Materials

Seventh-Generation Adhesive Systems

Dentistry Today November 1, 200211 Mins read5.6k Views

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Over the past two decades, the evolution of adhesive techniques has transformed the scope of dental practice. In North America, the majority of direct and indirect restorations are bonded to natural tooth structure rather than cemented or mechanically retained. Extensive research and product development have consistently improved the adhesive armamentarium available to the dentist, broadening its applications and range. Patient interests and demands have reflected a newfound interest in oral appearance and health, most commonly associated with adhesive procedures.

The widespread demand for, and use of, dental adhesives has fueled an intensive development of better and easier dental adhesives in rapid succession; dentists have literally been inundated with successive “generations” of adhesive materials. While the term “generation” has no scientific basis in the realm of dental adhesives, and is to a great extent arbitrary, it has served a useful purpose in the organization of the myriad of materials into more comprehensible categories.

The “generational” definitions help in the identification of the chemistries involved, the strengths of the dentinal bond, and the ease of use for the practitioner. Ultimately, this type of classification benefits the dentist and the patient in terms of simplifying the clinician’s chairside choices.

THE GENERATIONAL DEVELOPMENT OF ADHESIVE SYSTEMS
The first generation adhesives in the late 1970s were really nothing of the sort. While their bond strength to enamel was high (generally, all the generations of adhesives bond well to the microcrystalline structure of enamel; it is their bond strength to the semi-organic dentin that is the major problem facing dentists), their adhesion to dentin was pitifully low, typically no higher than 2 MPa. Bonding was achieved through chelation of the bonding agent to the calcium component of the dentin. While tubular penetration did occur, it contributed little to the retention of the restoration. It was common to see debonding at the dentinal interface within several months.1 These bonding agents were recommended primarily for small, retentive class III and class V cavities.2 Postoperative sensitivity was common when these bonding agents were used for posterior occlusal restorations.3

In the early 1980s, a distinct second generation of adhesives was developed. These products attempted to use the smear layer as a bonding substrate.4 This layer is bonded to the underlying dentin at a negligible level of 2 to 3 MPa. The weak bonding strengths of this “generation” (2 – 8 MPa to dentin) meant that mechanical retention form in cavity preparations was still required. Restorations with margins in dentin saw extensive microleakage, and posterior occlusal restorations were likely to exhibit significant postoperative sensitivity. The long-term stability of second-generation adhesives was problematic, and for restorations, 1-year retention rates were as low as 70%.5,6

In the late 1980s, two component primary/adhesive systems were introduced. The marked improvement that these bonding agents represented warranted their classification as third-generation adhesives. Significant increases in bonding strength to dentin (8 – 15 MPa) diminished the need for retention form in the cavity preparation. Erosion, abrasion, and abfraction lesions were treatable with minimal tooth preparation, hence the beginning of ultraconservative dentistry. A noticeable decrease in postoperative sensitivity with posterior occlusal restorations was very welcome. Third-generation adhesives were the first “generation” that bonded not only to tooth structure, but to dental metals and ceramics as well. The downside to these bonding agents was their longevity. Various studies demonstrated that adhesive retention with these materials started to decrease after 3 years intraorally. However, despite the elevated levels of postoperative sensitivity, patient demands for tooth-colored restorations convinced some dentists to begin providing posterior composite fillings on a routine basis.7-9

In the early 1990s, fourth-generation bonding agents transformed dentistry. The high bond strength to dentin (17 – 25 MPa) and decreased postoperative sensitivity in posterior occlusal restorations encouraged many dentists to begin the switch from amalgam to direct posterior composite fillings.10-12 This generation is characterized by the process of hybridization at the interface of the dentin and the composite. Hybridization is the replacement of the hydroxyapatite and water in the surface dentin with resin. This resin, in combination with the remaining collagen fibers, constitutes the hybrid layer. Hybridization involves both the dentinal tubules and the intratubular dentin, dramatically improving bond strength to dentin.13-16 Total etching and moist dentin bonding, concepts developed by Fusayama17 and Nakabayashi18 in Japan in the 1980s, introduced to North America by Bertollotti, and popularized by Kanca, are innovative hallmarks of

the fourth-generation adhe-sives.19-22 The materials in this group are distinguished by their components; there are two or more ingredients that must be mixed, preferably in precise ratios. This is easy enough to accomplish in the research laboratory, but rather more complicated chairside. The number of mixing steps involved and the requirement for exact component measurements tend to confuse the process and reduce the bonding strengths to dentin.

This led to the development and the great popularity of the fifth-generation dental adhesives. These materials adhere well to enamel, dentin, ceramics, and metal, but most importantly, are characterized by a single component, single bottle. There is no mixing, and thus less possibility for error. Bond strengths to dentin are in the 20 to 25+ MPa range, suitable for all dental procedures (except in conjunction with self-curing resin cements and self-curing composites). Dental procedures tend to be both stressful and technique sensitive. Where some of this stress can be eliminated, dentists, staff, and patients all benefit. Fifth-generation bonding agents, easy to use and predictable, are the most popular adhesives today. There is little technique sensitivity in a material that is applied directly to the prepared tooth surface. Postoperative sensitivity has been reduced appreciably.

Dentists and researchers have sought to eliminate the etching step, or to include it chemically in one of the other steps. The sixth-generation adhesives require no etching, at least at the dentinal surface. While this generation is not universally accepted, there are a number of dental adhesives, introduced since 2000, that are designed specifically to eliminate the etching step. These products have a dentin-conditioning liquid in one of their components; the acid treatment of the dentin is self-limiting, and the etch by-products are incorporated into the dentinal-restorative interface permanently. There have been some questions raised by researchers as to the quality of the bond after aging in the mouth. Interestingly, the bond to the dentin (18 – 23 MPa) remains strong after time, while it is the bond to the unetched, unprepared enamel that is suspect. Additionally, the multiple components and multiple steps in the various sixth-generation techniques can cause confusion and lead to error. There have also been some concerns about the efficacy and predictability of various innovative mixing procedures.

A new, simplified adhesive system has been introduced that is the first representative of the seventh generation of adhesive materials. Just as the fifth-generation bonding agents made the leap from previous multicomponent systems to a rational and easy-to-use single bottle, the seventh generation simplifies the multitude of sixth-generation materials into a single component, single-bottle system. Both the sixth- and seventh generation adhesives are available for self-etching, self-priming adhesion for dentists who are seeking improved procedures with minimal technique sensitivity and little or no postoperative sensitivity.

iBOND (Heraeus Kulzer), the first no-mix, self-etching, self-priming, single-bottle adhesive, represents the most current formulation of dentinal adhesives on the market. It eliminates the uncertainty of mixing, and thus, any resulting technique sensitivity. It also eliminates the etching step, and by accomplishing the priming and the bonding of the dental surfaces simultaneously, simplifies the adhesive procedure tremendously. This is a true, one-step, one-bottle system for the complete etching and bonding of both enamel and dentin surfaces. Its qualifications include an excellent bonding strength to dentin (18 – 25 MPa) and similar adhesion to both prepared and unprepared enamel. iBOND contains a desensitizing agent based upon Gluma. It can be used effectively for both direct and indirect composite restorations, and it adheres well to ceramic and metal. But most important of all, it is a single component, single-bottle product.

In addition to these characteristics, iBond is also insensitive to the amount of residual moisture on the surface of the preparation. The bond strength to both dentin and enamel is essentially the same, regardless of the moisture or lack of moisture on the prepared surfaces. It is also interesting to note that whether or not the clinician etches the preparation prior to application of iBond, no essential differences can be detected for either the enamel or dentin. The shear bond strengths for both substrates under both conditions are essentially similar. The shear bond strength of iBond to dentin is relatively unaffected by the type of curing light used to polymerize the material, whether halogen, LED, or plasma arc.

SEVENTH-GENERATION ADHESIVE TECHNIQUE
The following is an abbreviated technique description for the use of seventh-generation adhesives:


Figure 1. Distal surface decay.	Figure 2. Decay removed.

Figure 3. Conservative preparation.	Figure 4. Preparation is bonded,

Figure 5. Bonding agent is light cured.	Figure 6. Cavity is restored and interproximal contact is made.

Figure 7. The occlusal surface is preshaped.	Figure 8. Rough and intermediate polishing.

Figure 9. Final surface polishing.	Figure 10. The completed restoration.

(1) Decay is noted on the distal surface of the right mandibular second bicuspid (Figure 1).
(2) The decay is accessed and removed with the Great White No. 2 bur (SS White) (Figure 2).
(3) The conservative cavity preparation is complete. The tooth is matrixed (OmniMatrix, Ultradent) and wedged (Flexi Wedge, Garrison Dental) (Figure 3).
(4) The preparation is bonded with iBOND (Heraeus Kulzer) (Figure 4).
(5) The bonding agent is light cured (Figure 5).
(6) The cavity is restored with Venus (Heraeus Kulzer), and the interproximal contact is made with the CCI (Contact Curing Instrument, Hu-Friedy) (Figure 6).
(7) The occlusal surface is preshaped prior to curing of the surface layer with the “Duckhead” instrument (Hu-Friedy) (Figure 7).
(8) Rough and intermediate polishing are done with the Posterior Composite Finishing Kit (Brasseler USA). The instrument shown is the VisiFlex disk for marginal ridges and embrasures (Figure 8).
(9) Final surface polishing is accomplished with the POGO polisher (DENTSPLY Caulk) (Figure 9).
(10) The completed restoration (Figure 10).

CHEMISTRY OF DENTIN BONDING AGENTS
While the currently available dentin bonding agents effectively join composites to the surface of dentin, they can be improved. When manipulated under carefully controlled conditions, the clinical longevity of the bonded resin is as good as any other material used by the restorative dentist. Unfortunately, some of these systems have been found to be somewhat more technique sensitive than originally supposed. In a study with fourth-generation dental adhesives (which may possibly apply to fifth-generation products as well), Hashimoto has demonstrated that gradual debonding from the dentinal surface can occur over a period of time.23 The bond strength of posterior composite resin restorations adhered with fourth-generation materials decreased by nearly 75% aging over a 3-year period. In addition, scanning electron microscopy demonstrated that some of the collagen fibers beneath the hybrid zone had undergone levels of degradation. While this study was conducted on primary posterior teeth, the same conclusion could be extended to restored permanent teeth. This rationale is based on the fact that the mechanism of bonding to collagen, and the formation of the hybridized zone, are similar for both types of dentition.21

While the specific reasons for these findings have not been demonstrated, the most probable cause can be attributed to the manipulative procedures associated with the bonding process itself. Specifically, it is probable that once the decalcification process is completed, the bonding agent primer fails to penetrate completely into all of the evacuated spaces among the collagen fibers. Without the protection of the naturally occurring hydroxyapatite or the resin component of the dentin-bonding agent, the exposed collagen fibers simply undergo a process of biological degradation. This problem can be related in part to the manner in which either the fourth- or fifth-generation bonding agents are used. In both procedures, the acid-etching agent is first used to demineralize the dentin. Once this is completed, the clinician then applies the dentin-bonding agent to the tooth in order to reverse the process that has been accomplished by the acid-etching agent. Unless the dentist is very careful about the number of applications of the primer, and the time required to allow complete diffusion of the adhesive into the denatured dentin, adequate penetration may not be achieved.

Obviously, there are other factors that may influence this level of penetration. Overdrying the preparation, and thus the failure to leave some residual water on the surface (moist bonding), may discourage the primer from penetrating the dentin. Excess water on the surface may also prevent the influxing of the bonding agent. Another potential source for inadequate diffusion may be related to the premature vaporization of the alcohol or acetone solvent within the bonding agent.

The relatively recent introduction of the so-called self-etching dentin bonding agents has been met with a great deal of enthusiasm. There are several reasons for this. The most important seems to be the relative ease of use of these products. Many clinicians have viewed the self-etching adhesives as materials that can etch both dentin and enamel with a single application. They also perceive these bonding agents as systems that can simultaneously apply the primer in the same step. A secondary reason for the rapid acceptance of these materials can be related to the postoperative sensitivity that is associated with them—little or none. Together, these two factors have convinced many practitioners to abandon their traditional adhesive systems for a process that they have perceived to offer better, faster, easier, and more predictable bonding to tooth structure.

CONCLUSION
The inherent advantage of the self-etching dentin bonding agents is that they etch and deposit the primer simultaneously. With this procedural sequence, it is likely that the underfilling of the inorganic depleted zones will not occur. Consequently, the possibility of both long-term bond strength reduction and postoperative sensitivity are diminished considerably. Furthermore, technique sensitivity is reduced, as are the number of steps normally required for bonding composites to the dentin surface. The latest “generation” adhesive makes bonded dental procedures easier, better, and more predictable.

References

Harris RK, Phillips RW, Swartz ML. An evaluation of two resin systems for restoration of abraded areas. J Prosthet Dent. 1974;31:537-546.
Albers HF. Dentin-resin bonding. Adept Report. 1990;1:33-34.
Munksgaard EC, Asmussen E. Dentin-polymer bond promoted by Gluma and various resins. J Dent Res. 1985;64:1409-1411.
Causlon BE, Improved bonding of composite resin to dentin. Br Dent J. 1984;156:93.
Joynt RB, Davis, EL Weiczkowski G, Yu XY. Dentin bonding agents and the smear layer. Oper Dent. 1991;16:186-191.
Lambrechts P, Braem M, Vanherle G. Evaluation of clinical performance for posterior composite resins and dentin adhesives. Oper Dent. 1987;12:53-78.
Christensen GJ. Bonding ceramic or metal crowns with resin cement. Clin Res Assoc Newsletter. 1992;16:1-2.
O’Keefe K, Powers JM. Light-cured resin cements for cementation of esthetic restorations. J Esthet Dent. 1990;2:129-131.
Barkmeier WW, Latta MA. Bond strength of Dicor using adhesive systems and resin cement. J Dent Res. 1991;70(Abstract):525.
Holtan JR, Nyatrom GP, Renasch SE, Phelps RA, Douglas WH. Microleakage of five dentinal adhesives. Oper Dent. 1993;19:189-193.
Fortin D, PerdigaoJ, Swift EJ. Microleakage of three new dentin adhesives. Am J Dent. 1994;7:217-219.
Linden JJ, Swift EJ. Microleakage of two dentin adhesives. Am J Dent. 1994;7:31-34.
Barkmeier WW, Erickson RL. Shear bond strength of composite to enamel and dentin using Scotchbond multi-purpose. Am J Dent. 1994;7:175-179.
Bouvier D, Duprez JP, Nguyen D. Lissac M. An in vitro study of two adhesive systems: third and fourth generations. Dent Mater. 1993;9:355-369.
Gwinnett AJ. Shear bond strength, microleakage and gap formation with fourth generation dentin bonding agents. Am J Dent. 1994;7:312-314.
Swift EJ, Triolo PT. Bond strengths of Scotchbond multi-purpose to moist dentin and enamel. Am J Dent. 1992;5:318-320.
Fusayama A, Kohno A. Marginal closure of composite restorations with the gingival wall in cementum/dentin. J Prosthet Dent. 1989;61(3):293-296.
Nakabayashi N. Bonding mechanism of resins and the tooth. Kokubyo Gakkai Zasshi. 1982;49(2):410.
Kanca J. Effect of resin primer solvents and surface wetness on resin composite bond strength to dentin. Am J Dent. 1992;5:213-215.
Kanca J. Resin bonding to wet substrate. I. Bonding to dentin. Quintessence Int. 1992;23:39-41.
Gwinnett AJ. Moist versus dry dentin; its effect on shear bond strength. Am J Dent. 1992;5:127-129.
Pashley DH. The effects of acid etching on the pulpodentin complex. Oper Dent. 1992;17:229-242.
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. Freedman is a past president of the American Academy of Cosmetic Dentistry and currently associate director of the Esthetic Dental Education Center at the State University of New York at Buffalo. He is also director of postgraduate programs in aesthetic dentistry at the University of Florida, University of California at San Francisco, University of Missouri (Kansas City), Eastman Dental Center (Rochester), university programs in Seoul, South Korea, London England, and Schaan, Liechtenstein, and Scientific Chairman of the World Aesthetic Congress (London, England). Dr. Freedman is the author of 7 textbooks, more than 170 dental articles, and numerous CDs, and video and audiotapes. 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.

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 has served 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.

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