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 groove within the femur (thigh bone) in the knee (Figure 1). The patellar tendon attaches on the tibial crest, a bony prominence located on the tibia (shin bone), 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 (dislocation) is a condition where the knee cap rides outside the femoral groove when the knee is flexed (Figure 1). 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 respectively.
Patellar luxation is one of the most common orthopedic conditions in dogs, diagnosed in 7% of puppies. The condition affects primarily small dogs, especially breeds such as Boston and Yorkshire terriers, Chihuahua, Pomeranian, and miniature poodle. 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, potentially resulting in discomfort and loss of function.
Patellar luxation occasionally results from a traumatic injury to the knee, causing sudden severe lameness of the limb. However, the precise cause remains unclear in the majority of dogs but is likely multifactorial. The femoral groove into which the knee cap normally rides is commonly shallow (Figure 2a, Figure 2b) 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 that patellar luxation results from a congenital or developmental misalignment of the entire extensor mechanism. Developmental patellar luxation is therefore no longer considered an isolated disease of the knee, but rather a consequence of a complex skeletal abnormalities affecting the overall alignment of the limb, including:
- abnormal conformation of the hip joint, such as hip dysplasia (link to health topic)
- malformation of the femur, with abnormal angulation and torsion (rotation) (Figure 3)
- 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
Because there is evidence that this condition is at least in part genetic, dogs diagnosed with patellar luxation should not be bred.
Symptoms 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 (“skip”), 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.
The diagnosis of patellar luxation is essentially based on palpation of an unstable knee cap (Figure 4) on orthopedic examination. Additional tests may be required to diagnose conditions that can be associated with patellar luxation and help your ACVS board-certified veterinary surgeon recommend the most appropriate treatment for your pet. These may include:
- palpation of the knee under sedation to assess damage to ligaments
- x-rays of the pelvis, knee and occasionally tibias to evaluate the shape of the bones in the rear leg and evaluate for hip dysplasia (Figure 5)
- a three-dimensional computed tomography (CT or CAT Scan) to provide an image of the skeletal features of the entire rear legs. This advanced imaging technique helps the veterinary surgeon plan surgery in cases where the shape of the femur or tibia needs to be corrected (Figure 3)
- blood tests and urine analysis as a precaution before anesthesia
Overtime, the knee cap may luxate more and more often out of its groove, eroding cartilage and eventually exposing areas of bone which leads to arthritis and associated pain. In puppies, the abnormal alignment of the patella may also lead to serous deformation of the leg.
The veterinary surgeon that has operated on your pet will best be able to advise you and establish a personalized postoperative treatment plan.
Over 90% of owners are satisfied by the progress of their dog after surgery. The prognosis may be less favorable in large dogs, especially when patellar luxation is combined with other abnormalities, such as angulation of the long bones or hip dysplasia.
Recurrence of knee cap instability is uncommon. Migration or breakage of surgical implants used to maintain bones in position and infections occurs rarely.