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.
Intermittent or consistent lameness; bowlegged stance; reluctance to walk or jump; occasionally holding a rear leg out to the side when walking.
Lameness that is often intermittent, and may be unilateral or bilateral; thick, swollen stifles; pain on range-of-motion; crepitus; palpable luxation; inability to jump or walk normally; medial displacement of quadriceps muscle group; lateral bowing of the distal third of the femur.
Clinical signs associated with patellar luxation vary greatly with the severity of the disease: this condition may be an incidental finding detected by your veterinarian on a routine physical examination or may cause your pet to carry the affected limb up all the time. Most dogs affected by this disease will suddenly carry the limb up for a few steps, and may be seen shaking or extending the leg prior to regaining its full use.
As the disease progresses in duration and severity, this lameness becomes more frequent and eventually becomes continuous. In young puppies with severe medial patellar luxation, the rear legs often present a “bow-legged” appearance that worsens with growth. Large breed dogs with lateral patellar luxation may have a “knocked-in knee” appearance, combining severe lateral patellar luxation and hip dysplasia.
Causes of Patellar Luxation
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. The femoral groove into which the kneecap normally rides is commonly shallow or absent in dogs with non–traumatic patellar luxation.
Early diagnosis of bilateral disease in the absence of trauma and breed predisposition supports the concept of patellar luxation resulting from a congenital or developmental misalignment of the entire extensor mechanism. Congenital patellar luxation is therefore 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