Adhesion, or bonding is the process of forming an adhesive joint, consisting of two substrates joined together. Most adhesive joints involve only two interfaces; a bonding composite restoration is an example of a more complex adhesive joint.1 The word “adhesion” is derived from the Latin roots that translate as “to stick together,” and is defined as the molecular attraction exerted between the surfaces of bodies in contact; the force referred to as adhesion occurs when unlike molecules are attracted.2
Dental adhesives are used for a wide range of clinical applications in restorative dentistry. Direct composite resin restorations all require bonding, and indirect resin inlays, onlays, and veneers require bonding and—depending on their design—crowns, bridges, and endodontic posts and cores may require and/or benefit from the use of dental adhesives in conjunction with resin luting materials.3
Adhesion technology has allowed for more freedom for dentists and a better way to treat teeth without the need for extensive preparations, preserving the original sound tissue. However, the constant battle with adhesive dentistry becomes making procedures more efficient while not sacrificing bond strengths. In this way, dental adhesives have become so much of a changing force that it establishes a new way of thinking and treating cavities, orthodontics, and other treatments that are crucial to patient’s overall health. It’s important for future dentists to learn more about how these different generations of adhesives have both improved in quality over time and have grown to be a standard of care for many dentists.
A recent presentation, “A Primer on Dental Adhesion,” by Dr. Howard Glazer discussed the evolution of adhesive bonding in dentistry, the indications of total-etch and self-etch adhesives, and other related topics. Here’s a rundown of the differences involved in the generations of dental adhesives as Dr. Glazer explains:
The 4th generation of adhesive bonding in dentistry achieves bond strengths of over 18 megapascals in the enamel in order to have a strong adhesive bond. If a system requires more than one bottle and requires a separate etch step, it is commonly referred to as a "4th generation" adhesive. In a three-step total etch procedure, the etch is first applied, then primer, and then the adhesive agent.
On the other hand, if there is a separate etch step with only one bottle, it is regarded as a "5th generation" system. In the 5th generation, the procedure is now condensed to an etch, then a bottle that has the combined primer and adhesive. As one step in the process is eliminated from the 4th-generation adhesives, you can see how the bonding procedure can become simpler, faster, and more efficient.
In the 6th generation, the separate etch step is eliminated, and a primer adhesive, two-bottle system is now utilized. This approach has an acidic primer that is built into its system, and a separate bottle for the adhesive. Bond strengths in the 6th and 7th generation are among the strongest of their kind for enamel and dentin, with upwards of 20 megapascals.
A 7th-generation adhesive is a one-step, self-etch, bonding agent, and several options are available as HEMA-free formulations and with improved bond strengths. Here, adhesive dentistry has come to a single-bottle approach. By combining the acidic primer with the adhesive in one bottle, the 7th generation is a single-application system that allows it to be an effective and popular choice for dentists. Despite this, 7th generation bonding can still use a separate-etch technique.
As adhesive systems in dentistry continue to evolve, bond strengths become stronger and postoperative sensitivities are minimized for patients. Still, the name of the game is a better-quality product that is more efficient for use. While earlier systems have had acceptable bond strengths but were technique sensitive, today’s adhesive systems are less complex to use. The continuing balance between strength and efficiency is such an important aspect to being a dentist working chairside. As quality increases, adhesive dentistry becomes easier for the healthcare professional and more effective for the patient. It will be interesting to see how even more simplified and effective dental adhesives become as generations continue to advance.
 Perdigao J, Breschi L. Current perspectives on dental adhesion. In: Aesthetic Restorative Dentistry. 1st ed. Mahwah, NJ: Montage Media Corporation; 2008:319.
 Terry DA. Adhesion: A micro-mechanical bonding. In: Natural Aesthetics With Composite Resin. 1st ed. Mahwah, NJ: Montage Media Corporation; 2004:86.
 Latta M. Recent advances in dental adhesives: An overview. THE NEXTDDS. http://www.thenextdds.com/Articles/Recent-Advances-in-Dental-Adhesives/. Accessed November 22, 2016.