Can Patellar Tendon Angle reveal sagittal kinematics in total knee arthroplasty?

The Patellar Tendon Angle is strongly correlated with both patello-femoral and tibio-femoral joint kinematics. Thus, the Patellar Tendon Angle might be regarded as a concise indicator of the antero-posterior translation of the femur with respect to the tibia. Twelve subjects, who had undergone total knee arthroplasty, were investigated by video-fluoroscopy during step-up/down. The kinematics at the replaced knee was reconstructed by means of 3D fluoroscopy. The Patellar Tendon Angle and the knee-flexion angle were measured on sagittal fluoroscopic images with software designed for the purpose. The slope of the linear trend of the Patellar Tendon Angle versus knee-flexion angle relationship correlated significantly with the mean (p = 0.001), most anterior (p = 0.001), and most posterior (p = 0.016) position reached by the femur over the tibia. The Patellar Tendon Angle versus knee-flexion angle relationship provides relevant information about the anterior or posterior positioning of the femur over the tibia, but it cannot be interpreted as quantification of the actual antero-posterior translation of the femur over the tibia.