The patellofemoral joint
The knee joint is made up of three separate and distinct articulations. The patellofemoral joint is the articulation of the kneecap (patella) over the thigh bone (femur). This joint transmits the forces of the thigh muscles (quadriceps) over the front of the knee down to the front of the shin bone (tibia). The kneecap increases the lever arm of the quadriceps thereby reducing the force needed to straighten the knee (the longer the lever the lighter the load). The added advantage is that the kneecap forms a joint with the thigh bone that is lined with cartilage which reduces friction. This joint has the thickest cartilage of any joint in the body (up to 7mm thick) due to the forces transmitted.
The stability of the kneecap is dependant upon the shape of the joint, the ligaments and the muscles with an overall general tendency for the kneecap to move outwards due to the alignment of the thigh bone on the shin bone.
What is a dislocation?
Dislocation means complete disruption of a joint where the two bone surfaces move away from each other. A subluxation is a partial dislocation.
When the kneecap dislocates it moves off the front of the thigh bone to sit on its outside.
When does it occur?
The kneecap will generally dislocate in one of two ways: A traumatic dislocation involves a significant force often during sport knocking the kneecap out of joint. Occasionally the kneecap will dislocate with a powerful twisting movement of the knee similar to that which often causes an ACL rupture (link to that section please) and the diagnosis can therefore be missed. Traumatic dislocation usually occurs in an entirely normally shaped joint and rarely occurs again. More commonly the kneecap will dislocate with very much less force ( atraumatic dislocation) due to an underlying abnormality of the shape of the joint (similar to a clicky or dislocated hip in a newborn). This will usually become a recurrent problem often in the mid to late teens. The most common abnormality is a shallow or non-existent groove (trochlea) on the front of the thigh bone (called trochlea dysplasia), with less commonly a high kneecap (patella alta) or twisted thigh and shin bones (femoral and tibial torsion). Frequently there will be a history within the family and generalised flexibility of the joints.