Experimental Validations of VFEM Models for PVZ and PVLD Crowns

Objectives: Porcelain-veneered lithium disilicate (PVLD) crowns have become a popular prosthetic option due to their improved esthetics and biocompatibility. Unfortunately, they are still challenged by chipping of the porcelain veneer layer due to internal stresses created during the firing process. Measuring and mapping residual stresses created during manufacturing can further our understanding of the clinical behavior and failure of porcelain-veneered prostheses. Experimental determination of internal stresses are limited by techniques; thus, VFEM analysis have been adopted to predict stresses in dental restorations. This study aims to validate VFEM stress analyses for PVLD crowns using experimental validation and provide a basis for comparison with their porcelain-veneered zirconia counterparts.
Methods: PVLD anatomical crowns (n = 3) were CAD/CAM fabricated with core thickness of 0.7 mm, these were veneered with porcelain to a thickness of 1.4 mm. Vickers indentations were performed at various depths by removing material incrementally (4.9 N load, 5 s dwell time). Crack lengths were measured under optical microscopy. Analogous truncation and indentation were simulated on the VFEM model, and these values were compared.
Results: Similar stresses (20 MPa), mainly in compression, were seen in experimental measurements through the layers within one-third of porcelain thickness. The agreement between experimental measurements and VFEM predictions boosted our confidence in VFEM stress analysis. Re-construction of the VFEM model without lift-off demonstrated the majority of the porcelain veneer is in tension. The superficial layer of the veneer features compressive stresses to a depth of 200 mm. Additionally, higher stress concentrations can be noted across portions of the crown with sharp curvature.
Conclusions: A validated VFEM model is a powerful tool to elucidate internal residual stresses of anatomic prosthetics made from porcelain, zirconia and lithium disilicate. Stress maps of the crown without lift-off sectioning provide true stress values in both PVZ and PVLD crowns.