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1002 the specimens that survived specimen processing with an explicit note of the number of pre-testing failures. Study design All specimens were randomly divided into nine groups (3 adhesives!3 experimental groups, Fig. 1) and subjected to a bonding treatment strictly according to the respective manufacturer’s instructions (Table 1). After adhesive procedures, all teeth were stored in water for 24 h at 37 8C. For each adhesive, three teeth were subjected to 20,000 thermal cycles (group 1: thermo-cycling/cavity), i.e. the restored cavity was changed between two water baths of 5 and 55 8C with a dwell time of 30 s at each temperature extreme (Thermocycler, Willytec, Munich, Germany). From six other teeth per adhesive, four mTBS specimens (per tooth) were prepared. Two of these specimens were also subjected to the same thermo-cycling regimen (group 2: thermo-cycling/ stick). The other half of these specimens were stored for 20 days, the time needed for the thermo-cycling procedure, in 100% humidity to serve as control (group 3: control). Statistical analysis The results were analyzed at a significance level of 0.05 using a two-way ANOVA and post hoc Tukey– Kramer multiple comparisons. All statistics were performed using the statistical software package (StatSoft, Tulsa, OK, USA). Failure analysis The mode of failure was determined light-microscopically at a magnification of 50! using a stereomicroscope, and recorded as either ‘failure within dentin’, ‘interfacial failure’ or ‘failure within resin’. From each group, representative mTBS-specimens were processed for field-emission gun scanning electron microscopy (Feg-SEM, Philips XL30, Eindhoven, The Netherlands) using common electron microscopic specimen processing techniques including fixation, dehydration, chemical drying, and gold-sputter coating [12]. Results The mean mTBS, SDs, the number of pre-testing failures (ptf) and the total number of specimens (n) are summarized per adhesive and experimental condition in Table 2, and graphically presented in box-whisker plots in Fig. 2. Thermo-cycling of neither the mTBS specimens, nor the restored cavities decreased the bond strength of the adhesives tested. Pre-testing failures were only recorded for the one-step self-etch adhesive (iBOND). All pre-testing failures occurred during specimen processing (mostly during preparation of the sticks with the diamond saw). No additional pre-testing failures were produced by thermo-cycling. Because of the high number of pre-testing failures, the data of iBOND were excluded from the statistical analysis, as too few valid data were available to perform an adequate analysis and thus to draw a valid conclusion regarding degradation of this adhesive. The two-way ANOVA analysis disclosed no significant difference in mTBS between OptiBond FL and Clearfil Protect Bond (pZ0.321), nor between the different experimental conditions (control, thermo-cycling/cavity and thermo-cycling/stick; pZ0.111). The bonding effectiveness of the onestep self-etch adhesive tested, iBOND, was however already compromised at baseline, given the high number of pre-testing failures (Table 2). For none of the adhesives, were morphological changes induced by thermo-cycling observed using light-microscopy (Table 3) or Feg-SEM (Fig. 3–5) of the fracture surfaces. For the one-step self-etch adhesive, most specimens failed within the resin (Table 3; Fig. 5). Especially in the areas that fractured very close (a few mm) to the interface, the resin appeared very porous, and at higher magnifications many porosities could be noticed. The porosity amount and density was clearly higher in the area near to the interface with dentin, but also in the adhesive resin itself, some larger porosities could be observed (Fig. 5). Table 2 mTBS to dentin. mTBS (SD) ptf/n Control no thermocycling Thermo-cycling (20,000 cycles) Cavity Stick OptiBond FL 20.0 (3.6) 27.0 (11.5) 18.3 (9.8) 0/11 0/11 0/13 Protect Bond J. De Munck et al. 23.8 (8.3) 24.7 (9.9) 23.1 (7.5) 0/11 0/14 0/12 iBOND 14.7 (11.9) 12.1 (4.9) 12.6 (3.8) 6/9 8/12 11/20 mTBS, micro-tensile bond strength, value in MPa; ptf, pretesting failure; n, total number of specimens; SD, standard deviation.
Micro-tensile bond strength of adhesives bonded to class-I cavity-bottom dentin after thermo-cycling 1003 Figure 2 mTBS after thermo-cycling. The box represents the spreading of the data between the first and third quartile. The central vertical line represents the median. The whiskers denote the range of variance and outliers are represented by a dot. Discussion The hypothesis that thermo-cycling of restored occlusal class-I cavities (repetitive contraction/ expansion stress) as well as of mTBS sticks (diffusion-dependent hydrolysis and elution) does not decrease mTBS was confirmed, as for none of the adhesives a decreased mTBS was recorded after thermo-cycling (20,000 cycles). Because of the specific study design, in which thermo-cycled as well as non-thermo-cycled sticks originated from the same teeth (Fig. 1), an additional paired analysis was carried out to compare the control and thermo-cycling/stick group. This analysis is more powerful than the standard ANOVA analysis, because the variable ‘tooth’ was statistically excluded. Also using this more powerful analysis, no significant effect of thermo-cycling was recorded for OptiBond FL (pZ0.68), nor for Clearfil Protect Bond (pZ0.8624). Storage of small mTBS specimens in water for relatively short periods (3 months and longer) can significantly reduce the mTBS [13]. Given the long time needed to implement the thermo-cycling Table 3 Failure analysis under the light microscope. Experimental group Interfacial Mixed failure failure a Failure in Total (n) resin OptiBond FL Control 1 5 5 11 Thermo-cycling Cavity 2 5 4 11 Stick 4 7 2 13 Protect Bond Control 0 6 5 11 Thermo-cycling Stick 1 5 8 14 Cavity 0 8 4 12 1 3 Thermo-cycling Stick 0 3 b 1 4 Cavity 1 b 5 b 3 9 iBOND Control 0 2 b a Mixed failure, interfacial failure and failure within resin. b Feg-SEM evaluation revealed that some interfacially failed areas, actually failed within resin (Fig. 5).
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