The effect of marginal ridge thickness on the fracture resistance of endodontically-treated, composite restored maxillary premolars.

This study evaluated the effect of varying thicknesses of marginal ridge on the fracture resistance of endodontically-treated maxillary premolars restored with composite. Ninety non-carious maxillary premolars, extracted for orthodontic reasons, were selected for this experimental in vitro study. The teeth were randomly assigned to six groups (n=15). Group 1 received no preparation. In groups 2 through 6, the premolars were root filled and DO preparations were created, while MOD preparations were also created for group 2. The condition of the boxes was: the gingival seat was 1.5 mm above the CEJ and the buccolingual dimensions were 3.5 mm in gingival and 3 mm in occlusal. In groups 3 through 6, the dimensions of the mesial marginal ridge were measured using a digital caliper as follows: 2 mm, 1.5 mm, 1 mm and 0.5 mm, respectively. All samples in groups 2 through 6 were restored with a dentin bonding system (DBS: Single Bond, 3M) and resin composite (Z 250, 3M). Subsequently, premolars from all six groups were subjected to a thermocycling regimen of 500 cycles between 5 degrees C and 55oC water baths. Dwell time was 30 seconds, with a 10-second transfer time between baths. The premolars were submitted to axial compression up to failure at a 45 degrees angle to the palatal cusp in Universal Test Equipment (Tinius Olsen, Ltd, H5K-S model). The mean load necessary to fracture the samples was recorded in newtons (N), and data were subjected to analysis of variance (ANOVA) and LSD post-hoc test. According to these results, the mean loads necessary to fracture the samples in each group were (in N): group 1: 732 +/- 239, group 2:489 +/- 149, group 3: 723 +/- 147, group 4: 696 +/- 118, group 5: 654 +/- 183 and group 6: 506 +/- 192). Differences between group 1 and groups 2 and 6, and also differences between groups 3, 4 and 5 compared with group 2 and 6 were statistically significant (p < 0.05).