Adhesion to Enamel and Dentine

• 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

-Holds two substances together (adherend) without failure of the bond

• Intimate contact with the surface - Adhesive layer thickness

• Mechanical • Chemical • Intermolecular bonding • Diffusion

-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

• 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

-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

• 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

• 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

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

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

• Substrate 1 • Interface between Structure 1 and Adhesive • Adhesive • Interface between Adhesive and Substrate 2 • Substrate 2

• 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

Structural failure: within the material Cohesive failure: within the adhesive itself Adhesive failure: at the interface

Cohesive failure

• 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

• 37% OP acid for 20s on cut enamel - 37% OP acid for 30-60s on uncut enamel (ortho treatment)

-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

• Studies show you get more uniform spreading if it is a gel • More homogenous • Compared to liquid phosphoric acid

• Interrod region • Different orientation to enamel rods • Path of local fracture in most instances is through the interrod and parallel to the rod head

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

• 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

• 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

-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