Dental bonding

Question 1

What are the properties of a dental adhesive?6

Answer 1

Provides a high bond strength to tooth tissues, immediate high strength bond, durable and impermeable bond, easy to use and safe

Question 2

Is it hard to bond to enamel? Why?

Answer 2

Easy due to the structure of enamel

Question 3

What is the structure of enamel?

Answer 3

Heterogenous structure: densely packed prismatic

Question 4

Is enamel wet or dry?

Answer 4

Dry

Question 5

What does acid etching do to enamel prisms?

Answer 5

Roughens the surface of the enamel producing a characteristic etched pattern (keyhole shape)

Question 6

What does acid etching allow?

Answer 6

Micromechanical interlocking of resin filling materials

Question 7

How does acid etching lead to better wettability of the enamel and WHY is this important?

Answer 7

Etching increases the surface energy of the enamel surface by removing surface contaminants- leads to better wettability of the enamel- allows resin to adapt better to roughened enamel surface.

Question 8

Why must enamel be dry when bonding to resin?

Answer 8

Moisture contamination will prevent flow of the resin into the etched surface.

Question 9

What is normally used for acid etching?

Answer 9

30-50% aqueous solution of phosphoric acid

Question 10

Why is it bonding to dentine difficult?4

Answer 10

Dentine is constantly wet- fluid pumps up from pulp to dentine floor, inconsistent material ( aged dentine is more mineralised) and low surface energy, smear layer

Question 11

What are the requirements for a dentine bonding agent?

Answer 11

Ability to flow into surface, potential for intimate contact with dentine surface, low viscosity, and adhesion to substrate MECHANICALLY, CHEMICALLY AND TO THE VAN DER WAALS FORCES

Question 12

How to bond to dentine mechanically?

Answer 12

DBA and the dentine surface meshing and interlocking with minimum gaps.

Question 13

In dentine, how is chemical adhesion achieved?

Answer 13

At a molecular level, mineralised constituent of dentine bonds ionically and the organic constituent of dentine bonds covalently

Question 14

When is best adhesion to dentine achieved?

Answer 14

When van der waals forces are optimised

Question 15

What is van der waals adhesion and what does the strength of interaction depend on?

Answer 15

Electrostatic or dipole interaction- depends on contact angle which is a good indication of wettability of a solid by a specific liquid.

Question 16

What does a contact angle of less than 90 degrees mean?

Answer 16

Solid surface is hydrophilic- spreads out flat

Question 17

What quality must a liquid have for it to flow and stick to a surface?

Answer 17

Lower surface energy than the surface

Question 18

Why will dentine not bond to composite directly?

Answer 18

Dentine has a low surface energy lower than composite

Question 19

How do we increase the surface energy of dentine to bond to composite?

Answer 19

Dentine bonding agents (surface wetting agents)

Question 20

How does the adhesive bond to dentine?

Answer 20

Adhesive absorbed onto the surface but can also penetrate into the surface of the dentine.

Question 21

What does the absorbed component on the dentine bonding agent form?

Answer 21

Long chain polymer

Question 22

What is the smear layer?

Answer 22

Adherent layer of organic debris that remains on the dentine surface after the preparation of the dentine during the restoration of a tooth.

Question 23

Does the smear layer interfere with adhesion?

Answer 23

Yes

Question 24

How thick is the smear layer?

Answer 24

0.5-5 microns

Question 25

How do we remove the smear layer?

Answer 25

Total etch or self etch

Question 26

What used to be used to control the smear layer? Why is this not used anymore?

Answer 26

Phosphorylated esters of unfilled resins- ionic bond to the calcium in dentine by chlorophosphate or hydroxyl group- bond was only on to the top of the smear layer-inherently weak low bond strength and moisture caused hydrolysis of the bond resulting in leakage of the filling.

Question 27

What are the components of a total etch bonding agent?

Answer 27

Dentine conditioner : 35% phosphoric acid, primer: coupling agent with a hydrophilic end and hydrophobic end, adhesive: resin which penetrates into the surface of the dentine attaching to the primers hydrophobic surface.

Question 28

What does the dentine conditioner do? What is exposed to the next 2 components ready to be penetrated?

Answer 28

Removes smear layer, opens dentinal tubules by removing smear plugs and decalcifies the uppermost layer of the dentine. The collagen network in the top 10 micrometres of the dentine is exposed.

Question 29

In primer, which aspects bond to what?

Answer 29

C=C opens, forming a strong covalent bond on the hydrophobic side with the next resin layer. The hydroxyl group can combine with similar polar groups on the hydroxyapatite and can react with amine groups on the collagen protein. (Van der Waal not a intramolecular bond for the hydrophilic part)

Question 30

Why must primer have a spacer group?

Answer 30

Make it long enough to be flexible when bonding. Lack of flexibility reduces bonding sites and bond strength.

Question 31

Which coupling agent is found in many primers?

Answer 31

HEMA- hydroxy ethyl methacrylate

Question 32

What is the adhesive's components and is it hydrophilic/hydrophobic?

Answer 32

Mixture of resins- Bis-GMA and HEMA, predominantly hydrophobic

Question 33

What does the adhesive contain to make it stronger and what does it contain to allow light curing?

Answer 33

Filler and camphorquinone

Question 34

What does the adhesive penetrate and form?

Answer 34

Penetrates the primed dentine which now has a hydrophobic surface- forms a micromechanical bond within the tubules and exposed dentinal collagen fibres (molecular entanglement), forming a hybrid layer of collagen plus resin.

Question 35

What are some problems with total etch?

Answer 35

Over etching- collapse of collagen fibres so no resin can penetrate, too deep an etch and primer cannot penetrate to the full depth leading to sensitivity. If too dry- dentine surface collapses, decrease in porosity, poor penetration of dentine by primer, if too wet- primer dilutes and there is reduced strength

Question 36

Which texture is best for dentine to get a good bond? Why?

Answer 36

Moist dentine-expanded dentine surface is porous and will absorb primer hence giving a good bond

Question 37

What is in primer and sealer combined bottles?

Answer 37

HEMA/ GPDM/MDP/4-META combined with a resin Bis-GMA, a solvent and camphorquinone

Question 38

How do self etching primers work?

Answer 38

They don't remove the smear layer- infiltrate it and incorporate themselves into it.

Question 39

Why does a strong self etch not bond well to dentine? 3

Answer 39

Etching by-products are not washed away- soluble and weaken the bond integrity, and if too much HA is dissolved the exposed collagen is vulnerable to breakdown and the bond will fail.

Question 40

How does mild self etch work? Disadadvantages?

Answer 40

Partially demineralises dentine, HA crystals remain around the collagen, protecting it against hydrolytic breakdown and the remaining Ca ions allow bonding. If smear layer is thick then mild self etch won't penetrate.