The following paper provides a disussion of Hip Dysplasia to the interested reader:
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The presence of hip dysplasia can have a severe impact on the quality of life, level of performance, and reproductive potential of an athletic or working dog. Unfortunately, canine hip dysplasia is a common orthopedic disease, especially in larger breeds of dogs. Systematic screening for this disorder combined with responsible breeding has lowered, but not eradicated, the occurrence of hip dysplasia.
Canine hip dysplasia is an inherited and developmental condition that involves a lack of conformity between the femoral head and acetabulum. Affected dogs have hips that are structurally and functionally disease free at birth. As these puppies mature, they develop laxity in the coxofemoral joints. Laxity means that there is incongruity in the joint surfaces that allows the head of the femur to be partly displaced (subluxated) from the acetabulum.
The consequences of subluxation are abnormal loading in weight bearing and remodeling of cartilage and bone. The end result is degenerative joint disease (DJD), characterized by joint cartilage erosion, inflamed joint fluid, swelling of the round ligament, thickening of the joint capsule, and osteophyte formation. Abnormal hip joint laxity has been demonstrated in puppies as young as a few weeks of age.
The degree of joint laxity appears to have a hereditary basis, in that the progeny of dogs without hip dysplasia have less laxity than those from dysplastic parents. However, not all puppies with high laxity will develop degenerative changes in the hip joints. The heritability of hip dysplasia is estimated to be 25 to 40; in other words the appearance of hip dysplasia in a puppy is not 100 percent due to that dog’s genetic composition.
There are environmental factors that contribute to the disease and one of these is diet. Several investigators have concluded that the growth rate and the associated weight of the dog influence the expression of hip dysplasia. Overabundant feeding has been shown to shorten the time to first appearance and also increase the severity of canine hip dysplasia. In contrast, in a study of Labrador retrievers with a genetic predisposition for hip dysplasia, those fed 25 percent less food than their counterparts had less hip joint laxity when they were 30 weeks old. Furthermore, by maintaining the dogs on this feeding regimen until they were 2 years old, this beneficial effect was still present.
Controlled feeding will not eliminate hip dysplasia in those dogs that have it in their bloodlines but it may modulate the severity of the disease. An adult maintenance diet or large breed canine growth diet, versus regular puppy food, is recommended for puppies who are at risk for hip dysplasia.
The correlation between rapid weight gain and development of hip dysplasia does not necessarily mean that increased weight as such is the damaging factor. Other factors that may contribute to hip dysplasia include decreased pelvic muscle mass and abnormal bone maturation. It has been suggested that muscles develop and mature more slowly than does the skeleton. The relative decrease in muscle tone and strength, in turn, is thought to be responsible for laxity of the hip joint which initiates secondary degenerative changes. In a study comparing the relative muscle mass of greyhounds and German shepherd dogs, diminished pelvic muscle mass and altered muscle fiber size and composition were detected in German shepherds with hip dysplasia. The pelvic muscle atrophy was selective, not diffuse, suggesting that the atrophy was a primary problem and not secondary to disuse.
Alterations in skeletal development have been identified in dogs with hip dysplasia. In a study done on Labrador retriever puppies, onset of mineralization of the proximal (upper) femur was significantly later in dysplastic than in radiographically normal puppies, suggesting that bone maturation (endochondral ossification) may be abnormal in dogs with hip dysplasia. Great Danes that were fed ad libitum had more severe hip dysplasia and earlier radiographic closure of the acetabular growth plate than did their diet-restricted counterparts. Similar findings were obtained in a group of Labrador retriever pups.
Most cases of hip dysplasia develop between 4 and 12 months of age. Affected dogs may have a swaying gait and may draw their hind limbs forward or hyper extend their hock joints, perhaps to redistribute some weight to their forelegs. With time, affected dogs may have difficulty rising, climbing up stairs, or getting into a car. They may seem to have less energy and will refrain from normal activities such as running or playing with other dogs. On physical examination, thigh muscle atrophy and reduced range or motion in the hip joints may be detected. A dysplastic dog may have less than 180 degrees of hip extension or will display discomfort when fully extended. An Ortolani-sign, representing subluxation followed by reduction of the hip joint, may be elicited. This test is most reliable when performed on a sedated dog.
Radiographic techniques used to diagnose hip dysplasia include the standard ventro dorsal (VD) projection of the pelvis and the distraction index (DI). The standard VD pelvis is the view used by the Orthopedic Foundation for Animals’ (OFA) certification. In 1972 it was reported that canine hip dysplasia could be definitively diagnosed using the VD view in 16 percent of dogs at 6 months of age and 95 percent of dogs at 24 months of age. More recently, the low reliability of radiographic examination of dogs less than 2 years of age has been questioned. In a retrospective study analyzing date from over 2,000 dogs from the OFA database, the reliability of detecting mild to moderate hip dysplasia in dogs 3 to 18 months of age was 84.4 percent for mild dysplasia and 97.4 percent for moderate dysplasia. Based on these results, preliminary evaluations can be of value, and are recommended for potential breeding dogs or those with clinical signs, prior to 2 years of age.
The conventional VD pelvis view is an unnatural position and may actually cause “tightening” of the soft tissues supporting the joints. The distraction index (DI) involves positioning anesthetized dogs with their hind limbs perpendicular to the table and applying a distraction force to the femurs. A DI is calculated from the radiograph. Dogs having DI values of 0.9 or greater are more susceptible to developing degenerative changes from hip dysplasia. In a recent study evaluating the reliability of the hip distraction index, the DI at 4 months of age was correlated with the DI at 12 months of age. Each 0.1 increase in the DI at 4 months of age is associated with a 2.6-fold increase in the risk of developing DJD by 3 years of age. The DI can be useful in diagnosing laxity in very young dogs, prior to the development of degenerative changes in the hip joints.
Both conservative and surgical options exist. Conservative therapy should always be tried before considering surgery; it consists of behavior modification and possibly medical therapy. Behavior modification entails a schedule of controlled activity that is appropriate for the individual with an emphasis on lower-impact exercise such as swimming and walking. Weight control is essential. Medical therapy may consist of chondroprotective agents and nonsteroidal anti-inflammatory drugs such as buffered aspirin, carprofen, or erodolac. Ideally, nonsteroidals are given on an every-other-day or as needed basis to reduce side-effects. The most commonly recommended surgical options are a triple pelvic ostectomy (TPO), total hip replacement, or femoral head and neck ostectomy (FHO). The only one of these procedures that retains the dogs own hip joint is the TPO. Success with a TPO surgery is predicated on early detection. In one study, TPO surgery at 12 months of age was seven times more likely to result in postoperative DJD than surgery performed at 6 months. Total hip replacement usually yields excellent results with appropriate case selection and femoral head and neck ostectomy is a viable option that can eliminate pain and allow functional movement
The cause of hip dysplasia is multifactorial and still under investigation. A dog diagnosed with hip dysplasia may be virtually asymptomatic with insignificant pain, depending on a variety of factors. The clinical symptoms do not always correlate with radiographic signs. Early detection can be very advantageous to successful breeding programs and treatment regimens.
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