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Introduction

Several contemporary dental adhesives have been documented to provide adequate immediate bond strengths to enamel and dentin . However, the clinical longevity of bonded restorations is still too short due to degradation of the adhesive tooth–composite interface . In laboratory circumstances, the durability of this bond is tested using different kinds of artificial aging methods like water storage, thermo-cycling, mechanical loading, degradation by enzymes and various chemical substances . These in vitro durability tests give detailed information regarding the mechanisms of degradation. Although it is not correct to generalize that laboratory studies can predict the durability of the bond in clinical circumstances, there are some associations between laboratory and clinical data on bonding effectiveness. In a review article of Van Meerbeek et al. a possible relationship was searched for between laboratory bond-strength data obtained in a systematic review, and the clinical retention rates collected in a systematic review on the clinical effectiveness of contemporary adhesives in non-carious Class-V lesions. A significant, quite reasonable correlation was found between the aged bond-strength data and the 5-year clinical data. The number of medium to long-term clinical trials in the literature, however, is limited. There is certainly a need for these longer-term clinical trials as they remain the ultimate way to collect scientific evidence on the clinical effectiveness of a restorative treatment.

According to a systematic literature review of non-carious Class-V clinical trials, published between 1998 and 2009, the lowest annual failure rates (expressed as retention loss) have been recorded for the glass-ionomers (2 ± 2.03%) and the so-called ‘mild’ and ‘intermediately strong’ 2-step self-etch adhesives (1.9 ± 3.3%) . Among the self-etch adhesives, the mild 2-step self-etch adhesive Clearfil SE (Kuraray) is considered as the golden-standard because of its highly adequate dentin bonding effectiveness in vitro and in vivo . The Clearfil SE adhesive system, the primer of which has a pH 2, provides a uniform submicron hybrid layer (0.5–1 μm) with substantial hydroxyapatite crystals still protecting the collagen fibrils . The functional monomer 10-methacryloyldecyl dihydrogen phosphate (10-MDP), present in the Clearfil SE primer, has been proven to interact with this residual hydroxyapatite through primary ionic bonding . The resulting calcium salts are hydrolytically stable, which contributes to the long-term durability of the resin/dentin interface . Regarding the durability of the bond of Clearfil SE to dentin in vitro , most studies reported a slight decrease in dentin bonding effectiveness , while in some studies the bonding effectiveness did not change after different types of aging . Until now, no long-term clinical trials are available in the literature that can give information about the clinical bond durability of this adhesive.

In non-carious Class-V lesions, the major part of the bonded tooth surface consists of dentin, while only at the incisal side the adhesive restorative material is bonded to enamel. Literature so far indicates that the most durable bond to enamel is obtained following an etch-and-rinse approach, signifying that the distinct enamel etch pattern created by phosphoric acid-etching is most important to achieve a durable bond to enamel . In contrast, Clearfil SE, produces a very mild superficial etch pattern . Although some studies reported a bond strength to ground enamel similar to that of etch-and-rinse adhesives , in most studies the bonding effectiveness to enamel was significantly lower . Even though in vitro durability studies showed a stable strength to enamel over time , the marginal integrity of Clearfil SE to enamel deteriorated resulting in increased microleakage . Selective etching of enamel with phosphoric acid before application of the adhesive has been proposed to improve the durability of the enamel bond . In 2000, we started a Class-V clinical trial to evaluate the effect of adjunctive/selective enamel etching on the clinical performance of the restorations . After 5 years, we observed a significant higher number of small marginal defects at the enamel side in the non-etch group compared to the etch group . We decided to continue the follow-up of the study for a longer period to determine if this marginal deterioration will become worse with time and will negatively influence the clinical performance of the restorations.

Therefore the objective of this randomized clinical trial was to evaluate the clinical performance of the mild 2-step self-etch adhesive, Clearfil SE, in non-carious Class-V lesions after 8 years of clinical functioning. The hypothesis tested was that selective enamel etching with phosphoric acid had a significant influence on the clinical performance of Class-V restorations.