Nonlocking types of semiprecision attachments.

The history of the nonlocking type of intracoronal semiprecision rest has been traced from its origins with Neurohr in 1930, up to its present use as the Thompson dowel rest. The advantages of the use of this deep rest design and the clasp arm design for primary retention have been discussed. The Blatterfein modification of the lingual clasp arm has been introduced to overcome the problems of breakage and lack of stabilization that are present with the Thompson, Van Dam, and Wands designs. Several clinical situations have been illustrated demonstrating the variety of applications of the Thompson dowel rest type of semiprecision prostheses.