Kinematic modeling of the knee requires an accurate method to identify the rotational axes [1]. The tibio-femoral joint has two fixed rotational axes: the flexion-extension axis in the femur (FE) and the longitudinal rotational axis in the tibia (LR) [2–4]. Because the knee naturally produces coupled rotation about the LR axis during flexion, attempting to identify the axes from natural flexion alone creates an underdetermined system. A previous study using a mechanical axis finder identified the FE and LR axes by initially identifying the LR axis from pure internal-external (I/E) rotations [4]. Because the major source of error in this method was the visually-based alignment of the axis finder, it should become more repeatable and reliable by utilizing a mathematical optimization to situate the axis finder. Therefore, the two objectives were to develop and validate a new axis finding method, which is based on a mathematical optimization, for identifying the two fixed rotational axes of the tibio-femoral joint.

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