Influence of Rigid Bar and Nonrigid Double Crown Attachments on Maxillary Implant Overdentures: An In Vitro Study...

To evaluate the bending strain exerted on a maxillary implant overdenture supported by rigid bar and nonrigid double crown connectors in models of high- and low-quality bone.

Four implants were embedded in acrylic resin models of an edentulous maxilla simulating good- and poor-quality bone at the canine and second premolar regions bilaterally. A bar (cross-section dimension: 5 × 5 mm; bilateral distal extension: 10 mm) and double crown connectors with a metal framework (same morphology as the bar) were attached to these implants and placed under a constant 49 N vertical load at the anterior and cantilever regions of both superstructures. The bending strain on the implants was measured by strain gauges attached to the implant surfaces.

In good-quality bone with cantilever loading, the strain--which was highest near the load for both attachment systems--was highly concentrated in the rigid bar system but dissipated through the double crown connector. The directionality of the bending moment was homogenous on the two attachments. In poor-quality bone, strains were significantly lower and more variable, especially for double crown connectors, and there was heterogeneity in the directions of the bending moment.

Within the limitations of this study, bar and double crown connectors have identical biomechanical features in good-quality edentulous maxillary bone, but have divergent properties in poor-quality bone.