Stress distribution and abutment tooth mobility of distal-extension removable partial dentures with different retainers: an in vivo study.

Three types of retainers (wrought wire clasp, Aker's cast clasp, and conical crown telescopic retainer) designed for distal-extension removable partial dentures (RPDs) were assessed in two Kennedy class I patients' mouths. The assessment, included the ratio of denture base shearing load and mobility of the terminal abutments when loaded on a free-end RPD occlusal surface. The mean values of denture base shearing ratios of wrought wire clasp, Aker's clasp and conical crown telescope were 60, 42 and 20%, respectively. The abutment mobility of the three types of retainers were all within the 'mobile ability area' except the wrought wire clasp for patient A's right side. The greatest tooth mobility was observed with the wrought wire clasps, followed by Aker's clasp and the conical crown telescopic retainer. From the analysis the following was concluded: (i) different retainers do influence the occlusal load distribution; (ii) the occlusal load distributed to the free-end saddle is closely related to the connecting rigidity of the retainer; (iii) mucosal support has an indispensable role in sharing the occlusal load with various retainers, even the rigid telescopic retainer.