Evaluation of resistance form of dislodged crowns and retainers

The concept of limiting taper has been described as a boundary between tapers that do and those that do not provide resistance form for a preparation. There is controversy as to whether this boundary that divides preparations with from those without resistance form translates clinically into a boundary for success.

This investigation evaluated the resistance form of abutments of crowns or retainers that have been dislodged to determine the clinical correlation between restoration dislodgment and lack of resistance form in the preparation.

Dies were fabricated from single crowns and retainers of fixed partial dentures that came loose and evaluated for resistance form. A total of 44 abutments were evaluated and included 1 incisor, 15 premolars, and 28 molars. Data from a previous study on the percentage of abutments lacking resistance form for restorations leaving a large dental laboratory was used for comparative statistical tests.

Forty-two of the 44 abutments (95%) lacked resistance form. All molar abutments and 93% of premolar abutments lacked resistance form in one or more directions. The one incisor abutment did not lack resistance form. Chi-square test revealed a statistically significant difference in the percentage lacking resistance form between the group composed of clinical failure (uncemented crowns) and the group leaving a dental laboratory with P = .0005 for the molars, and P = .0005 for the premolars.

The clinical dislodgment of cast restorations is associated with the lack of resistance form in the preparations. In this study, there was a relationship between clinical success or failure and the all-or-none nature of resistance form; dislodged crowns come almost exclusively from preparations with tapers that did not provide resistance form.