Effects of attachment design on strains around implants supporting overdentures.

The objective of this study was to compare strain magnitudes around implants supporting ball-retained, bar-retained, and distal cantilevered bar-retained mandibular overdentures subjected to vertical and oblique loads. Twelve edentulous mandibular acrylic resin casts with 2 implants in the parasymphyseal region were randomly divided into 3 groups (n = 4). Ball-retained, bar-retained, and distal cantilevered (8 mm) bar-retained overdentures were fabricated. A linear strain gauge was bonded distal to each supporting implant on the surface of the cast. As static loads of 20, 40, and 60 N vertically were applied 20 degrees obliquely and unilaterally to the overdenture, the generation of strains was recorded by a data acquisition system and corresponding software at a sample rate of 1,000 Hz. The absolute strain data of each group were compared within each loading condition by Kruskal-Wallis test followed by post hoc tests (a = .05). Under vertical and oblique loading conditions, strain magnitudes around implants supporting ball-retained overdentures were higher than those induced around bar-retained and distal cantilevered bar-retained overdentures (P <.01). On the contralateral side from load application, strains around implants supporting ball-retained overdentures were lower than those around bar-retained and distal cantilevered bar-retained overdentures (P <.01). Force distribution does not occur for implants supporting ball-retained overdentures, and such implants are subjected to more load than those supporting bar-retained and distal cantilevered bar-retained overdentures.