31
Q

What increments can be placed for bulk fill and why is it more than conventional RBCs?

A

5mm or less
Bulk fill materials have less pigment

32
Q

What factors will affect the cure?

A

Adequate light irradiance
Exposure time
Radiant exposure
Increment thickness
Tip to material distance

33
Q

How can we ensure adequate light irradiance for best possible cure?

A

Ensure light is working properly, ensuring no contamination or crack of light
>300mW/cm2
Modern LED lights - 1000mW/cm2 (high output)

34
Q

Can a low intensity light be compensated for?

A

Yes, dose of light is the product of intensity and time, so increase time

35
Q

What should exposure time be for the best possible cure for the conventional composite of 2mm with the standard modern light?

A

Normally 20-40s

36
Q

What should exposure time be for the best possible cure for the bulk fill composite with the standard modern light?

A

10-20s

37
Q

Can a material be over cured?

A

No but light on for too long can cause excess heat generation, so be careful with mucosa and pulp.

38
Q

How can bright light affect the eyes?

A

Can cause ocular damage so don’t look at it

39
Q

What is the radiant exposure for the best possible cure?

A

Irradiance x time
About 20J/cm2

40
Q

How is the tip to material distance varied for the best possible cure?

A

Stabilise and concentrate the light

41
Q

When is gap formation inevitable?

A

If stress exceeds adhesive layer bond strength

42
Q

What is post-gel shrinkage stress?

A

The initial stages of reaction have enough flow of the material and propagation of radicals etc to absorb some of the stress generated. When cross-link density gets too high, it cannot compensate for the amount of stress = post-gel shrinkage

43
Q

Which stage of polymerisation has the largest effect on stress generation?

A

Latter stages of polymerisation (high conversion) have the largest effect on stress generation (graph pg79)

44
Q

Is stress an intrinsic material property?

A

No it is a complex interaction of several factors. Will occur is stress generated on cue exceeds strength of the adhesive layer between tooth and restoration.

45
Q

What factors effect polymerisation stress?

A

Volumetric polymerisation shrinkage
Resin composite elastic modulus
Configuration factor
Compliance
Speed of curing

46
Q

How does volumetric polymerisation shrinkage affect polymerisation stress?

A

Contributes to stress magnitude
Does not directly affect gap formation

47
Q

How does resin composite elastic modulus affect polymerisation stress?

A

More flexible liner may reduce stress by absorbing it
Stiffer more rigid material won’t be able to compensate for stress generation

48
Q

What is configuration factor?

A

Ratio of free to bonded surface area, e.g., a deep class I cavity will have a higher ratio of unbounded to bonded cavity than a class IV

49
Q

How does configuration factor affect polymerisation stress?

A

With lots of restriction and less bond, there is nowhere for the stress to be compensated, so it acts at the interface more. Image pg80. As you increase C factor, number of bonded to unbounded surfaces, you increase shrinkage stress.

50
Q

How does compliance affect polymerisation stress?

A

Rigidity of supporting tooth structure

51
Q

How does speed of curing affect polymerisation stress?

A

So-called, soft start regimes

52
Q

Effects of ramped intensity on stress (graph pg81)

A

High intensity - 600mW/cm2 x 40s, fastest cure
Low intensity - 200mW/cm2 x 120s, reduced rate but achieves similar stress values
Pulse delay - 600 x 1 s for 3min + 600 x 39s, slowest cure, substantial decrease in rate translates to significant decrease in stress

53
Q

Will soft start regimes reduce stress?

A

Unlikely

54
Q

Should we use lower cures to reduce stress then?

A

Energy at lower cures might not reach high enough to cure the material

55
Q

Advantages of resin composites summary

A

Cosmetics
Single paste
Command set
Versatile
Clinical proven

56
Q

Disadvantages of resin composites summary

A

Technique sensitive adhesive application
Stress on setting
Limited cure depth
Time consuming to place
Toughness < amalgam
Susceptible to failure in high caries risk patients

Y3 Dental Materials (16 decks)

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