I mentioned in my previous post that the options and results for hip testing are complicated. I want to lay out the basics for the current options, so you can make an informed decision when choosing a breeder, and choosing your puppy. There are currently two options for testing hips in the US: the Orthopedic Foundation for Animals (OFA) and the University of Pennsylvania Hip Improvement Program (PennHIP). They utilize different methods and measure different things, and choosing to use one or both is dependent on the breeder. Intelligent people can disagree on which is better, but at least one should be performed on any dogs intended to be bred. It’s even better if siblings not intended to be bred are also x-rayed, to give a better sample size of hip inheritance for a particular dog/line.
Hip dysplasia is a common condition in all dogs, influenced by both genetic causes (nature) and environmental causes (nurture). The hip joint functions as a ball and socket. In dogs with hip dysplasia, the ball and socket do not fit together or develop properly, and they rub and grind instead of sliding smoothly. This results in deterioration over time and an eventual loss of function of the joint itself.
OFA (Orthopedic Foundation for Animals)
OFA evaluates the structure of the hip, whether the parts are shaped correctly, and how they fit together. A healthy hip has a deep seated ball (femoral head) which fits tightly into a well-formed socket (acetabulum) with minimal joint space. There should also be almost complete coverage of the socket over the ball. Hips susceptible to or affected with dysplasia have a shallow socket, a femoral head that is not well seated (meaning there is space between the two bones) into the socket, and there are arthritic bone changes caused by those parts not being formed correctly and not fitting together well (such as remodeling, osteophytes, bone spurs, and sclerosis).
Passing scores from OFA are rated as Excellent, Good, and Fair. Failing scores are Borderline, Mild, Moderate, and Severe. This determination is made by 3 expert canine radiologists, and can be considered more subjective than PennHIP scores. You may not agree with a given score, but you also have the actual x-ray image to consider what part of the hip to improve with a better structured mate. OFA X-rays can be done after 2 years of age, and preliminary tests can be done as early as 4 months, but preliminary scores do not count as the “final/official score” until the dog has matured. As discussed ahead, there is reasonable reliability that a dog who passes their preliminary evaluation will pass their final evaluation, but it can be problematic if a breeder only ever performs preliminary x-rays.
PennHIP (University of Pennsylvania Hip Improvement Program)
PennHIP is intended to measure the laxity of the hip joint. The evaluation measures Distraction Index. DI is a measurement of the amount of maximal passive hip laxity. A low DI indicates a small amount of laxity or a "tight" hip, while a high DI indicates a "loose" hip. Dogs with tight hips (DI's less than or equal to 0.30), are at extremely low risk for developing degenerative joint disease (hip dysplasia). As the DI increases, the hip is more likely to develop DJD (degenerative joint disease). Dogs with DI's that are 0.70 and above are considered to be at high risk for developing hip dysplasia.
The way PennHIP evaluates the hip joint is through a compression view. The x-ray is taken with the femoral heads seated tightly in the acetabula congruency between the two joint surfaces. A second x-ray is taken with a distraction view, which shows the maximum separation distance of the femoral head center from the acetabular center. According to the University of Pennsylvania, this protocol has been shown to reveal 2.5 times more joint laxity than the standard hip-extended radiograph.
The program utilizes a database to determine the overall scores of a breed, so a dog can be measured as having better, worse, or average hips compared to the particular breed at large (apples to apples). This arguably makes PennHIP more objective - dogs are being measured against other members of their breed, rather than the opinion of 3 expert veterinarians.
PennHIP touts its accuracy as early as 16 weeks, meaning there is a fairly high level of consistency between the evaluations done at 16 weeks and those done 1-2 years later, meaning one benefit of PennHIP is earlier data to use to make breeding decisions. However, that data is not made publicly available, which means if you want to find out the scores of a particular dog, you would have to ask the breeder to share that information.
There are pros and cons to weigh for both methods, but you should expect a responsible breeder to use one or the other (or both!) to help determine their breeding decisions, and for that information to be made available to you before purchasing a puppy.