Adhesion: Past, Present, and Future

The evolution of adhesive techniques has transformed the scope of dental practice; most direct and indirect restorations are bonded to natural tooth structure rather than cemented or mechanically retained. This has fueled an intensive development of better and easier dental adhesives in rapid succession.

Successive adhesive material advances have been denoted as "generations." 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 commercially available materials into more comprehensible categories. This classification assists in identifying the chemistries involved, the strengths of the dentinal bond, and the ease of clinical use, simplifying the clinician's chairside choices.

The 1st generation adhesives (1970s) were largely unsuccessful. High enamel bond strength (micromechanically to the microcrystalline structure of enamel) was offset by the 2 MPa chelation to the semi-organic dentin, where debonding failure was typically observed within several months. By the early 1980s, 2nd generation adhesion attempted to use the smear layer as a bonding substrate. The weak, 2-8 MPa dentinal bonding strength required mechanical retention. Dentinal margins saw extensive microleakage, and posterior restorations exhibited significant postoperative sensitivity.

Two-component 3rd generation adhesives (late 1980s) exhibited improved dentin adhesion (8-15 MPa), offered decreased postoperative sensitivity, and permitted minimal tooth preparation. Intraorally, adhesion decreased within 3 years, limiting longevity. The 4th generation bonding agents (early 1990s) transformed dentistry, as their high bond strength to dentin (17-25 MPa) and limited posterior postoperative sensitivity enabled many dentists to switch from amalgam to direct posterior composite fillings.

The 5th generation (mid 1990s), with its premixed adhesive component, simplified chairside dentistry, while 6th generation adhesives (2000) eliminated the separateetching step. The 7th generation adhesives (2002) represented the first single-component, single-bottle system. They required no mixing, offered self-etching, self-priming, no technique sensitivity, and no postoperative discomfort, and eliminated the need for "moist" bonding.

Each "generation" has simplified the adhesion process significantly: fewer components, fewer steps, less chairside time, easier use, and better predictability. How can a single-component, single-step, total-comfort process be improved upon? The answer is as simple as it is difficult to develop: zero-step adhesives.

The only possible evolution is to eliminate the remaining component and single step entirely. The 8th generation adhesive has no bottle and no components, at least as a distinct, separate, clinical step. The adhesive is incorporated into the restorative material. As the practitioner places the restorative composite resin, the contained adhesive etches, primes, desensitizes, and bonds both the dentin and enamel surfaces, requiring only polymerization to integrate directly into the tooth-restorative complex. The technique is identical to current practice, except that one significant step (adhesion) is eliminated.

This direct integration means there is no adhesive film thickness, allowing the direct diffusion of fluoride ions into dentin and enamel. The 8th generation system combines the benefits of chemical adhesion at the tooth-restorative interface with the esthetics and durability of hybrid composite.

Eliminating steps eliminates technique sensitivity. The etching, priming, bonding, and desensitization of tooth surfaces are accomplished simultaneously with the insertion of the composite restorative material. Fewer clinical steps mean better predictability, less chairside time, and greater practice efficiency.

Science fiction? Actually, these chemistries are currently available in self-etching, self-priming, and self-bonding one-step resin cements and post-and-core composites. The next major evolution of dental adhesives, the 8th generation, will see the elimination of adhesion as a separate step. The only task that remains is to incorporate these innovative adhesives into direct restorative materials. We expect to see them soon.

- George Freedman, DDS, Dipl ABAD
Adjunct Professor, Western University College of Dental Medicine, Pomona, California

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