Biomechanical Response of MOD Class II Restorations Considering Polymerization Shrinkage and Transient Occlusal Loading: A Finite Element Study

Class II mesio-occluso-distal (MOD) restorations are routinely used to rehabilitate molars with extensive structural loss, where cavity bases are often employed to protect dentin and modulate stress transfer. However, polymer-based restorative materials undergo polymerization shrinkage during curing, generating residual stresses that are subsequently superimposed on functional occlusal loads during mastication. The interaction between these two stress fields is rarely considered explicitly in finite element studies of MOD restorations. This study evaluates the biomechanical influence of cavity base type and the absence of a cavity base in Class II MOD restorations by means of three-dimensional finite element analysis (3D-FEA). A three-dimensional human molar model was reconstructed from cone-beam computed tomography (CBCT) data. A transient occlusal loading cycle from 0 to 0.25 s was simulated, with a maximum load of 600 N applied at 0.15 s. Restorations with cavity bases were modeled using combinations of ceramic and glass–ceramic restorative materials with different base materials, whereas restorations without cavity bases were modeled using reinforced composite resins with different silica contents. Mechanical behavior was assessed through von Mises stress and maximum principal stress. The results show that stress distribution and stress magnitude depend on the interaction among restorative material properties, cavity base configuration, and the overall stiffness of the system. In particular, certain reinforced composite configurations without cavity bases exhibited a more homogeneous stress distribution and a narrower stress range than some multilayer configurations. However, this behavior should not be generalized to all cavity-base-free restorations, as it depends on the specific mechanical properties of the evaluated materials and the structural configuration of the restorative system. These findings highlight the importance of jointly considering polymerization shrinkage and transient occlusal loading in the biomechanical assessment of MOD restorations, and suggest that, under specific conditions, optimization of restorative material properties may be as relevant as the incorporation of intermediate layers.