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Hip Testing 201 - The Best Test


TL;DR There are pros and cons to both methods. It's more important that hip x-rays are performed and scored with either method (or both!), and how that information is used, than which method is used.

I want to delve a little bit into the details of pros and cons for both testing methods. Because OFA and PennHIP are taking different measurements and using their data to make different conclusions, it’s hard to compare the two, and even harder to determine which is more useful and reliable. There are valid critiques for both methods, but a responsible breeder will be able to explain their choice. Some breeders believe that other breeders only use PennHIP when dogs do not receive a passing score from OFA. Other breeders don’t value the evaluation method of OFA because the results are more subjective. Other breeders may do BOTH PennHIP and OFA, because the more information, the better.

Both tests measure different aspects of the functionality of the hip joint, and that information can be used by knowledgeable breeders to make the best breeding decisions possible. Both tests struggle with sample size - not all dogs are tested, therefore the population being measured may not be entirely representative of the breed as a whole. There is also the issue that dwarf anatomy and biomechanics (how all of the pieces of their body machine fit together and operate in tandem with the rest of the machine) are vastly different from other breeds.

OFA bases their ideal hip conformation OFA based off the structure of the Greyhound. How certain can we be that a corgi with hips like a Greyhound (with entirely different structure and function) will benefit their health and longevity? That answer is not clear, especially with a polygenic condition influenced by environment (nutrition and a tendency to permit pudginess in puppyhood; play on hard surfaces, growth plate injuries, etc). Those confounding variables are why breeders should x-ray hips, but the matter of score is only a piece of the puzzle for evaluating breeding stock.

One of the biggest benefits of OFA is that their data is publicly available. If you want to know if a particular breeder x-rays hips, you can mosey on over to OFA and check for yourself. For every aspect of picking a breeder, trust but verify. You don’t even need to ask the breeder to provide that proof - you can search their kennel name on OFA's website and it will show you those results. ALL passing results are made public; failing scores are not made public unless the breeder agrees to publish that information. This is why it’s suspicious if a breeder has excuses for not having them scored.

There’s another grey area in terms of breeding young dogs. Dogs need to be at least 2 years old (and OFA is specific that it can’t even be a few days before the dog’s birthday), breeding before that age is deemed irresponsible, because they cannot have received those scores prior to that age. OFA does offer preliminary scores, which a breeder can use to determine if they want to move forward with breeding a young dog. I want to note that you should raise your eyebrows if EVERY dog in a program is being bred with only preliminary scores.

OFA states that a breeder can have a reasonable measure of confidence that if a dog has a passing preliminary score, they will mature to have a passing score. OFA claims there is 100% reliability for a preliminary grade of Excellent to receive a passing score - meaning that if they are scored as Excellent in their preliminary x-ray, they are 100% likely to receive a passing score of Excellent, Good, or Fair from the official hip x-ray evaluation. They further claim there is 97.9% reliability for a preliminary grade of Good to receive a passing final score; and 76.9% reliability for a preliminary grade of Fair being normal at age 2. Reliability of scores increased the closer a dog gets to 2 years, for both passing and failing scores. For normal hip conformations, the reliability was 89.6% at 3-6 months, 93.8% at 7-12 months, and 95.2% at 13-18 months.*

False positives (a score being rated failing when it is actually passing) can occur. This can be confusing, as you might think a positive score is, well, a positive thing. In this case, a dog being “positive” for hip dysplasia means they were evaluated as having the condition, not that the condition is a positive thing to have. False negatives are more problematic, because it means a dog receives a passing score (they were “negative”, as in, lacking the presence of dysplasia) but later are determined to be dysplastic.

As noted above, if a dog receives a Fair rating in their preliminary score, there is a 24% chance a 4-month-old puppy will not receive a passing score in their final evaluations. PennHIP claims their compression-distraction method has only a 12% false positive rate for 4-month-old puppies. OFA-type predictive tests gave false negative results (receiving a failing preliminary score, but will eventually receive a passing score) in 15% of cases in 6-month-old puppies, and 12% in 12-month-old puppies. PennHIP’s method showed zero false negatives for 6- and 12-month old dogs.

What about subjectivity in OFA evaluations? Each hip is examined in 7 anatomical dimensions. OFA analyzed the results of 1.8 million radiographic evaluations by 45 radiologists; they found that all three radiologists agreed as to whether a dog’s scores were passing or failing 94.9% of the time. 73.5% of the time, all three radiologists agreed on the specific score a hip should receive; 21% of the time, two radiologists agreed on the same hip grade and the third radiologist was within one hip grade of the other two. Two radiologists agreed on the same hip grade and the third radiologist was within two hip grades of the other two 5.4% of the time. Despite the subjective nature of the test, OFA considers this a high level of reliability in their scores.

Other complications include the fact that OFA statistics are skewed because not all films taken are submitted, whereas PennHIP requires all films, regardless of score, be submitted and entered into the database. There’s also the issue of what a PennHIP score actually means. PennHIP uses a percentile calculated by the scores submitted from all dogs within a breed. 0% is the highest percentile (the loosest hips in the breed database) and 99% is the lowest (the tightest hips in the breed database). But those numbers will depend entirely on the number of dogs submitted. What good is it if all the dogs submitted have a very low variety of scores? If one breed has dogs scoring between .2 and .8, you would feel confident that there is a difference between a dog in the lowest percentile, compared to a dog in the top percentile. However, what if a breed’s range is between .50 - .55? The difference between the absolute best scores and the absolute worst scores may not be different enough to justify choosing to breed one dog and not another.

The shortcoming for both tests is that neither are 100% accurate in determining which dogs will suffer from dysplasia. There is a correlation between the hip structure and the likelihood of developing hip dysplasia, but it is possible for a dog to receive poor evaluations and never show outward, clinical symptoms of dysplasia their entire lives. It is also possible that symptoms that appear to be dysplasia are caused by other conditions (such as (luxating patella, rheumatoid arthritis, degenerative spinal disease, and many others). Rather than viewing that as the grey area/mystery of any scientific endeavor, irresponsible breeders will use this as an excuse to not x-ray hips at all. Instead, a responsible breeder keeps up to date with all puppies they produce, and study how their dogs' scores correlate with quality of life as the dogs age.

*Corley, EA, et al. Reliability of Early Radiographic Evaluation for Canine Hip Dysplasia Obtained from the Standard Ventrodorsal Radiographic Projection. JAVMA. Vol 211, No. 9, November 1997.

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