The taper of clinical preparations for fixed prosthodontics.

Convergence angles of full-coverage preparations were measured in a clinical environment and compared with each other and the ideal taper of 4 to 10 degrees. Despite educational emphasis, the practical application of preparation design routinely exceeds the ideal taper and casts a different light on retention and resistance characteristics described in both laboratory and theoretical work. Comparison of preparations done by residents and by prosthodontists in this study showed that ideal preparation taper is seldom achieved. Given the complex interrelationships of clinical, theoretical, and mechanical factors that determine the retention and resistance characteristics of a preparation in vivo, it is advisable to design preparations that blend retentive characteristics with functional demands. Because it is difficult to assess preparation taper intraorally, efforts should be directed to using other retentive devices, especially on posterior preparations where ideal taper is difficult to achieve.