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June 2009 | Dentalproductsreport.com Understanding the generational divide A quick history of adhesive technology. Over the past 60 years, the art and science of dental adhesive technology has exploded and forever changed the practice of clinical dentistry. Beginning in the 1950’s with the introduction of acid etching of enamel and the realization that the micromechanical retentive undercuts would permit the adaptation of hydrophobic resins, the pursuit of the ideal adhesive has been ongoing.1,2 It took less than a year after the acceptance of enamel bonding to prompt researchers and chemists to begin exploring the possibility of similar bonding mechanisms to hydrophilic dentin.3 Historically, the label of adhesives by “generation” began during the early 1960’s with the introduction of bi-functional resins that possessed both hydrophilic and hydrophobic functional groups that in theory served as “coupling agents” to unite hydrophobic methacrylates to the moist dentin substrate.4 Since these now called “First Generation” adhesives attempted to chemically bond to the smear layer (debris resulting from rotary instrumentation of the dentin surface), bond strengths were less than 3 megapascals and the clinical results were poor. By the mid 1970’s, “Second Generation” dentin coupling agents advanced but still relied on ionic bonds to calcium found in the smear layer and hydroxyapetite and failed to produce a reliable bond.5 By the end of the 70’s, Fusayama et al. realized that chemically bonding to an intact smear layer had to be abandoned. This brought forth the “Third Generation” of dentin adhesives which relied on the partial removal or modification of the smear layer through acid conditioning and the evolution of many of the modern ambiphilic dentin primers still used today. By the early 1980’s, ground breaking research on the hybridization of dentin by Nakabayashi and Pashley opened the door for the controversial concept of simultaneously etching both enamel and dentin with phosphoric acid and the complete removal of the dentin smear layer.6 This set the stage for what is now called the “Fourth Generation” adhesives which advocated the concept of “total etching” and the subsequent infiltration of polymerized acrylic monomers. The proper clinical technique of total-etch adhesives would soon be developed by Kanca and the necessity of “wet bonding” became established in the early 1990’s.7 These Fourth Generation materials, many of which have been improved and are still on the market today, involve numerous technically demanding steps to be exacted with precision in order to be effective. Though bonding to wet dentin would be broadly accepted by the mid 1990’s, the proper method of achieving the exact level of moisture in dentin continues to prove difficult for many practitioners. The “over-wet” phenomenon and the detrimental effect of over-drying dentin which leads to collagen fibril collapse led many adhesive manufacturers explore formulations which were less sensitive to the level of post-etch dentin moisture levels.8 This included, but was not limited to the use of various solvents such as acetone, ethanol and water. Though some Fourth Generation materials are considered more user-friendly, the quest for simplified systems continues. The desire to reduce the number of steps and simplify the clinical technique spawned the “Fifth Generation,” single-bottled total etch adhesives. This generation of adhesives is characterized as having both the ambiphilic primer and hydrophobic resin adhesive in the same bottle. By combining primer and adhesive, these systems reduced the number of steps required, creating less margin for clinical error and immediately gained widespread acceptance. But since these systems still require a total-etch technique, the level of dentin moisture remains critical and though some formulations boast a larger window of error through the varied use of solvents to reduce volatility and technique sensitivity, many practitioners continued to struggle with post-operative sensitivity. CONTINUED ON NEXT PAGE
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