Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, Philadelphia 19104-6010, USA.
The DI method was developed, at least in part, because of perceived variations among radiologists in regard to subjective hip scores assigned to radiographs. It also was recognized that the prevalence of CHD among many dog breeds was disturbingly high, despite efforts to lower the prevalence using subjective score as a selection criterion. The DI method was developed on the basis of results of mechanical testing of cadaver hip joints, which accomplished 3 principal objectives. First, it showed that when the hip joint is in the standard, hip-extended, internally rotated position, passive laxity is minimized. Second, it showed that passive laxity was maximized when the hip joint is in the neutral position. Third, it demonstrated the sigmoidal nature of the load/displacement curve for the hip joint in the neutral position, which suggested that high repeatability from examiner to examiner in regard to DI could be expected without the need to standardize applied force. The mean and range of DI were shown to vary from 1 breed to the next, but within an individual dog, DI appears to remain constant (within limits of scientific acceptability and clinical applicability) from 16 weeks of age. Passive hip laxity measured on the distraction view is, on average, 2.5 to 11 times greater than that measured on the standard, hip-extended radiographic view. Performance Borzois and Greyhounds, which have an extremely low prevalence of CHD, uniformly have tight hip joints (DI < 0.3), and mean DI for dog breeds that have a high prevalence of CHD is significantly greater than mean DI for Borzois and Greyhounds. However, individual dogs that have DI < 0.3, even though members of breeds prone to CHD, have a low risk of developing DJD. Some, but not all, dogs with DI > 0.3 will develop DJD by 3 years of age, and DI has been shown to be the principal risk factor for development of DJD. Susceptibility to development of DJD appears to be breed-specific. For example, given equivalent DI, German Shepherd Dogs are more at risk for developing DJD than are Rottweilers. Heritability of DI is higher than that of the subjective hip score; thus, selection pressure based on DI should result in faster genetic change than selection pressure based on subjective hip scores. Finally, the DI method has been performed on approximately 14,000 dogs, some of which were evaluated multiple times. It is apparent that this method is no more harmful than the standard, hip-extended radiographic method or palpation performed as part of a routine orthopedic examination.
Advances in diagnosing canine hip dysplasia
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