Composite bonding

Question 1

How does CR bond to enamel?

Answer 1

Enamel:
• Resin is hydrophobic
• It bonds well to enamel’s crystalline structure which is dry via micromechanical retention
• Acid etch with orthophosphoric acid to create enamel tags, apply bond agent which CR bonds too

Question 2

Describe dentine

Answer 2

• Dentine is made of different things; peritubular dentine, intratubular dentine, tubules and odontoblastic process
• Peritubular dentine and intratubular dentine are highly mineralised. Point: dentine is structurally different to enamel
• Dentine is hydrophilic and it more wet
• Water interferes with bonding

Question 3

Explain CR’s bonding mechanism with dentine

Answer 3

• Bonding to dentine involves not only etch, but something called a primer
• The primer is a methacrylate monomer that is dissolved in a solvent
• Primer bonds to both dentine and resin
• After acid etch and smear layer removal, the primer flows into the open tubules and the decalcified dentine
• The bonding resin covers the primed network within dentinal tubules
• The primer, containing hydrophobic and hydrophilic monomers, copolymerizes with the adhesive (unfilled resin)
• A hybrid layer forms that is composed of resin tags intermingled with bonding resin

Question 4

What are some dentine adhesive systems?

Answer 4

• More simplified systems exists now where primers have been combined with adhesives and/ or etchants
• Some options may be self-etching instead of the need to etch and rinse

Question 5

What are issues with self-etch?

Answer 5

• The infiltration of resin occurs simultaneously with the self-etching process
• Is the enamel adequately etched?
• What are the long term effects of incorporating dissolved smear layer remnants?
• Leakage??, ↓ bonding, hydrolytic degradation
• Dissolved calcium and phosphate ions in the adhesive attract more water than the 3 step adhesives: ↓ resin-dentine bond
• This results in nano-leakage, which is nano- sized pores that form within hybrid layers
• Acid in etch & rinse and self-etch bonding agents can activate matrix metalloproteinases (MMP’s) in dentine which results in the breakdown and loss of collagen fibres within the hybrid zone

Question 6

What are factors affecting the placement of CR?

Answer 6

Depth of cure

Oxygen inhibition layer

Unreacted C=C bonds

Question 7

Describe depth of cure as a factor affecting the placement of CR?

Answer 7

Filler type and composition
• Smaller filler particles scatter light more= difficult to penetrate the deeper regions of the material. Greater exposure times are required
• The higher the filler = more difficult for the light to penetrate the composite

Shade and Translucency
• Darker shades require a longer curing time because the light is more readily absorbed by the dark colour and does not transmit through the material

Intensity and duration of light curing 
• Light should be placed as close as possible to  composite, otherwise, inverse square law. 
• Cure 2mm material for 40 seconds
• Halogen light: 450 nW/cm² 
LED: 470nm.

Question 8

Describe the oxygen inhibition layer as a factor affecting the placement of CR?

Answer 8

• When the surface is exposed to oxygen in the air, it results in a thin layer of uncured layer of bonding resin. This stops polymerisation
• Once another layer of composite is placed over the bonding resin, the oxygen inhibited layer and the new layer are chemically bonded when the second layer is cured
• The uncured layer is usually removed during finishing and polishing or can be removed by cotton pellet as with fissure sealants.

Question 9

Describe the unreacted C=C bonds as a factor affecting the placement of CR?

Answer 9

• Some C=C bonds may remain unreacted even after curing
• This allows old composite bond to new composite
• Some of the unreacted C=C bonds in the old material will react with the setting matrix of the new material
• Obviously the strength of the bond is much lower than that of a newly placed material