L15 Resin Composites 2 Flashcards
What is the role of filler?
Reduces polymerisation shrinkage
Reduces coefficient of thermal expansion
Improves mechanical properties
Increases elastic modulus
Provides radiopacity
Adjusts handling
Controls translucency and aesthetics
How does filler reduce polymerisation shrinkage?
More filler means less resin means less shrinkage
But can still be enough to generate sufficient stress at the interface to cause gaps
Compare the TEC of the resin monomer and the tooth
Expansion at different rates is unwanted. Resin thermal properties do not match the tooth. Monomer - 80ppm/0C, enamel - 11ppm;0C, glasses - 8ppm/0C
Which mechanical properties does filler improve?
Fracture toughness and overall strength
How does filler increase elastic modulus?
Increases stiffness and reduces flexibility of the polymer. Creates appropriate wear resistance.
What does filler making resin more radioopaque mean?
It is harder to spot on radiographs than amalgam
(Zirconia is added to help this)
What are the indications of using a relatively large filler?
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.
What is the issue with acid-modified resin?
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.
What are the issues with light curing?
Cannot place more than a certain amount to cure because you won’t get effective light transmission through the material.
What can be filled more overcoming the light cure issues?
Bulk fill
What is the order and development of light-curing units?
Self-cured, UV, visible light
Order in development of fillers
Microfilled, hydrids,munivsersal flowables and packables, microhybrids, nano filled and nanohybrid
Order in development of resins
Low-shrinkage, self-adhesive, bulk-fill
How has particle size of fill restorative resins changed over time?
Decrease in average particle size range since the introduction of RBCs
Filler particle sizes manipulated to achieve desired mechanical and physical properties
In hybrid resin composites, how are properties optimised?
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.
How would you increase viscosity of a resin composite?
Increase amount of filler or decrease amount of resin
When might a flowable composite be used?
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.
What are the key aspects of curing efficiency?
Degree of conversion, cross-linking
Depth of cure
Shrinkage and shrinkage stress
Mechanical properties - strength characteristics, modulus
Trapped radicals, residual monomer
What is degree of conversion and how is it optimal for curing efficiency?
The degree to which you’ve converted monomer to polymer units - want to maximise this and not have excess uncured material
How does depth of cure affect curing efficiency?
DC with depth? Not only amount of cross-linking creating, but how much is created with depth.
How does shrinkage affect curing efficiency?
Stress generation is more critical. Gaps can cause secondary infection.
How might trapped radicals and residual monomer be formed, from curing efficiency?
Consequences of monomer elation
Always be some residual monomer
Why is there a limited depth of cure?
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.
What affects absorption of light?
Photoinitiator chemistry
Pigments
How do pigments affect absorption?
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.
Why does light scatter?
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.
How can resin composites be optimised optically then?
Can optimise resin composites by trying to change the density of the filler to match the optical property of the resin (to increase translucency)
What factors affect interfacial scattering?
Porosity, filler density, resin density, refractive index mismatch
What can vary the depth of cure? (Graph pg78)
How well it is cured, light intensity, material itself etc
Why do we place in increments of 2mm or less for conventional RBCs?
After about 2mm there is an exponential drop off in hardness of the material, so don’t place increment in more than 2mm.
What increments can be placed for bulk fill and why is it more than conventional RBCs?
5mm or less
Bulk fill materials have less pigment
What factors will affect the cure?
Adequate light irradiance
Exposure time
Radiant exposure
Increment thickness
Tip to material distance
How can we ensure adequate light irradiance for best possible cure?
Ensure light is working properly, ensuring no contamination or crack of light
>300mW/cm2
Modern LED lights - 1000mW/cm2 (high output)
Can a low intensity light be compensated for?
Yes, dose of light is the product of intensity and time, so increase time
What should exposure time be for the best possible cure for the conventional composite of 2mm with the standard modern light?
Normally 20-40s
What should exposure time be for the best possible cure for the bulk fill composite with the standard modern light?
10-20s
Can a material be over cured?
No but light on for too long can cause excess heat generation, so be careful with mucosa and pulp.
How can bright light affect the eyes?
Can cause ocular damage so don’t look at it
What is the radiant exposure for the best possible cure?
Irradiance x time
About 20J/cm2
How is the tip to material distance varied for the best possible cure?
Stabilise and concentrate the light
When is gap formation inevitable?
If stress exceeds adhesive layer bond strength
What is post-gel shrinkage stress?
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
Which stage of polymerisation has the largest effect on stress generation?
Latter stages of polymerisation (high conversion) have the largest effect on stress generation (graph pg79)
Is stress an intrinsic material property?
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.
What factors effect polymerisation stress?
Volumetric polymerisation shrinkage
Resin composite elastic modulus
Configuration factor
Compliance
Speed of curing
How does volumetric polymerisation shrinkage affect polymerisation stress?
Contributes to stress magnitude
Does not directly affect gap formation
How does resin composite elastic modulus affect polymerisation stress?
More flexible liner may reduce stress by absorbing it
Stiffer more rigid material won’t be able to compensate for stress generation
What is configuration factor?
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
How does configuration factor affect polymerisation stress?
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.
How does compliance affect polymerisation stress?
Rigidity of supporting tooth structure
How does speed of curing affect polymerisation stress?
So-called, soft start regimes
Effects of ramped intensity on stress (graph pg81)
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
Will soft start regimes reduce stress?
Unlikely
Should we use lower cures to reduce stress then?
Energy at lower cures might not reach high enough to cure the material
Advantages of resin composites summary
Cosmetics
Single paste
Command set
Versatile
Clinical proven
Disadvantages of resin composites summary
Technique sensitive adhesive application
Stress on setting
Limited cure depth
Time consuming to place
Toughness < amalgam
Susceptible to failure in high caries risk patients
