Basic kinematics and biomechanics of the patello-femoral joint. Part 1: The native patella.

The patello-femoral joint (PFJ) enhances our ability of knee flexion and extension and is assumed to have evolved through men's ability of having adopted a bipedal gait. This articulation between patella and femur is relatively complex and displays intricate biomechanical behaviour. Forces in the patello-femoral joint are a function of the quadriceps force, and the angle of flexion of the knee. They are highly dependent on the distance between the patello-femoral joint and the centre of gravity, which explains why different activities despite equivalence in tibio-femoral angle may exert wide variations in patello-femoral reaction forces and contact pressures. During normal daily activities the PFJ becomes exposed to force values between 0.5 to 9.7 x body weight, whilst sporting activities create force values that approach up to 20 x body weight. Those forces are considerable and it is therefore not surprising that the PFJ is particularly susceptible to degenerative disease especially if its mechanical equilibrium is disturbed through injury, muscle weakness and congenital or developmental abnormalities. The clinician must be aware of biomechanical and kinematic specifics, the high patellofemoral load values, small patellofemoral contact areas, and resultant high stress magnitudes when trying to remedy such abnormalities.