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

Background: Limited data is present upon 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. 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 is significant. Results: There were 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 -8o digitally blocked specimen (315.6µ).