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What Is It?

Medial patellar luxation is the medial displacement of the patella or knee cap from its normal anatomic position. It is sometimes referred to as a luxating patella, a ‘trick knee’, or a ‘knee dislocation’. It is thought to be hereditary as it is seen more often in certain small and toy breeds, but the exact inheritance pattern has not been established. It can be seen in any breed or sex of dog, but is more common in some breeds including the Affenpinscher, Brussels Griffon, Chihuahua, English Toy Spaniel, Japanese Spaniel, Maltese, Manchester Terrier, Miniature Pinscher, Papillon, Pekingese, Pomeranian, Poodle, Pug, Shih Tzu, Silky Terrier, and Yorkshire Terrier. It can also be seen in cats, with the Devon Rex and Abyssinian being overrepresented.

Medial patellar luxation can vary from a mild instability of the patella diagnosed during a routine examination to a complete permanent luxation associated with severe skeletal deformities and lameness. It may be unilateral or bilateral. It may also occur acutely secondary to trauma.

Diagnosis

Diagnosis of medial patellar luxation begins with a complete medical history and a thorough physical examination. Radiographs of both pelvic limbs are necessary to identify concurrent skeletal abnormalities and grade the condition appropriately. Medial patellar luxations can be divided into four grade classifications:

  • Grade I: The patella can be luxated manually on examination, but reduces immediately when pressure is released. Spontaneous luxation or lameness rarely occurs.
  • Grade II: The patella can be easily luxated manually on examination, and there may be frequent luxation of the patella with lameness during normal movement. Reduction of the patella is not always immediate and may remain luxated until reduced by the examiner or spontaneously reduced when the leg is extended. There may be mild angular and torsional deformities of the tibia and femur.
  • Grade III: The patella remains luxated medially most of the time, but can be manually reduced with the leg in extension. After manual reduction, however, normal flexion and extension of the stifle usually results in reluxation. There may be abnormalities of the supporting soft tissues of the stifle joint and deformities of the tibia and femur.
  • Grade IV: The patella is permanently luxated and cannot be manually repositioned. There are usually significant abnormalities of the supporting soft tissues of the stifle joint and deformities of the tibia and femur including a medial rotation of the tibia.

Treatment

Treatment of this condition depends on the clinical signs, the degree of luxation, the degree of skeletal deformities, and age of the patient. Grade I luxations are generally treated medically and monitored for progression of disease. Medical treatments may include chondroprotectants such as ucosamine and chondroitan (Cosequin), nonsteroidal anti-inflammatories as needed, and weight management. Most patients with Grade I medial patellar luxation require no treatment.

For all other grades, surgery is generally recommended. Surgery is aimed at reestablishing the alignment of the femur, patella, and tibia and usually involves a combination of soft tissue and orthopedic procedures. Prognosis after surgery is usually good to excellent. Reluxation after surgery is not uncommon, but it is usually reduced to an asymptomatic Grade I luxation. Arthritis may still develop in the joint even after surgical intervention.