The stifle (knee) is essentially a hinge joint, allowing the major muscles of the upper leg to produce the normal swinging movement of the lower leg with walking or running. The patella (knee cap) is a small bone within the patellar tendon of the quadraceps muscle group that rides in a groove in the femur at the knee joint acting as a pulley mechanism, stabilizing the knee. The patellar tendon attaches to the tibial crest below the stifle. Occasionally the patellar mechanism is not well aligned during development and the end result is that the patella luxates, flipping in and out of the groove.
The primary goal of surgery is to realign the patellar mechanism and thus prevent luxation/dislocation of the knee cap. Most cases of patellar luxation may be surgically corrected with deepening the groove that the patella slides in and realignment of the attachment of the patellar tendon on the crest of the tibia. Occasionally the malalignment of the quadraceps group is severe enough to require a corrective osteotomy of the femur to help 'straighten' the patellar tendon.