Biomechanical performance of molars restored with 3D-printed composite crowns with different root-canal retentions

The aim of this in vitro study was to evaluate the effects of root-canal retention combined with core build-up, fiberglass posts (FGPs), and endocrown retention on the fatigue survival rate, fracture resistance, and stress distribution of 3D-printed resin composite crowns (3D-RC crowns) in endodontically treated molars. Sixty human molars were randomly assigned to four groups: FGP, core build-up and 3D-RC crown (CcFGP); core build-up without an FGP and 3D-RC crown (Cc); 3D-RC endocrown restoration (EndC); and sound teeth (SD). The specimens were subjected to accelerated fatigue testing until failure or up to a maximum load of 2800 N (161,000 cycles). The surviving specimens were then subjected to static load-to-failure tests. A 3D finite element analysis (FEA) was performed, and the stress was evaluated using the modified von Mises criterion. Groups were compared via the Kaplan-Meier survival analysis log-rank test and pairwise post hoc comparisons, along with life table analysis for load-to-failure followed by Wilcoxon pairwise comparison (α = 0.05). No statistically detectable difference was observed among the tested groups in terms of the number of fatigue cycles or fracture load under the evaluated conditions. Fracture strength and failure mode were not influenced by the restorative technique. Most failures were restricted to the 3D-RC crowns or core build-up and were considered repairable. FEA showed comparable stress magnitudes among the restorative groups. Core build-up, FGP, and endocrown retention techniques exhibited similar fatigue survival, fracture strength, and failure modes. Although 3D-RC crowns showed favorable biomechanical performance for restoring endodontically treated molars without a ferrule, further clinical studies are required to confirm these results.