Resin Composites Flashcards
what are the 3 components of resin based composites
• Resin forms the matrix of the composite material, binding the individual filler particles together though the coupling agent
what is the resin matrix
initially a fluid monomer but is converted to a rigid polymer by a radical addition reaction
what is the most common monomer
Bis-GMA
what is bis-GMA derived from
reaction of bisphenol-A and glycidylmethacrylate
what is the advantage of using bis-GMA over MMA
○ Higher molecular weight than MMA which helps to reduce the polymerization shrinkage
what is another common resin that is often used instead of Bis GMA
UDMA
urethane dimethacrylate resin
why are bis-GMA and UDMA highly viscous fluids
because of their high molecular weights
what does UDMA and Bis-GMA being highly viscous mean
the addition of even a small amount of filler would produce a composite with a stiffness that is excessive for clinical use
how is the problem of over viscosity in UDMA/BisGMA composites overcome
low viscosity monomers known as viscosity controllers are used
what are common examples of low viscosity monomers
TEGDMA
EGDMA
MMA
what ensures an adequate shelf life
nhibitor is also used to prevent premature polymerisation
what does the resin matrix contain that achieves the cure
The resin matrix also contains the activator/initiator systems for achieving the cure - components depend on which kind of reaction is employed, may be either chemical curing or visible-light activated curing
what can be done to reduce PM shrinkage
To reduce polymerisation shrinkage, can incorporate large amounts of glass filler particles as filler does not take part in polymerisation process
why is a coupling agent required
For composite to have acceptable mechanical properties, it is of the utmost importance that the filler and the resin are strongly bonded to each other
what happens if there is a breakdown in the interface between resin and GF
the stresses of the load will not be effectively distributed throughout the material and this interface will act as a primary for fracture leading to the subsequent disintegration of the composite
what are coupling agents
silanes
what is the most common coupling agents
y-MPTS
what happens if there is not a good bond between resin and glass
stress transfer between resin and glass will be inefficient and most of the stress will be will have to be carried by the resin matrix -this will result in excessive creep and eventually fracture and wear of the restoration
what is the fundamental problem with resin and glass
resins are hydrophobic whereas silica-based glasses are hydrophilic due to a surface layer of hydroxyl groups bound to the silica hence the resin does not have a natural affinity to bond to the glass surface - the solution to the problem lies in a suitable coupling agent
how do coupling agents work
• The silane coupling agent has been so chosen as to have hydroxyl groups on one end which are attracted to the hydroxyl groups on the glass surface and the other end consists of a methacrylate group that is able to bond to the resin via the carbon double bond
what is the oxygen inhibition surface
When there is an air interface with resin, the resin will not cure and a sticky surface is readily discernible - this is of benefit when carrying out an incremental procedures as it ensures that layers of composite will be well bonded together
why is a high degree of conversion of C=C desirable
to achieve the optimum mechanical properties and this relates to the curing time and the power of the light unit
what does a lack of cure result in
• Any lack of cure provides a poor foundation for the restoration and may leaed to fracture - this is due to a lack of support at the cervical margin
what can affect the depth of cure
type of composite
quality of light source
method used
how can type of composite affect the depth of cure
as light hits the composite, it is reflected, scatters and absorbed and this limits the amount of penetration that is achieved. There is particular concern for darker shades - importance of incremental technique and curing time!
how can quality of light source affect depth of cure
light source should be designed so as to produce its maximum light output at approximately 460-480 nm where the maximum of the camphorquinone absorption coefficient is located
how does the method used affect the depth of cure
tip should be placed as close as possible to the restoration
what is the limitation of composite
One limitation of composites is that there will be a marginal gap as the composite shrinks away from the cavity wall on setting - PM SHRINKAGE
why is composite more prone to secondary caries than amalgam and GIC
• Composites do not have any intrinsic defence mechanisms against caries attack unlike GICs and amalgam and hence once a gap is formed, microleakage will occur which can quickly lead to the spread of recurrent caries
what does a higher degree of conversion also lead to
greater polyermisation shrinkage
what is polymerization shrinkage dependent on
type of resin and the amount of resin present in its unpolymerized form
what does a higher proportion of glass filler particle mean
lower final shrinkage
why should shrinkage of composite be a low as possible
since this enhances marginal adaption and reduces the possibility of breakdown of the bond to tooth tissues and inhibits the development of recurrent caries
what happens during the setting process
shrinkage stresses develop because the material is constrained by the adhesion to the cavity walls and these stresses can be sufficient to cause breakdown of the interfacial bond whereby the advantage of an adhesive procedure is lost
why is dentine more at risk in PM shrinkage
the bond to dentine which is less strong than that achieved to acid etched enamel and as a consequence the shrinkage tends to occur towards the acid-etched enamel bonded interface if the bond to the dentine breaks down
what will the gap formed between the restoration and dentine lead to
post op sensitivity due to hydrodynamic effect
marginal leakage
what does incremental placement of composite encourage
polymerisation shrinkage towards rather than away from the cavity walls
what are the clinical considerations for composite
avoid large restorations avoid deep gingival preparations lack of peripheral enamel replacement or inlays of load bearing cusps poor moisture control bruxism
why do we avoid composite in large restorations
larger the restoration the greater the problem of PM shrinkage and lower the chances of achieving a good marginal seal
why do we avoid composite deep gingival preparations
proximal restorations can extend sub gingivally such that the base of the box extends into root dentine and it is extremely difficult to ensure close marginal adaption and to obtain a perfect marginal seal even with DBA
what do we avoid composite when there is a lack of peripheral enamel
acid etch bond to enamel is extremely effective - when a tooth is badly broken down, there will be little enamel left to bond to and the restoration has to rely more and more on the bond to the remaining dentine and this bond is highly unreliable and increases the possibility of a breakdown of the marginal seal when subjected to stresses generated by polymerization shrinkage, thermal mismatch and occlusal loading
why do we avoid composite in the replacement of onlays and load bearing cusps
composites suffer from much higher rates wear when they are in occlusal contact with opposing teeth. This is a problem for posterior teeth where the loads are generally much higher, excessive wear of the composites is likely to occur
why do we avoid composite in poor moisture control
impossible to obtain an adhesive bond between tooth tissues and composites when the tooth surfaces are contaminated with moisture
why do we avoid composite in bruxism
aggressive wearing action associated with bruxism will cause the composite restoration that is in occlusal contactor one that is in contact with an implement such as a pipe to wear down extremely rapidly
