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Monday, February 14, 2011

Knee cap dislocation in dogs, a common but easily fixable problem.

Your Pet

The overall anatomy of mammals is pretty constant. But for some reasons, different names are used when it comes to animals. For instance, the dog knee is called the stifle, although all the structures are the same as in the human knee. And just as is the case with humans, dogs can suffer from a dislocation or a luxation of their kneecap, the patella in medical terms.

But this is where the similarities stop. An orthopedic surgeon friend of mine once told me that patella luxations in humans are more likely to have a traumatic origin. Although traumatic luxations in dogs do exist, it is fairly unusual. Most of the time, patella luxation is spontaneous and present at birth or at an early age as the result of an abnormal anatomy of the stifle. And surgical stabilization nearly always requires some restoration of a more normal anatomy as part of the treatment.

But first what is this normal anatomy? In a normal knee, every time the joint bends or extends, the kneecap should remain centered at the end of the thigh bone, the femur, in a specific area called the trochlea. The trochlea is simply a groove where the patella smoothly slides up and down. Above the patella is the very powerful quadriceps femoris muscle, and below the patella is the patellar tendon, which attaches it to the tibia on another specific structure called the tibial tuberosity. For the whole mechanism to function normally, the quadriceps-patella-tendon unit has to be on a straight, centered line when the muscle contracts during extension of the knee. In the case of an abnormal anatomy, the patella will dislocate intermittently or permanently, based on the severity of the problem. As a result, the femoral trochlea may be either flat or convex, instead of concave. The direction of the luxation, or the dislocation, can be either on the inside of the knee (medial luxation, more common in small- to medium-size dogs) or the outside (lateral luxation, rarer and found mainly in large-size dogs).

Classically, a dog diagnosed with a patella luxation shows an intermittent skipping nonweight-bearing lameness of one or both rear legs. But this does not necessarily mean a visit to the orthopedic surgeon.

The need for surgery all depends on the frequency of the lameness or the amount of pain. For instance, when I was a child, my family had a toy poodle who, once in a blue moon, would hold his leg up for a few steps. He was otherwise perfectly normal with no sign of pain. He never had surgery, died happily of old age, and only when I became a vet many years later did I learn about patella luxation. But in some dogs, this purely mechanical and pain-free initial lameness can progress and become more frequent, even permanent. This is usually due to the erosion of the articular cartilage and the development of secondary arthritis.

Another reason could be a rupture of the cranial cruciate ligament, which is placed under increased stress in the presence of a patella luxation. This progression is more rapid in larger dogs, and in my mind, any large dog diagnosed with a patellar luxation, whether it is clinically silent or a cause of lameness, whether it is lateral or medial, is a surgical candidate, since it is just a matter of time for that knee to become trouble.

During the surgical procedure, different manipulations of the soft tissues and the bones of the joint are performed. The minimum treatment usually requires the creation of a femoral trochlea of adequate depth and the re-alignment of the quadriceps mechanism in a more central position. The soft tissues on the side of the luxation are released, and the soft tissues on the side opposite to the luxation are instead tightened by sutures. In case of extreme anatomy, the end of the femur or the top of the tibia or both have to be cut and reset in a more normal configuration. These femoral and tibial corrective osteotomies are major surgical procedures but, fortunately, are not commonly required.

From an owner’s point of view, whatever is needed on the inside is not that important; the length of the incision and the post-operative care are practically the same.

The good news is that after submitting the knee to such a “surgical beating,” the prognosis after surgical stabilization of a luxating patella is excellent, especially if post-operative rehabilitation is used and if the procedure was done prior to the development of arthritis or rupture of the cranial cruciate ligament. In fact, surgery for a patella luxation is a breath of fresh air in an orthopedic surgeon’s day.

Because of our experience with the procedure and because of the consistently favorable outcome, we usually look good at the end. And who does not need a stress-free day with a happy ending?

Your Pet is published on the second and fourth Mondays of each month. Bruno Massat, DVM, is a veterinary surgeon, board certified by the American and European Colleges of Veterinary Surgeons. He also is a certified canine rehabilitation practitioner. Massat is associated with the Animal Medical Center of New England in Nashua. Call 821-7222 or visit for more information or to make an appointment.