Effect of Wall Thickness and Support on Fired CAD/CAM Restortations

Objective: To determine if proper support during crystallization and crown wall thickness have an effect on the fit of Lithium Disilicate CAD/CAM restorations.

Method: A second molar model was scanned with Cerec InLab software. Four crown groups (N=10) were milled from e.max lithium disilicate: two groups were milled with the recommended wall thickness (Tk) and two groups with a reduced wall thickness (Tn). The crowns were ultrasonicated in distilled water for 10 minutes. One thick and one thin group were fired with a proper support (P) using Object-Fix flow in a Progromat P500 porcelain furnace using the manufacturer’s instructions. The remaining twenty crowns were fired utilizing the same firing program with an under-supported (U) deposit of Object-Fix on SiN firing pins to keep the restoration stable. Following crystallization, crowns were cleaned ultrasonically. The initially scanned model was replicated using FujiRock, and each crown was cemented to a prep using SpeedCEM using finger pressure then cured according to the manufacturer’s instructions. All specimens were embedded in clear acrylic, and cut buccal-lingually along the occlusal fossa to expose cement film. Cement thickness was measured at eight constant pre-determined locations (both margins, axial walls, perpendicular to cusp tips and adjacent to occlusal facia) on each of the forty specimens using a light microscope and electronic measuring software. The cement thicknesses at the eight locations were added together to give a total gap thickness for each specimen.

Result: The mean total thicknesses for each group (mm) were Tk/P:1.181±0.170, Tk/U:1.873±0.183, Tn/P:1.684±0.340, and Tn/U:2.142±0.391. Using two-way ANOVA (p≤0.05), a statistically difference was found between Tk/P and the other groups.  No statistical significance was found between Tk/U, Tn/P and Tn/U.

Conclusion: Within this study, lithium disilicate crowns properly supported during crystallization produced a total overall more accurate fit than restorations not fully supported.