A novel approach to knee kinematics.

We describe our experience with in vivo dynamic fluoroscopy that uses simple 2-plane video fluoroscopy of subjects performing maneuvers such as deep knee-bends, gait, and stair climbing after total knee arthroplasty (TKA). Kinematic analysis has evolved from 2-dimensional vector calculations to automated 3-dimensional computer-assisted design matching techniques that are accurate to 0.75-mm translation and 0.75 degrees rotation and that allow simultaneous determination of medial and lateral condyle contact positions. TKAs that retain the posterior cruciate ligament (PCL) have consistently shown posterior contact in extension and anterior translation with flexion with a large variability among subjects. PCL-stabilized implants and mobile bearings with high conformity have posterior femoral rollback more consistent with normal knees--which is a function of prosthetic geometry. Condylar liftoff and screw-home rotation are typical kinematic features of all TKAs.