Effect of Abutment Screw Design and Crown/Implant Ratio on Preload Maintenance of Single-Crown Screw-Retained Implant-Supported Prostheses

Purpose: New solutions need to be developed for cases in which implants that were placed years ago are now presenting prosthetic complications. A conical head screw design for a single-tooth abutment was developed to preserve the initial applied torque. The aim of this study was to assess the preload maintenance of different screw design sets (a conical head screw set and a flat head screw set) for single-tooth abutments in external hexagon implants, verifying whether reverse torque changes after mechanical loading at different crown/implant ratios and to understand if the use of the tested conical head screw set design can help clinicians solve loosening torque. Materials and Methods: Forty external hexagonal implants, 40 singletooth abutments, 20 conical head screws, and 20 flat head screws were split into four groups with different crown/implant ratios (crown/implant ratio > 1 or crown/implant ratio 1). The abutments were attached to the implants by applying a torque of 35 Ncm; the specimens were mechanically loaded for 1 million cycles, and the loosening torque was checked and recorded with a digital torque wrench. The Kruskal-Wallis test (P = .05) and Wilcoxon test were performed to assess the results. Results: In all groups, at least one specimen kept 100% of the initial applied torque before mechanical loading (t0). After mechanical loading (t1), all specimens presented torque reduction. The Kruskal-Wallis test was performed, and the flat head screw t0 group presented lower torque maintenance and a significant difference (P .05) compared with the initial applied torque and with the conical head screw t0 group. The conical head screw t0 group presented a higher torque maintenance and no significant difference (P > .05) compared with the initial applied torque. For the flat head screw, the crown/implant ratio affected the torque maintenance. For the conical head screw, the crown/implant ratio did not affect the torque maintenance (P > .05). Conclusion: The conical head screw set presented a higher maintenance of applied preload than the flat head screw set. As far as reverse torque is concerned, the crown/implant ratio affects the torque maintenance only in association with a flat head screw set. The use of the tested conical head screw set can help clinicians solve loosening torque, mainly in a situation with a crown/implant ratio > 1. Keywords: dental implantation, dental prosthesis, implant-supported, torque