Resin Composites

Question 1

what are the 3 components of resin based composites

Answer 1

• Resin forms the matrix of the composite material, binding the individual filler particles together though the coupling agent

Question 2

what is the resin matrix

Answer 2

initially a fluid monomer but is converted to a rigid polymer by a radical addition reaction

Question 3

what is the most common monomer

Answer 3

Bis-GMA

Question 4

what is bis-GMA derived from

Answer 4

reaction of bisphenol-A and glycidylmethacrylate

Question 5

what is the advantage of using bis-GMA over MMA

Answer 5

○ Higher molecular weight than MMA which helps to reduce the polymerization shrinkage

Question 6

what is another common resin that is often used instead of Bis GMA

Answer 6

UDMA

urethane dimethacrylate resin

Question 7

why are bis-GMA and UDMA highly viscous fluids

Answer 7

because of their high molecular weights

Question 8

what does UDMA and Bis-GMA being highly viscous mean

Answer 8

the addition of even a small amount of filler would produce a composite with a stiffness that is excessive for clinical use

Question 9

how is the problem of over viscosity in UDMA/BisGMA composites overcome

Answer 9

low viscosity monomers known as viscosity controllers are used

Question 10

what are common examples of low viscosity monomers

Answer 10

TEGDMA
EGDMA
MMA

Question 11

what ensures an adequate shelf life

Answer 11

nhibitor is also used to prevent premature polymerisation

Question 12

what does the resin matrix contain that achieves the cure

Answer 12

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

Question 13

what can be done to reduce PM shrinkage

Answer 13

To reduce polymerisation shrinkage, can incorporate large amounts of glass filler particles as filler does not take part in polymerisation process

Question 14

why is a coupling agent required

Answer 14

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

Question 15

what happens if there is a breakdown in the interface between resin and GF

Answer 15

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

Question 16

what are coupling agents

Answer 16

silanes

Question 17

what is the most common coupling agents

Answer 17

y-MPTS

Question 18

what happens if there is not a good bond between resin and glass

Answer 18

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

Question 19

what is the fundamental problem with resin and glass

Answer 19

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

Question 20

how do coupling agents work

Answer 20

• 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

Question 21

what is the oxygen inhibition surface

Answer 21

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

Question 22

why is a high degree of conversion of C=C desirable

Answer 22

to achieve the optimum mechanical properties and this relates to the curing time and the power of the light unit

Question 23

what does a lack of cure result in

Answer 23

• 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

Question 24

what can affect the depth of cure

Answer 24

type of composite
quality of light source
method used

Question 25

how can type of composite affect the depth of cure

Answer 25

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!

Question 26

how can quality of light source affect depth of cure

Answer 26

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

Question 27

how does the method used affect the depth of cure

Answer 27

tip should be placed as close as possible to the restoration

Question 28

what is the limitation of composite

Answer 28

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

Question 29

why is composite more prone to secondary caries than amalgam and GIC

Answer 29

• 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

Question 30

what does a higher degree of conversion also lead to

Answer 30

greater polyermisation shrinkage

Question 31

what is polymerization shrinkage dependent on

Answer 31

type of resin and the amount of resin present in its unpolymerized form

Question 32

what does a higher proportion of glass filler particle mean

Answer 32

lower final shrinkage

Question 33

why should shrinkage of composite be a low as possible

Answer 33

since this enhances marginal adaption and reduces the possibility of breakdown of the bond to tooth tissues and inhibits the development of recurrent caries

Question 34

what happens during the setting process

Answer 34

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

Question 35

why is dentine more at risk in PM shrinkage

Answer 35

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

Question 36

what will the gap formed between the restoration and dentine lead to

Answer 36

post op sensitivity due to hydrodynamic effect | marginal leakage

Question 37

what does incremental placement of composite encourage

Answer 37

polymerisation shrinkage towards rather than away from the cavity walls

Question 38

what are the clinical considerations for composite

Answer 38

``` avoid large restorations avoid deep gingival preparations lack of peripheral enamel replacement or inlays of load bearing cusps poor moisture control bruxism ```

Question 39

why do we avoid composite in large restorations

Answer 39

larger the restoration the greater the problem of PM shrinkage and lower the chances of achieving a good marginal seal

Question 40

why do we avoid composite deep gingival preparations

Answer 40

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

Question 41

what do we avoid composite when there is a lack of peripheral enamel

Answer 41

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

Question 42

why do we avoid composite in the replacement of onlays and load bearing cusps

Answer 42

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

Question 43

why do we avoid composite in poor moisture control

Answer 43

impossible to obtain an adhesive bond between tooth tissues and composites when the tooth surfaces are contaminated with moisture

Question 44

why do we avoid composite in bruxism

Answer 44

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