Variations between Visually Estimated and Actual Convergence Angles of Tooth Preparations

Missing teeth and lost or defective tooth structures can be restored by various types of fixed prostheses, the retention of which can be achieved by many ways. The most important retention technique is minimizing the convergence angle (CA) to an optimal taper that allows proper seating and retention of the dental prosthesis. The CA, measured in degrees, is defined as the taper of a crown preparation or the angle formed between opposing axial walls when teeth are prepared for crowns or fixed dental prostheses. This term is best described as the total occlusal convergence. CAvalues ranging from 4 to 38 degrees have been measured in the literature by various methods, including using a tool-maker microscope, geniometer, three-dimensional (3D) laser scanner, white-light 3D scanner, digitizer, AutoCAD photographs, Lava design, 3D-inspection software, and a protractor to measure a traced silhouette from a photo of the projection of dies . Only one study mentioned using visual estimation to measure CA.

The operator should be able to assess the CA visually during tooth preparations. However, interoperator or interevaluator variation in the assessment and estimation of CA values has not been discussed much in the literature. Therefore, the purpose of this study was to compare actual CA values to those visually estimated by experienced prosthodontists, and to compare performance between prosthodontists. The null hypothesis was that the visually estimated CA does not vary from the actual CA or between operators. Read more..................