The patella is a large bone called a sesamoid that is embedded in the knee joint, which also is referred to as the femorotibial joint or the stifle. The patella actually sits in the tendon of the quadriceps muscle as it passes over the knee joint. Patellar luxation, commonly known as a "slipped knee cap," occurs when the patella is displaced from the joint. It is one of the most common orthopedic stifle abnormalities in domestic dogs. Miniature and toy breeds most commonly are affected by medial luxation associated with structural abnormalities of the long bones in their hind limbs. Although less common, lateral patellar displacement is seen more often in large and giant breeds. Most veterinarians use a grading system to classify the severity of a dog's condition, ranging from normal up to Grade IV, which is permanent, severe patellar luxation and non-weight-bearing lameness.
Causes of Patellar Luxation
Patellar luxation can be congenital (present at and existing from the time of birth), or it can be acquired later in life. Hereditary and developmental factors, such as poor hind limb musculoskeletal conformation and/or cranial cruciate ligament instability, can contribute to luxating patellas by altering the alignment of the long hind leg bones as they meet at the knee. Trauma to the femur or to the soft tissue that supports and surrounds the patella can also cause acquired patellar luxation. Most cases of patellar luxation in domestic dogs are thought to have a strong genetic component.
Prevention of Patellar Luxation
The best way to prevent patellar luxation is to remove affected animals from the breeding population. Conscientious breeders typically will not breed affected dogs, nor will they repeat sire-dam breedings that resulted in one or more affected offspring. Short of that, at-risk dogs should be carefully managed in terms of weight and exercise regimens, and they may require surgical intervention to return to pain-free function.
Patellar luxation is a common, mildly to severely painful condition in domestic dogs. Basically, the patella (kneecap) slips out of place for some reason, resulting in lameness, weakness and pain. It is most commonly seen in young miniature and toy breeds, although any age, gender or breed of dog can develop the disorder.
Symptoms of Patellar Luxation
Patellar luxation causes intermittent and chronic hind limb weakness, lameness and pain. The clinical signs of this condition can vary from dog to dog depending upon their pain tolerance and the severity of the displacement, but the symptoms typically include one or more of the following:
Intermittent lameness, which may range from partial to complete non-weight-bearing lameness in one or in both hind legs
Loss of range of motion in one or both hind legs
Abnormal carriage of one or both hind legs
Abnormal function of one or both hind legs
Temporary paralysis of the stifle (knee) joint
Pain when moving (or when the stifle joint is manipulated manually)
Reluctance to run or jump
Swelling at or around the stifle
The signs seen by owners will vary depending on the severity of the patellar displacement, the amount of degenerative osteoarthritis involved, the particular dog's pain tolerance level and the existence and extent of any other stifle abnormalities, such as cranial cruciate ligament rupture or tear. Some dogs develop an unusual "skipping" gait that waxes and wanes depending on the degree of pain they are experiencing.
Dogs At Increased Risk
All dogs are susceptible to patellar luxation. However, it is most frequently seen in young toy and miniature breeds - especially Poodles, Yorkshire Terriers, Pomeranians, Pekingese, Chihuahuas and Boston Terriers. Females are at a slightly higher risk of becoming affected than are males, although the reason for this is unclear.
Patellar luxation, which tends to be seen most often in young tiny dogs, is not difficult for a skilled veterinarian to diagnose using physical manipulation and radiology.
How Patellar Luxation is Diagnosed
While many competent veterinarians routinely perform a complete blood count, a serum biochemistry profile and a urinalysis on dogs displaying lameness or other obvious signs of pain, the results of these tests are typically inconclusive when patellar luxation is the culprit, because displacement of the patella is not a disease or systemic disorder. This condition involves only the knee joint(s) and some form of anatomical malalignment of muscle, tendons, supporting soft tissue and bones. Manual palpation of the knee will usually reveal patellar instability in affected animals. The patella may "pop" in and out of place when manipulated. Sedation may be required during this procedure, because it can be quite painful.
Another diagnostic tool available to veterinarians is arthrocentesis, which is the surgical puncture of a joint capsule and aspiration of a synovial fluid sample for laboratory analysis. Radiographs (x-rays) usually are recommended to assess the conformation of the knee joint and of the femoral and tibial long bones of the hind legs. The attending veterinarian may want to radiograph the hip and "ankle" joints as well, especially in advanced or chronic cases. Finally, a computed tomography scan (known as a CT scan or a CAT scan) can be used to visualize the stifle joint in three dimensions; this can be helpful as an aid in formulating a treatment protocol.
Owners of limping or lame dogs are cautioned against manipulating their dogs' legs without the guidance of a veterinarian or other competent, skilled professional. Stretching, bending, twisting, pulling and pushing on an already-damaged knee joint can worsen the pain and prognosis for a dog with luxating patellas.
The goals of treating dogs with luxating patellas are to relieve pain and to improve function of the knee joint by physically realigning the pertinent bones and stabilizing the patella in its proper anatomical position. When an owner notices her dog limping, reluctant to rise or showing other signs of hind limb discomfort, she should contact her dog's veterinarian for a thorough assessment of its condition. There are a number of treatment options if patellar luxation turns out to be the problem.
Non-surgical medical management typically includes administration of nonsteroidal anti-inflammatory drugs (NSAIDs) to minimize pain and reduce inflammation, together with particular physical rehabilitation exercises to strengthen the quadriceps muscles. Weight control is quite important in affected animals to reduce undue stress on the stifle (knee) joint. Other techniques that may or may not be appropriate in addition to medical treatment might include massage therapy to stimulate healing and reduce overall stress, hydrotherapy (swimming and other water exercises), possible application of acupressure techniques and other forms of follow-up supportive care which may help to ease pain, increase circulation, speed healing and otherwise promote wellness and comfort.
Surgical treatment options are also available. These include reconstruction of the soft tissue surrounding the stifle to provide proper support for the patella. There also are a number of surgical techniques to deepen the groove in the femur bone within which the patella normally nestles. Cryotherapy (ice packing) is usually done following surgery, with passive stifle range-of-motion exercises as soon as they can be tolerated. The veterinarian or skilled technician typically shows the owner how to perform these exercises safely.
A common complication of luxating patellas is the development of degenerative joint disease, also called osteoarthritis, in affected stifles. A number of chondroprotective supplements are available, both over the counter and by prescription, to help sooth the signs of arthritis and the pain associated with degenerative joint disorders. These supplements presently include polysulfated glycosaminoglycans, glucosamine, chondroitin sulfate and hyaluronan, among others. Weight control and moderate but regular exercise are also valuable.
In most cases, dogs with luxating patellas have a good prognosis. It is important for owners of dogs with this condition to discuss the possible risks and benefits of NSAIDs and all other forms of medical and supportive therapy with their veterinarian. Some side effects can include gastrointestinal, liver, kidney or other adverse reactions, and more severe and even permanent musculoskeletal damage can occur depending upon the therapy protocol used. There also is an increased risk of recurrence following corrective surgery, so the healing and recovery process should be carefully managed. Generally, however, the prognosis is good to excellent for return to normal or almost-normal hind limb function in mild to moderate cases that are treated surgically with appropriate follow-up care. Unfortunately, as mentioned above, degenerative joint disease usually continues despite treatment, and dogs with luxating patellas tend to be at greater risk of developing cranial cruciate ligament disease down the road.