Effect of Stiffness of Cement on Stress Distribution in Ceramic Crowns.

Objective: To analyse the stress distribution in monolithic- and bilayer-structured ceramic crowns by means of the finite element method (FEM), as a function of elastic modulus of the core ceramic, Ecor, and that of the cement used to lute the crown, Ecem, with a view to identifying an ideal stiffness for the cement.

Methods: A two-dimensional axisymmetric FEM model was created to represent tooth structure with a cemented ceramic crown in place. The value of Ecor was set at 70, 100, 150 and 200  GPa representative of the range of commercially available materials. For the veneer, Even, it was set at 70 GPa, while that of the cement, Ecem, was varied from 0.2 to 200  GPa, in a 1-2-5 sequence. The tensile stress along the x-direction was calculated as an indication of the local sensitivity of the model to failure at a given load.

Results: The stiffness of both the core ceramic and of the cement strongly affected the tensile stress distribution. With an increase in Ecor, the stress was increased for low Ecem. Also, the stress in the cement tended to increase with an increase in Ecem. However, the stress in the dentine varied little over the ranges studied here. For Ecor > Ecem, the stress in the core for low Ecem was higher than for high Ecem.

Conclusion: It is suggested that the modulus of elasticity for the cement used to lute the ceramic crown plays a critical role in improving the fracture resistance of ceramic restorations.