This study compares the mechanical strengths of bulk-fill composite resin and amalgam material to investigate the stress distribution and capacity to mitigate stress of restored Class I and Class II teeth under chewing loads, using finite element analysis. A 3D model of a human mandibular first molar and four Class I (C1) and Class II (C2) caries, including 95-degree cavity-margin angles, were created. Different material combinations were simulated: model C1-A and C2-A, with an amalgam material; and model C1-C and C2-C, with a bulk-fill composite resin. Solid 3D elements with four grid points were employed for modeling the tooth. A vertical occlusal load of 600 N was applied, and nodal displacements on the bottom cutting surfaces were constrained in all directions. All materials were assumed to be isotropic and elastic, and a static linear analysis was performed. The highest maximum principal stress was observed in C2-C, followed by C1-C, C2-A, and C1-A, respectively. The maximum principal stress load on the lingual cusp was recorded at the junction of the lingual margin (C1-C and C2-C), and stress was recorded on the line of restoration and enamel (C1-A and C2-A). Restoration materials and cavity preparations influence the stress distribution at the restoration-tooth interface and, consequently, the measured bond strength.
