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Medially luxating patella a condition where the patella (knee cap) pops in and out of place in response to extension, flexion, or weight bearing of the knee joint. The disease is congenital in origin, resulting from inherited shallow patellar grooves (the groove in which the patella normally sits in). The disease is sometimes complicated by an inherited abnormally medially located attachment point for the patellar tendon. The luxations almost always occur in the medial direction (toward the center of the body). For example, in the left limb, the tendency is for the patella to luxate to the right. The opposite is true for the right limb. The majority of luxating patella cases are bilateral.

Luxations cause inflammation and pain in the knee joint. Also, because the chief purpose of the patella is to redistrubute tendon forces to best handle weight bearing and stress, patellar luxations tend to throw off the physics of the entire leg, causing excessive wear on the hips and ankles. This predisposes to degenerative joint disease in these other joints. What's more, over time, as the luxations occur, the medial ridge of the patellar groove becomes worn, increasing the frequency of luxations. Eventually, if left untreated, the disease can progress to the point where the patella spends all of its time out of place. Patellar luxations are graded on the following scale:

Grade I: Patella luxates occasionally. Pain, discomfort, and lameness intermittent.

Grade II: Patella luxates regularly. Pain, discomfort, and lameness more frequently observed than normal gait.

Grade III: Patella spends more time luxated than in place. Pain, discomfort, and lamness almost always apparent.

Grade IV: Patella always luxated. Pain and discomfort severe to the point of severe to non-weight bearing lameness.

Medially luxating patella most commonly occurs in toy to small sized dogs. However, the disease is occasionally reported in larger dogs and in cats. Treatment is surgical and geared toward deepening the patellar groove and anchoring the patella laterally. Several surgical procedures effectively acheive this objective. Surgery is rewarding, but best prognosis is associated when performed early in the disease process before severe degenerative joint disease sets in.

By: Roger L. Welton, DVM

Founder, Web-DVM

President Maybeck Animal Hospital

Author Canine and Feline 101

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