Numerous factors have been hypothesized as contributing to mechanically-assisted corrosion at the head-neck junction of total hip prostheses. While variables attributable to the implant and the patient are amenable to investigation, parameters describing assembly of the component parts can be difficult to determine. Nonetheless, increasing evidence suggests that the manner of intraoperative assembly of modular components plays a critical role in the fretting and corrosion of modular implants. This study was undertaken to measure the magnitude and direction of the impaction forces applied by surgeons in assembling modular head-neck junctions under operative conditions where both the access and visibility of the prosthesis may potentially compromise component fixation. A surrogate consisting of the lower limb with overlying soft tissue was developed to simulate THR performed via a 10cm incision using the posterior approach. The surrogate was modified to match the resistance of the body to retraction of the incision, mobilization of the femur and hammering of the implanted femoral component. An instrumented femoral stem (SL PLUS) was surgically implanted into the bone after attachment of 3 miniature accelerometers (Dytran Inc) in an orthogonal array to the proximal surface of the prosthesis. A 32mm cobalt chrome femoral head was mounted on the trunnion (12/14 taper, machined) of the femoral stem. 15 Board-certified and trainee surgeons replicated their surgical technique in exposing the femur and impacting the modular head on the tapered trunnion. Impaction was performed using an instrumented hammer (5000 Lbf Dytran impact hammer) that provided measurements of the magnitude and temporal variation of the impact force. The components of force acting along the axis aof the neck and in the AP and ML directions were continuously samples using the accelerometers.Introduction
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