Principles of Adhesion to Enamel and Dentine Flashcards
- What does adhesion depend on?
An adhesive making intimate physical contact with the substrate (wetting the surface)
What are the substances adhesive holds together called?
Adherend
What is meant by a wetting and non-wetting adhesive?
- Wetting
An adhesive that adheres completely to a surface making it high energy
- Nonwetting
An adhesive that adheres only partially to a surface, making it low energy:
Give 4 types of adhesion.
- Mechanical
- Chemical bonding
- Intermolecular bonding
- Diffusion
What surfaces are preferred for mechancial adhesion and why is this?
Rough or porous surfaces due to:
- Large interfacial area
- Micromechanical retention of the adhesive into the surface micro-voids
Give 2 examples of mechanical adhesion.
- Acid etching of enamel- creates a rough surface for adhesive.
- Orthodontic brackets.
How do some adhesives and adherends chemically bond?
Via Ionic and Covalent bonding:
- Ionic
An ionic bond is formed when atoms donate electrons to another (dissimilar electronegativity)
- Covalent
A covalent bond is a chemical bond in which 2 atoms share 1 or more pairs of electrons (similar electronegativity)
Give 2 examples of chemical bonding adhesion.
- Free radical reaction of vinyl groups (veneers)
- Reaction of isocyanates to hydroxyl groups
How do some adhesives and adherends bond via diffusion?
If the molecules of the adhesive and adherend are similar it may allow the molecules to move across the interface.
Give 2 examples of diffusion adhesion.
- Composite repairs
- Dentine bonding agents.
Give 4 ideal properties of an adhesive.
- Low contact angle (under 90)
This allows the adhesive to wet fully wet the surface of the adherend.
- The surface energy of the adhesive must be lower then the surface energy of the substrate.
A lower surface energy material will wet a higher energy surface.
- Initial low viscosity.
- Air must be displaced from the interface.
This needs to occur to avoid air pockets which would inhibit polymerisation.
What are the 3 modes of failure between a material and adhesive?
- Structural failure
Failure within the material - Adhesive failure
Failure at the adhesive interface - Cohesive failure
Failure within the adhesive itself
How does etching enamel lead to an irregular surface and what does it allow?
- As acid etch selectively erodes certain hydroxyapatite formations.
- This allows the penetration of adhesive forming resin tags- leading to micromechancial retention.
Due to the exposure of the enamel prism/rod structure.
What are the 3 types of pattern etched enamel can be?
- Preferential dissolution of the prism core.
- Preferential dissolution of the periphery of the prisms.
- A mix of type 1 and 2.
Why is it important to not agitate strongly acidic etchants placed on enamel?
When agitated, strongly acidic etchants placed on enamel can cause enamel to be removed more than intended, leading to a weakened bond.
What does rinsing off the etch and drying enamel do to the surface?
Creates a high energy micro-rough surface.
What etch and percentage is used to etch enamel?
37% phopshoric acid.
How long is etch used on cut and on uncut enamel?
- Cut - 15 seconds
- Uncut - 30-60 seconds.
How much does etch remove from the surface of enamel and what does this removed surface contain?
- 5 microns
- Adsorbed pellicle
- Amorphous CaP layer (cut surfaces)
What is the difference in bond strength when etching perpendicular to rod direction and parallel?
40% reduction in bond strength when parallel to rods:
Is liquid or gel phosphoric acid better and why
Gel phosphoric acid is generally preferred because it is easier to control and less likely to run or pool, resulting in more precise etching and a more consistent surface. It is also homogenous
What are the 2 different types of enamel bonding?
- Single bond (etch and bond)
Uses phosphoric acid etching and rinsing before application of acidic monomers and resin in solvent.
- Clearfil SE bond
Uses an acidic monomer primer to dissolve the surface HA followed by application of resin in solvent.
Which out of bond and etch and self etch systems provide more uniform bonds?
Self etch when all cavity walls are considered.
When are sealants used?
As a caries preventative measure on molars.
Give 3 types of sealants.
- Filled, unfilled
- Self-cured
- Light cured
What etch is used with sealants and why?
Phosphoric acid- most effective based on studies.
Why is bonding composite and dentine difficult?
- Dentine is hydrophilic
- Composites are hydrophobic
What covers dentine that also affects its bonding capabilities?
A smear layer.
What are 2 different compositions of a dentine bonding agent?
- Etchant - this opens up dentinal tubules that causes the flow of dentinal fluid.
- Primer
- Sealant/Bonding agent.
OR
- Weak self etching primer/strong self etching primer.
- Bond.
What is the role of adhesive monomers in dentine bonding?
They enhance dentine bonding.
How do adhesive monomers for dentine bond the dentine and composite together?
- They are bifunctional monomers.
- With a hydrophobic end that interacts with composite.
- And a hydrophilic end that interacts with dentine.
- They are separated by a hydrocarbon chain.
How is a 4-META monomer formed, and what does it bond to?
- n presence of moisture the anhydride reacts to form 2 carboxyl groups.
- It requires a special cataylst for polymerisation.
- It bonds to metal (base) and enamel/dentine.
What is the most commonly used dentine bonding agent?
10-MDP
What else in 10-MDP used in?
Cements adhesive to metal.
What percentage by weight (concentration) does 10-MDP need to be sufficient for adhesion?
5-10%.
In general percentage by weight (concentration) do adhesive monomers need to be sufficient for adhesion?
2-10%
What’s another term for smear layer?
Acid labile layer.
What causes a smear layer on dentine?
Any mechanical manipulation of dentine causes one:
- Bur
- Hand instrument
- Wooden stick.
What does a smear layer on dentine inhibit?
The flow of adhesive.
- What is the role of acid etching on dentine?
- Removes smear layer
So dentinal tubules open so adhesive can flow.
In terms of proximity to the pulp where are higher bond strengths found?
- Tubules near DEJ - higher bond strengths.
- Tubules near pulp - lower bond strengths.
Overall bonding is increased superficially.
What does etching and washing also do to the dentine that allows monomer infiltration?
Demineralises the dentine
How is a hybrid layer formed after acid removes the smear layer?
Primer and resin can penetrate the collagen fibers in demineralised dentine to form a hybrid layer.
Give the 4 types of dentine bonding agents.
- 3 step - scotchbond MP+
- 2 step - optibond solo - total etch
- 2 step - SE bond - self etch
- 1 step - iBond
What can be done to ceramics before cementing for better bonding?
Sandblasting.
How does composite shrinkage in class I restorations come about?
When a dental composite is placed in a tooth cavity and exposed to a curing light, it undergoes a chemical reaction called polymerization.
As the reaction progresses, the composite material undergoes a volumetric contraction or shrinkage. This shrinkage occurs because the monomers within the composite resin are converting into a more densely packed polymer network, resulting in a reduction in the overall volume of the material.
In class I restorations, which involve filling small to moderate-sized cavities within the tooth, the composite material is typically placed in direct contact with the cavity walls. As the composite undergoes shrinkage during polymerization, it pulls away from the cavity walls, leading to several potential consequences:
- Deformation of tooth places additional stress on the bonding and weakest bond (pulpal floor) fails.
- This opens a fluid flow path to pulp leading to post op sensitivity:
What is a common defect in adhesively bonded restorations which is commonly mistaken for caries?
Margin discolouration:
