The effect of loading direction and restorative material on stress distribution in endodontically treated maxillary molars restored with endocrowns: a three-dimensional finite element analysis

This study aimed to evaluate the effects of different computer-aided design/computer-aided manufacturing (CAD/CAM) and 3D-printed endocrown restorative materials on stress distribution in endodontically treated maxillary molars under vertical and oblique loading conditions using three-dimensional finite element (FE) analysis. A three-dimensional FE model of a sound human maxillary first molar was developed for biomechanical evaluation. Six endocrown models were generated using different restorative materials while maintaining identical geometry. The standardized endocrown preparation included rounded internal line angles, 10° internal wall divergence, and a 3-mm pulp chamber depth. The evaluated groups included CAD/CAM milled materials and 3D-printed resin composites. In the first scenario, a vertical static load of 200 N was applied parallel to the long axis of the tooth on the buccal and lingual cusp tips. In the second scenario, an oblique static load of 200 N was applied at 45° to the long axis of the tooth on the lingually inclined plane of the buccal cusp. The biomechanical response of the tooth–endocrown complex was evaluated by extracting peak stress and deformation values from the FE models. All reported values correspond to the maximum nodal stress observed in each component and are expressed in megapascals (MPa). Under vertical loading, all models exhibited relatively low and symmetrical stress distributions along the long axis of the tooth. Oblique loading generated higher stress concentrations, particularly at the cervical tooth structures and adhesive interface, with stress distribution patterns varying according to material stiffness. Higher stress concentrations within the resin cement layer were observed in materials with lower elastic modulus. Materials with higher elastic modulus showed greater stress concentration within the restoration, whereas materials with lower elastic modulus increased stress transfer toward the adhesive interface and cervical tooth tissues. Material stiffness influenced stress localization under oblique loading. Materials with higher elastic modulus concentrated stress within the restoration and reduced cement layer stresses, whereas materials with lower elastic modulus shifted stress toward the adhesive interface and cervical tooth structures.