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Enamel and Dentine Bonding

Q: What is the composition of enamel?

96%mineral, 1% organic material and 3% water by weight

Q: What is the purpose of acid etching?

Mechanical interlocking of composite and enamel by removing surface film and debris to produces micro irregularities into which resin tags flow

Q: What is the surface energy of enamel without acid etching?

Enamel has lower surface energy due to its pellicle layer

Q: List the clinical procedures involved in acid etching

Patient selection (patient and tooth selection) Enamel prophylaxis (removing pellicle with prophylactic paste and water) Application of the etchant Application of the resin composite

Q: How do surface contaminants hinder the bond between enamel and composite?

Polar nature of surface contaminants prevent nonpolar resin from closely adapting to enamel surface

Q: Why do primary teeth enamel require longer etching time?

Longer etching time is necessary to create the etching pattern on enamel as primary teeth contains more prismless enamel that is also less calcified, resulting in lower bond strength overall.

43 important questions on Enamel and Dentine Bonding

What is the composition of enamel?

96%mineral, 1% organic material and 3% water by weight

What is used for enamel bonding?

Acid etch

What is the acid solution used for acid etching?

37% phosphoric acid

What is the purpose of acid etching?

Mechanical interlocking of composite and enamel by removing surface film and debris to produces micro irregularities into which resin tags flow

Explain how acid etching works

Acid-base reaction by phosphoric acid causes...
1. Hydroxyapatite to go into solution (loss of enamel prism periphery and prism core)
2. Cleanses debris from enamel leading to better wetting
3. Increases surface energy providing better wetting

What is the surface energy of enamel without acid etching?

Enamel has lower surface energy due to its pellicle layer

List the clinical procedures involved in acid etching

Patient selection (patient and tooth selection)
Enamel prophylaxis (removing pellicle with prophylactic paste and water)
Application of the etchant
Application of the resin composite

How do surface contaminants hinder the bond between enamel and composite?

Polar nature of surface contaminants prevent nonpolar resin from closely adapting to enamel surface

How long is instrumented, uninstrumented, and primary teeth enamel etched for?

Instrumented enamel - 15-30 seconds
Uninstrumented enamel - longer etching time
Primary teeth enamel - longer etching time

Why do primary teeth enamel require longer etching time?

Longer etching time is necessary to create the etching pattern on enamel as primary teeth contains more prismless enamel that is also less calcified, resulting in lower bond strength overall.

How long should acid etch be rinsed off for?

5-10 seconds of thorough rinsing with copious amounts of water

What can insufficient washing of acid etch do?

Leave debris hindering flow and resin and tag formation

How much of the enamel does acid etch remove?

4-10 μm of enamel

How deep are the micro porous layers formed from acid etching?

5-50 μm deep

How should a properly etched enamel look like?

Frosty white appearance

What is the relationship between etching efficiency and
concentration of phosphoric acid?

Inverse relationship

What is the optimum phosphoric acid concentration for acid etch?

30-50% phosphoric acid

What are the results of phosphoric acid concentrations higher than optimal range of 30-50%?

Higher concentrations aren't as effective at producing ideal etch pattern. Excessively high concentration leads to rapid saturation of the solution with by products, slowing down the rate of dissolution.

Explain what the results of overetching a tooth is

Greater removal of enamel prism sheath
Not as many porosities produced
Loss of etch pattern
Ability of the resin to produce tags is reduced
Bond strength lower

When can a tooth be re-etched?

Re-etching a tooth can only happen when the surface becomes contaminated.

How many times can a tooth be re-etched?

Maximum once

How can bevelling of enamel improve the quality of etching pattern?

Due to the angulation of the prisms, no unsupported prisms remains

What viscosity of unfilled resin penetrates the microscopic spaces from acid etching better?

Low viscosity

What is the composition of dentine?

70% inorganic ‘living tissue’ (mainly hydroxyapatite)
20% organic (mainly collagen)
10% water
with dentinal tubules transversing throughout

What are the challenges that dentine pose?

1. Dentine is hydrophilic whereas most adhesives are hydrophobic
2. Dentine is a vital tissue
3. Dentine consists of both inorganic and organic material
4. Dentine is covered by a smear layer (layer of enamel and dentine debris 0.5-5um thickness)

What are the three steps of dentine bonding

1. Etching
2. Priming
3. Adhesive layer/sealing

What is the purpose of the first step of dentine bonding?

To remove the smear layer and demineralise intertubular dentine via use of dentine conditioners/etch, which creates micro mechanical roughness of collagen fibrils

What does a stronger acid in dentine etch do?

The stronger the acid the more pronounced the effects, which is especially effective for sclerosed dentine which requires longer exposure to the etch

How does micro mechanical roughness of collagen fibrils form on dentine from etching?

Acid-base reaction with hydroxyapatite and etch results in opening of dentinal tubules and creates a demineralised surface layer up to 10 μm deep, leaving the collagen matrix intact and uncollapsed

What variables can affect the quality of dentine etch?

concentration and time of application of etch

What are difficulties encountered with dentine etching?

1. Varying thickness of the smear layer = differential opening of the dentinal tubules
2. Over aggressive etching (particularly in deep cavities) cause inflammatory response in the pulp
3. If dentine is overtreated or excessively dried = collapse of the collagen scaffolding

What is the purpose of the second step of dentine bonding? (priming)

To facilitate bonding to vital dentin and form the ‘hybrid layer’

How does a primer work on dentine to form the hybrid layer?

Bifunctional monomers in primer ionically links the hydrophobic methacrylate groups in the sealer/adhesive (step3) to the collagen in hydroxyapatite in the hydrophilic dentine.

What is the general formula of a primer and what do each components do?

methacrylate group-spacer group-reactive group
1. methacrylate group - bonds to composite resin

2. spacer group - provides flexibility for reactive group to form a satisfactory conformational arrangement

3. reactive group - a polar end group attracted to hydroxyapatite in the tooth

Why is the coupling agent (bifunctional monomer) dissolved in a solvent?

A solvent, such as ethanol or acetone, helps chase water and penetrate the layer of demin collagen remains, creating a better hybrid layer

What is the hybrid layer that forms from priming dentine?

A layer of collagen fibrils that is impregnated with the hydrophilic resin of the primer

What happens from overetching of dentine?

The demineralised layer of dentine becomes too thick, insufficiently impregnating the hydrophilic resin of the primer and leaving unimpregnated layer of dentine. This overall reduces bond strength.

In what environment should dentine bonding happen in?

It should be carried out in a wet environment formed by displaced water from solvents employed within the primers. A dry environment causes collapse of collagen from dessication and scaffolding.

What is the purpose of the third step of dentine bonding? (adhesive layer/sealing)

To facilitate the attachment of the restorative resin to the hybrid layer and aid in the adaptation of the restorative resin in the cavity

How does a sealer (low viscosity resin) work?

Cross linking with methacrylate chain in the coupling agent stabilises the hybrid layer and forms resin tags.

What is a resin tag?

A microscopic projection that extends into the dentinal tubules, playing an essential role in the bonding procedure.

Classify the bonding agents by the number of stages involved and list the stages

1. Three stage bonding agents - conditioner, primer, sealer
2. Two stage bonding agents - condition + mixture of primer and sealant OR mixture of conditioner and primer + sealer
3. One step self etch adhesive - mixture of etch, primer, and bond

What are the three scenarios of bond failure?

1. Adhesive
2. Cohesive
3. Mixed

The question on the page originate from the summary of the following study material:

Topic 3: Dental Materials

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