Effect of blocking reverse taper preparation methods on the vertical marginal and internal fit of CAD/CAM monolithicZirconia crowns : (In-vitro study) /

Abstract

Undercuts are often considered a mistake in the preparation and conveyed to be a source of problem during impression taking and crown fabrication. Treatment methods to such problem include either sacrificing the tooth structure or blocking of the undercut. However, there is minimal scientific evidence regarding whether the digital algorithm will be able to substitute the conventional wax blocking in terms of producing tooth supported zirconia dental prosthesis with predictable vertical marginal accuracy and internal fit. So, The aim of this study was to evaluate the effect of blocking methods of reverse taper preparation (conventional wax blocking and digital software’s algorithm) on the accuracy of fit (vertical marginal and internal fit) of CAD/CAM monolithic zirconia crowns and to compare the accuracy of fit between zirconia crowns constructed over blocked preparation and 12o total occlusal convergence (TOC). Therefore, in the present study 3D printed dies mimicking a reduced upper molar were engineered with predetermined total occlusal convergence to assess the digital and conventional blocking methods. Thirty dies in total ten in control group with 12 o taper, ten with -4 o taper and ten with -8 o taper as shown

in figure 1. Groups containing undercuts were further divided in equals and exposed to the two types of undercut blocking methods, digital and conventional. Groups IIA and IIIA were blocking conventionally using wax blocking and then scanned utilizing CEREC OMNICAM intra-oralscanner to provide a STL file for the zirconia fabrication. Groups IIB and IIIB were scanned first using CEREC OMNICAM intra-oral scanner and the designing software, EXOCAD, was used to digitally identify and block the undercut. Finally, the milled zirconia crowns were assessed in terms of the vertical marginal fit and internal fit. Vertical marginal fit was assessed by a stereooptical microscope on the 3D printed model. Five equidistant measurement points were taken from each surface (buccal, lingual, mesial and distal) with a total of 20 points and with the aid of an image analysis software vertical marginal values were acquired. Internal fit was assessed using silicone replica technique, after the sectioning of the specimen, measurements were taken as the mean of five equidistant 5 points of single proximal wall.