Clinical evidence indicates that resistance form is one of the essential elements in crown preparation design to ensure clinical success. The on/off nature of resistance form lends itself to the theoretical determination of minimally acceptable tapers. The boundary between resistive and nonresistive tapers can be determined at each point on a preparation (limiting taper) or for the entire preparation (limiting average taper). Using average height-to-base ratios for incisor, canine, premolar, and molar preparations, minimally acceptable guidelines can be determined for symmetrical preparations. The short wide molar with the small height-to-base ratio is the most difficult tooth preparation to achieve resistance form, and grooves should be used routinely. The average guidelines for taper do not apply to preparations with uneven margins, which are frequently seen with tipped mandibular molars. It is possible to have perfectly parallel opposing mesial and distal walls and not have resistance form. Buccal and lingual grooves solve the problem. Every tooth must be analyzed individually. The Lewis, Zuckerman and "direction of arc" techniques for evaluating resistance form are consistent, but the results do not agree with those of the Weed method. Laboratory studies have produced a linear relation between measured failure loads and taper but have failed to provide tapers in the "off" region of resistance form. Thus, the relation of failure load to taper has not been evaluated over the "on or off" boundary. It is anticipated that in properly designed laboratory studies the continued linear relationship will not be seen across this boundary and that adding grooves to nonresistive preparations will require much larger loads to produce failure. The clinical data indicate that there is a relationship between clinical success or failure and the on/off nature of resistance form.
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