360-Degree Analysis of Convergence Angle in Crown Preparations: a Prospective Follow-Up

Objectives: Convergence angle (CA) of tooth preparations is crucial to crown retention. However, CA is clinically assessed by two-dimensional visual inspection only and valuable information is still missing. This study aimed to demonstrate the use of a computer-aided method to analyze CA three-dimensionally as well as its relationship with crown retention and to prospectively investigate the crown longevity.
Methods: A computer program that automatically analyzed the CA of crown preparation in 360 degrees was developed. The axial walls and the long axis of the tooth were identified using the piece-wise linear fitting algorithm. Crown preparation was sectioned 180 times and individual CA was determined. A standard test object with known CA (8-degree) was used to assess the accuracy of the program. Definitive casts of maxillary first permanent molars, which were prepared for metal-ceramic crowns by undergraduate dental students, were collected, digitized, and analyzed using MATLAB® (version 9.8; The Mathworks, Inc.). A curve of CA was plotted against the tooth sections. The longevity of these crowns was clinically reviewed at a mean observation time of 55 months.
Results: Thirty-five tooth preparations were analyzed. Mean CA was 24.8±7.6-degree. Only three preparations (8.5%) fulfilled the recommended CA of 6-degree. Eighteen (50.1%) crowns were clinically examined and the majority (n=17) of these crowns survived.
Conclusions: 360-degree analysis of tooth preparation allows operators to evaluate their work and investigate areas of improvement critically. A longer clinical follow-up of crown survival is needed.