Correction of axis misalignment in the analysis of knee rotations.

The Cardanic or Eulerian description is the most commonly used method for the description of in vivo knee rotation. It is based on the determination of external anatomical landmarks used for the decomposition of the position of the tibia relative to the femur by three rotations about three pre-defined axes. However, the in vivo localisation of external anatomical landmarks is known to be difficult and subjective. Even a small mislocalisation may lead to dramatic consequences: the Cardanic description may become irreproducible and angle values may be overestimated. This error is well documented in the literature and known as the "cross-talk effect". Therefore this study proposes an additional calibration step of the classic Cardanic description by a reorientation procedure of rotation axes. The procedure is based on biomechanical constraints of knee kinematics as they appear during a knee squat exercise using the finite helical axis (FHA) method and is independent of anatomical landmark. The method was validated with the help of a special set-up modelling a perfect knee. Furthermore, an inter-session reliability study was performed involving tests on two healthy subjects during knee squat exercises. We found that the reorientation procedure was more reproducible than the classic Cardanic description. We observed a maximum inter-session difference of 37.1 degrees for the adduction angle obtained with the classic Cardanic description. In contrast, the maximum angle difference obtained with the reorientation procedure was less than 10 degrees.