State of the art of self-etch adhesives
Introduction
Adhesive technology has evolved rapidly since it was introduced more than fifty years ago. The main challenge for dental adhesives is to provide an equally effective bond to two hard tissues of different nature. Bonding to enamel has been proven to be durable. Bonding to dentin is far more intricate and can apparently only be achieved when more complicated and time-consuming application procedures are followed. Consequently, today's adhesives are often regarded as technique-sensitive with the smallest error in the clinical application procedure being penalized either by rapid de-bonding or early marginal degradation. As a consequence, the demand for simpler, more user-friendly and less technique-sensitive adhesives remains high, urging manufacturers into developing new adhesives at a rapid pace.
Today's adhesives either follow an ‘etch-and-rinse’ or a ‘self-etch’ (or ‘etch-and-dry’) approach, which differ significantly in the manner they deal with tooth tissue. Nevertheless, it should be stated that both approaches have performed successfully in laboratory as well as clinical research, while obviously there also exists a high product-dependency. Following the previous presentation (and paper) by David Pashley on ‘The state of the art of etch-and-rinse adhesives’, the main objective of this presentation (and paper) is to present the latest developments with regard to the self-etch approach.
Section snippets
General characteristics of self-etch adhesives
Different from etch-and-rinse adhesives, self-etch adhesives do not require a separate etching step, as they contain acidic monomers that simultaneously ‘condition’ and ‘prime’ the dental substrate. Consequently, this approach has been claimed to be user-friendlier (shorter application time, less steps) and less technique-sensitive (no wet-bonding, simple drying), thereby resulting in a reliable clinical performance [1], [2], [3], [4], though this appeared very product-dependent. Another
Major shortcomings of one-step adhesives
The latest generation of most simple-to-use one-step adhesives are intricate mixes of hydrophilic and hydrophobic components. These ‘difficult’ mixtures should so far be considered as ‘compromise’ materials that have consequently been documented with several shortcomings (Fig. 2) [15], [16], [17]. Generally, a reduced ‘immediate’ bond strength is recorded in comparison to that measured for multi-step adhesives [15], [18]. In addition, any kind of ‘aging’ demonstrates a lower long-term bonding
The ‘AD-concept’ revisited as basis of durable bonding
The fundamental mechanism of bonding to enamel and dentin is essentially based on an exchange process, in which minerals removed from the dental hard tissues are replaced by resin monomers that upon polymerization become micro-mechanically interlocked in the created porosities [27]. This process, which is called ‘hybridization’ on dentin, involves infiltration and subsequent in situ polymerization of resin within the created surface porosities, and thus is a process primarily based upon
Avoid ‘strong’ self-etch adhesives
‘Strong’ self-etch adhesives present rather deep demineralisation effects at both enamel and dentin (Fig. 1c). The interfacial ultra-structure produced by these adhesives resembles that of etch-and-rinse systems, but differs for the fact that the dissolved calcium phosphates are not rinsed away. These embedded calcium phosphates are expected to be very unstable in an aqueous environment, thereby seriously weakening the interfacial integrity. Laboratory as well as clinical data have undeniably
Importance of keeping hydroxyapatite at the interface to protect collagen and generate chemical interaction receptiveness
Indeed, ‘mild’ self-etch adhesives demineralise dentin only partially, leaving a substantial amount of HAp-crystals around the collagen fibrils (Fig. 1b). Dentinal collagen exposed by an etch-and-rinse procedure has been documented to be highly vulnerable to hydrolytic and enzymatic degradation processes [38], [39], [40]. Smart biomimetic remineralisation procedures have been developed by Tay and Pashley [41], [42], [43], [44] to ‘repair’ the natural HAp-protection of collagen. However, the
Importance of the smear layer on dentin
It is well known that during cavity preparation using rotary instruments, the surface to bond to will be covered by a smear layer [57], [58]. Depending on the preparation technique, this smear layer varies significantly in size and structure. Unfortunately, the smear layer is not attached firmly to the tooth surface, and self-etch adhesives should be able to dissolve it to obtain a satisfactory bond to the underlying tooth surface [59].
There are indications that the bonding effectiveness of
Self-etching enamel
At enamel, an etch-and-rinse approach using phosphoric acid remains the choice of preference, since it not only guarantees the most durable bond to enamel, but also seals and thus protects the more vulnerable bond to dentin against degradation [48], [63], [64], [65], [66], [67]. As mentioned above, while ‘strong’ self-etch adhesives generally perform not that unfavorably at enamel, bonding of ‘mild’ self-etch adhesives to enamel (and certainly to unground, aprismatic enamel) remains so far
Conclusion
Further optimization of the self-etch approach is achievable by synthesis of functional monomers tailored to exhibit good chemical bonding potential following a mild self-etch approach. This approach appears to guarantee the most durable bonding performance at dentin provided that it deals adequately with the debris smeared across the surface by bur. Micro-mechanical interlocking is still the best strategy to bond to enamel. Selective phosphoric-acid etching of the enamel cavity margins is
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