Angle of convergence of posterior crowns prepared by under-graduate students

Objectives: Angle of convergence (AC) of crown preparations significantly influences their retention.  The aim of this study was to measure AC of posterior crowns prepared by under-graduate dental students at University of Toronto. Methods: Ninety-one cast-gold crown preparations made by final-year students as part of clinical competency exam were included. Trimmed stones dies of  preparations were digitally-scanned with in-Eos-Blue scanner (Sirona). Created images were virtually-sliced at three locations (middle, buccal, lingual aspects of mesiodistal plane); and at three similar locations of buccolingual plane. Virtual protractor was used to determine AC of each section. Means and SDs were calculated and data statistically-analyzed with ANOVA and Student’s t-test for operator’s gender, experience, tooth type. Results: Fifty five crowns were prepared by female students while 36 were prepared by males. Mean mesiodistal AC by females was 22°(9) and 23.2°(8.6) by males, while mean buccolingual AC by females was 24°(8.6) and 24°(7.6) by males. Student’s t-test revealed no significant differences among these groups (p>.14). Among 91 students 25 were identified as foreign-trained dentists (FTD). Mean mesiodistal AC by students was 22.4°(7.7) and by FTD-students was 22.8°(10.3), while mean buccolingual AC by students was 23.3°(8.3) and by FTD-students was 25.6°(7.6). Student’s t-test revealed no significant differences among these groups (p>.21). When analyzed by tooth type mean mesiodistal AC was 16°(6.8) for maxillary premolars (n=11), 19.8°(6.4) for maxillary molars (n=22), 19.6°(7.6) for mandibular premolars (n=14), 26.4°(8.2) for mandibular molars (n=44). ANOVA revealed significant difference among AC means (p<.000). Conclusions: Mean AC values were greater than ideal range of 4-6°, however, they were within ranges published for dentists/prosthodontists. Gender and experience had no significant effect on AC, however, tooth type significantly-affected AC with mandibular molars having highest AC and maxillary premolars having lowest values. Acknowledgement: Patterson Dental Canada.