Stress analysis of mandibular implant-retained overdenture with independent attachment system: effect of restoration space and attachment height.

In this in vitro study, 2 implants were embedded in the interforaminal region of an acrylic model. Two kinds of retention mechanisms were used to construct complete overdentures: ball type and direct abutment (Locator). The ball-type retention mechanism models included 3 different collar heights (1, 2, and 3 mm) with 15 mm occlusal plane height, and 3 different occlusal plane heights (9, 12, and 15 mm) with 1 mm collar height. The direct abutment models included 3 different occlusal plane heights (9, 12, and 15 mm) with 1 mm cuff height. Vertical unilateral and bilateral loads of 150 N were applied to the central fossa of the first molar. The stress of the bone around the implant was analyzed by finite element analysis. The results showed that by increasing vertical restorative space, the maximum stress values around implants were decreased in both unilateral and bilateral loading models. The results also showed that the increase in maximum stress values around implants correlated with the ball attachment collar height. The Locator attachment with a 1 mm cuff height and 9 mm occlusal plane height demonstrated 6.147 and 3.914 MPa in unilateral and bilateral loading conditions, respectively. While a reduction in the collar height of a ball-type retention mechanism and an increase in the vertical restorative space in direct abutment retention mechanisms are both biomechanically favorable, and may result in reduced stress in peri-implant bone, a ball attachment seems to be more favorable in the stress distribution around an implant than a Locator attachment.