Penn study looks at risks associated with hip dysplasia

Philadelphia — A study comparing a University of Pennsylvania method for evaluating a dog's susceptibility to hip dysplasia with the traditional American method has shown that 80 percent of dogs judged to be normal by the traditional method are actually at risk for developing osteoarthritis and hip dysplasia.

The survey results, researchers say, indicate that the standard Orthopedic Foundation for Animals (OFA) model for scoring of radiographs that certify dogs for breeding underestimates their osteoarthritis susceptibility.

The traditional OFA screening method relies heavily on conventional hip-extended, or HE, radiographs, which the study contends do not provide critical information needed to accurately assess passive hip joint laxity and, therefore, osteoarthritis susceptibility.

The PennHIP method quantifies hip laxity using the distraction index (DI) metric that ranges from a low of .08 to greater than 1.5. Smaller numbers mean better hips. The PennHIP method considers a DI of less than .3 to be the threshold below which there is a near-zero risk to develop hip osteoarthritis later in life. In contrast, dogs having hip laxity with DI higher than .3 show increasing risk to develop hip osteoarthritis, earlier and more severely, as the DI increases.

Dr. Gail Smith, professor of orthopaedic surgery, lead author and director of the PennHIP Program, explains, "The reason the PennHIP method measures more hip laxity is the specific position in which the hips are placed. A small distraction force is applied with the hips in this position, and 2.5 to 11 times more joint laxity is revealed. It is this laxity that is quantified by the DI, and which multiple studies have shown to be significantly correlated with OA risk."

According to the OFA's Dr. G.G. Keller, director of veterinary services, that assertion misses the mark. "(The study) is comparing apples to oranges because the OFA method is not designed to measure joint laxity. They are different methodologies," he says, adding that joint laxity is not the only factor to consider in evaluating susceptibility for developing hip dysplasia.

Comparing the overall results of the study, 52 percent of OFA-rated "excellent," 82 percent of OFA-rated "good" and 94 percent of OFA-rated "fair" hips all fell above the PennHIP threshold of .3, making them all susceptible to the osteoarthritis of canine hip dysplasia (CHD) though scored as "normal" by the OFA. Of the dogs the OFA scored as "dysplastic," all had hip laxity above the PennHIP threshold of .3, meaning there was agreement between the two methods on dogs showing CHD or the susceptibility to CHD.

According to Penn researchers, even if breeders were to selectively breed only those dogs having OFA-rated "excellent" hips, the study suggests that 52-100 percent of the progeny, depending on breed, would be susceptible to hip dysplasia based on the Penn scoring method.

In the study, both methods were applied to a sample of 439 dogs older than 2 years. The four most common breeds included in the study were German Shepherds, Labrador Retrievers, Golden Retrievers and Rottweilers, all breeds commonly susceptible to hip dysplasia.

The findings, the researchers contend, point to a weakness in current breeding practices. If breeders continue to select breeding candidates based upon traditional scores, then breeders will continue to pair susceptible dogs and fail to improve hip quality in future generations.

Keller stands behind OFA's methods and believes the transparency of data available from OFA is invaluable. "We have an open display of data, and we track results and trends within the breeds," he explains. Anyone in the public domain has open access to the results, which is essential to ensure the breeding standards remain high. For example, a breeder searching for potential dams or sires can refine their search to select only those dogs that have "excellent" rated hips to help decrease susceptibility for CHP.

Smith says despite well intentioned hip-screening programs to reduce the frequency of the disease, the reason little progress has been made is that CHD is erroneously considered an "all or nothing" disease—the pet either has it or it doesn't.

The key feature of the PennHIP radiographic method, he says, is its ability to determine which dogs may be susceptible to osteoarthritis later in life.

"We have to put aside the 'all or nothing' notion. There is a variable degree of susceptibility for the disease," he explains.

"If the dog has a DI above .3, the susceptibility of having hip dysplasia increases—but that doesn't mean it's a guarantee that the dog will develop the condition."

To achieve genetic control of CHD, researchers say, an accurate test must minimize false-negative diagnoses that mistakenly permit the breeding of dogs that carry genes coding for CHD. Particularly for a late-onset disease such as CHD, dogs remaining in the gene pool must not only be free of obvious signs of CHD at the time of evaluation (2 years for OFA) but ideally should not be susceptible to the osteoarthritis of CHD that occurs later in life.

"We don't want people to think that if the dog is at risk that it isn't breedable," Smith says. "But if we can tighten the gene pool and only breed dogs that are better than the average, it can make a difference."

The study was conducted by Smith, Michelle Y. Powers, Georga T. Karbe, Thomas P. Gregor, Pamela McKelvie, William T. N. Culp and Hilary H. Fordyce of the Department of Clinical Studies at Penn Vet. Culp is currently with the School of Veterinary Medicine at the University of California, Davis.

The study was funded by the University of Pennsylvania, the National Institutes of Health, The Seeing Eye Inc., the Morris Animal Foundation and Nestle Purina Co. The article was published in the Journal of the American Veterinary Medical Association.

Smith, who is the inventor, and the University of Pennsylvania, which holds the patent, have a financial interest in the PennHIP method.