Biomechanics of the patella in total knee replacement

An anatomical shape for the trochlea of the femoral component is an advantage in total knee design because in many cases the natural patella is preserved. The trochlea profile can be described as having a V-shape, with a wide angle and a circular base. In such cases the plastic patella should have the same profile, resulting in a generally conical shape. One of the questions regarding this design is whether the natural or artificial patella tilts in the femoral trochlea, producing wear and deformation of the plastic. We analysed the forces between the trochlea and the patella for various Q-angles and flexion angles. We found that the patella is fully stable and without tilting at all flexion angles, for a Q-angle of 14 degrees or less. Q-angles greater than this represent severe misalignment, and hence the cone-shaped patella is considered to be stable design.