Adhesion

Question 1

Adhesion vs cohesion

Answer 1

Adhesion is a chemical reaction that bonds different materials

Cohesion is between similar materials

Question 2

3 key criteria for good adhesion

Answer 2

1. Sufficient close contact
2. No debris which would prevent direct contact between adhesive and adherent
3. Adequate wetting of adherent by adhesive (so that surface has surface energy to pull liquid droplet down, increasing strength of adhesive junction

Question 3

What should contact angle be for adequate wetting by adhesive

Answer 3

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

Question 4

How does viscosity affect strength of adhesive

Answer 4

Decreased viscosity, increased strength

Because easy flow on adherent surface vs needing mechanical irritation to spread if more viscous

Question 5

How does thickness of adhesive affect its strength

Answer 5

Decreased thickness, increased strength

Because fewer air voids ie defects

Question 6

How does surface irregularities affect strength

Answer 6

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!)

Question 7

Is adhesive hydrophobic or hydrophilic

Answer 7

Hydrophobic

Question 8

What does acid etch do to enamel

Answer 8

• 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

Question 9

How deep are the microporosities in enamel after etching

Answer 9

5-50um

Question 10

How much of enamel surface is removed after etching

Answer 10

10um

Question 11

What does acid etch do to dentine

Answer 11

• remove smear layer

- open dentinal tubules and expose collagen network

Question 12

What is the hydrophobic group in primer molecule

Answer 12

Methacrylate group (to bond to composite resin matrix)

Question 13

Water based vs alcohol based primer

Answer 13

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

Question 14

How does adhesive attach to various components

Answer 14

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

Question 15

Depth of smear layer and smear plug

Answer 15

Smear layer: 1-5um (loosely attached to underlying dentin)

Smear plug: up to 40um (in dentinal tubules)

Question 16

Components of smear layer

Answer 16

Inorganic: tooth structure, inorganic tooth contaminants

Organic: heated coagulated proteins, odontoblastic processes, microorganisms

Question 17

How does smear layer encourage bacteria growth

Answer 17

Smear layer is substrate for bacteria growth, bacteria entrapped in smear layer can survive and multiply beneath restoration

Question 18

What are some problems with bonding dentin

Answer 18

• 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

Question 19

At which generation was hybrid layer introduced

Answer 19

Generation 4

Question 20

What did generation 3 introduce

Answer 20

Acid etching of dentin (no longer just enamel or not at all)

Removal of smear layer

Separate primer

Question 21

Why did generation 3 fail

Answer 21

Clinical failure due to margin staining around composite restoration

Question 22

What did generation 5 do

Answer 22

Combine primer and adhesive

Question 23

What did generation 6 introduce

Answer 23

Self etching primer to get a consistent homogenous hybrid layer

Question 24

Which generation introduced the all in one etch+prime+bond?

Answer 24

Generation 7