Crown Preparation Parameters Produced by New Zealand Dental Students

Objectives: Dental schools advocate metal-ceramic crown preparations to have 12° total occlusal convergence (TOC), 1.3-1.5 mm (metal-ceramic collar) and 0.5 mm (metal collar) margin widths. This is thought to directly influence restorations retention and resistance forms, and clinical survivability. The aim of this study is to present the TOC angles and margin widths of metal-ceramic molar preparations made by 4^th and 5^th year dental students at the University of Otago, New Zealand in 2014.
Methods: Forty-two digital files of student preparations for metal-ceramic molar restorations were collected for the year 2014. The files were uploaded onto a custom software (Preppr™) for measuring various tooth preparation geometries. It enables the measurement of buccolingual (BL) and mesiodistal (MD) cross-sections by objectively selecting 6 points to calculate TOC and margin widths using coordinate geometry. Distribution of values were analysed and presented as box and whisker plots.
Results: The TOC values for the BL aspects from quartile 0 to 4 respectively are as follows; -4.28°; 6.59°; 15.48°; 20.51°; 50.82°. For the MD aspects; -1.87°; 18.45°; 25.00°; 32.64°; 67.37°. The combined marginal widths in mm are as follows; 0.00; 0.50; 0.74; 1.02; 2.00.
Conclusions: Students are able to achieve TOC angles close to that recommended for the BL cross-section but not the MD cross-section. However, there was a large range from negative values (undercuts) to the extreme over preparations of > 60°. Majority of preparations meet the recommended marginal widths.