Historic perspective

Seven and possibly eight distinct generations of dentin bonding agents have evolved. The first generation was developed in the late 1950s and early 1960s and was composed of polyurethanes, cyanoacrylates, glycerophosphoric acid dimethacrylate, and NPG-GMA (N-phenyl glycine and glycidalmethacrylate). All these materials were disappointing clinical failures. In vitro shear bond strengths were only approximately 10 to 20 kg/cm². Nearly two decades later, second-generation dentin bonding materials were introduced (Scotchbond, Dentin Bonding Agent, Creation Bonding Agent, Dentin-Adhesit, Bondlite, and Prisma Universal Bond). Most were halophosphorus esters of bis-GMA that were designed to adhere to the mineral portion of the dentin as a phosphate-calcium bond. In vitro bond strengths of these materials were reported to be 30 to 90 kg/cm.^2, 3 The bond, however, was hydrolyzed over time in the oral environment, which contributed to their poor clinical success.^3–7

The third-generation dentin bonding agents flooded the market in the early 1980s. Bowen et al.⁴ introduced a novel oxalate dentin bonding system in 1982. Originally this system was cumbersome and unpredictable, but it demonstrated a marked improvement, with bond strengths of 100 to 150 kg/cm². The acidified ferric oxalate in this system was believed to be a source of marginal discoloration, and the complicated series of reagents made the system clinically cumbersome. Bond strengths improved, with modifications close to that of composite resin bonded to etched enamel at 200 to 220 kg/cm². Nevertheless, clinical success was not satisfactory.

Fourth-generation dentin bonding agents are probably the closest to an ideal dentin bond. The effect on the pulp of conditioning the dentinal surface was long an issue.^5, 6

Clinical procedures were simplified by the simultaneous etching of the enamel surface and conditioning of the dentin. This “total etch technique” improved the bond strength to dentin as well.⁷

Fifth-generation bonding agents are essentially a modification of the fourth generation materials. They are self-priming “one-bottle” systems allowing faster and easier clinical application.⁸ The self-etching bonding systems demonstrate some chemical bonding but they do not show enough improvement to surpass those of the bond created when etch-and-rinse systems with phosphoric acid etched enamel are used.⁹

In the late 1990s, sixth-generation “self-etching” primers were introduced. These bonding adhesives incorporated an acidic primer that eliminated a separate acid-etching step.¹⁰ Some systems require mixing the primer with an adhesive before placement, whereas others require a separate adhesive to be placed over previously applied primer. Although they reduced the incidence of posttreatment sensitivity¹¹ their bond strengths are lower than those of previous fourth- and fifth-generations systems.¹²

Introduced in late 2002, seventh-generation “all-in-one” systems combine materials for etching, priming, and bonding in a single solution. Studies show that these agents exhibit bond strengths and margin sealing equal to sixth-generation systems.¹³ All-in-one bonding solutions are available in bottles and unit doses.

In 2010, VOCO America introduced VOCO, Futurabond DC, a nano-reinforced, self-cured, light-cured, and dual-cured one-step, self-etch adhesive in a single-dose delivery system that they have designated as an eighth generation system.¹⁴ With a manufacturer’s reported adhesive strength of more than 30 MPa to both dentin and enamel when used with light-cured composite resins, Futurabond DC exhibits bond strengths similar to those of fourth- and fifth-generation adhesives.¹⁵

Ideal characteristics of a dentin bonding agent

The ideal dentin bonding agent should achieve the following:

Despite dramatic improvements in dentin bonding agents, clinical techniques are still confusing, even though research and product information on the subject have dramatically increased. An electronic Medline search using the phrase “dentin adhesive” as of July 2012, lists more than 50 publication abstracts. A Google web search using the keywords “dentin” and “adhesive” produced more than 24,900 scholarly works and 322,000 web search results with up-to-date research, discussion groups, and promotional information.¹⁶

Product selection

Sufficient laboratory and animal data to predict clinical performance of dentin bonding agents are currently lacking. In addition, no large-scale controlled clinical trials comparing the performance of these materials in humans have been done. The most important long-term data include only 100 restorations, but demonstrated a 93% success rate for the three-step bonding approach over a 12-year period.¹⁶ This study implies the validity of the three-step approach. Fortunately, however, some progress has been made in attempting to support scientifically the clinical choice between bonding agents. The American Dental Association (ADA) Acceptance Program was extended for professional products that included “Dentin and Enamel Adhesive Materials;” however, this Acceptance Program was discontinued in 2007. The ADA Standards Committee on Dental Products (SCDP) develops standards for dental materials, but standards are still pending for dentin bonding agents. However, the Acceptance Program with guidelines for materials that treat hypersensitivity is still current, and two dentin bonding materials are listed by the ADA (Table 3-1). Even though the Acceptance Program for dentin and enamel adhesive materials was eliminated, the ADA lists current products in the dentin bonding category, as shown in Table 3-2.