Does a low-shrinking composite induce less stress at the adhesive interface?

Objectives Polymerization shrinkage of a composite in a high configuration-factor (C-factor) cavity leads to polymerization shrinkage stress. In the prevention of high polymerization shrinkage stress, a low-shrinking silorane composite (Filtek Silorane, FS, 3M ESPE) was recently marketed. Therefore, we investigated the effect of C-factor (flat surface vs. class-I cavity) and different composite application protocols on the bonding effectiveness of a low-shrinking composite to human dentin. Methods A low-shrinking silorane-based composite (FS) and a conventional methacrylate-based composite (Filtek Z100, 3M-ESPE) were bonded to standardized occlusal class-I cavities (4 mm x 4 mm x 2.5 mm) and to flat mid-coronal dentin surfaces using the two-step self-etch adhesive 'Silorane System Adhesive' (3M-ESPE). Eight experimental groups were formed, according to the following treatment protocols: Z100 'flat'(1), Z100 'cavity'(2), FS 'flat'(3), FS 'flat/bulk-filled'(4), FS 'cavity/bulk-filled'(5), FS 'cavity/layered-filled'(6), FS 'cavity/flowable cured'(7) and FS 'cavity/flowable uncured'(8). For each group, at least 5 teeth were used. After 1 week of water storage, the teeth were sectioned to 1 mm x 1 mm (non-trimmed) sticks to measure the micro-tensile bond strength (microTBS). Results No statistical difference in microTBS was recorded between Z100 'flat'(1) and FS 'flat'(3). Bonding to class-I cavity dentin lowered the microTBS in all groups, but this was only statistically significant for FS 'cavity/bulk-filled'(5) and FS 'cavity/flowable uncured'(8). Significance Using the two-step self-etch Silorane System Adhesive, the conventional composite Filtek Z100 and the low-shrinking composite Filtek Silorane bonded equally well to dentin. Bulk-filling with Filtek Silorane (FS 'flat/bulk-filled'(4)) significantly decreased the microTBS, suggesting that factors other than polymerization shrinkage influenced the microTBS.