Low-shrinking composites have recently been introduced, but the effect of using them in constrained conditions is still unclear. Objectives: To investigate the effect of curing time and filling method on the micro-tensile bond strength (µTBS) to cavity-bottom dentin in occlusal Class-I cavities. Methods: Two low-shrinking composites (Kalore, GC; N'Durance, Septodont) and a conventional composite (Filtek Z100, 3M-ESPE) were bonded into standardized occlusal Class-I cavities(4.2x4.2x2.5mm) using the one-step self-etch adhesive G-Bond (GC). The cavity was filled in bulk or in 3 separately cured layers. Bulk-filled cavities were cured for 20sec, as recommended by the manufacturer, or for 80 sec. After 1-week water storage, teeth were sectioned to 1x1 mm sticks for µTBS.
Results:
µTBS (MPa) | Failure analysis (%) | ||||||
Composite | Group | µTBS (SD) | Ptf/n/N | Substrate | Interface | Bond | Composite |
Kalore | 20s/Bulk | 17.4a,b,c (13.4) | 0/20/5 | 4 | 35 | 36 | 25 |
80s/Bulk | 14.1a,b,c (7.8) | 4/18/5 | 0 | 39 | 51 | 10 | |
80s/Layers | 21.4a,b (6.8) | 0/21/5 | 3 | 2 | 27 | 68 | |
N'Durance | 20s/Bulk | 17.1a,b,c (10.4) | 0/22/5 | 0 | 1 | 90 | 9 |
80s/Bulk | 21.5a,b (8.5) | 6/20/5 | 7 | 4 | 70 | 19 | |
80s/Layers | 24.4a (11.2) | 0/20/5 | 0 | 9 | 54 | 37 | |
Z100 | 20s/Bulk | 8.3 c (1.9) | 10/18/5 | 0 | 50 | 50 | 0 |
80s/Bulk | 11.8b,c (5.1) | 7/21/5 | 0 | 50 | 50 | 0 | |
80s/Layers | 20.7a,b,c (11.0) | 4/21/5 | 1 | 34 | 38 | 27 | |
LM=Light microscopy; SD=Standard deviation; ptf=pre-testing failures; n=number of specimens; N=number of teeth; Means with the same superscript are not significantly different (Tukey HSD, p>0.05); ptf's were excluded from further analysis.
Two-way ANOVA revealed that composite as well as application methodology affected the µTBS (both p<0.0001). While no significant difference was observed between 20- and 80-sec bulk-curing, a significantly higher µTBS was observed for the layered groups (p=0.0001).
Conclusions: Overall, the conventional composite bonded least effective in constrained conditions and bond strength dropped by about 50% when applied and cured in bulk. Low-shrinking composites were less affected, though a combination of increased curing time and layered application consistently revealed the most favourable results.