Objectives The aim of this study was to evaluate the influence of inlays/onlays with or without pulp extension from different materials on stress distribution in endodontically treated molars by three-dimensional finite element analysis (3D FEA). Materials and Methods We used 3D mandibular molar models in this study. The models represented mesio-occluso-distal (MOD) cavities restored by inlays, onlays that covered buccal cusps, and onlays that covered all cusps with pulp extension (modified inlay/onlay) or without pulp extension (conventional inlay/onlay). Three materials (L: lithium disilicate glass-ceramic, P: polymer-infiltrated ceramic network [PICN], and C: nanofilled composite resin) were utilized. A force of 600 N was applied vertically and obliquely. Stress distribution in FEA models was analyzed using the von Mises theory. Results The results revealed that an oblique load generated higher stresses than vertical load. Composite resin restorations transmitted almost all the stress to the neighboring tooth structures, while lithium disilicate ceramic restorations absorbed most of the stresses. Moreover, modified inlays and onlays with pulp extension proved better than conventional inlays/onlays in terms of stress redistribution in dental structures. Onlays showed a better pattern of stress distribution than inlays within the restoration and the restored tooth. Conclusions According to stress distribution in dental structures, modified lithium disilicate ceramic onlays with pulp extension have been found to be the best choice to restore endodontically treated molars among the studied restorations.

Background: This study aimed to evaluate the influence of inlays/onlays and their material on stress distribution in mandibular molars with large cavities, using finite element analysis (FEA). Methods: 3D models of the first mandibular molar were created. Then, a mesio-occluso-distal cavity was created, and cusps were reduced (1.5 mm for buccal cusps and 1 mm for lingual cusps). The restorations were: inlay, onlay that covered buccal cusps (B models), and onlay that covered all cusps (LB models). Inlays and onlays were represented by two materials: nanofill composite resin and polymer-infiltrated ceramic network (PICN). Vertical load of 600 N was applied and von Mises stresses were calculated. Results: Inlay models showed higher stress concentration in tooth structures than onlay models. Composite resin inlays and onlays transmitted most of the stress to adjacent structures. On the other hand, PICN inlays/onlays absorbed most of stress and transmitted less stress to dental structures than composite resin restorations. Moreover, stress concentrations in PICN onlay models (B-buccal cusps and LB-all cusps) were similar, while composite resin LB onlay showed higher stress concentration in dental structures than composite resin B onlay. Conclusions: Onlays showed better stress distribution than inlays. PICN might be a suitable choice as a restorative material of inlay/onlay for large cavities in molars, while the composite resin is unfavorable material for such restorations in terms of stress redistribution in dental structures.