Influence of Rigid Bar and Nonrigid Double Crown Attachments on Maxillary Implant Overdentures: An In Vitro Study with Differential Bone Quality

PURPOSE: 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. MATERIALS AND METHODS: 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. RESULTS: 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. CONCLUSION: 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.