Resin Based Composite Materials - disadvantages Flashcards
What is the most important shortcoming of resin based composites?
polymerisation shrinkage, lead to microshrinkage
What is microleakage?
leakage of minute amounts of fluids, debris and microorganisms through the microscopic space between a dental restoration or its cement and the adjacent surface of the prepared tooth
What are the effects of polymerisation shrinkage?
debonding
inflammation of the pulp
secondary caries
microleakage
marginal staining
What is more significant, shrinkage or shrinkage stress?
shrinkage stress
microfracture of the tooth tissue
How can you overcome polymerisation shrinkage stress?
can be minimised by;
material selection
method of material placement into the cavity
- amount
- position
use of the correct matrix system when constructing the approximal wall
employing various curing techniques
What is the problem with adding the lowering molecular weight monomers to make the material more manipulatable?
they exhibit the greatest shrinkage on polymerisation
What do you mix bis-GMA with to minimise shrinking?
bisphenol A polyethylene glycol diether dimethacrylate (bis-GMA) and UDMA as their resin component
In bis-GMA, bis-EMA and IDMA, is the resin hydrophobic or hydrophilic?
hydrophobic
What is the configuration factor?
C factor - ratio of bonded to unbonded surfaces
What is the effect of a high ratio of bonded to unbonded surfaces?
more stress potential
How would you build up a cavity?
incremental build up
Why build up a cavity in increments?
with each cured increment shrinkage is minimised and compensated for to some extent
Do you want to increase or decrease the C factor?
decrease
What is the compliance of the tooth?
ability of the tooth to withstand flexure
What does compliance of the tooth depend upon?
amount of the remaining tooth tissue
quality of the remaining tooth tissue
position of the remaining tooth tissue
Is resin composite materials photophilic or photophobic and what does this mean?
photophilic, light loving
What is command cure?
up to us to determine when the material sets
What is the effect of increased tertiary amine in the paste?
quicker it will set
What is the oxygen inhibition layer?
material only partially cured in air
surface of the restoration remains tacky
has inferior mechanical properties
What do you do with the oxygen inhibition layer? (2 options)
remove and restorative trimmed back to fully set material
create anaerobic environment such as using a matrix strip or cover with glycerine or dentine binding agent (probs easier)
Are resin-based composites hydrophobic or hydrophilic?
hydrophobic
Do you need good moisture control for placing resin based composites?
yes
resin is sensitive to water contaomination
What happens if resin is contaminated with water?
water sorption
causes irreversible degradation of properties of resin composite
affects wear resistance and colour stability (staining)
What is hydroscopic expansion?
material swells with water sorption
When can hydroscopic expansion occur?
start 15 mins after initial polymerisation and continues for up to 10 weeks
What is coefficient of thermal expansion?
resin has a COTE similar to that of the tooth
What is the effect of bisphenol A? (biocompatibility)
known to mimic the hormone oestrogen in vitro in the monomeric form
linkage with male infertility and prostatic and breast carcinomas
after polymerisation the risks are reduced
What is the problem with HEMA? (in many resin and bonding agents)
POWERFUL DERMATOLOGICAL SENSITISER CAUSING CHEMICAL DERMITITIS
Is HEMA cytotoxic after curing?
no, only before
When are Bis-GMA, TEGDMA and UDMA cytotoxic?
in vitro in their pure form
What is the drawback of composites?
the resin is relatively much weaker than the filler
moisture control hard in the mouth, so results more variable
undergo slow but steady degradation after placement
When would you use a dual-cured composite?
for crowns etc
What are the advantages of dual-cured resin composites?
if you cant get all the light to the area you can use a chemical
