A comparative in vitro study on the retention and stability of implant-supported overdentures

Objectives: Implant-supported overdentures have become the treatment of choice in restoring complete edentulism, but the number of implants required and type of connectors to assure retention and stability are still controversial. The aim of this study was to evaluate the effect of connector type and implant number and location on the retention and stability of implant-supported overdentures by measuring retentive forces during vertical and 2 types of rotational dislodgment. Method and Materials: Two model designs were selected based on the number and location of the inserted implants: In a first setup, 2 implants were placed in the canine regions; in a second setup, 2 implants were placed in the canine regions and 2 in the premolar regions. Three types of connector were used in each model: magnets, balls, and locators; 10 resin bases were fabricated and 3 hooks fixed at tripodal locations for chain testing. Vertical dislodging forces and 2 aspects (oblique and posterior rotational dislodging forces) of stability were tested. Results: Two-way analysis of variance showed significant differences in retention among the 3 connectors in the 2 models, with the lowest values obtained with the magnet group (mean [SD]: 2.15 N [0.09]) and highest values with the locator group (31.30 N [0.12]). Posterior rotational dislodging forces showed higher values than vertical or lateral forces in both models; 4-implant models required higher dislodging forces than did 2-implant models. Conclusion: Locator connectors provide significantly higher retention and stability of implant-supported overdentures, followed by ball connectors and then magnets. The 2-implant design offers less retention and stability than the 4-implant model. Number of implants and type of connector significantly affected retention and stability of implant-supported overdentures. Keywords: ball, implant, locators, magnet, overdenture, retention, stability