A new technique for determining proper mechanical axis alignment during total knee arthroplasty: Progress toward computer-assisted TKA

Successful total knee arthroplasty (TKA) relies on proper positioning of pr osthetic components to restore the mechanical axis of the lower extremity. This report presents and analyzes a new noninvasive method using the Optotr ack (Northern Digital Inc, Ontario, Canada) to accurately determine the cen ter of the femoral head. This method, together with direct digitization of the bony landmarks of the knee and ankle intraoperatively, permits placemen t of the lower extremity in proper alignment intraoperatively. It also perm its the surgeon to follow all the angles of movement or rotation and all di splacements that occur at each step of the operative procedure. The Optotrack is an infrared tracking system that has been modified to be m ounted on the distal femur and proximal tibia to record the motion and alig nment of the knee intraoperatively via a customized Windows-based program. In addition to presenting our first case, which, importantly, represents th e first computer-assisted TKA in a patient, we report on the accuracy and r eproducibility of the technique for locating the center of the femoral head obtained during an extensive series of cadaver studies. Location of the fe moral head, a major aspect of effecting neutral mechanical axis alignment, appears to be possible to within 2-4 mm, which corresponds to an angular ac curacy of better than 1 degrees. This method requires no computed tomograph y scans or other preliminary marker placement. The only basic requirement o ther than the instrumentation described is a freely mobile hip, which is ge nerally present in TKA patients.