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Principles of Bonding Chapter 5 Basic Principles of Bonding In dentistry, the term bond, or bonding, is used to describe the process of attaching restorative materials to the tooth by adhesion. When describing cosmetic restorations such as porcelain or composite veneers, patients often use the term bonding. Bonding The basic principles of the bonding process involve preparing the surface of the tooth or restoration. Preparing the tooth surface includes removing decay, plaque, and debris. The tooth then is etched or conditioned with a 10% to 38% solution of phosphoric acid. This conditioning removes microns of tooth to provide a rough surface to which bonding can occur. Bonding to an Etched Surface When a resin bonding agent or primer is flowed over the etched surface, it penetrates into the microscopic pores. When a resin bonding agent cures, it forms resin tags that adhere to the etched surface of the tooth. The resin bonding agent then will chemically bond to other resins placed over it, such as composite resin. Primary and Secondary Bonds The primary or chemical bond occurs through adhesion when the bonding agent and the composite resin material adhere to each other. The chemical bond is stronger than a secondary or physical bond. Example: adhesion of paint to a metal surface Surface Wetting Etching increases the ability of the bonding material to wet the tooth surface by creating a high surface energy. Good wetting increases the contact of the material and the tooth; poor wetting prevents complete contact. For this reason, bonding agents are typically made of a low-viscosity material. Bond Strength The strength of the bond is determined by the amount of force needed to break the two materials apart. This is accomplished by pulling the two materials apart or by applying force until the bond fails. An enamel bond is typically stronger than a dentinal bond. Enamel Etching Etching of the enamel removes a small portion of the surface, reduces enamel rods, and opens porosities among the rods. Etching also removes the smear layer, which is made up of debris on the enamel or dentinal surface that results from the cutting of tooth structure during the cavity preparation. The smear layer comprises bacteria, salivary components, and tooth dust. Etching Etching should take approximately 20 to 60 seconds, depending on the manufacturer’s instructions and the type of procedure to be completed. Some teeth are more dense and must be etched for a longer time; others respond well in as little as 10 seconds. Etch materials are available in liquid or gel. The area is rinsed for approximately 10 seconds to remove the etch material. Smear Layer The dentin has a higher water content because of its organic makeup. It also contains collagen and other fluids from the pulp chamber. The smear layer, which is made up of debris on the enamel or dentinal surface resulting from the cutting of tooth structure during cavity preparation, must be removed before the bonding procedure is performed. The smear layer comprises bacteria, salivary components, and tooth dust. Dentin Etching Etching dentin with phosphoric acid dissolves the smear layer first, then portions of the hydroxyapatite crystals from the surface of the dentin, creating a porous surface and exposing collagen fibrils that are part of the dentin matrix. This opens the dentinal tubules, which leaves a rough, porous surface. Dentin has a lower mineral content and needs to be etched for only about 10 seconds. Over-etching leaves a weaker bond. Bonding Agents Bonding agents are low-viscosity resins dissolved in solvents, typically acetone or ethyl alcohol, that penetrate porosities in the tooth surface created by etching. Bonding of agents for enamel requires only a single component—low-viscosity liquid. Bonding agents for dentin are viewed as two types: Primer Bond Preparing to Bond If only the enamel is etched, the surface is dried to a frosty finish. When dentin is bonded, it is left moist to keep the collagen fibrils from collapsing, which could prevent penetration of the bonding material. The primer is placed first and is dried before curing, to remove the solvent and the remaining water. Oxygen-Inhibited Layer On the surface of the polymerized bonding resin is a very thin coating of uncured resin. The resins used for composites and sealants also form this layer of uncured resin on their surfaces. Polymerization is inhibited when the surface is exposed to oxygen in the air. Once the composite resin is placed over the bonding resin, the uncured layer on the bonding resin will cure when the composite is cured. Types of Bonding Agents Self-etching: Some two-component bonding agents have incorporated acidic groups in the primer that will etch enamel and dentin and allow penetration of the resin without the need for rinsing and drying. These are called self-etching primers. These materials work well on prepared surfaces but are not as effective on unprepared surfaces. Sealants and veneers Modes of Cure Three modes of curing are used for resin bonding agents: Self cure Light cure Chemical reaction occurs between two resins (base and catalyst) are mixed together Light in the blue wave range activates a chemical (photoinitiator) that sets off the polymerization reaction or curing process Dual cure Uses a combination of self-cure and light-cure ingredients Microleakage When restorations are not completely sealed at the margins, they can leak. This permits fluids, bacteria, and other contaminants to enter the cavity preparation. This microleakage can result in recurrent decay and failure of the existing restoration. Tooth Hypersensitivity Some patients may experience transient tooth sensitivity after a bonded restoration. Usually lasts only a short time Common causes of tooth hypersensitivity include the following: Tooth is over-dried during the bonding process (desiccation). Tooth has been over-etched and not properly sealed. Composite resin is cured in too large segments. Clinical Application of Bonding Porcelain: stronger retention if bonded rather than cemented Metal: used to create a stronger bond before cementation of a crown or bridge Amalgam: helps seal out microleakage Orthodontic Bonding Brackets have replaced bands for orthodontic treatment. Conventional cements cannot be used, so the brackets must be bonded. Conventional cements are difficult to cure through brackets, so a dual cure material is used. Endodontic Posts A post is placed within the roots of endodontically treated teeth to retain dental materials used to build up missing tooth structure. Posts may be metal or nonmetal. Posts are bonded with the use of bonding agents and resin cements. Summary Bonding of resins permits restorative materials to stick to tooth structure by way of micromechanical retention or chemical bonds. Phosphoric acid is used on both enamel and dentin to remove the smear layer and provide a rough surface to which to bond. Tooth sensitivity may occur but should be minimal when correct techniques are used.