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Fundamental Concept Of Enamel And Dentin Adhesion 1 By: Dr. Rehab Alwakeb Operative Dentistry Division Refrence: Art and Science of operative dentistry 6th edition, chapter 4; page: 114. ILOs: I. Know adhesion mechanisms of dental resins. II. Determine enamel bonding...

Fundamental Concept Of Enamel And Dentin Adhesion 1 By: Dr. Rehab Alwakeb Operative Dentistry Division Refrence: Art and Science of operative dentistry 6th edition, chapter 4; page: 114. ILOs: I. Know adhesion mechanisms of dental resins. II. Determine enamel bonding procedure. III. Detect Challenges in Dentin Adhesion. IV. Discuss Current Strategies for Adhesion of Resins to Dentin. I. Introduction  Adhesion is one of the ideal requirements of a restorative material.  The development of bonded tooth-colored restorations was a great advantage to restorative dentistry with no more use of “extension for prevention concept” which was adopted by G.V black  So, Having a bonded restoration encourages the following: Initial seal: Absence of marginal gap between the restoration and tooth structure this minimizes microleakage, post operative sensitivity and the possibility of secondary caries. Strengthening the remaining tooth structure (one block concept). Conservation :Minimize the need of preparing additional mechanical retentive means, thus preserving the sound tooth structure (adds for both retention and resistance of restoration). II. Basic Terminology ADHESION/ BONDING The state in which two surfaces are held together by interfacial forces which may consist of valance forces or interlocking forces or both. ADHESIVE This refers to the bonding agent that when applied to the surfaces or substances, can join them together, resist separation and transmit loads across the bond. ADHEREND This refers to the surface or substrate that is adhered to. Adhesive joint/bonded interface III. Bonding Mechanisms In dentistry bonding to the tooth structure is as follows : 1. Micro-Mechanical : penetration of resin and formation of resin tags within the tooth surface. 2. Adsorption : chemical bonding to the inorganic component (hydroxyapatite) or organic components (mainly type I collagen) of tooth structure 3. A combination of the previous mechanisms IV. Requirement of good adhesion 1. The surface of the tooth substrate should be clean and wettable with high SURFACE ENERGY. 2. The adhesive should have low SURFACE TENSION therefore: Have a low contact angle Wet the substrate well and spread onto the surface. Requirement of good adhesion 3. There should be intimate adaptation between the adhesive and the adherent. 4. The adhesive should be well cured/polymerized. 5. The bond strength between Requirement of good the adhesive adhesion and the adherent should be strong enough to resist de-bonding. Bonding to Enamel I. Enamel bonding Acid etching Bonding agents Acid etching Buonocore in 1955 introduced the acid-etch technique for the adhesion of resin to the tooth. He envisioned the use of acids to etch enamel for sealing pits and fissures. Acid etching transforms the smooth enamel into an irregular surface (with micro-pores)and increases its surface free energy. Un etched Enamel surface Etched Enamel surface Bonding to enamel 1. Acid Concentration Presently a 37% concentration of phosphoric acid is preferred. 2. Etching time: 15 sec. 3. Rinsing time: After etching, the enamel surface should be thoroughly rinsed with a continuous stream of air water spray for double the etching time. This should be followed by proper drying which will produce a frosty, white appearance. N.B: Optimum acid concentration and etching time produces (mono-calcium phosphate monohydrate) precipitate which can be easily rinsed off Acid etching Available forms of acid etchants: Acid may be available as a liquid or gel form. Gel etchants are preferred due to better control in placement over the enamel surface. The acid gel is applied by means of syringe which makes placement easy and precise Bonding agents These agents flow easily into the microporosities of the enamel surface and when polymerized by light activation form ‘resin tags’ which lock them into the enamel surface. Bond strength of composite resins to etched enamel is in the range from 20 to 50 Mpa Such bond strengths provide: ✓adequate retention for a broad variety of procedures. ✓prevent leakage around enamel margins of restorations.  Dentin adhesion primarily relies on the penetration of adhesive monomers into: the filigree of collagen fibers left exposed by acid etching forming HYBRID LAYER. Dentinal tubules forming RESIN TAGS Bonding to Dentin Challenges in dentin bonding compared to Enamel. 1. Chemical composition: i. Less mineral content than enamel: dentin consist of a substantial proportion of water and organic material as compared to the 90% mineral composition of enamel where type I collagen is the principal organic content of dentin. This results in decrease of surface energy after etching. ii. Water content: Dentin is an intrinsically hydrated tissue, penetrated by a maze of fluid filled dentinal tubules which connects pulp with the DEJ. The constant pressure from the pulp causes the fluid to move towards the DEJ. (transudation) 2. Microstructure: i. Diameter of dentinal tubules increases as we move from the periphery towards the pulpal side so the relative area occupied by the tubules increase. Therefore: Adhesion can be affected by the remaining dentin thickness after tooth preparation. Bond strengths are generally less in deep dentin than in superficial dentin ii. Dentin is a dynamic tissue that shows changes due to ageing, caries, or restorative procedures e.g. Sclerotic Dentin which resist etching. 3. Smear layer: Is unique structure formed on cut dentinal surface. It is composed of debris of hydroxyapatite crystals and denatured collagen. The smear layer fills the orifices of dentin tubules, forming ‘smear plugs’ and decreases dentin permeability by 85%. Thank You