Investigating the tightening torque of an osseointegrated transfemoral implant system

Abstract

Osseointegrated transfemoral implants have been introduced as an alternative to conventional, socket-based prostheses for above knee amputees. This method is showing great promise; however, the frequency of device failure is of concern. As a means to understand some of these failures, this study investigates the manufacturer's specified tightening torque and the preload stress experienced by the retention bolt of the transfemoral system. This has been explored using a protocol proven during tests of dental implants with a similar assembly. Elongation of the bolt was recorded after applying known torques using a digital torque wrench, to a maximum of the manufacturer's specified torque. Using the elongation data, preload stress and force acting on the bolt were calculated. At the manufacturer's specified tightening torque, preload stress peaked at 23.8% of the material's yield strength, with the retention bolt tensile preload measured to be 3.5 kN. This suggests that the retention bolts may be tightened beyond the manufacturer's recommended tightening torque to increase assembly strength without inducing plastic deformation of the components. © 2009 Springer Berlin Heidelberg.