Resin–dentin bond strength durability testing has been extensively used to evaluate the effectiveness of adhesive systems and the applicability of new strategies to improve that property. Clinical effectiveness is determined by the survival rates of restorations placed in non-carious cervical lesions (NCCL). While there is evidence that the bond strength data generated in laboratory studies somehow correlates with the clinical outcome of NCCL restorations, it is questionable whether the knowledge of bonding mechanisms obtained from laboratory testing can be used to justify clinical performance of resin–dentin bonds. There are significant morphological and structural differences between the bonding substrate used in in vitro testing versus the substrate encountered in NCCL. These differences qualify NCCL as a hostile substrate for bonding, yielding bond strengths that are usually lower than those obtained in normal dentin. However, clinical survival time of NCCL restorations often surpass the durability of normal dentin tested in the laboratory. Likewise, clinical reports on the long-term survival rates of posterior composite restorations defy the relatively rapid rate of degradation of adhesive interfaces reported in laboratory studies. This article critically analyzes how the effectiveness of adhesive systems is currently measured, to identify gaps in knowledge where new research could be encouraged. The morphological and chemical analysis of bonded interfaces of resin composite restorations in teeth that had been in clinical service for many years, but were extracted for periodontal reasons, could be a useful tool to observe the ultrastructural characteristics of restorations that are regarded as clinically acceptable. This could help determine how much degradation is acceptable for clinical success.
Attempts to determine the effectiveness of adhesive systems must include durability testing. The pioneer work of Buonocore evaluated the quality of adhesion by determining the “survival” time of bonds of acrylic resin made to enamel and dentin . As stated by Buonocore (1955), “ At this time we feel that because evidence of this nature has not been previously reported, the reasons for the increased adhesion are less important than the finding that the adhesive bond attained on treated (i.e. acid-treated) as compared to untreated (i.e. control), surfaces survived oral conditions for relatively long periods of time ”, it became clear that the value of the newly developed technique was because the improved adhesion was more durable than previous adhesion techniques.
The issue of bond durability has dominated most current research in both resin-enamel and resin–dentin bonding. Because bonds made to enamel are regarded as reliable and durable , most of the attention has been devoted to understand why bonding to dentin does not match the durability of its neighboring hard tissue. Several reasons have been given to explain why bonding to dentin is still a challenge, despite of the improvements in dental adhesive technology and advances in bonding knowledge. These include the heterogeneity of the structure and composition of dentin, the dentin surface characteristics after bur cutting and chemical treatments; and bond strategy and physicochemical properties of the adhesives, among other variables . Most of the current knowledge of bonded interfaces originated from laboratory studies. The question as to whether these laboratory outcomes are somehow related or can be predictive of clinical performance remains dubious. Except for a few weak relationships , most of the attempts to correlate laboratory and clinical data are inconclusive .
While it is widely recognized that the characteristics of the bonding substrate plays a major role on the quality of adhesion , and that clinically relevant substrates include caries-affected, caries-infected, sclerotic, deep, and bur cut dentin, major new insights in bonding mechanisms are often generated from laboratory studies using sound, freshly cut and sand-paper abraded dentin as the testing substrate. Clinical effectiveness of adhesives is assessed from the performance of restorations placed in Class V, non-carious cervical lesions . This approach provides direct evidence of the ability of the adhesive to effectively bond, because the restorations fail by loss of retention. However, the type of sclerotic substrate encountered in such lesions is rather unique and may not reflect how adhesives bond to other clinically available surfaces for bonding. Class II composite restorations fail frequently because of marginal leakage that leads to secondary caries . The breakdown of interfacial sealing poses a challenge to the longevity of restoration . If longevity of these restorations are mainly affected by leakage of oral fluids and bacteria along the interface , studies on this phenomenon should be more clinically relevant to better predict the clinical performance of adhesive restorations . Instead, bond strength data are generally used for such predictive analysis, even though no correlation seems to exist between bond strength and marginal leakage . All this may account for the difficulties in establishing a reliable and predictive relationship between durability of bonds measured in the laboratory and clinical success of adhesive restorations. Several clinically possible adjunctive procedures have been suggested to improve short-, and perhaps long-term adhesion to dentin. These include ethanol wet-bonding , extended adhesive application time use of warm air to accelerate solvent evaporation , use of protease enzyme inhibitors , use of collagen cross-linkers , and rubbing action during the adhesive application . While these strategies have been proved quite effective under laboratory and short-term in vivo conditions , only a few have been translated to a controlled clinical testing .
While durability testing in the laboratory has consistently demonstrated bond degradation within a relatively short period of time , clinical data indicate that resin–dentin bonds last much longer . This suggests that the mechanisms involved in the degradation of bonds observed in laboratory may not apply at the same rate clinically, or the effects of the degradation of the bonds have a secondary role in the clinical success of restorations.
This article will not provide an exhaustive review the topic on durability of bonds and the respective clinical outcome. Several excellent review articles have been published within the last 2 years that cover the current knowledge on that topic in detail . Rather, this review intends to critically analyze some of the approaches used to evaluate the effectiveness of adhesive systems and, perhaps, stimulate new approaches to this topic.