Hip dysplasia is a problem where a dog’s hip joint doesn’t fit together well. This can cause pain and arthritis later in life. Veterinarians take special X-rays to look at dog hips so breeders and owners can make better choices.
There are two common ways to check dog hips with X-rays, but an important difference between them is the age when they can be used.
OFA scoring
This method looks at a picture of the dog’s hips with the legs stretched out. The hips are graded as excellent, good, fair, or worse. While OFA X-rays can be taken earlier, the results are only considered official when the dog is 24 months (2 years) of age.
PennHIP distraction index (DI)
This method uses a special device and X-rays to measure how loose the hip joint really is. The result is a number called the distraction index. Higher numbers mean looser hips, which will lead to more hip problems in the future. PennHIP testing has been shown to be valid as early as 16 weeks of age, long before a dog reaches adulthood.
Why age matters
The difference in timing between the two tests is very important. Waiting until a dog is 2 years old to get official OFA results means hip looseness may go undetected during puppyhood and adolescence. PennHIP allows veterinarians and breeders to measure hip looseness much earlier, when decisions about training, breeding, or career paths can still be adjusted.
A large study looked at hundreds of dogs that had both an OFA score and a PennHIP distraction index. The researchers found that many dogs judged as “normal” by OFA showed clinically important joint laxity, or looseness, when tested with PennHIP. Clinically important laxity means the dog has a higher risk of developing arthritis later in life, even if the OFA X-ray looks okay.
Here is what the results showed.
If OFA said “Excellent” hips (60 dogs)
About 52 percent, or 31 out of 60 dogs, still had loose hips based on PennHIP measurements.
If OFA said “Good” hips (223 dogs)
About 82 percent, or 183 out of 223 dogs, had loose hips by PennHIP standards.
If OFA said “Fair” hips (84 dogs)
About 94 percent, or 79 out of 84 dogs, had loose hips based on PennHIP measurements.
All dogs that OFA called borderline or dysplastic
100 percent of these dogs had loose hips when measured with PennHIP.
Quick points to remember
Many dogs that look normal on OFA X-rays still have hip looseness that PennHIP can detect.
More than half of dogs with “excellent” OFA hips still had joint laxity on PennHIP testing.
PennHIP can be performed as early as 16 weeks of age, while official OFA results are not available until 24 months of age. Earlier testing allows earlier decisions that may reduce pain, arthritis, and long-term joint damage.
What this means for dog owners and breeders
OFA X-rays are useful and widely used, but this study shows they do not always detect hip joint looseness. PennHIP provides additional information by directly measuring how loose the joint is, and it can do so much earlier in a dog’s life. This makes PennHIP especially helpful for breeders, working-dog programs, and owners who want to understand future joint health as early as possible.
In short, only looking at how the bones line up on an X-ray is not enough to tell if a hip joint is truly tight and healthy. Measuring joint looseness with PennHIP, and doing so early in life, can reveal hidden risks that matter for both breeding decisions and a dog’s long-term comfort and mobility.
The entire paper is here:
https://avmajournals.avma.org/view/journals/javma/237/5/javma.237.5.532.xml
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