Labrador Hip Dysplasia Diagnosis
How is hip dysplasia diagnosed?
Diagnosis of hip dysplasia in dogs that are showing clinical signs of arthritis and pain is usually made through the combination of a physical exam and radiographs (x-rays). If a dog is showing outward signs of arthritis, there are usually easily recognized changes in the joint that can be seen on radiographs. In addition, the veterinarian may even be able to feel looseness in the joint or may be able to elicit pain through extension and flexion. Regardless, the results are straightforward and usually not difficult to interpret.
However, about half of the animals that come in for a determination on the health of their hip joints are not showing physical signs, but are intended to be used for breeding. The breeder wants to ensure that the animal is not at great risk for transmitting the disease to his or her offspring. There are two different testing methods that can be performed. The traditional and still most common is OFA testing. The other newer technique is the PennHip method.
OFA: The method used by the Orthopedic Foundation for Animals (OFA) has been the standard for many years. The OFA was established in 1966, and has become the world’s largest all-breed registry. The OFA maintains a database of hip evaluations for more than 475,000 dogs. Radiographs are taken by a local veterinarian under specific guidelines and are then submitted to the OFA for evaluation of hip dysplasia and certification of hip status. Since the accuracy of radiological diagnosis of hip dysplasia using the OFA technique increases after 24 months of age, the OFA requires that the dog be at least two years of age at the time the radiographs are taken. They also recommend that the evaluation should not be performed while the female is in heat. To get the correct presentation and ensure that the muscles are relaxed, the OFA recommends that the dog be anesthetized for the radiographs. OFA radiologists evaluate the hip joints for congruity, subluxation, the condition of the acetabular margins and acetabular notch, and the size, shape, and architecture of the femoral head and neck. The radiographs are reviewed by three radiologists and a consensus score is assigned based on the animal’s hip conformation relative to other individuals of the same breed and age. Using a seven point scoring system, hips are scored as normal (excellent, good, fair), borderline dysplastic, or dysplastic (mild, moderate, severe). Dogs with hips scored as borderline or dysplastic are not eligible to receive OFA breeding numbers.
|When dogs born in 1972 to 1980 were compared with dogs born in 1989 and 1990, 60% of the breeds demonstrated a statistically significant decrease in hip dysplasia. At the same time, 68% of breeds had a statistically significant increase in the number of hips scored as excellent.|
The OFA will also provide preliminary evaluations (performed by one OFA radiologist) of dogs younger than 24 months of age to help breeders choose breeding stock. Reliability of the preliminary evaluation is between 70 and 100% depending on the breed. Results published by the OFA suggest that the incidence of hip dysplasia in certain breeds has decreased as a result of selective breeding programs. When dogs born in 1972 to 1980 were compared with dogs born in 1989 and 1990, 60% of the breeds demonstrated a statistically significant decrease in hip dysplasia. At the same time, 68% of breeds had a statistically significant increase in the number of hips scored as excellent. This information may suggest progress is being made to decrease the frequency of hip dysplasia, but it may simply be that only radiographs from dogs thought to have normal hips are being submitted to the OFA, while those with dysplasia are being screened out by referring veterinarians.
PennHIP: The diagnostic method used by the University of Pennsylvania Hip Improvement Program (PennHIP) uses distraction/compression radiographic views to more accurately identify and quantify joint laxity. Radiographs of the hip joints are taken with the dog under heavy sedation. Two views are obtained with the hind limbs in neutral position to maximize joint laxity. Weights and an external device are used to help push the head of the femur further into or away from the acetabulum. The amount of femoral head displacement (joint laxity) is quantified using a distraction index (DI). The DI ranges from 0 to 1 and is calculated by measuring the distance the center of the femoral head moves laterally from the center of the acetabulum and dividing it by the radius of the femoral head. A DI of 0 indicates a very tight joint. A DI of 1 indicates complete luxation with little or no coverage of the femoral head. A hip with a distraction index of .6 is 60% luxated and is twice as lax as a hip with a DI of .3. When the DI was compared to the OFA scores for 65 dogs, all dogs scored as mildly, moderately, or severely dysplastic by the OFA method had a DI above .3.
Hip joint laxity as measured by the DI is strongly correlated with the future development of osteoarthritis. Hips with a low DI are less likely to develop osteoarthritis. Hips with a DI below .3 rarely develop osteoarthritis visible on radiographs. Although hips with a DI above .3 are considered “degenerative joint disease susceptible” not all hips with a DI greater than .3 eventually develop osteoarthritis. It is known that some hips with radiographically apparent laxity do not develop osteoarthritis. A means of differentiating lax hips that develop osteoarthritis from those that will not is important in developing a prognosis and making treatment recommendations. In one study, the DI obtained from dogs at four months of age was a good predictor of later osteoarthritis, though the 6 and 12-month indices were more accurate.
To assure quality and repeatability among diagnostic centers using the PennHip technique, veterinarians must take a special training course to become certified. As this technique gains popularity more and more veterinarians are becoming certified.
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