It is well known that the human knee is a complex structure that joins the thigh with the shank and, because of the presence of three bones that articulate within this anatomical plexus, it consists of two joints, the lemoral (TFJ) and the patello-femoral joint (PFJ), the latter being the smaller of the two. Regardless of its size and sesamoid development, the patella plays two crucial roles within the knee: the transmission of tensile forces generated by all heads of the quadriceps to the patellar tendon and the tibia, and the increase of the lever arm of the extensor muscles during TFJ flexion-extension, i.e. ultimately the increase of the efficacy of the whole extensor mechanism of the knee [18, 25, 50, 53, 55]. The motion of the patella relative to the distal femur is generally called either PFJ kinematics or patellar tracking, and the important biomechanical functions cited above are successfully achieved only when this motion occurs correctly [18, 50, 53]. Patellar tracking is a full six-degree-of-freedom motion, i.e. the patellar bone is not constrained in its motion. This motion can be described as translation and rotation along and about, respectively, predefined axes [15, 18, 26, 60]. Among all kinematics variables, only a few are generally considered of clinical importance, and these are PFJ flexion, rotation and tilt, these being assessed on the knee sagittal, coronal and transverse plane, respectively, and patella translation along the medio-lateral axis of the distal femur. All references, both on the patella and the femur, have variable definitions .
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