Composite Flashcards
What are the ideal properties of direct filling materials
mechanical – strength, rigidity, hardness
bonding to tooth / compatible with bonding systems
thermal properties
aesthetics
handling / viscosity
smooth surface finish/ polishable
low setting shrinkage
radiopaque
anticariogenic
biocompatible
What types of glass filler particles are in composite resin
microfine silica
quartz
borosilicate glass
lithium aluminium silicate
barium aluminium silicate
What is BIS-GMA a reaction product of
bisphenol-A and glycidyl methacrylate
What are the key characteristics needed to be monomers in resin
- difunctional molecule
(C=C bonds – facilitate crosslinking)
undergoes free radical addition polymerisation
How is composite resin able to be cured by light
Camphorquinone
What process occurs under blue light to cause the composite to cure
Camphorquinone –
-activated by blue light
-produces radical molecules
-these initiate free radical addition polymerisation of BIS-GMA
-leading to changes in resin properties (ie increased molecular weight, so increased viscosity, and strength)
causes degree of conversion of resin:
35-80%
Why do composite resins contain low weight dimethacrylates (eg TEGDMA)
added to adjust viscosity & reactivity
Why does composite contain silane coupling agent
good bond between filler particle and resin is essential
normally water will adhere to glass filler particles, preventing resin from bonding to the glass surface
a coupling agent is used to preferentially bond to glass and also bond to resin
How does a coupling agent act with the fillers and resin
Surrounds the glass fillers allowing better bonding between them and the resin
What handling characteristics are desirable
condensable - “amalgam feeling” - greater porosity
syringeable - good adaptation, less porosities, easy to apply
flowable - lower filler content, more shrinkage, difficult to apply, place for them -with fibre ribbons
What effect does adding filler particles have on composite resins
improved mechanical properties
-strength, rigidity, hardness, abrasion resistance etc
lower thermal expansion (still not perfect)
lower polymerisation shrinkage (still a problem)
less heat of polymerisation (BUT not negligible)
improved aesthetics
some radiopaque
What types of composite curing are there
self curing (two pastes)
UV activation (obsolete, one paste)
Light curing
-blue light 440nm(one paste)
direct curing (in mouth)
indirect curing / post curing(in laboratory)
How does a composite self cure
Free radicals from the reaction break the resin c=c bonds
polymerisation (formation of chain)
What reactants are present in self cure composite
benzoyl peroxide + aromatic tertiary amine
What are the advantages of light curing systems
extended working time
-on-demand set, triggered when light activated
less finishing
immediate finishing
less waste
higher filler levels (not mixing two pastes)
less porosity (not mixing two pastes)
What part of composite sets quickest
Most of blue light absorbed close to surface which sets readily and becomes hard
How is depth of cure value calculated according to ISO 4049
Column of composite light cured, soft comp scraped off, measured distance of hard comp is then divided by 2 giving the depth of cure
How is depth of cure defined
DoC is defined as depth at which material HARDNESS is about 80% that of the cured surface
the depth to which the composite resin polymerises sufficiently
At what depth does the composite resin usually polymerise sufficiently
typically 2mm
What composite increment thickness is best to use in restorations
2mm as greater than this wikk result in under-polymerised bases
-soggy bottom and poor bonding to tooth
What is an example of a bulk fill comp
Tetric evoceram
How is a bulk fill composite different to hybrid composites
Lucerin initiator as well as camphorquinone
Has different optical absorption spectrum
hence UV and blue light needed to polymerise (cure) material fully
What are some potential problems of light curing
light / material mismatch - overexpose
premature polymerisation from dental lights - avoid exposure
optimistic “depth of cure” values
-product, shade, light exposure & intensity
-use small increments - 2 mm max
recommended setting times too short
-product, light used, light/ material distance,
-contamination or damage to light guide,
-timer accuracy,
-variations in light output (eg over repeated use, between different units)
use > 30 s
polymerisation shrinkage
-affects bond to tooth,
-potential for cuspal fracture, microleakage
- use small increments - light from different angles
What dangers occur with light curing
Exothermic reaction so adjacent enamel and dentine can conduct heat up to 16 degree rise
Divergent light beam - unless optical rod is ALWAYS close to the composite resin surface
SOME blue light MAY illuminate patient’s soft tissues and may cause thermal trauma
two clinical incidents in 2017
What temperature can cause potentially irreversible trauma to dental pulp
5.5 degrees
How is occular damage prevented when curing
Safety sheilds or safety glasses
Why is hybrid composite preffered
Various glass filler sizes to maximise percentage of composite resin made up of particles which improve mechanicalproperties
In a large posterior cavity what clinical requirements are there of the composite
High strength, high YM, high abrasion resistance
What does hardness refer to
material surface
resistance to scratching
indentation resistance
What is abrasion (wear)
removal of surface layers when two surfaces make frictional contact
What are the affects of surface roughness
appearance
plaque retention
sensation when in contact with tongue
What are the surface roughness depths of conventional and microfine
Conventional 80um
Microfine 10um
What material factors affect wear
filler material
particle size distribution
filler loading
resin formulation
coupling agent
How can clinical factors affect wear
cavity size & design
tooth position
occlusion
placement technique
cure efficiency
finishing methods
How is composite bonded to tooth
enamel - acid etch technique
dentine - dentine / universal bonding systems
What percent phosphoric acid is in etch
30%
What is the typical bond strength of composite to enamel and dentine
40MPa
What does bond strength depend on
Surface prep, composite brand and method
What is the value of shear bond strength
40MPa
Why is an uneven cavity floor with ridges undesirable
Concentrates stress at the interface increasing liklihood of failure
What material has a greater
Compressive strength (MPa), Tensile strength (MPa), Elastic modulus (GPa) and Hardness
Amalgam or composite
Amalgam
What is the thermal conductivity of composites
should be low to avoid pulpal damage from hot & cold foods/fluids - it is low
What is thermal diffusivity
How readily a material transmits heat when exposed to a short/transient stimulus
What is the thermal diffusivity of composite (hybrid)
Low - similar to dentine
What is the ideal thermal expansion of composite
should be equal to tooth, to reduce microleakage - BUT it’s high - which is poor
What material has the lowest thermal expansion coefficient
Ceramic
What is the thermal expansion coefficient of compostie compared to glass ionomer
Comp 25-68ppm (highest of materials)
GI 10-11ppm
What factors are important for aesthetics
shade range
translucency
maintenance of properties over lifetime
resistance to staining
surface finish
What are the potential problems of light curing
Must ensure blue light is delivered correctly (intensity, exposure time, proximity of optical rod to tooth/material) to ensure increment is fully curedand proper bond made
What is the most common failing material in posterior composites
Micro-filled composites ahead of fine hybrid composites
What are the choices of material for high, medium and low risk caries patients
Resin Modified GI - high caries risk
- frequent attenders
Compomer - medium caries risk
- caries under control
- regular attenders
Composite resin - low caries risk patients
