Statement of problem. When 2 implants are used to support a removable overdenture, optimal stress distribution to minimize forces to the implants is desired.

Purpose. This study used photoelastic analysis to compare the stress patterns generated around implants with 2 retention mechanisms.

Material and methods. Two root form implants were anchored in photoelastic mandible and a removable overdenture was fabricated. The denture was lined with a resilient layer of impression material to simulate oral mucosa. Attachment of the denture to the implants was varied by using ball/O-ring and bar-clip attachments. Vertical and oblique oclusal forces from 10 to 200 lb were applied unilaterally to the right and left first molars.Resultant stress distributions in the photoelastic mandible were photographed and evaluated.

Results. With vertical forces, the ball/O-ring attachment transferred minimal stress to both implants. Vertical forces applied to the bar-clip overdenture created immediate stress patterns of greater magnitude and concentration on both implants. A posterior oblique load resulted in similar stress patterns to both systems with greater stress to the bar-clip overdenture.

Conclusions. Within the limits of this study, ball/O-ring attachments transferred less stress to implants than the bar-clip attachments when the photoeslastic model was subject to a posterior load.