The patella, or kneecap, is a small bone buried in the tendon of the extensor muscles (the quadriceps muscles) of the thigh. The patella normally rides in a femoral groove within the stifle. The patellar tendon attaches on the tibial crest, a bony prominence located on the tibia, just below the knee. The quadriceps muscle, the patella and its tendon from the “extensor mechanism” and are normally well-aligned with each other. Patellar luxation is a condition where the knee cap rides outside the femoral groove when the stifle is flexed. It can be further characterized as medial or lateral, depending on whether the knee cap rides on the inner or on the outer aspect of the stifle.
When the patella is in its normal position, its cartilage surface glides smoothly and painlessly along the cartilage surface of the trochlear groove with little or no discomfort.
As the patella “pops out” of its groove these cartilage surfaces improperly rub each other. The animal may cry and try to straighten (extend) the leg to “pop it back in” or may hold the limb up until muscle relaxation allows the kneecap to reposition itself. This resembles an intermittent lameness.
There is little or no discomfort until the cartilage is effectively “rubbed off” or eroded to a point where bone touches bone. From this point on, each time the patella “pops out” into its abnormal, luxated position it will cause pain. This explains why many individuals have no clinical lameness until they reach adulthood. Often progressive cartilage wear creates an acutely painful condition.