Influence of restorative technique on the fracture resistance of molar teeth restored with composite overlays

Aim: To evaluate the resistance to load of maxillary molars restored with composite overlays manufactured with three different techniques, namely the traditional indirect technique, a chairside semi-direct technique, and a CAD/CAM technique. Methods: Sample size was calculated considering the results obtained in preliminary experimental studies (α=0.05; β=0.20; δ =500.0; σ =300.0). Eighteen sound maxillary molars of comparable size were selected from a pool of freshly extracted teeth and received a standardized preparation for a full-coverage overlay with two proximal boxes. The prepared teeth were randomly divided into three experimental groups of 8 elements each: traditional indirect technique with polyvinyl siloxane impression and stratification on a stone model; chairside semi-direct technique with alginate impression and stratification on a silicon model; CAD/CAM technique with digital impression and production with the Cerec 3 system. All tested materials were resin-based composites of the same brand (Voco). For the CAD/CAM production process, a custom-designed phantom model was used. The cementation protocol was the same for all groups and involved the use of sandblasting, self-etch adhesive procedures with selective enamel etching, and dual cure resin cement. All the restored teeth underwent thermomechanical aging (1,250,000 cycles, 1Hz, 5-55°C); afterwards, they were axially loaded to fracture with a universal testing machine with a round-head stainless steel stylus. The type of fracture was registered. The groups were compared in terms of maximum load to fracture with a one-way analysis of variance and Scheffè post-hoc test (p<0.05). Results: No defects were appreciable on the occlusal surface of the specimens after the thermomechanical aging. In all the tested groups, the mean registered values of maximum load exceeded the threshold of the masticatory forces (≈800 N). The CAD/CAM technique exhibited the greatest resistance values (2136.2±295.1 N). The traditional indirect technique (1599.8±252.8 N) and the chairside semi-direct technique obtained significantly lower resistance values (1472.9±574.8 N), which were statistically similar. The most prevalent type of failure was fracture with fragment detachment below the cemento-enamel junction. Conclusion: Under the conditions of the present study, all the tested restorative techniques showed satisfactory performance in terms of resistance to load. In the comparison among groups, composite CAD/CAM overlay.