Between 1988 and 1997, amalgam placement in the United States decreased by 32%, from 85%1 to 58% of all restorations.2 This trend would indicate that the profession is moving toward use of composite resin material. Further, the cementation of indirect restorations is routinely performed with adhesive, not luting, cements. The lining of cavities, the basing of restorations, and the sealing of pits and fissures are all bonding procedures. Thus, it is essential that every clinical dentist be in a position to evaluate the quality and analyze the long-term ramifications of adhesive procedures.
This article discusses the specifics of dentin bonding and reviews three technical principles that must be strictly followed to achieve long-term clinical success.
MECHANISMS OF DENTIN BONDING
The discovery of the mechanisms whereby composite resin bonds to human tooth structure has had profound effects on the practice of dentistry and on the expectations of both patients and dentists. The concept of bonding to tooth structure has led to major modifications in cavity preparation, with a general trend towards the increased preservation of healthy, natural tooth structure. More importantly, bonding has contributed substantially to the widespread increase of patient interest in aesthetic procedures. This, in turn, has fueled the rapid development and enhancement of aesthetic restorative techniques. Overall, bonding has promoted cooperation between the patient and the dentist, and has improved oral health.
In the mid-1950s, Buonocore first described the concept of bonding resins to tooth enamel.3 The process of bonding resins consists of removing selected portions of the ends of individual enamel rods, thereby providing micromechanical retention of the composite resin to the enamel. It was 15 years later, however, that this technique of bonding to enamel was adapted for clinical procedures and incorporated into general practice. The process of bonding resin to dentin took considerably longer. For nearly 40 years, the dental biomaterial researchers attempted to develop methods to chemically join polymers to collagen. Many different approaches were tried, but Nakabayashi was the first to develop a successful method.4
Nakabayashi approached the problem in an unconventional manner. Rather than try to bond composite to the dentinal surface chemically, he developed the process of bonding to it through micromechanical retention. Applying citric acid to the dentinal surface, he succeeded in removing the mineral content of the superficial dentin. The chemical removal of the hydroxyapatite crystals in the surface dentin is known as etching. This technique of acid etching created a structural network of exposed collagenous fibers surrounded by empty spaces that had once been occupied by hydroxyapatite.4
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| Figure 1. Process of bonding at the cut dentinal surface. |
Nakabayashi then used a low-viscosity resin (dentin-bonding agent) to fill in these empty spaces and the areas between and around the collagen fibers. The net effect was the removal of the mineral and water components of the superficial dentin, and subsequently replacement with a polymer.4 The hydroxyapatite substitution sites, commonly referred to as hybrid zones, are limited to the superficial layers of the cut dentinal surface and the peritubular regions. At the dentinal surface, the reaction sites of the dentin are typically less than 10 µm deep. Dentinal tubules, however, are filled to a depth of 75 to 100 µm by the dentin-bonding agent4 (Figure 1). Thus, the overall bond to dentin is dependent on two components: the intratubular adhesion within the dentinal tubules, and the intertubular adhesion to the surface dentin between the tubule orifices.
The incorporation of a low-viscosity resin into the spaces between the collagen fibers not only develops a micromechanical bond to the dentinal surface, but also creates a mechanism for sealing the deeper vital layers of the tooth from the external environment. The degree of bonding is such that the bond may actually be greater than the tensile strength of the dentin itself.5
ENSURING SUCCESS OF THE BONDING PROCEDURE
Successful bonding procedure is dependent on precise clinical technique. There are several cardinal steps in the bonding process that must be carefully performed. Failure to do so will jeopardize the outcome. These steps include:
(1) Time of acid etching
(2) Tooth wetness prior to bonding
(3) Thoroughness of the diffusion of the bonding agent onto and into the etched, decalcified dentin.
Time of acid etching. The time required for effective acid etching has been established by current research at 15 seconds.6 This provides optimal removal of hydroxyapatite without damaging the collagen fibers, and without causing any irreversible effects on the microcirculation within the pulp,6 even in very deep cavities.
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| Figure 2. Appropriate tooth wetness prior to bonding. |
Tooth wetness prior to bonding. The wetness of the dentinal surface is best described as a “glossy” appearance that precludes the matte look of dry dentin, and the pooling of water in overwet preparations5 (Figure 2).
Thoroughness of the diffusion of the bonding agent onto and into the etched, decalcified dentin. After the first layer of the dentin-bonding agent is rubbed lightly and continuously into the acid-etched surface over 15 seconds, gentle air dispersal of the remaining solvent (usually acetone or alcohol) prepares the surface for the second layer, applied in exactly the same manner. Light curing the adhesive between the layers is contraindicated. Light curing of the dentin-bonding agent after the second layer is mandatory.
Unfortunately, the routine and established use of bonding techniques for more than a decade led to a degree of laxness in technique. Less-than-precise technique can result in an increased potential for postoperative sensitivity and future debonding.
Interestingly, current clinical research has demonstrated that the bonding to dentin may not be as routinely stable as formerly assumed. For example, Hashimoto and colleagues have reported on the degradation of the dentin-bond interface.7 This clinical study of composite resin restorations was performed during a 3-year period. A series of posterior composite resins were inserted in primary molars using ScotchBond Multi Purpose dentin bonding agent (3M Company). One to 3 years later, in association with the eruption of the permanent tooth, the composite resin filled teeth were extracted. Immediately after extraction, these teeth were sectioned perpendicular to the adhesive interface and then prepared for micro-tensile bond testing. In addition to determining the tensile bond strengths, each fractured surface was evaluated in detail by means of scanning electron microscopy (SEM). Both the tensile bond strength values as well as the SEM evaluations were compared with a control. The control consisted of measuring the adhesion immediately after placement.
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| Figure 3. Effect of aging on dentin-bonding strength. |
The bond strength measurements from this study are presented in Figure 3. Note that the adhesion values decreased considerably over the 3-year period. While the initial bond strength values were 28.3 MPa, the values at the end of 3 years of clinical service decreased to 9.1 MPa, or a loss of nearly 70% of the original value.
The SEM evaluation of the clinical specimens also revealed interesting findings. Specifically, the proportions of demineralized dentin (that had been etched but never fully sealed by bonding) at the fractured surfaces increased with time. Furthermore, where exposed to oral fluids, the degradation of the composite resin, and the digestion and depletion of the collagenous fibrils increased as the restorations aged. In other words, this study indicated that degradation of collagen fibers that were not sealed with resin dentin-bonding agents occurred after aging under routine conditions in the oral cavity.
This study suggests that in cases where there is no strict adherence to proven methodology, there is a greater potential for dentin-bonding failure than has been commonly assumed. In situations where the dentin-bonding agent fails to completely saturate the surface, serious clinical problems can occur in the future. Should the bonding agent fail to completely fill in all the regions previously occupied by the hydroxyapatite crystals that were evacuated by acid etching, a considerable number of the collagen fibers will remain exposed and unprotected. This, in turn, can result in the eventual biodegradation or deterioration of the collagen fibrils.7
In most cases, it is necessary to apply two layers of the dentin-bonding agent to the acid-etched surface. The surface of the cavity preparation is lightly and continuously rubbed with the adhesive-saturated applicator. This process takes approximately 15 seconds. After lightly air dispersing the remaining solvent, a second application is placed in the same manner as the first. Under no circumstances should the bonded surface be light cured between applications of the dentin-bonding agent. Doing so may harden the diffusing bonding agent before it has an opportunity to penetrate into the deepest portions of the demineralized dentin. Premature curing of the first layer also prevents an effective diffusion of the second application of bonding agent.
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| Figure 4. Thoroughly diffused dentin-bonding agent. |
It is easy for the clinician to determine the number of applications necessary to accomplish a complete diffusion of the bonding agent. The last application (after air dispersing) should produce a surface that is glossy or highly reflective of light (Figure 4). The bonded dentin is then ready for light curing. Should the surface be dull (matte) or nonreflective, another application is mandatory.
CONCLUSION
While dentin bonding has been shown to be a safe and effective technique for bonding composite resins to the dentinal surface, it is incumbent upon the clinician to be very conscientious in the applications of these systems. Specific care must be given to: (1) time of acid etching; (2) amount of residual moisture on the preparation surface; and (3) proper application of the dentin-bonding agent. Failure to do so may result in an incomplete penetration of the bonding agent into the decalcified (etched) zones. This in turn may cause a deterioration of the bond strength over a period of years, as well as promote biodegradation of the unsealed collagenous fibers.
References
1. Berry TG, Summit JB, Chung AK, et al. Amalgam at the new millenium. JADA. 1998;129:1547-1556.
2. Hickel R. Restorative materials: an evidence-based review. Dent Prod Rep. 2001;35:36-45.
3. Buonocore MG. A simple method of increasing the adhesion of acrylic fillings to enamel surfaces. J Dent Res. 1955;34:849-853.
4. Nakabayashi N, Nakamura M, Yasuda N. Hybrid layer as a dentin-bonding mechanism. J Esthet Dent. 1991;3:133-138.
5. Freedman G. Fifth generation bonding systems: state of the art in adhesive dentistry. J Can Dent Assoc. 1997;63:347-350.
6. Ivanyi I, Kispelyi B, Fazekas A, et al. The effect of acid etching on vascular diameter of pulp-vessels in rat incisor (vitalmicroscopic study). Oper Dent. 2001;26:248-252.
7. Hashimoto M, Ohno H, Kaga M, et al. In vivo degradation of resin-dentin bonds in humans over 1 to 3 years. J Dent Res. 2000;79:1385-1391.
Dr. Leinfelder is professor emeritus, University of Alabama and adjunct professor, University of North Carolina. He is the recipient of the Dr. George Hollenbeck award (1995) as well as the Norton N. Ross award for outstanding clinical research (1997), and the American College of Prosthodontists Distinguished Lecturer Award (1998). He presently serves as associate editor of the Journal of the American Dental Association and as a dental materials research consultant for numerous materials companies. Dr. Leinfelder has been published extensively and lectures nationally and internationally on clinical biomaterials.
Dr. Freedman is a past president of the American Academy of Cosmetic Dentistry and currently associate director of the Esthetic Dental Education Center and visiting clinical associate professor at the State University of New York at Buffalo. He is also director of post-graduate programs in aesthetic dentistry at the University of Florida; University of Missouri (Kansas City); Eastman Dental Center (Rochester); and universities in Seoul, South Korea, London England and Schaan, Liechtenstein. Dr. Freedman is the author of 7 textbooks, more than 150 articles, and numerous video and audio tapes. A diplomate of the American Board of Aesthetic Dentistry, he lectures internationally on dental aesthetics, dental technology, and photography. Dr. Freedman maintains a private practice limited to aesthetic dentistry in Toronto, Canada, and can be reached at (905) 513-9191.
