Finite Element Study on: Zirconia Overlay Restorations Made with Different Tooth Preparation Designs

Background : This study aimed to compare and evaluate the effect of different tooth overlay preparation designs on deformation and stress distribution under average masticatory load. Methods : Four three-dimensional (3D) finite element models were specially created to simulate the clinical cases of using four different overlay restoration designs as: Model 1 (ACC) anatomical cusp reduction with chamfer finishing line. Model 2 (FCC) flat cusp with chamfer finishing line. Model 3 (ACS) anatomical cusp reduction with proximal steps. Model 4 (FCS) flat cusp reduction with proximal steps were tested. All models were restored with translucent CAD/CAM zirconia crowns. Geometries were obtained via laser scanning, then manipulated by Boolean operation to finalize the models. All models were subjected to vertical loading of 600N compressive load located at central fossa. Results : variation in deformation results did not exceed six microns between anatomic and flat cusp reduction with chamfer finish line, and also between anatomic and flat cusp reduction with proximal steps. Mucosa and spongy bone "based on stress values" were not sensitive to crown type or its preparation. On the other hand, FCC showed better behavior in comparison to ACC. Conclusions : Flat cusp designs showed better behavior by showing slight less stresses on crown body, cement layer and cortical bone. In addition, ACS, FCS designs exerts less stresses on remaining tooth. Therefore, in case of availability of the four preparations, the ACS, FCS design will be preferred, and flat cusp preparation might showed slightly better behavior.