Adhesion to Enamel and Dentine Flashcards
Definition of adhesion
- Complex physiochemical process occuring at the interface of 2 (solid) materials brought into intimate contact
- Results in the formation of an attractive force between two materials
- Covalent or ionic bonds are not necessary
Definition of adhesive
-Holds two substances together (adherend) without failure of the bond
Requirements of an adhesive to make a good bond
- Intimate contact with the surface
- Adhesive layer thickness
Different forms of adhesion
- Mechanical
- Chemical
- Intermolecular bonding
- Diffusion
Mechanical adhesion definition and examples
-Rough or porous surface provides stronger adhesion due to:
Large interfacial area
Micromechanical retention of the adhesive into the surface microvoids
Eg. acid etching of enamel
orthodontic brackets
Chemical adhesion definition and examples
Distinguishing between types of chemical bonds
- Molecules of the adhesive may form chemical bonds with the molecules of the adherend
- Chemical bonds can be ionic or covalent
- Amount of electronegativity determines the amount of covalent/ionic that occurs
- Ionic is when there is dissimiliar electronegativity
- Covalent is when there is similiar electronegativity
- Examples:
- Free radical reaction of vinyl groups (impression materials)
- Reaction of isocyanates to hydroxyl groups
Diffusion adhesion definition and examples
-If the molecules of the adhesive and adherend are similiar, it may allow for the molecules to move across the interface
-For example,
Composite repairs (placing a composite on a previously broken composite)
Dentine bonding agent
Ideal properties of an adhesive
- Should wet the surface of the adherend
- Low contact angle and surface energy appropriate to allow wetting
- Surface energy of the adhesive must be lower than the surface energy of the substrate
- A lower surface energy material will wet a higher surface energy surface
-Flow over the substrate
Initially you want a low viscosity adhesive
-Thickness of adhesive layer very important
Should be as thin as possible to effectively cover all the surface
Allows intimate contact between the two materials
-Displacement of air from interface
Almost all adhesives undergo polymerisation so all have an oxygen inhibition layer
Helps minimize stress concentration
Avoid inhibition of polymerisation
Adhesion test methods
- Depends upon adhesive application
- Utilize shear and tensile tests
- Basically work out the force to break the two adherends
Peel tests
Wedge tests
MMDB and End Notch Flexure (ENF) tests
Microtensile Testing
Best way to calculate composite bond strength to enamel/dentine in a tooth
Microtensile bonding
- Bond composite occlusally
- Cur up the tooth into slices
- Cut each slice into matchsticks
- Calculate tensile force required to pull composite from each matchstick
- Can do the same thing but bond the resin to the enamel outside and make the slices that way
- Basically it can be sectioned whichever way you want
Best way to calculate composite bond strength to coronal dentine/cervical dentine/middle root dentine
Microtensile bonding
- Cut cavity in the long axis of the tube
- Build up composite along the long axis of the tooth
- Section the tooth into slices perpendicular to the long axis (enamel, coronal dentine, cervical dentine etc)
- Trim the bonding surfaces further to give a bonded surface area of 1mm^2
- Measure tensile bond strengths at a cross head spead of 1mm/min
Structure of an adhesive joint
- Substrate 1
- Interface between Structure 1 and Adhesive
- Adhesive
- Interface between Adhesive and Substrate 2
- Substrate 2
Features of the adhesive joint and what they control
- Physical and chemical properties of the adhesive may differ at the non contact points (Especially the core as it will be homogenous)
- Generally believed that the boundary layer composition controls the durability and strength of an adhesive joint
- Also mainly responsible for the transfer of stress from one adherend to another
- Frequently the sit of environmental attack leading to joint failure
Modes of failure of adhesion
Structural failure: within the material
Cohesive failure: within the adhesive itself
Adhesive failure: at the interface
What type of failure is likely to result from a thick adhesive layer
Cohesive failure
Enamel etching process and resultant surface
- Leads to an irregular surface due to selective dissolution of hydroxyapatite
- Allows fluid adhesive to penetrate, which leads to the micromechanical retention
- Acid etching of the enamel selectively erodes certain hydroxyapatite formations that facilitates penetration of the adhesive forming resin tags
- Standard etchant is 37% orthophosphoric acid for 20 seconds
- Very large increase in surface area
-High energy micro-rough surface is created after rinsing and drying
Difference in etching procedures with uncut and cut enamel
- 37% OP acid for 20s on cut enamel
- 37% OP acid for 30-60s on uncut enamel (ortho treatment)
What does acid etching achieve
-Removes about 5 microns from the surface including the adsorbed pellicle, amorphous CaP layer, smear layer if present (cut surfaces)
- Exposes enamel prism/rod structure
- When dry a high energy surface results
- Incrreases surface area for bonding
- Produced micro-irregularities/micro-porosities in the prismatic enamel surface. Surface roughness will provide the micro-mechanical retention for the resin
Liquid v gel prosphoric acid
- Studies show you get more uniform spreading if it is a gel
- More homogenous
- Compared to liquid phosphoric acid
When etching enamel, where are you most likely to get a failure and why
- Interrod region
- Different orientation to enamel rods
- Path of local fracture in most instances is through the interrod and parallel to the rod head
How does enamel etching change with rod orientation and clinical relevance
How was this measured
What if you used self-etching systems instead of etch and bond
If you etch parallel to the rods, there is a 40% reduction in bond strength compared to if you etch perpendicular to the rod direction
-How you cut your enamel affects bond strength
-But, sometimes you cant avoid cutting parallel to the rods
Microtensile bonding when you cut the enamel in different planes
-Self etching systems provide more unifrom bonds when cavity walls are considered no matter of the plane in which you are measuring
Sealant definition and types
- Cost effective caries preventive measurement
- Care in placement is critical to success
- Filled, unfilled
- Self cured
- Light cured
- No difference in efficacy
- Phosphoric acid etching required
Difficulty in bonding to dentine and contents of a DBA
- Dentine hydrophilic
- Composites hydrophobic
- Dentine is vital and has organic and inorganic components
- Dentine is covered by a smear layer
-Dentine bonding agent may consist of an etchant, primer, sealant/bonding agent
or
Weak self etching primer/strong self etching primer
bond
Adhesive monomers for dentine
Examples of hydrophilic end
Examples of adhesive monomers
-Di/bifunctional monomers with hydrophobic and hydrophilic end groups separated by a hydrocarbon chain
-Hydrophobic end undergoes polymerisation
-Chain length between groups is important
-Hydrophilic groups:
Hydroxyl
Carboxyl
Phosphate
Sulphate
- For example HEMA and 4-META
- In presence of moisture, the anhydride reacts to form two carboxyl groups
- Responsible for adhesion
- Monomer requires special catalyst for polymerization
- Bonds to metal (base) and enamel/dentine
- Component of dentine bonding agents
- MDP (methacrylate demethylphosphate) is a component of dentine bonding agent
- It is an etch and bond molecule
- Phosphate group has a mild etching effect but can also bond to hydrophilic part of dentine
