Method: Abutment tooth models made of composite resin (E-modulus of 11 GPa and Poisson ratio of 0.43) with specified chamfer widths and occlusal reductions were prepared. Sixty monolithic zirconia crowns (Lava Zirconia Plus, 3M ESPE, USA) and 6 lithium disilicate crowns (IPS emax press, Ivoclar/Vivadent, Germany) were manufactured using a dental CAD/CAM system and a press sintering technique, respectively. The crown thickness was measured using a micro-CT. Then, the crowns were cemented onto the abutments with 20 N pressure. After water storage, the crowns were loaded until fracture. Based on the measurement of crown thickness and the fracture load, multiple regression analysis was performed.
Result: It was revealed that the occlusal thickness significantly affected the fracture load (p<0.01) but the axial thickness did not (p=0.2828). According to the multiple regression analysis, an increase in occlusal thickness with 1 mm contributes to an augmentation of the fracture load with 3465 N. Although the reduction of occlusal thickness decreased the fracture resistance of monolithic zirconia crown, the fracture load of the zirconia crowns with the occlusal thickness of 0.5 mm (5558±522 N) was significantly higher than that of lithium disilicate crowns with an occlusal thickness of 1.5 mm (3147±409 N).
Conclusion: Within the limitations of the present study, it is suggested that monolithic zirconia crown with chamfer width of 0.5 mm and occlusal thickness of 0.5 mm can be used in the molar region in terms of fracture resistance.