Patellar luxation

The patella, or knee cap, 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 form 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.

Patellar luxation is one of the most common congenital anomalies in dogs, diagnosed in 7% of puppies. The condition affects primarily small dogs, especially breeds such as Boston terrier, Chihuahua, Pomeranian, miniature poodle and Yorkshire terrier. The incidence in large breed dogs has been on the rise over the past ten years, and breeds such as Chinese shar pei, flat-coated retriever, Akita and Great Pyrenees are now considered predisposed to this disease. Patellar luxation affects both knees in 50% of all cases, resulting in discomfort and loss of function.

Patellar luxation occasionally results from a traumatic injury to the knee, causing sudden non-weight-bearing lameness of the limb. It may also develop subsequent to cranial cruciate deficiency in dogs that will typically have a chronic history of lameness. However, the cause remains unclear in the majority of dogs. Congenital patellar luxation is no longer considered an isolated disease of the knee, but rather a component/consequence of a complex skeletal anomaly affecting the overall alignment of the limb, including:

• Abnormal conformation of the hip joint, such as hip dysplasia
• Malformation of the femur, with angulation and torsion
• Malformation of the tibia
• Deviation of the tibial crest, the bony prominence onto which the patella tendon attaches below the knee
• Tightness/atrophy of the quadriceps muscles, acting as a bowstring
• A patellar ligament that may be too long

You should seek veterinary surgical advice if you have any concern about the gait of your pet or if your primary veterinarian advises you to do so. The severity of patellar luxation has been graded on a scale of 0 to 4, based on orthopedic examination of the knee. Surgical treatment is typically considered in grades 2 and over:

Grade I	Knee cap can be manipulated out of its groove, but returns to its normal position spontaneously
Grade II	Knee cap rides out of its groove occasionally and can be replaced in the groove by manipulation
Grade III	Knee cap rides out of its groove most of the time but can be replaced in the groove via manipulation
Grade IV	Knee cap rides out of its groove all the time and cannot be replaced inside the groove

Thank you American College of Veterinary Surgery for making this information available for all who are interested!