Effect of two Blocking Reverse Taper Preparation Methods on the Internal Fit of CAD/CAM Monolithic Zirconia Crowns (An In-vitro study)

Abstract

Background: Limited data is present on the accuracy of algorithms present inside CAD/CAM software regarding its effect on reverse taper blocking and, furthermore, on its effect on the internal fit. Aim: To evaluate the effect of two blocking methods of reverse taper preparation (conventional wax blocking and digital software's algorithm) on the internal fit of CAD/CAM monolithic zirconia crowns, and to compare the internal fit between zirconia crowns constructed over blocked preparation, and 12º Total Occlusal Convergence (TOC). Material and methods: Thirty CAD/CAM monolithic zirconia crowns were constructed over 3D printed dies. They were divided into three groups, each (n=10) according to the degree of TOC ;12 º, -4 º and -8 º reverse taper.Group II and Group III will be further subdivided into 2 sub-groups (n=5) according to the method of reverse taper blocking: A, conventional wax blocking technique, and B, digital method. 3D models were created to simulate a reduced upper molar. All dies were scanned to create STL files.Internal fit was assessed with the silicone replica technique. The Kruskal-Wallis test was used to compare between the groups. Dunn's test was used for pair-wise comparisons when Kruskal-Wallis or Friedman's test was significant. Results: There was no statistically significant difference between internal gap distance measurements of the five groups and between methods. Conclusion:Under the conditions of this study, the internal fit of the zirconia crowns constructed over reverse tapered preparations were within the clinically accepted values (300µ) except for -8 o digitally blocked specimen (315.6µ).