Patellofemoral instability refers to an unstable relationship between the patella (kneecap) and femur (thigh bone). It is commonly encountered in sportspersons, especially in women, and may disrupt normal movement, causing pain and inflammation.
Knee joint anatomy:
Four bones join together to form the knee joint. The thigh bone (femur) joins at its lower end to the lower leg bone (tibia), forming the femorotibial joint, while at the front it articulates with the kneecap or patella, forming the patellofemoral joint. The two lower leg bones, tibia and fibula, are also joined together forming the tibiofibular joint.
Bones in a movable joint are not attached surface-to-surface but are bound together by cord-like fibrous extensions called ligaments which extend between the bone ends that hold the bones together while allowing movement at the same time.
Ligaments are tough, fibrous bands that ensure a stable relationship between the articulating bone surfaces in all positions and movements. Any ligament abnormality or trauma leading to loose or torn ligaments affects joint stability. An unstable joint leads to pain and disability.
The articulating end of the femur has two rounded heads, separated by a small groove. When the knee is extended or flexed, the patella slides vertically up and down in this groove. In the case of patellofemoral instability, the patella moves in an uncontrolled way, leaving its normal path of motion, giving the impression that it is sliding away.
Patellofemoral instability may vary in intensity. There may only be lateral maltracking of the patella in the femoral groove. As it moves, the patella slides outwards but remains inside the groove.
There may be recurrent partial dislocations referred to as subluxation, which correct automatically with the patella coming to normal alignment without external help. In severe cases, complete dislodgment of the patella may occur. The patella dislocates usually to the lateral side completely out of the groove and is locked there. When dislocated, the patella needs to be placed back into position.