Although decayed/fractured teeth can be reconstructed with minimally invasive techniques and nearly invisibly using adhesive technology, the clinical longevity of adhesive restorations is still too short. Nowadays, many studies on dental adhesive technology are empirical, testing the bond strength of different cocktails of adhesive solutions to enamel and dentin in the laboratory. Adhesive technology has evolved rapidly from multi-step adhesive systems to more simple to use one-step adhesives.
This lecture focused on the latest developments in adhesive technology, as well as providing an in-depth discussion on the underlying mechanisms of adhesive interaction at the biomaterial–hard tissue interface following both the etch-and-rinse and self-etch approaches, especially in respect to adhesive protocols with improved bond durability.
These are still often referred to as “total-etch” systems. The “3-step” system indicates that the etch/primer/bond are applied separately. The “2-step” system denotes etching, then priming and bonding at the same time.
In dentin bonding, phosphoric acid-etching removes hydroxyapatite (HAp) and exposes collagen fibrils. This produces a deep hybrid layer at the interface with the tooth, providing micromechanical retention. There is no (primary) chemical interaction with the tooth structure involved.
In this system, insufficient resin infiltration may occur, by which unprotected collagen fibrils may be exposed. The exposed collagen fibrils are vulnerable to degradation (decomposition) by enzymes.
In this system, etching and resin penetration occur at same time. Mild pH (pH > 2) and strong pH (pH < 1) self-etch bonding agents are available.
Mild pH self-etch systems, such as CLEARFIL™ SE BOND/SE PROTECT, result in a thinner hybrid layer than etch-and-rinse systems and do not expose collagen fibrils. There is (primary) chemical retention between the adhesive monomer and HAp remaining around collagen.
On the other hand, strong pH self-etch bonding agents work more like etch-and-rinse systems. They can etch the tooth deeply, and produce a less stable hybrid layer without (primary) chemical interaction involved.
Along with the pH, the kind of adhesive monomers within bonding agents significantly influences bond durability. According to research findings from Van Meerbeek and co-workers, MDP—Kuraray’s adhesive acid monomer—showed a high chemical bonding potential to HAp (lowest water solubility).
According to long-term clinical research conducted by Van Meerbeek and co-workers, and hundreds of literature reviews, a 2-step mild self-etch approach as used by CLEARFIL SE BOND appeared the “most reliable approach, certainly with respect to bonding to dentin.” Selective phosphoric acid-etching only at enamel helps marginal adaptation.
In these reviews, he focused on Class V cervical lesions, because they do not provide any macromechanical retention. Any malfunctioning of the adhesive is penalized by de-bonding of the restoration, the most objective criterion of failure in clinical trials.
The team reviewed 1019 articles on dentin bond strength. The adhesive most tested in literature was CLEARFIL SE BOND.
The micro-tensile bond strength of different bonding systems were reviewed. The 3-step etch-and-rinse adhesives were the strongest, but not significantly better than the 2-step self-etch adhesives, followed by the 2-step etch-and-rinse adhesives and finally the 1-step self-etch adhesives that scored the lowest. After artificial aging, the bond strength of 1-step self-etch adhesives was affected the most, followed by that of 2-step etch-and-rinse bonding agents.
Images were presented demonstrating that the enamel margin of restorations bonded with an etch-and-rinse adhesive remained intact, but that the dentin margins discolored and showed clinical signs of microleakage. This degradation in marginal adaptation is related to the lower durability of the etch-and-rinse bonding approach at dentin, mainly because of insufficient resin envelopment of unprotected collagen fibrils that on the long term are decomposed by hydrolysis and an enzymatic (MMP) degradation process.
This bond degradation was further illustrated with TEM photos of the adhesive–dentin interface. Phosphoric acid-etching removes most HAp surrounding the collagen fibrils, as most likely etch-and-rinse bonding agents cannot coat these fibrils adequately, as was also demonstrated by Tay and Pashley.
In this presentation, two adhesive monomers from Kuraray—MDP and Phenyl-P—and their different features were discussed. Products that contain Phenyl-P can etch tooth structure more deeply than MDP. However, MDP-containing products demonstrate a better long-term bond durability. One reason for this improved bond durability is that the salt of MDP–calcium formed at the interface with HAp is more stable than that of Phenyl-P. In addition, Phenyl-P exposes collagen fibrils in the same manner (although less aggressively) as etch-and-rinse adhesives, while MDP does not. Therefore, the kind of adhesive monomer in the self-etch bonding agents is of primary importance with regard to bond durability.
