What is Patellar Luxation?

The patella, or kneecap, is part of the stifle joint (knee). In patellar luxation, the kneecap luxates, or pops out of place, either in a medial or lateral position.

Bilateral involvement is most common, but unilateral is not uncommon. Animals can be affected by the time they are eight weeks of age. The most notable finding is a knock-knee (genu valgum) stance. The patella is usually reducible, and laxity of the medial collateral ligament may be evident. The medial retinacular tissues of the stifle joint are often thickened, and the foot can be seen to twist laterally as weight is placed on the limb.

 

Patellar Luxation Grades

The Patellar Luxation Database is for dogs 12 months and over. Examinations performed on dogs less than 12 months will be treated as consultations and no OFA numbers will be assigned.

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A method of classifying the degree of luxation and bony deformity is useful for diagnosis and can be applied to either medial or lateral luxations by reversing the medial-lateral directional references. The position of the patella can easily be palpated starting at the tibial tubercle and working proximally along the patellar ligament to the patella.

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Grade 1: Manually the patella easily luxates at full extension of the stifle joint, but returns to the trochlea when released. No crepitation is apparent. The medial, or very occasionally, lateral deviation of the tibial crest (with lateral luxation of the patella) is only minimal, and there is a very slight rotation of the tibia. Flexion and extension of the stifle are in a straight line with no abduction of the hock.

Grade 2:  There is frequent patellar luxation, which, in some cases, becomes more or less permanent. The limb is sometimes carried, although weight bearing routinely occurs with the stifle remaining slightly flexed. Especially under anesthesia, it is often possible to reduce the luxation by manually turning the tibia laterally, but the patella reluxates with ease when manual tension of the joint is released. As much as 30 degrees of medial tibial torsion and a slight medial deviation of the tibial crest may exist. When the patella is resting medially the hock is slightly abducted. If the condition is bilateral, more weight is shifted onto the forelimbs. Many dogs with this grade live with the condition reasonably well for many years, but the constant luxation of the patella over the medial trochlear ridge of the trochlea causes erosion of the articulating surface of the patella and also the proximal area of the medial lip. This results in crepitation becoming apparent when the patella is luxated manually.

Grade 3:  The patella is permanently luxated with torsion of the tibia and deviation of the tibial crest of between 30 degrees and 50 degrees from the cranial/caudal plane. Although the luxation is not intermittent, many animals use the limb with the stifle held in a semi-flexed position. The trochlea is very shallow or even flattened.

Grade 4:  The tibia is medially twisted and the tibial crest may show further deviation medially with the result that it lies 50 degrees to 90 degrees from the cranial/caudal plane. The patella is permanently luxated. The patella lies just above the medial condyle and space can be palpated between the patellar ligament and the distal end of the femur. The trochlea is absent or even convex. The limb is carried, or the animal moves in a crouched position, with the limb flexed.

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