Dental Adhesion PDF Fall 2024

Course Title: Dental Biomaterials I Course Code:BDS011 Dental Adhesion F A C U L T Y O F D e n t i s t r y T h e F u t u r e S t a r t s H e r e F a l l 2 0 2 4 Topics to be covered Basic concepts and definitions I...

Course Title: Dental Biomaterials I Course Code:BDS011 Dental Adhesion F A C U L T Y O F D e n t i s t r y T h e F u t u r e S t a r t s H e r e F a l l 2 0 2 4 Topics to be covered Basic concepts and definitions Importance of adhesion in dentistry and dental considerations Surface tension and surface energy Classification of adhesion Factors affecting the strength of an adhesive junction and method of testing Bonding to enamel and dentin Adhesion: Bonding between atoms and molecules of dissimilar materials. Cohesion: Bonding between atoms and molecules similar materials. Example: gold foil dental restoration Adhesive: is the liquid materials used to produce adhesion Adherend : is the solid substance to which adhesive is applied The key to adhesion Materials being joined must be in close (intimate) contact Adhesive must be liquid state low viscosity produce a thin layer.  Types of Adhesion: 1- Chemical (true adhesion) 2- Mechanical (Macro-mechanical, Micro-mechanical) 3- Physical  1.Chemical Adhesion : is chemical bonding between dissimilar materials. Example: Glass ionomer and zinc polycarboxylate with the tooth Glass ionomer and polycarboxylate cements contain COOH group that react chemically with calcium in the tooth. 2. Mechanical bonding (attachment) Bonding between dissimilar materials through mechanical interlocking. No actual bond is formed. Examples : implants, amalgam, composite and zinc phosphate cement.  Mechanical interlocking Macro-mechanical Micro-mechanical Attachment by interlocking. Penetration of a liquid into pores or irregularities Example: in the surface of a solid and hardening. Clasp tip engage undercut Example: Amalgam restoration Bonding agents Pin for amalgam restoration 3. Physical adhesion By physical force which is weaker than chemical force Example: Complete denture Factor Affecting The Strength Of Adhesive Junction: 1. Wetting 2. Stresses due to setting contraction of adhesive 3. Thermal stresses 4. Cleanliness of the adherent 5. Thickness of adhesive 6. The type of bond formed 1. Wetting Is the ability of an adhesive to spread over the surface of the adherend. It is measured by the contact angle. Contact angle: It is the angle between the surface of liquid and the surface of solid at their interface. Contact angle : it is the angle formed between adhesive and adherent at their interface. Factor affecting wetting : 1-Surface energy 2-Surface tension 3-Viscosity 4-Surface irregularities (roughness) of the adherend 1. Surface energy : ▪ It is the force of attraction between surface atoms of the solid. ▪ Atoms on the surface of solids have greater energy than atoms inside it because the outer most atoms are not equally attracted in all direction. Increasing Surface Energy of the solid, increases wettability. surface energy contact angle Wetting Examples: 1. Metals usually have a higher surface energy and are easy to wet by suitable adhesive. 2. Waxes are not easily wetted because they have low surface energy. 3. Teflon used in non stick cooking utensils has low surface energy. 2. Surface tension ▪ It is the force of attraction between the surface atoms of the liquid. ▪ Atoms on the surface of a liquid are strongly attracted to the inside. Good wetting can be achieved if the molecules of adhesive are attracted to the molecules of the adherent more than they are attracted to each other which means → spreading of liquid on the solid surface. 3. Viscosity : ▪ It is the resistance of a fluid to flow. 4. Irregularities on the adherend’s surface: Deep Irregularities may prevent an adhesive from completely wetting the adherend. Air pockets may be formed in small pit or crack and prevent the adhesive from penetrating into that area. Irregular and deep roughness Air pockets prevent adhesive from penetrating no intimate contact between adherent and adhesive weak adhesive bond. Regular and shallow surface roughness Increases surface area of the adherend good adhesive bond.  Importance of wettability in dentistry: 1.Good wetting is important in soldering. 2.Good wetting for better denture retention. 3.A more natural appearance is achieved if restorative materials are wetted by a thin film of saliva. 4. Smooth surface of casting, the wax pattern is coated by surface acting agent (wetting agent) before investing. This will improve wax wettability thus, producing a smoother surface. 5. Ceramo-metallic restorations. 6. Bonding agents. 7.Decrease marginal leakage between restoration and cavity walls. Wax pattern coated with surfactant (Debubblizer)  Good wetting should occur between wax pattern and investment material Wax pattern coated with surfactant (Debubblizer) 2. Stresses due to setting contraction of adhesive: Contraction of liquid adhesives during setting → creation of stresses at the interface → decreases the strength of adhesion. 3. Thermal stresses: If the adhesive and the adherent have large difference in the coefficient of thermal expansion and contraction, change in temperature will produce stresses in the bond. Close matching is required to minimize stresses and increases the strength of adhesion. 4. Cleanliness of the adherent Debris or surface contaminations prevent intimate contact which is necessary to produce adhesion. ** Adhesion to clean and dry surface of enamel and dentin is better than adhesion to wet contaminated one. 5. Thickness of the adhesive Thinner adhesive film → Stronger adhesive junction, ✓ Less air voids are present. ✓ Less thermal stresses ✓ Less stresses due to setting contraction of adhesive.  6. Type of bond formed Primary bonds between adhesive and adherent produce stronger adhesion than if secondary bonds are formed (Soldered Joint is stronger than glued joint).  Failure of adhesive junction 1. Adhesive failure (adhesive-adherent separation). 2. Cohesive failure of the adhesive. 3. Cohesive failure of the adherent. Adhesive Failure between Cohesive Failure within the adhesive and the adherent adhesive or within the adherend Obstacles To Adhesion In The Oral Cavity  Obstacles To Adhesion In The Oral Cavity The following condition prevent ideal adhesion in the oral cavity : 1- The inhomogeneous composition of enamel and dentin 2- Surface irregularities in the prepared cavity 3- Debris in prepared cavity 4- Presence of water in the prepared cavity 1. The inhomogeneous composition of enamel and dentin: Enamel and dentin are inhomogeneous in their composition. Part is organic and part is inorganic. The material that adhere to enamel will not be able to adhere to dentin and vice versa. The adhesion is not uniform over the entire surface 2-Surface irregularities in the prepared cavity : The surface of the prepared cavity is full of pits and fissures. Increase roughness with dental bur. It is difficult to design an adhesive that flow into those minute irregularities and wet the entire surface. 3-Debris in the prepared cavity: 4. Presence of water in the prepared cavity Water from the dentinal tubules can form a thin film of water that can prevent the adhesive from coming into intimate contact with the tooth.  Bonding To Tooth Structures Two mechanisms of adhesion (bonding) may be distinguished: chemical and mechanical. Example for chemical adhesion: Chemical bond of polycarboxylate or glass ionomer cements with tooth structure. Carboxylic group (COOH group of polycarboxylate cement or glass ionomer) reacts with the calcium of the tooth structure forming a chemical bond. Example for mechanical (micromechanical adhesion): using acid etch to create microporosities in tooth structure, where the adhesive interlocks with the tooth structure forming microtags. A. Bonding to enamel 1. Acid etching 2. Bonding 1. Acid etching 30-50 % phosphoric acid for 30 seconds to remove 5 um of enamel and form the micropores (selective decalcification) The acid removes about 5 microns of the surface of enamel and produce micropores. Acid etching help bonding to enamel by: a. Removal of surface debris b. Increase the surface energy of enamel c. Producing pores in the surface d. Increasing the surface area of enamel Enamel adhesive bond to enamel by mechanical interlocking. Then composite is applied to bond with the adhesive chemically After etching enamel become chalky white appearance. 2. Application of the adhesive The acid etching of the surface of enamel produces microtags into which the adhesive will penetrate, then resin composite (filling material) bond to the adhesive 3. Application of the composite filling materials Apply composite filling material that bonds to the adhesive B. Bonding to Dentin: Dentin poses greater obstacles to adhesive bonding than does enamel due to the : i) Presence of higher amount of water so it is strongly hydrophilic. ii) Presence of smear layer, which will prevent proper adhesion. N.B. Smear layer (debris layer) is a 5-10 microns thickness layer formed of collagen, cut tooth structure, blood, saliva and bacteria (resulting during cavity preparation). 1. Etching of dentine (15 seconds) 1- Remove the smear layer (partial or complete removal) 2- Selective Demineralize the surface (that produce porous dentin surface) 3- Reduce surface energy of dentin due decrease in mineral content and exposure of collagen fibers that have low surface energy N.B.:The same acid etch used for enamel and dentin 2. Primer : -Used to elevate the surface energy of dentin to improve wetting. -The primer contains a hydrophilic and hydrophobic part. - The hydrophilic part is designed to interact with the moist dentin surface and the hydrophobic part bonds to the resin. 3. Dentin bonding agent (Adhesive): -Low viscosity resin -Bond with primer (chemically) and collagen fibers (micromechanical mechanical). - The resultant bond is micromechanically rather than true chemical adhesion 4. Application of the composite filling materials Apply composite filling material that bonds to the adhesive chemically (But still the main bonding mechanism to the tooth structure micromechanical)  Formation of hybrid layer: Resin infiltrated dentine, This layer is a resin-reinforced layer, part of it is tooth and part is resin and resin tags