5- adhesion

What is adhesion?

Force/intermolecular attraction between molecules of 2 unlike substances when in contact

Adhesive- substance added to produce adhesion

Adherend- material to which it’s applied

What are the 3 types of adhesion mechanisms?

Chemical- primary forces- covalent, ionic , metallic- weakest

Physical- secondary forces- occur at molecular dipoles (van der walls) or interaction of induced dipoles (dispersion forces) or electron clouds (h bonds)

Mechanical- strongest, e.g formation of hybrid dentin- resin polymers entangled with collagen fibrils

What forces do most adhesives rely on?

Van der Waals + mechanical adhesion

If surface rough and porous- mechanical adhesion predominates

If smooth and polished- van der waals

What are the 3 factors that affect adhesion?

Surface energy

Wetting

Contact angle

How does surface energy affect adhesion?

Harder surface, higher surface energy- more adhesion

What is wetting and what does it depend on?

How much does adhesive stretch over the surface

Depends on surface energy (higher better) and cleanliness of surface

Metals tend to have high surface energy

What is a contact angle and how does it affect adhesion?

Angle formed between adhesive and adherend at interface

Bigger angle- less wettability, smaller angle- better

When large contact angle- liquid doesn’t wet surface completely

Important to bevel to penetrate all enamel prisms

What are 7 requirements for an ideal bonding agent?

Biocompatible

Bond effectively to E + D

Sufficient strength to resist masticatory forces

Mechanical properties close to tooth structure

Resist degradation in oral environment

Easy to use

Prevent micro leakage

Surface energy of enamel is…

Low surface energy

Pellicle (organic layer) covers untouched enamel

Smear layer covers ground enamel

What is conditioning?

Removal of organic/smear layer to make enamel surface more reactive

What is etching?

Procedure that leads to demineralisation of superficial calcium ion

What is the smear layer?

Caused by instrumentation

Covers normal dentin structure by 1-2um, penetrates 1-5um into tubules to form smear plugs

How does acid conditioning/etching affect enamel?(5)

Removes residual pellicle to expose the inorganic crystallite

Removes approx 10 μm of surface enamel

Creates porous layer with depth of 5 - 50μm— when low viscosity resin applied- flows into the microporosities and polymerises to form resin tags

Increases the wettability and surface area of the enamel substrate

Raises surface energy of enamel by creating reactive polar sites

How to etch- what to do before, objectives and material

Place bevel in all cavities

Clean surface, create microporosities, dilute hydroxypatite crystals

30-37% orthophosphoric acid 30s

If a surface is left without acid…

Will remain unconditioned- won’t adhere- causing micro leakage

Why is bonding to dentin more difficult than to enamel?

Structure- contains a lot of fluids

50% inorganic content

30% organic

20% fluid- must stay moist

Hybrid layer formed

Vs in enamel

96% inorganic

Dry bonding surface

Micro mechanical retention

What is the hybrid layer?

Micro mechanical attachment between resin and demineralised, primed layer of intertubular dentin

Achieved by etching to remove smear layer and produce demineralisation of 3-6um

What are the steps of bonding to dentin?

1. Conditioning of dentin

2. Primer- monomer- HEMA and 4-META dissolved in acetone or ethanol- promotes resin diffusion into demin dentin

3. Bonding- stabilised hybrid layer formed, production of resin tags

How can dentin adhesive systems be classified based on their clinical approach to smear layer?

1- modifies in 1 or 2 steps, doesn’t remove smear layer- natural barrier to pulp, prevents invasion, limits outward flow of pulpal fluid that might impair bonding efficiency

2- Removes smear layer- 2 or 3 step application- (type 1) dentin conditioning (acid) + combined primer and adhesive, acid + primer + bond

3- Dissolve not remove smear layer- (type 2) combined acid and primer + adhesive resin

What are characteristics of 3 step technique?

Etch > prime > bond

Requires most technique

Great bond strength and durability

Total-etch technique (can also be 2 step)

What’s the 3 step total etch technique?

Etch

Primer for 10secs + dry

Adhesive for 10secs + dry

Light cure for 10-20 secs

The 2 step total etch TYPE 1 technique characteristics?

Etch > prime + bond

More susceptible to water degradation

Most cases of post operative sensitivities

Total-etch technique

How do you do 2 step (type 1) total etch technique?

Etch, rinse with water

Prime and bonding (in same bottle)

What are 2 step (type 2) characteristics?

Self etch technique

Etch + prime > bond

Less effective at bonding to enamel– may need to selectively etch enamel first

Less dependant on hydration state of dentin

If deep cavity close to pulp- use self etch- doesn’t remove smear layer can be used with adhesive to create hybrid layer

What does 2 step type 2 technique include?

1 bottle with self-etch primer 10 secs

Dry with air and can use aspiration too

Put adhesive for 10s and light cure for 15s

Total etch (etch and rinse) protocol

Isolate

35-37% phosphoric acid on enamel 15-30secs, rinse for 15-20secs- should be frosty/chalky white

On dentin 10-15secs, rinse, dry- must be moist (glistening)

Apply primer- scrubbing motion 15-20secs, evaporate solvent, surface should be shiny

Apply adhesive- light cure 10-20sec

What is the protocol recommended for universal adhesives?

Selective etching of enamel- etch orthophsophoric acid 15-30s, rinse, dry

Apply universal adhesive to enamel + dentin,  active scrubbing for 20s, gentle drying then light cure

What does selective enamel etch + universal adhesive provide?(3)

Strong enamel bond

Reduce post op sensitivity

Simple technique

What 5 common mistakes to avoid when using universal adhesives?

Over-etching dentin (>15 sec)

Overdrying dentin

Not evaporating solvent properly

Inadequate light curing

Contamination with saliva (if contaminated → re-etch enamel 5 sec)