An In Vitro Comparative Study to Assess Minimal Thickness Required for Monolithic Zirconia Crowns to Resist Fracture under Load on Rapid Prototyped Models

Abstract

To evaluate and validate minimal thickness required for computer-aided designed (CAD) and computer-aided manufactured (CAM) monolithic zirconia crowns to withstand occlusal load. The study compares two systems. Forty-eight rapid prototype die models with varying occlusal reductions were fabricated. Group I samples had an axial wall height of 7.0 mm with occlusal reduction of 0.5 mm, group II had axial wall height 6.5 mm with occlusal reduction 1.0 mm, group III had axial wall height 6.0 mm with occlusal reduction of 1.5 mm. Control group IV had axial wall height 5.5 mm with occlusal reduction of 2.0 mm. Laboratories A (Czar) and B (3M) were provided with 24 samples each, 6 samples in each group for fabricating CAD/CAM monolithic zirconia crowns of 0.5, 1.0, 1.5, and 2 mm occlusal thickness, respectively, and cemented using resin-modified glass ionomer cement over the die models. Samples were loaded on a universal testing machine for fracture testing. Surface topography analysis of fractured specimens was done under the scanning electron microscope (SEM). The results were subjected to one-way analysis of variance (ANOVA) and honestly significant difference (HSD) Tukey test to analyze statistical significance at 0.05 levels. Samples of laboratory A performed superior to laboratory B. The t test showed fracture resistance of group AI (0.5 mm) > group BII (1.0 mm) and also group AIII (1.5 mm) > control of Lab B (2 mm). Monolithic zirconia crowns showed a favorable mechanical property to withstand occlusal load with minimal tooth preparation. The occlusal thickness of Czar with 0.5 mm is found to resist fracture under physiological masticatory load. Scanning electron microscope revealed increased voids in the microstructure of 3M, which lead to decreased fracture resistance. Preservation of tooth structure can be considered using monolithic crowns with minimal thickness of 1 mm.