‘From Theory and Practice’ of Retention and Resistance form of Tooth Preparations for All Ceramic Restorations: A Digital Imaging Study

Background: Retention and resistance forms are the characteristics of a preparation that prevent castings from becoming uncemented, debonded, or cement failures, which are one of the top three reasons for the replacement of castings

Aim: This study aims to discern an actually achieved degree of taper in a clinical situation, and the textbook recommended degree of taper. Furthermore, it also assessed how resistance and retention form correlate with the degree of taper of tooth preparation.

Materials and Methods: Scanned digital images of 114 die preparations for all ceramic restorations (n = 114) were collected from a dental laboratory. All the images were also analyzed digi- tally using Adobe Photoshop® software to analyze the degree of taper (angle of convergence) of each preparation and then applied the Zuckerman’s circle, and the Lewis perpendicular methods were used to measure the resistance form.

Results: For the current study, the overall average degree of taper was found to be 20.9° (range, 2–80°), which is more than what is recommended by most previous studies and also sharply greater than the textbook ideal of 3 to 6°. Mean degree of taper for maxillary was 17.56° (anterior—10.50°, posterior—23.7°), and for mandibular teeth, it was 25.22° (anterior—15°, posterior—28.45°). Out of the 64 analyzed images of maxillary teeth, 61 presented resistance form, while 3 were without it. Out of the 50 mandibular teeth analyzed, 38 possessed resistance form, whereas 12 were without. All the anterior teeth showed resistance form irrespective of the arch.

Conclusion: The degree of taper showed a significant relation- ship with resistance and retention form, which was inversely proportional to each other. The recommended “degree of taper” is not always the clinically achievable as advocated in textbooks, as it is modified by various factors in the actual clinical situation.

Clinical Significance: The study provides scientific background regarding the relationship between the degree of taper with resistance and retention form, and the relationship was found to be inversely proportional to each other. The recommended “degree of taper” is not always the clinically achievable as advocated in textbooks, and it is modified by various modifying or limiting factors in the actual clinical situation.