Fig. 1

Definition of the proximal and distal tibia axes in 3D shown on a model of the proximal tibia (orange) and distal femur (transparent grey) on the left and the distal tibia and fibula (both orange) on the right side. Left: the most distal points were identified for each femoral condyle, and PCA was applied to the corresponding points on the tibial plateau (blue dots). The PTA was defined as the first principle component. Right: A plane (DTJP, blue) was fitted to nine manually selected points on the articular surface (blue dots). A bone slice of 10 mm thickness was extracted distal from the DTJP (green), and PCA was applied to find the DTA (yellow line). DTA was defined as the second principle component. The DTA was calculated in two different adaptations (with and without consideration of the fibula). PCA principle component analysis, PTA proximal tibia axis, DTJP distal tibia joint plane, DTA distal tibia axis