Canine hip dysplasia is a very common degenerative joint disease seen in dogs. There are many misconceptions surrounding it. There are many things that we know about hip dysplasia in dogs, there are also many things we suspect about this common cause of limping, and there are some things that we just do not know about the disease. We will cover all of those here and hope to separate out fact, theory, hypothesis, and opinion.
What is hip dysplasia?
To understand what hip dysplasia really is we must have a basic understanding of the joint that is being affected. The hip joint forms the attachment of the hind leg to the body and is a ball and socket joint. The ball portion is the head of the femur while the socket (acetabulum) is located on the pelvis. In a normal joint, the ball rotates freely within the socket. To facilitate movement, the bones are shaped to perfectly match each other, with the socket surrounding the ball. To strengthen the joint, the two bones are held together by a ligament. The ligament attaches the femoral head directly to the acetabulum. Also, the joint capsule, which is a very strong band of connective tissue, encircles the two bones adding further stability. The area where the bones actually touch each other is called the articular surface. It is perfectly smooth and cushioned with a layer of spongy cartilage. In the normal dog, all of these factors work together to cause the joint to function smoothly and with stability.
Hip dysplasia results from the abnormal development of the hip joint in the young dog. It may or may not be bilateral, affecting both right and left sides. It is brought about by the laxity of the muscles, connective tissue, and ligaments that should support the joint. Most dysplastic dogs are born with normal hips, but due to genetic and possibly other factors, the soft tissues that surround the joint start to develop abnormally as the puppy grows. The most important part of these changes is that the bones are not held in place, but actually move apart. The joint capsule and the ligament between the two bones stretch, adding further instability to the joint. As this happens, the articular surfaces of the two bones lose contact with each other. This separation of the two bones within a joint is called subluxation and this, and this alone, causes all of the resulting problems we associate with the disease.
What are the symptoms of hip dysplasia?
Dogs of all ages are subject to the symptoms of hip dysplasia and the resultant osteoarthritis. In severe cases, puppies as young as five months will begin to show pain and discomfort during and after vigorous exercise. The condition will worsen until even normal daily activities are painful. Without intervention, these dogs may be unable to walk at all by a couple years of age. In most cases, however, the symptoms do not begin to show until the middle or later years in the dog's life.
The symptoms are typical for those seen with other causes of osteoarthritis. Dogs may walk or run with an altered gait, often resisting movements that require full extension or flexion of the rear legs. Many times, they run with a 'bunny hopping' gait. They will show stiffness and pain in the rear legs after exercise or first thing in the morning. Most dogs will warm up out of the muscle stiffness with movement and exercise. Some dogs will limp and many will decrease their level of activity. As the condition progresses, the dogs will lose muscle tone and may even need assistance in getting up. Many owners attribute the changes to normal aging, but after treatment is initiated, they are shocked to see much more normal and pain-free movement return.
Who gets hip dysplasia?
Hip dysplasia can be found in dogs, cats, and humans, but for this article, we are concentrating only on dogs. In dogs, it is primarily a disease of large and giant breeds. The disease can occur in medium-sized breeds and rarely even in small breeds. It is primarily a disease of purebreds although it can happen in mixed breeds, particularly if it is a cross of two dogs that are prone to developing the disease. German Shepherds, Labrador Retrievers, Rottweillers, Great Danes, Golden Retrievers, and St. Bernards appear to have a higher incidence, however, these are all very popular breeds and may be over represented because of their popularity. On the other end Greyhounds and Borzois have a very low incidence of the disease.
What are the risk factors for the development of hip dysplasia?
Hip dysplasia is caused by looseness in the hip joint. The looseness creates abnormal wear and erosion of the joint and as a result pain and arthritis develops. The disease process is fairly straightforward; the controversy starts when we try to determine what predisposes animals to contract the disease. Almost all researchers agree that there is a genetic link involved. If a parent has hip dysplasia, then the offspring are at greater risk for developing hip dysplasia. Some researchers feel that genetics are the only factor involved, where others feel that genetics contribute less than 25% to the development of the disease. The truth probably lies in the middle. If there are no carriers of hip dysplasia in a dog's lineage, then it will not contract the disease. If there are genetic carriers, then it may contract the disease. We can greatly reduce the incidence of hip dysplasia through selective breeding. We can also increase the incidence through selectively breeding. We cannot, however, completely reproduce the disease through selective breeding. In other words, if you breed two dysplastic dogs, the offspring are much more likely to develop the disease, but will not all have the same level of symptoms or even necessarily show any symptoms. The offspring from these dogs will, however, be carriers and the disease may show up in their offspring in later generations. This is why it can be difficult to eradicate the disease from a breed or specific line.
Nutrition: Experimentally, we can increase the severity of the disease in genetically susceptible animals in a number of ways. One of them is through obesity. It stands to reason that carrying around extra weight will exacerbate degeneration of the joint in a dog with a loose hip. Overweight dogs are therefore at a much higher risk. Another factor that may increase the incidence is rapid growth in a puppy during the ages from three to ten months. Experimentally, the incidence has been increased in genetically susceptible dogs when they are given free choice high protein, high calorie diets. In a large study done in 1997, Labrador Retriever puppies fed a high protein, high calorie diet free choice for three years had a much higher incidence of hip dysplasia than their litter mates who were fed the same high calorie, high protein diet, but in an amount that was 25% less than that fed to the dysplastic group. As might be expected, however, the free choice group was significantly heavier at maturity and averaged 22 pounds heavier than the control group. Because obesity is also a risk factor, this study may be difficult to interpret.
I have yet to see a study that links an increased incidence of hip dysplasia in dogs fed a normal diet of commercial puppy food versus a specialty diet formulated for just large breed dogs. There have also been studies looking into protein and calcium levels and their relationship to hip dysplasia. Both of these studies were able to increase the level of hip dysplasia by feeding increased amounts of calcium and protein. But once again, the studies of puppies fed greatly increased amounts over normal recommended values and compared them to animals fed decreased amounts. They failed to compare puppies fed a normal amount of food that had the recommended amount of protein, fat, and calcium to those fed a diet with slightly less protein, fat, and calcium (similar to those 'large breed puppy foods' that are now flooding the market). I have yet to see a study that links an increased incidence in hip dysplasia in dogs fed a normal diet of commercial puppy food versus a specialty diet formulated just for large breed puppies.
Exercise: Exercise may be another risk factor. It appears that dogs that are genetically susceptible to the disease may have an increased incidence of disease if they over-exercise at a young age. But at the same time, we know that dogs with large and prominent leg muscle mass are less likely to contract the disease than dogs with small muscle mass. So exercising and maintaining good muscle mass may actually decrease the incidence of the disease. Moderate exercise that strengthens the gluteal muscles, such as running and swimming, is probably a good idea. Whereas, activities that apply a lot of force to the joint are contraindicated. An example would be a jumping activity such as playing Frisbee.
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