Adhesion vs cohesion
Adhesion is a chemical reaction that bonds different materials
Cohesion is between similar materials
3 key criteria for good adhesion
- Sufficient close contact
- No debris which would prevent direct contact between adhesive and adherent
- Adequate wetting of adherent by adhesive (so that surface has surface energy to pull liquid droplet down, increasing strength of adhesive junction
What should contact angle be for adequate wetting by adhesive
Less than 90
Zero degrees: small contact angle, good wetting, adhesive spreads to form a film
Less than 45 degrees: low/moderate contact angle
More than 105 degrees: high contact angle, poor wetting, droplet formation
How does viscosity affect strength of adhesive
Decreased viscosity, increased strength
Because easy flow on adherent surface vs needing mechanical irritation to spread if more viscous
How does thickness of adhesive affect its strength
Decreased thickness, increased strength
Because fewer air voids ie defects
How does surface irregularities affect strength
Decreased surface irregularities, increased strength
Irregularities cause air pockets to form, preventing complete wetting
Note: irregularities can increase bonding surface and increase mechanical interlocking to maximise adhesion (not about strength!)
Is adhesive hydrophobic or hydrophilic
Hydrophobic
What does acid etch do to enamel
- remove surface debris
- produce pores into which adhesive penetrate ie resin tags for mechanical interlocking (≠adhesion)
- increase surface energy for increased wetting
- make rougher and increase surface area of enamel
How deep are the microporosities in enamel after etching
5-50um
How much of enamel surface is removed after etching
10um
What does acid etch do to dentine
- remove smear layer
- open dentinal tubules and expose collagen network
What is the hydrophobic group in primer molecule
Methacrylate group (to bond to composite resin matrix)
Water based vs alcohol based primer
Water based: when all water is removed. Rehydrate and expand collagen to keep collagen fibrils from collapsing. Improving penetration
Alcohol based: when there is excess water. Water evaporate faster due to alcohol and leave only primer behind
How does adhesive attach to various components
Enamel:
- mechanical interlocking
- resin tags (micromechanical interlocking)
Composite:
- adhesion ie chemical reaction
Dentine:
- hydrophilic molecules penetrate to bond to primer and collagen fibres, by mechanical interlocking, form hybrid layer
Depth of smear layer and smear plug
Smear layer: 1-5um (loosely attached to underlying dentin)
Smear plug: up to 40um (in dentinal tubules)
Components of smear layer
Inorganic: tooth structure, inorganic tooth contaminants
Organic: heated coagulated proteins, odontoblastic processes, microorganisms
How does smear layer encourage bacteria growth
Smear layer is substrate for bacteria growth, bacteria entrapped in smear layer can survive and multiply beneath restoration
What are some problems with bonding dentin
- bonding and etching chemicals may irritate pulp
- random arrangement of HA crystals vs regularly in enamel
- dynamic dentin structure (aging, caries, etc)
- dentinal fluid in tubules continuously flowing outwards which can degrade adhesive and reduce adhesion by preventing intimate contact of hydrophobic adhesive
At which generation was hybrid layer introduced
Generation 4
What did generation 3 introduce
Acid etching of dentin (no longer just enamel or not at all)
Removal of smear layer
Separate primer
Why did generation 3 fail
Clinical failure due to margin staining around composite restoration
What did generation 5 do
Combine primer and adhesive
What did generation 6 introduce
Self etching primer to get a consistent homogenous hybrid layer
Which generation introduced the all in one etch+prime+bond?
Generation 7
