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
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
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
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.