The effects of cavity-margin-angles and bolus stiffness on the mechanical behavior of indirect resin composite class II restorations

Objective. To study the influence of the different class II mesio-occlusal-distal (MOD) cavityshape on the stress and strain distributions in adhesive indirect restorations, using numer-ical finite element analysis (FEA). To investigate the relationship between restored teethfailure and stiffness of food, three values of Young’s modulus were used for the food.Methods. A 3D model of a sound lower molar and three class II MOD cavities with differ-ent shape were created. Slide-type contact elements were used between tooth surface andfood. An adhesive resin-based cement, modeled with fixed-type contact elements, and asingle restorative filling materials were considered. To simulate polymerization shrinkageeffect, which is basically restricted to the thin composite cement layer, shell elements wereemployed and the thermal expansion approach was used. A vertical occlusal load of 600 Nwas applied, while assigning fixed zero-displacements on the cutting surfaces below thecrevices. All the materials were assumed to be isotropic and elastic. A static linear analysiswas carried out.Results. In the lingual cusp, the displacements increased as the values of the stiffnessfood increased. In the restored teeth, the stress near the restoration-tooth interface wasstrongly dependent on the MOD cavity shape. The stress peaks were mainly located alongthe enamel–dentin interface at the lingual side; wedge-shaped MOD cavity with a low angle,in combination with the lowest food stiffness provided the best results.Significance. A more complex load application on the occlusal surfaces was introduced. Foodstiffness slightly affected the stress distribution of the restored and sound teeth. Teeth withadhesive class II MOD indirect resin composite restorations were potentially more suscep-tible to damage if the class II MOD cavity-margin-angle was higher than 95◦. Restored teethwith a higher cavity-margin-angle led to considerable stress concentration in the lingual cusp along the enamel–dentin interface. These models were more susceptible to fracture in the lingual cusps when compared to the buccal ones.