Reporting numeric values of complete crowns. Part 2: Retention and resistance theories.

STATEMENT OF PROBLEM: Determining the retention and resistance of a tooth preparation for a complete crown has only existed in theory, and these theories have never been measured on tooth preparations performed in vivo. PURPOSE: The purpose of this study was to measure the theoretical retention and resistance of clinically produced complete crown preparations by using an objective measuring method. MATERIAL AND METHODS: Stone dies from 236 complete crown preparations were collected from dental laboratories. The dies were scanned and analyzed with the coordinate geometry method. Cross-sectional images were captured, and the surface area was measured with a cone frustum and right truncated pyramid formula. Two different theories of resistance form, the "on" or "off" theory (limiting taper) and the linear model (resistance length), were calculated for premolar and molar preparations. RESULTS: The mean surface areas ranged from 33.97 mm(2) to 105.44 mm(2) for the cone frustum formula and 41.75 mm(2) to 117.50 mm(2) for the right truncated pyramid formula. The facial side of maxillary premolars exhibited the highest percentage of resistance form with the limiting taper, at 58%, and the mesial side of the mandibular molars exhibited the lowest percentage of resistance form, at 6%. CONCLUSIONS: The objective method used in this study provides a way for retention and resistance theories to be tested and for further clinical implications to be investigated.