Fracture Strength of CAD/CAM-Generated Composite Resin Molar Crowns

Objectives: Composite resin crowns have a relatively low cost and are indistinguishable from natural teeth; however, the fracture resistance of the composite resin must be considered for use in molar crowns. The aim of this study was to investigate whether different fabrication processes for composite resin molar crowns, i.e., a CAD/CAM system or the manual build-up technique, affect the fracture strength.
Methods: Flexural strength and fracture toughness of stick-shaped samples of composite resins [Lava Ultimate (LU: 3M/ESPE) and Estenia C&B (EC&B: Kuraray Noritake Dental)] and lithium disilicate glass-ceramic IPS e.max press (e.max: Ivoclar/Vivadent) were evaluated. For the fracture test, 4 types of molar crowns were fabricated: 1) CAD/CAM-generated composite resin crowns (LU crowns), 2) manually built-up monolayer composite resin crowns (EC&B-monolayer crowns), 3) manually built-up layered composite resin crowns (EC&B-layered crowns), and 4) e.max crowns. The internal voids in each crown were analyzed by micro-CT. Each type of molar crown was cemented to abutment models and the fracture strength was tested.
Results: For the stick-shaped samples, there was no significant difference in flexural strength or fracture toughness between LU and EC&B, whereas e.max showed significantly higher values (P<0.05). EC&B-layered crowns showed significantly lower fracture resistance than LU and EC&B-monolayer crowns. The micro-CT analysis showed that decreased strength likely resulted from internal voids in the EC&B-layered crowns introduced by the layering process. There was no significant difference in fracture strength among LU, EC&B-monolayer and e.max crowns. Both types of composite resin crowns showed fracture loads >2,000 N, which is higher than the molar bite force.
Conclusions: Composite resin crowns, especially CAD/CAM-generated crowns without internal defects, could be applied to molar regions with sufficient fracture resistance to static loading.