Concepts of Enamel adhesion.pptx

CONCEPTS OF ENAMEL AND DENTIN ADHESION The word adhesion = Latin adhaerere (‘to stick to’) Adhesion is ‘the state in which two surfaces are held together by interfacial forces which may consist of valence forces or interlocking forces or both’. In dentistry, bonding of resin-based materials to tooth...

CONCEPTS OF ENAMEL AND DENTIN ADHESION The word adhesion = Latin adhaerere (‘to stick to’) Adhesion is ‘the state in which two surfaces are held together by interfacial forces which may consist of valence forces or interlocking forces or both’. In dentistry, bonding of resin-based materials to tooth structure is a result of four possible mechanisms, as follows: I. Mechanical—penetration of resin and formation of resin tags within the tooth surface II. Adsorption—chemical bonding to the inorganic component (hydroxyapatite) or organic components (mainly type I collagen) of tooth structure III. Diffusion—precipitation of substances on the tooth surfaces to which resin monomers can bond mechanically or chemically IV. A combination of the previous three mechanisms USES OF ADHESIVES I. Restore Class I, II, III, IV, V and VI carious lesions or traumatic defects. II. Change the shape and the colour of anterior teeth (e.g. with full or partial resin veneers). III. Improve retention for porcelain-fused-to-metal (ceramometal) or metallic crowns. IV. Bond all-ceramic restorations. V. Seal pits and fissures. VI. Bond orthodontic brackets. VII. Bond splints for tooth luxations and periodontally involved anterior teeth and conservative toothreplacement prostheses. VIII. Repair existing restorations (composite, amalgam, ceramic or ceramometal). IX. Provide foundations for crowns. X. Desensitize non carious cervical lesions (NCCLs) and exposed root surfaces. XII. Bond fractured fragments of anterior teeth. XIII. Bond prefabricated fibre, metal and cast posts. XIV. Reinforce fragile endodontically treated roots internally. XV. Seal root canals during endodontic therapy. XVI. Seal surgically resected root apices. ENAMEL ADHESION Inspired by the use phosphoric acid to facilitate adhesion of paints and resins to metallic surfaces, Buonocore envisioned the use of acids to etch enamel. Etchant: phosphoric acid gels having concentrations of 30%–40%, with 37% phosphoric acid gel being the most common. Etch time. Currently, an etching time of 15 s is recommended. Bond Strength: shear bond strengths of composite to phosphoric acid-etched enamel usually exceed 20 Mpa. Such bond strengths provide retention and prevent leakage around enamel margins of restorations. RESIN MICROTAG MECHANISM OF ENAMEL ADHESION Acid etching transforms the smooth enamel into an irregular surface and increases its surface free energy Application of a fluid resin-based material to the irregular etched surface, facilitates penetration of the resin into the surface Monomers in the material are then polymerized, and the material becomes interlocked with the enamel surface The formation of resin microtags within the enamel surface is the fundamental mechanism of resin–enamel adhesion. DENTIN ADHESION Bonding to dentin presents a much greater challenge. Several factors account for this difference between enamel and dentin bonding: Enamel is a highly mineralized tissue composed of more than 90% (by volume) hydroxyapatite, whereas dentin contains a substantial proportion of water and organic material, primarily type I collagen. Dentin also contains a dense network of tubules that connect the pulp with the dentinoenamel junction. (DEJ). A cuff of hypermineralized dentin called peritubular dentin lines the tubules. The less mineralized intertubular dentin contains collagen fibrils. Dentin is an intrinsically hydrated tissue, penetrated by a maze of fluid-filled tubules. Dentinal tubules enclose cellular extensions from the odontoblasts and are in direct communication with the pulp. The relative area occupied by dentin tubules decreases with increasing distance from the pulp. Adhesion can be affected by the remaining dentin thickness after tooth preparation. Bond strengths are generally less in deep dentin than in superficial dentin SMEAR LAYER Whenever tooth structure is prepared with a bur or other instrument, residual organic and inorganic components form a ‘smear layer’ of debris on the surface. The smear layer fills the orifices of dentin tubules, forming ‘smear plugs’and decreases dentin permeability by nearly 90% in vitro. The composition of the smear layer is basically hydroxyapatite and altered denatured collagen. STRESSES AT THE RESIN–DENTIN INTERFACE Composites shrink as they polymerize, creating considerable stresses within the composite mass, depending on the configuration of the preparation. 1) CONFIGURATION FACTOR (C-FACTOR) Stress relief within a three-dimensional bonded restoration is limited however by its configuration factor (C-factor). In an occlusal preparation, composite is bonded to five tooth surfaces—mesial, distal, buccal, lingual and pulpal. The occlusal surface of the composite is the only ‘free’ or unrestrained surface. In such a situation, the ratio between the number of bonded surfaces and the number of unbonded surfaces is 5:1, giving the restoration a configuration factor equal to 5. Stress relief is limited because flow can occur only from the single free surface. Unrelieved stresses in the composite contribute to: - internal bond disruption and - marginal gaps around restorations that may increase microleakage - potential postoperative sensitivity 2)POLYMERIZATION SHRINKAGE STRESS In addition to the C-factor, the magnitude of the polymerization shrinkage stress depends on several other variables, including: rate of the polymerization reaction, degree of conversion, composite stiffness and its rate of acquisition of stiffness during polymerization nature and relative volume of the inorganic filler, type of monomers in the composite, composite insertion technique and opacity of the composite. 3) COEFFICIENT OF THERMAL EXPANSION Each time a restoration is exposed to wide temperature variations in the oral environment (e.g. drinking coffee and eating ice cream), the restoration undergoes volumetric changes of different magnitude compared with those of the tooth structure. This occurs because the linear coefficient of thermal expansion (CTE) of hybrid composites is about 2–3 times and microfilled composites is about 4 times greater than that of dentin. Microleakage around dentin margins is potentiated by this discrepancy in linear CTE between the restoration and the substrate 4) OCCLUSAL LOADING Studies have found that cyclic loading and preparation configuration significantly reduced the bond strengths of self-etch and etch-andrinse adhesives. CURRENT OPTIONS FOR RESIN–DENTIN BONDING I. THREE-STEP ETCH-AND-RINSE ADHESIVES Because they include three essential components that are applied sequentially, they are more accurately described as three-step etch-and rinse systems. Also called 4th Generation The three essential components are I. phosphoric acid–etching gel that is rinsed off; II. primer containing reactive hydrophilic monomers in ethanol, acetone or water and III. nonsolvated unfilled or filled resin bonding agent. MECHANISM OF ACTION OF THREE-STEP ETCH AND RINSE ADHESIVES Application of acid to dentin results in partial or total removal of the smear layer and demineralization of the underlying dentin Acids demineralize intertubular and peritubular dentin, open the dentin tubules and expose the collagen fibres, increasing the microporosity of the intertubular dentin Dentin is demineralized by up to approximately 7.5 μm, depending on the type of acid, application time and concentration The primer in the system is designed to increase the critical surface tension of dentin, which gets decreased after the acid etching step. Bonding mechanism I. When primer and bonding resin are applied to etched dentin, they penetrate the intertubular dentin, forming a resin–dentin interdiffusion zone or hybrid layer II. They also penetrate and polymerize in the open dentinal tubules, forming resin tags II. TWO-STEP ETCH-AND-RINSE ADHESIVES In order to simplify the clinical procedure, two-step etch-and-rinse adhesive system was introduced. Some authors refer to these as fifth-generation adhesives. They are sometimes called one-bottle systems because they combine the primer and bonding agent into a single solution. A separate etching step still is required. III. TWO-STEP SELF-ETCH ADHESIVES Introduced in Japan, two-step SEAs contain an acidic monomer that functions as a self-etching primer and a hydrophobic nonsolvated bonding resin. The acidic primers include a phosphonated and/or carboxylated resin molecule that performs two functions simultaneously— etching and priming of dentin and enamel. In contrast, to conventional etchants, the acidic primers are not rinsed off. Mechanism of action. The bonding mechanism of SEAs is based on I. simultaneous etching and priming of enamel and dentin, forming a continuum in the substrate and II. incorporating smear plugs into the resin tags Advantages The elimination of rinsing and drying steps has the following advantages: 1- Simplifies the bonding technique. 2- Reduces the possibility of overwetting or overdrying dentin, either of which can affect adhesion adversely. Limitations SEAs do not etch enamel as well as phosphoric acid, If enamel is not etched adequately, the seal of enamel margins in vivo might be compromised. When enamel bonds are stressed in the laboratory by thermal cycling, some of the first SEAs were more likely than etch-and-rinse systems to undergo deterioration. Strong one-step SEAs continue the demineralization of the adjacent dentin structure in the tubules which may also result in exposed collagen fibres. The presence of a hybrid layer that is not completely infiltrated by the adhesive may increase the stresses in resin–dentin interfaces. IV. ONE-STEP SELF-ETCH ADHESIVES One-step SEAs, which have etching, priming and bonding functions delivered in a single solution. V. UNIVERSAL ADHESIVES The major difference between traditional one-step SEAs and universal adhesives is that most universal adhesives contain 10MDP (and/or other monomers), which is capable of chemical bonding to calcium in hydroxyapatite through nanolayering. The 10-MDP (10-Methacryloyloxydecyl dihydrogen phosphate) molecule forms stable calcium-phosphate salts without causing strong decalcification. The chemical bonding formed by 10-MDP is more stable in water than that of other monomers used in the composition of SEAs, such as 4-META and phenyl-P. REFERENCE Sturdevant’s Art and Science of Operative Dentistry