Effect of digital blocking of reversed tapered preparations in comparison to conventional wax blocking on fracture resistance of monolithic zirconia crowns /

Abstract

The optimal taperness convergence of axial walls dictates retention, stability, resistance to fracture, and longevity. Reverse taper might affect fracture behavior of cemented restorations, they commonly occur 1n a lot of situations while preparing teeth which may be due to tilted tooth, difficult accessibility, and malocclusion. CAD/CAM technology has been developed to eliminate challenges in restoration production. In this study, preparing three different groups of 3D printed dies with TOC (12,-4,-8), all dies were made with 6mm height, 1.5 mm occlusal reduction, 1mm axial reduction and 0.8 mm chamfer finish line. Each group contains 10 samples of the same angle but different from other group. Blocking of reversed taper preparation was done by two techniques: conventionaly by wax which was done before scanning and digitally blocking. Scanning of the dies was done using intra-oral scanner (Omnicam), as for blocking dies digitally it was done by software algorithim and then designing of the restoration was done by Exocad software. Monolithic translucent zirconia crowns were milled for full coverage restoration with Roland DWX-51D Dental Milling Machine. Monolithic translucent zirconia is widely used for its high strength, translucency and excellent biocompatibility. It shows higher flexural strength with reduced occlusal thickness and does not detach from the core layer as in bilayered zirconia. Crowns were then polished, glazed and sandblasted by Al,O3 particles size 50 um. Self-adhesive resin cement (Thercem) was used for cementation as it contains 10 MDP for better bonding of zirconia using cementing device. Afterwards, fracture test was done using universal testing machine. For better interpretaion of the resutls fractographic analysis of the fractured parts was processed using SEM. After analysis it was found that only tapering angle had a significant effect on fracture resistance, while the effect of blocking technique and its interaction with tapering angle was not statistically significant.