Optimizing Cavity Shape with Margin Elevation via Numerical Fatigue Analysis

Objectives: The purpose of this study was to evaluate the durability of class II cavities restored with deep margin elevation (DME) procedures and optimize the cavity shape via a series of parametric studies.
Methods: A three-dimensional model of the intact premolar was reconstructed from CT images. The inlay cavity designs were defined by seven parameters, including the length (L_i), depth (L_p), and width (W) of the isthmus, the length (L_p) and depth (D_p) of the proximal box, the open angle of the cavity (A), and the thickness of DME (Le). The design of experiments for the seven parameters was performed using L₁₈ orthogonal arrays. Two materials of the inlay, Lithium Disilicate (L) and feldspathic porcelain (P), were tested. The indenter vertically applied a compressive force of 700 N to the tooth. All design points were solved using the software ANSYS. The main effect of the seven parameters on the service life of adhesive was evaluated by the Taguchi method, and the optimal parameter configurations were obtained.
Results: This study predicted the service life using the S-N curve of adhesive. For both materials, the results showed that L_p (P: 9.305 %, L: 12.93 %) and W (P: 82.45 %, L: 70.06 %) were the major factors that significantly affected the service life of adhesive. Also, the longer L_p (3 mm) combined with the wider W (2.4 mm) obtained the lowest failure risk, which led to a longest life of adhesive. The other parameters showed an insignificant effect on the service lift of adhesive.
Conclusions: The long-term stability can be achieved by reducing the amount of tooth preparation. Based on the findings of the present study, when one designs the cavity preparation clinically, the size selection of L_p and W should be more cautious and removal of tooth structure should be limited to the minimum. More importantly, the thickness of DME seemed not to influence the mechanical behavior of the structure.