Patellar Luxation

Patellar luxation describes a kneecap that slips out of the shallow groove in which it is meant to glide. In dogs it is a very familiar orthopaedic problem; in cats it is less often discussed, partly because cats are so good at hiding lameness and partly because many affected cats cope remarkably well with only subtle signs.

The patella, or kneecap, sits within the tendon of the big thigh muscles and runs in a groove at the lower end of the femur, acting like a pulley that lets the leg extend smoothly. When the groove is too shallow, or the alignment of the bones and soft tissues is off, the kneecap can pop to the inside (medial) or, less commonly, the outside (lateral) of the joint. In cats the cause is usually developmental — the joint simply forms with a predisposition to slip — though a fall or injury can occasionally be responsible.

The tell-tale sign is an intermittent skip or hop: the cat is walking normally, then briefly holds a hind leg up or gives a little hitch in its stride, then carries on as if nothing happened. This is the kneecap slipping out and then sliding back into place. Vets grade the condition from one to four, from a kneecap that only luxates when manipulated, up to one that is permanently out of place. Some breeds, including the Abyssinian and Devon Rex, are reported more frequently.

Diagnosis is largely a hands-on job: the vet feels the knee, gently manipulates the patella to see how easily it luxates, and grades the severity, sometimes supported by X-rays to assess the bones and check for early arthritis. Because cats mask discomfort, the impact is easy to underestimate, so a careful examination matters.

Management depends on severity. Many mild cases need nothing more than keeping the cat slim, providing joint support, and monitoring for change. More severe or clearly painful luxations, or those causing progressive arthritis, are treated surgically — deepening the groove and realigning the supporting structures so the kneecap tracks properly. With appropriate care, the outlook for an affected cat is generally very good.