A new measurement technique for the tibiofemoral contact point in normal knees and knees with TKR.

Flexion gap instability after cruciate retaining TKR allows paradoxical anterior movement of the femur during flexion. The tibiofemoral contact point (CP) moves anteriorly and produces a decrease in the lever arm of the extensor apparatus. This can provoke patellofemoral, tibiofemoral-joint pain and instability for the patient. In order to quantify the amount of paradoxical motion on a 90 degrees flexion radiograph of the knee, the average normal CP of the natural knee should be known. There are no known CP measurement methods suited for natural knees and knees with TKR that can be applied in daily practice, and only estimations for the CP position have been made. Therefore, a CP measurement technique on lateral radiographs that can be applied to natural knees and knees with a TKR has been developed. The reproducibility of this method was assessed. It was then used to determine the normal range of the CP in natural knees. The medial contact point in the natural knee in 90 degrees of flexion was determined to be at 68% (+/-6.6%) of the AP diameter of the tibia measured below the tibia-plateau simulating a bone resection with TKR. This reproducible CP measurement method can be used clinically to evaluate the CP after knee prosthesis and also in patients with suspected ligament lesions.