Influence of restorative technique on the fracture resistance of molar teeth restored with composite overlays after receiving cervical margin relocation

Aim: To evaluate the resistance to load of maxillary molars restored with composite overlays manufactured with a traditional indirect technique, a chairside semidirect technique, and a CAD/CAM technique after receiving cervical margin relocation (CMR). Methods: Sample size was calculated on the basis of the results obtained in preliminary experimental studies (α=0.05; β=0.20; δ=500.0; σ=300.0). Thirty 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 a 2mm proximal box below the cementoenamel junction (CEJ). All the boxes were restored with a composite resin CMR 1mm above the CEJ. The prepared teeth were randomly divided into three experimental groups of 10 elements each: traditional indirect technique with polyvinyl siloxane impression and stratification on a stone model; chairside semidirect technique with alginate impression and stratification on a silicon model; CAD/CAM technique with digital impression and milling with the Cerec 3 system. The same brand of resin-based composites (Voco) was used for the study. Moreover, a customdesigned phantom model was used for the CAD/CAM impression procedures. The cementation protocol involving sandblasting, self-etch adhesive procedures with selective enamel etching, and dual- cure resin cement was the same for all groups. The restored teeth underwent thermomechanical aging (1,250,000 cycles, 1Hz, 5-55°C) and then were axially loaded to fracture by using a universal testing machine with a round-head stainless steel stylus. The type of fracture was analyzed. A one-way analysis of variance and Scheffè post-hoc test (p<0.05) were used to compare the maximum load to fracture among the groups. Results: The specimens showed no detectable defects on the occlusal surface after the thermomechanical aging. The maximum load values exceeded the threshold of the masticatory forces (~ 800 N) in all the tested groups. The greatest resistance values were found in the CAD/CAM technique group (1807,60±511,61 N); the traditional indirect technique (1699,18±399,56 N) and the chairside semi-direct technique (1386,28± 434,95 N) showed a trend towards lower resistance values without any statistical difference. A fracture below the cemento-enamel junction was the most frequent type of failure. Conclusion: Under the limitations of this study, all the tested restorative techniques showed a good performance in terms of load resistance. The composite CAD/CAM overlay seems to offer the greater resistance in comparison to the considered indirect and semi-direct techniques. The CMR does not reduce the fracture resistance under the threshold of the masticatory forces.