Bonding agents and techniques prepared by : Mustafa moniem Bahoz Himdad Ari Aso

Bonding Bonding and adhesioncomprise a complex set of physical, chemical and mechanical mechanisms that allow the attachment and binding of one substance to another. Bonding also means of binding two adjoining materials, for example, dental hard tissues, metal, composite, or ceramic, and providing resistance to their separation.

functions of bonding provides resistance to separation of an adherendsubstrate (i.e., enamel, dentin, metal, composite, ceramic) from a restorative or cementing material; (2) distributes stress along bonded interface; 3) seals the interface via adhesive bonding between dentin and/or enamel and the bonded material, thus increasing resistance tomicroleakage and decreasing the risk for postoperative sensitivity, marginal staining, and secondary caries.

applications of bonding acid etching is one of the most effective ways to promote restoration retention and to ensure a sealed interfacial joint at restoration margins. This procedure has markedly expanded the use of resin-based restorative materials because it provides a strong, durable bond between resin and tooth structure and has formed the basis from many innovative dental procedures as diverse as orthodontic bracket bonding and porcelain laminate veneer bonding.

applications of bonding Additional applications include pit and fissure sealants; amalgam bonding; both enamel and dentin bonding; adhesive cements, including glass-ionomer restorative materials; and endodontic sealers.

mechanisms of adhesion If true adhesion of restorative materials to tooth structure is to be achieved, three conditions must be satisfied: 1. Sound tooth structure must be conserved. 2. Optimal retention must be achieved. 3. Microleakage must be prevented.

Oral hard tissues and their environment are complex Oral hard tissues and their environment are complex. However, the fundamental mechanism of adhesion to tooth structure can be regarded simply as an exchange by which inorganic tooth material (hydroxyapatite) is replaced by synthetic resins. This process involves two parts: (1) removing hydroxapatite to create micropores 2) infiltration of resin monomers into the micropores and subsequent polymerization.

As a result,resin tags are formed that micromechanically interlock or interpenetrate with the hard tissue. There may also be chemical interactions with the tooth substrate if monomers having acidic or chelating functional groups are present. In general, the following factors can play major or minor roles in achieving adhesive bonds: 1. Surface energy and wetting 2. Interpenetration (formation of a hybrid zone) 3. Micromechanical interlocking 4. Chemical bonding

wetting Wetting is the essential first step for the success of all adhesion mechanisms. An adhesive cannot form micromechanical interlocks, chemical bonds, or interpenetrating networks with a surface unless it can form intimate contact with the surface, spread over it and penetrate by capillary attraction into any microscopic and submicroscopic irregularities. These conditions are, by definition, achieved if the adhesive wets the surface.

Generally, wettability can be enhanced by increasing the surface energy of the substrates (e.g., dentin, enamel, and synthetic materials). Since a clean, microroughened tooth surface has higher surface energy than unprepared tooth surfaces, organic adhesives are inherently able to wet and spread over such a surface unless a lowsurface tension material contaminates it before the adhesive can be applied

smear layer Poorly adherent layer of ground dentin produced by cutting a dentin surface; also, a tenacious deposit of microscopic debris that covers enamel and dentin surfaces that have been prepared for a restoration.

smear layer Whenever both enamel and dentin tissues are mechanically cut, especially with a rotary instrument, a layer of adherent grinding debris and organic film known as a smear layer is left on their surfaces and prevents strong bonding. Different quantities and qualities of smear layer are produced by the various cutting and instrumentation techniques, as occurs, for example, during cavity or root canal

In dentin, the smear layer becomes burnished into the underlying dentinal tubules and lowers dentin permeability, which is a protective effect. However, it is also a very weak cohesive material and interferes with strong bonding. Therefore, various cleaning or treatment agents and procedures are employed to either remove the smear layer or enhance its cohesive strength and other properties.

Acid Etch techniques The process of cleaning and roughening a solid surface by exposing it to an acid and thoroughly rinsing the residue to promote micromechanical bonding of an adhesive to the surface. Surface of etched enamel in which the centers of enamel rods have been preferentially dissolved by phosphoric acid

Dentin etching Dentin etching is more technique sensitive than enamel etching because of the complexity of the dentin structure. Unlike enamel, dentin is a living tissue, consisting of 50 vol% (volume percentage) of calcium phosphate mineral (hydroxyapatite), 30 vol% of organic material (mainly type I collagen), and 20 vol% fluid. Acid etching removes hydroxyapatite almost completely from several microns of sound dentin, exposing a microporous network of collagen suspended in water

Whereas etched enamel must be completely dry to form a strong bond with hydrophobic adhesive resins, etched dentin must be moist to form a hybrid layer. The amount of water left in etched dentin is critical. If insufficient water is present, the collagen network will collapse and produce a relatively impermeable layer that prevents resin infiltration and subsequent hybridization

If too much water remains, resin infiltration cannot fully replace the water in the collagen network and, consequently, sets the condition for later leakage into those locations. Therefore, a priming step is required to maintain a hydrated collagen network 

process and procedural factors 1_Etching Time The optimal application time for the etchant may vary somewhat, depending on previous exposure of the tooth surface to fluoride and other factors. For example, a permanent tooth with a high fluoride content may require a somewhat longer etching time, as do primary teeth. In the latter, increased surface conditioning time is needed to enhance the etching pattern on primary tooth enamel that is more aprismatic than permanent tooth enamel. Currently, the etching time for most etching gels is approximately 15 seconds.

Etching Time . The advantage of such short etching times is that they yield acceptable bond strength in most instances, while conserving enamel and reducing treatment time.

2_Rinsing and Drying Stage Once the tooth is etched, the acid should be rinsed away thoroughly with a stream of water for about 20 seconds, and the rinsed water must be removed. When enamel alone is etched and is to be bonded with a hydrophobic resin it must be dried completely with warm air until it takes on a white, frosted appearance. 

Dentin, in contrast, cannot withstand such aggressive drying, which would cause bond failure because of the formation of impermeable, collapsed collagen fibers. In the total-etch technique, a dentin bonding agent and primer must be used that are compatible with both moist dentin and moist enamel.

3_Cleanness of the Bonding Surfaces The etched surfaces must be kept clean (free of contaminants) and sufficiently dry until the resin is placed to form a sound mechanical bond. Although etching raises the surface energy, contamination can readily reduce the energy level of the etched surface. Reducing the surface energy, in turn, makes it more difficult to wet the surface with a bonding resin that may have too high a surface tension to wet the contaminated surface

Thus, even momentary contact with saliva or blood can prevent effective resin tag formation and severely reduce the bond strength. Another potential contaminant is oil that is released from the air compressor and transported along the air lines to the air–water syringe. If contamination occurs, the contaminant should be removed, and the surface should be etched again for 10 seconds.

Dentin bonding agents Dental bonding agents are designed to provide a sufficiently strong interface between restorative composites and tooth structure to withstand mechanical forces and shrinkage stress. The success of adhesives is dependent on two types of bonding: 1. Micromechanical interlocking, chemical bonding with enamel and dentin, or both 2. Copolymerization with the resin matrix of composite materials

Before the total-etch technique was adopted, enamel bonding agents were used only to enhance the wetting and adaptation of resin to conditioned enamel surfaces. Generally, enamel bonding agents are made by combining different dimethacrylates from resins of composite materials (e.g., bis-GMA) with diluting monomers (e.g., triethylene glycol

a successful dentin bonding system must meet several requirements: 1. Adequate removal or dissolution of the smear layer from enamel and dentin 2. Maintenance or reconstitution of the dentin collagen matrix 3. Good wetting

4. Efficient monomer diffusion and penetration 5. Polymerization within tooth structure 6. Copolymerization with the resin composite matrix