Our experience with computer plans of kinematically aligned total knee arthroplasty showed that the anteroposterior (AP) axis of the tibial component when viewed in an axial plane did not consistently intersect either the medial border or the medial 1/3 of the tibial tubercle. The purposes were (1) to determine the variability in the mediolateral location of the tibial tubercle with respect to the medial tibia on the magnetic resonance image (MRI) of the knee and (2) to determine whether the AP axis of the kinematically aligned tibial component intersects either the medial border or the medial 1/3 of the tibial tubercle.
One hundred and fifteen knees in 111 consecutive subjects treated with total knee arthroplasty were studied. The mediolateral location of the tibial tubercle was measured from a magnetic resonance image (MRI) of the knee. The distances between the AP axis of the tibial component and the medial border of the tibial tubercle and between the AP axis and the medial 1/3 of the tibial tubercle were measured from a computer plan of the reconstructed knee.
On the MRI, the medial border of the tibial tubercle varied 15 mm from the medial border of the tibia. On the computer plan, the AP axis of the tibial component in an axial view of the tibia did not intersect either the medial border (p < 0.0001) or the medial 1/3 of the tibial tubercle (p < 0.0001). In 70 and 86 % of knees, the mediolateral distance of the AP axis of the tibial component was 2 mm or greater from the medial border of the tibial tubercle and the medial 1/3 of the tibial tubercle, respectively, which causes a clinically meaningful error in rotation of 5° or more.
IV.
Because the mediolateral location of the tibial tubercle varies, the medial border and medial 1/3 of the tibial tubercle are not reliable landmarks when the goal is to kinematically align the rotation of the tibial component on the tibia.