Ultrastructural correlates of in vivo/in vitro bond degradation in self-etch adhesives

Objective: The purpose of this study was to evaluate the bond durability and interfacial morphology of two self-etch adhesive systems aged in the laboratory and in the oral nvironment for 1 year. Materials and Methods: The adhesives tested in this study were Clearfil SE Bond and Clearfil Protect Bond (both from Kuraray, Tokyo, Japan). Both systems include a water- and ethanol-based self-etching primer and a filled bonding agent. The chemistry of the two systems is similar except that Protect Bond contains fluoride and a modified antibacterial monomer. Volunteer subjects with one or more erupted, unrestored third molars scheduled for extraction were included in the study. Standard Class I preparations (3 mm in depth) were made in the teeth and restored under rubber dam isolation. The preparations were restored using one of the adhesives, followed by a thin layer of a flowable resin liner and two increments of a hybrid restorative composite. The restored teeth were extracted following either 24 hours or 1 year. After extraction, each tooth was sectioned into 0.9 ¥ 0.9 mm beams for microtensile testing. Bond testing was done using a universal testing machine. Failure modes were examined at 30¥ magnification and were classified as adhesive, cohesive, or mixed. In vitro specimens were prepared and restored using extracted third molars. These were stored in artificial saliva at 37C for either 24 hours or 1 year. Microtensile specimens were obtained and tested in the same manner as the in vivo specimens. In addition to the microtensile specimens, other specimens were evaluated using transmission electron microscopy (TEM). The TEM was used specifically to examine nanoleakage at the resindentin interfaces. Results: For Clearfil SE Bond, the mean in vitro bond strengths were 33.9 MPa at 24 hours, and 21.4 MPa at 1 year. The corresponding means for in vivo specimens were 21.3 and 13.8 MPa. For Clearfil Protect Bond, the in vivo mean at 24 hours was 17.9 MPa, and the mean at 1 year was 18.6. In vitro, the respective means were 28.1 and 28.3 MPa. The proportion of adhesive failures for both materials was greater at 1 year than at 24 hours. Interfacial "water trees" were observed at 1 year but not at 24 hours. These channels were more extensive in Clearfil SE Bond and might have been formed by slow water sorption through the adhesives that expedited leaching of hydrolytic resin compounds. Conclusions: Degradation of self-etch adhesives has a similar mechanism in vitro and in vivo.