March 2020 ■ Journal of Dental Education 329 Predoctoral Dental Education Convergence Angle of Preparations for Lithium Disilicate Glass-Ceramic Crowns by Dental Students and Its Effect on Crown Retention Ahmed Aziz, Omar El-Mowafy Abstract: The aims of this study were to determine the convergence angles of posterior teeth prepared by dental students at the retention rate. A total of 280 preparations for posterior monolithic LDGC CAD/CAM crowns were performed on 270 patients (169 women and 101 men). Crowns were cemented with RelyX Unicem and Calibra Universal resin cements. Mesial, distal, and angle of convergence were measured on the bitewing radiographs. Cemented crowns were followed for up to six years. Data were analyzed for tooth type and location and for operator experience. The results showed the majority of convergence angles were greater than the recommended guidelines but fell within a clinically acceptable range (20 to 24 degrees). However, angles of convergence for mandibular molar preparations were highest (28.06±5.50 degrees), while maxillary premolars exhibited the low- Ahmed Aziz, BDS, MSD, PhD, is Instructor, Department of Restorative Dentistry, Faculty of Dentistry, University of Toronto; and Omar El-Mowafy, BDS, PhD, FADM, is Professor, Department of Restorative Dentistry, Faculty of Dentistry, University of Toronto. Direct correspondence to Dr. Ahmed Aziz, Department of Restorative Dentistry, Faculty of Dentistry, University of Toronto, 124 Edward Street, Toronto, ON, M5G 1G6, Canada; 647-671-5160; ahmed.aziz6181@gmail.com. Keywords: dental education, prosthodontics, tooth preparation, dental crowns, CAD/CAM doi: 10.21815/JDE.019.173 A ngle of convergence (AC) of crown prepara- tions is among several factors, such as abut- ment height, diameter, and margin width, that are important for retention of complete coverage crowns. 1-4 axial opposing surfaces of the prepared tooth. The values of those angles should ideally fall between 5 and 6 degrees. 5,6 In clinical settings, higher AC of crown retention. 7-10 This range is clinically achievable and eliminates the risk of creating undercuts that may interfere in seating of crowns. 11 In addition, studies for both practicing dentists and dental students. 12-16 A systematic review conducted by Tiu et al. reported that the mean buccolingual AC was up to 35.7 de- grees and, for mesiodistal, it was up to 37.2 degrees. 17 Due to the higher tendency of exceeding the theoretically prescribed AC (6 to 24 degrees), sev- eral recommendations have been made to reduce the crowns dislodgement rate. These include modifying the preparation design, increasing the abutment incorporating retentive grooves or boxes, and using adhesives rather than conventional cements. 1,18-20 For posterior lithium disilicate glass-ceramic (LDGC) computer-aided design/computer-aided manufactur- ing (CAD/CAM) crowns, an AC of 12 degrees, 1 mm shoulder with rounded inner edges, and at least 4 mm of tooth height are recommended. 21 A strong bonding between crown material and tooth structure can be achieved with adhesive cementation due to the presence of the etchable glass component in glass-ceramics. 22,23 This advantage compensates for the excessive tooth tapering often produced by dental students or inexperienced den- tists and for teeth with short clinical crown height or preparations lacking retention form. 24,25 Moreover, two studies found that resin bonding increased reten- tion and improved the fracture resistance of LDGC crowns. 26,27 Previous clinical studies on LDGC CAD/CAM crowns reported no incidents of loss of retention for a period ranging from two to four years, 28-30 aside