by Patricia Long
reviewed & edited by Dr. Gail Smith, and Dr. Peterson
Genetics is the one area of dog breeding that breeders try hardest to control with the least amount of information. Until genetic markers are found, and testing for those markers becomes standard, there is very little that gives a breeder the ability to control or eliminate genetic defects. For hip dysplasia, however, there now exists a tool that has been proven effective at greatly reducing the incidence of bad hips.
Canine hip dysplasia (HD) is caused when the femoral head does not fit properly in the hip socket, causing instability of the joint. Over time, this malformation can cause degenerative joint disease (DJD) which causes increased pain and immobility. Surgery can help to correct the problem, but can be expensive and stressful for dog and family. Obviously, this is a situation that everyone would like to see eliminated, but how?
Joint laxity has been the only statistically significant predictor of DJD. The tighter the joint, the less likely DJD will occur. Originally, the only means at the breeder’s disposal was to look at the dog’s movement in order to judge whether the hips seemed sound. But many dogs with wretched movement never develop hip problems, and dogs with excellent movement can develop DJD. Next came the x-ray and later the OFA registry. (Actually, a Swedish registry was the first to rank hip x-rays, followed by the UK, and eventually the US and finally Australia). The x-rays are taken with the hip extended, then sent to OFA to be read by 2 radiologists - 3 in the case of a tie. (GDC is similar, except that it is an open registry. It uses the same x-ray positioning as OFA.)
There are several problems with OFA. Positioning of the hips is critical for an accurate reading, and a great deal of variation is possible. Precision and accuracy are difficult to achieve on a seven point scale that is only a little better than a pass/fail where the reading is subjective. Readings done at an early age have little or no correlation to readings done at later ages. Certification cannot be received until the dog is two years old (one year for GDC). And the biggest drawback is that the predictive value of OFA has never been shown. Some dogs with OFA good develop hip problems, and some dogs with OFA bad never have any troubles at all.
PennHIP (from University of Pennsylvania Hip Improvement Program) is a closed registry that was developed by Dr. Gail Smith at the University of Pennsylvania School of Veterinary Medicine. In studies of the mechanics of the hip joint, Dr. Smith found that the hip extended view could actually mask some of the hip laxity. He determined that a neutral position, where the hips are positioned as though the dog were standing, best showed maximum laxity. An additional benefit of this position is that it produces a much more consistent reading. A distraction index (DI) is then calculated from the x-ray. This is an objective value measuring the percent of potential movement of the femoral head in the hip socket. The smaller the DI, the tighter the hips. So a DI of .23 would be a very tight hip, and a DI of .83 would be an extremely loose hip. But best of all, the readings can be done as early as 4 months; test have shown an extremely high degree of correlation between readings done at 4 months and those done at 2 years using PennHIP methods.
PennHIP requires that all x-rays taken be submitted for a reading; the vet is not permitted to pre-screen. It is hoped that this will allow the database to more accurately reflect the range of DI’s for each breed. The relationship between the DI and DJD is breed specific. In other words, a DI of .45 would a high probability for DJD in a breed like German Shepherds, but would just as certainly predict sound hips for a Rottweiler. Currently with 170 Bernese Mountain Dogs in the data base, the DI range is .23 to 1.0 with a median of .56 .
The DI as a measure of passive hip laxity has been shown to have a high heritability. This means that when breeding stock selection is based on hip laxity, the offspring will tend to resemble the parents with respect to hip conformation. In studies with German Shepherd Dogs and Labrador Retrievers, significant improvement in joint laxity was achieved in only 2 generations by always selecting dogs with DI’s in the low end of the range for that breeding population. For BMDs, if breeding stock is selected with DI’s markedly less than .56, meaningful selection pressure can be brought to bear.
For breeders, PennHIP offers the best tool to date for improving hips. Not only does it offer more sensitive, precise, and consistent readings, but predictability at a much younger age. How often are breeders tempted to breed an excellent bitch at 20 months based on preliminary OFA readings, only to have the same bitch fail to pass at 24 months? Or how about a dog that was easily finished, and campaigned for multiple Best of Breed wins, at considerable time and expense, only to fail OFA? (Any resemblance to actual dogs, living or dead, is entirely coincidental.)
Recent Advances in the Diagnosis of Hip Dysplasia and the Genetic Implications, Gail K Smith, VMD, PhD, 1997.
Genetics of the Dog, Malcolm B Willis, 1989.
Essentials of Small Animal Internal Medicine, Richard W Nelson & C Gullermo Couto, 1992.
Saunders Manual of Small Animal Practice, Stephen J Birchard & Robert G Sherding, 1994.
The Merck Veterinary Manual, seventh edition, 1991.