Current Status of Dentinal Adhesive Systems
Karl F. Leinfelder, DDS, MS; and Douglas A. Terry, DDS
The coupling of composite resins with the ability for bonding to tooth structure has forever changed the way restorative procedures are carried out. While bonding to enamel has been a standard procedure for nearly 4 decades, successful adhesion to dentin has been accomplished for considerably less time. Almost without exception, bonding to dentin is a process by which hydroxyapatite is removed and the resultant vacancies are impregnated with a low-viscosity resin. Adhesive bond strengths have been sufficiently great that retention of the restoration can be achieved without mechanical undercutting of the prepared tooth, while biomechanically reinforcing tooth structure.1,2
The first clinically successful bonding to dentin consisted of etching the dentin with phosphoric acid, followed by washing, light drying, and then diffusion of the bonding resin into the surface. Categorized as a “total etch” technique,3-5 the procedure consists of applying a primer and then an adhesive (fourth generation). The technique was subsequently modified so that the primer and adhesive could be applied in a single procedure (fifth generation).
Although the fifth-generation dentin bonding agent was simpler to use as compared to its predecessor, it was characterized by a number of shortcomings. These included the potential for incompatibility with self-cured resins (composite resin cores and luting agents) and an appreciably higher level of postoperative sensitivity.6,7 The incompatibility issue could be corrected by mixing an amine additive (sometimes included with the adhesive kit) to the bonding agent. The issue of postoperative sensitivity, however, was more complicated and not readily resolved.
In an effort to eliminate the potential for postoperative sensitivity, the etchant and primer were combined by the manufacturer and then dispensed from a single bottle or container. Identified as a sixth-generation dentin bonding agent, the demineralization of the dentin and diffusion of the resin occurred almost simultaneously. Under such a condition, there is a much greater chance of eliminating the potential for under-filling the voids created by the dissolution of the mineral phase. Complete diffusion of the resin bonding agent into the evacuated spaces in turn prohibits pressure changes on the underlying odontoblastic processes.8-10 This is postulated to result in the absence of postoperative sensitivity.
Incidentally, this newer generation of dentin bonding agents is identified as “self-etching” because it conditions the entire cavity preparation without the need for pretreatment with phosphoric acid. Unfortunately the so-called self-etching bonding agents are not as efficient as pretreatment with phosphoric acid at etching the enamel surfaces. While the lack of etching is most obvious on unprepared enamel, it is also a potential problem with cut or surfaced enamel. Failure to take special caution with the self-etching bonding agents may result in the formation of a thickened brown line near the preparation margins. This “line of demarcation” probably is caused by a thin section of the composite resin that may not have been properly removed during the finishing process. The relatively poor bonding of the composite resin to the surface results in the entrapment of dyes from various food substances.
The problem of under-etching the enamel surfaces can be resolved in two different ways. The first consists of pre-etching the enamel portion of the cavity preparation with a 37% solution or gel of phosphoric acid for a 15-second period. After washing and lightly drying for 2 seconds, the self-etching primer is applied to the entire surface of the cavity preparation for 15 seconds and then air-dispersed.
The second method is somewhat different. In this case, the self-etching bonding agent is applied first to the enamel portion of the cavity preparation over a period of 15 seconds. The applicator then is applied to the dentinal aspect of the preparation. In this way, the bonding agent is in contact with the enamel surface twice as long as it is to dentin. The extra time of application to the enamel will result in a satisfactory etching of its surface. Incidentally, the second method described here is preferred by the authors over the pretreatment technique. Pretreating may result in etching some of the adjacent dentin, which may result in inadequate diffusion of the self-etching bonding agent into the evacuated spaces.
Like the fifth-generation dentin bonding agents, some of the self-etching systems may be incompatible with self-cur-ing composite resin cores and self-setting luting agents. In the same way, the addition of a tertiary amine to the primer can offset the potential problem. If there is a question of incompatibility between the two systems, the clinician should consider carrying out the following test: lightly coat a glass slab (using a microbrush) with the dentin bonding agent and light-cure. Then place a thin layer (about 1 mm thick) of dual-cure luting agent over the surface of the dentin bonding agent and light-cure. If incompatibility does exist between the two agents, the surface in contact with the dentin bonding agent will remain uncured.
The most recent incarnation of self-etching dentin bonding agents is the one in which all of the components are found in one bottle. Shaking the container is all that is required to thoroughly mix all of the components. Also identified as seventh-generation dentin bonding agents, this latest advance in marketing has been quite popular among clinicians. Other than the advantage of convenience in use, however, it still possesses the same potential disadvantages inherent in the fifth- and sixth-generation dentin bonding systems.
Although dentin bonding agents have positively influenced the manner in which restorative dentistry is carried out, a substantial problem has been recognized with these relative newcomers to the dental profession. The problem is one of extended durability of the hybrid or resin diffusion zone. Several years ago, it was shown that the adhesion of posterior composite resin restorations to the walls of the preparation may actually undergo a significant decrease. Specifically, the authors demonstrated that at the end of 2 to 3 years, the bond strength decreased by as much as 50% to 68%. Furthermore, scanning electron microscopy evaluations determined that the collagenous structures to which the bonding resin was added actually underwent a process of biological degradation.11 A number of other investigations revealed similar findings.12,13
In light of the potential for collagenous degradation over an extended period of time, it is important to consider some form of micromechanical retention when the preparation is largely in dentin. In the case of cervical lesions, for example, it is recommended that retentive grooves be placed in the occlusal (incisal) and gingival portion of the preparation. In the case of ceramic or resin veneers it is suggested that the preparation be limited, if possible, to enamel. And for the same reason it is important to maximize as much as possible the enamel margin in the proximal box of class II cavity preparations.
Finally, it is important to remember that the long-term clinical effectiveness of the bond is affected by other factors. There are external factors that include the isolation method, type of light source, finishing instruments, and skill of the operator. In addition, there are patient-related factors that can influence the bonding effectiveness more than the material. These include: age, oral hygiene, and condition of the substrate (ie, sclerotic dentin).11,14,15 Thus, the long-term clinical outcome requires a careful attention to operative detail and the manufacturer’s recommendation for use. Failure to consider these factors can result in an increase in the potential for eventual deterioration of the hybrid zone and instability at the restorative interface.
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About the Authors
Karl F. Leinfelder, DDS, MS
Department of Operative Dentistry
University of North Carolina
Chapel Hill, North Carolina
University of Alabama School of Dentistry
Douglas A. Terry, DDS
Department of Restorative Dentistry and Biomaterials
University of Texas Health Science Center at Houston