Assessing the dysplastic hip

Millions of dollars have been expended over the past 50 y in the diagnosis, assessment, and treatment of canine hip dysplasia (CHD). Very little has been resolved in terms of reducing the incidence of the problem, determining how best to diagnose it, or even agreeing on a definition of CHD!

Most would agree that CHD is a genetically determined, multifactorial developmental abnormality of the coxofemoral joint in which the hip does not fit together properly (1). Hip laxity produces abnormal development of the femoral head and acetabulum resulting in degenerative joint disease and varying degrees of pain and dysfunction (1,2).

Diagnosis of the condition relies on history of hind limb pain or dysfunction and physical examination findings compatible with hip laxity (positive Ortolani sign or Barden’s test) or degenerative joint disease. Ultimately, a diagnosis of CHD relies on radiography…and that’s when the fun begins!

Even the most casual observer of canine hip radiographs will be able to pick out a severely subluxated coxofemoral joint or one with advanced degenerative changes; however, the detection of early hip changes can be a much more challenging proposition. Breeding decisions or, in some cases, even life or death choices for specific animals can hinge on relative radiographic subtleties.

Veterinarians and dog owners or breeders have 2 major choices when it comes to formal radiographic assessment of their dog’s hips. Certification is provided by the Orthopedic Foundation for Animals (OFA), or hip laxity assessment can be obtained from the University of Pennsylvania Hip Improvement Program (PennHIP).

The OFA (www.offa.org) was founded in 1966 as a not- for-profit organization primarily through the efforts of the Golden Retriever Club of America and the German Shepherd Dog Club of America. The OFA currently maintains registries of several inherited diseases in dogs and cats, including elbow dysplasia, hypothyroidism, congenital cardiac abnormalities, as well as hip dysplasia (2).

Due to its status as the oldest method of assessing dysplastic hips, and because of its origins and ongoing close relationship with the American Kennel Club and individual breed organizations, the OFA enjoys a position of prominence over its newer rival — PennHIP.

And “rival” is the correct word! Owing to personalities on both sides, the interaction between the OFA and PennHIP hasn’t exactly been collegial!

The OFA provides an assessment of a dog’s coxofemoral phenotype leading to “certification.” While radiographs from dogs under 24 mo of age can be submitted for an opinion, certification will not be provided until radiographs are taken at or after 24 mo. A hip-extended ventrodorsal radiograph of diagnostic quality, marked to identify the animal and veterinarian, and properly positioned including both ilia and stifles is required. The OFA advises that the radiographs be taken with the animal under sedation or general anesthesia but there is no requirement that this be done. Submissions to the OFA are blindly assigned to 3 veterinary radiologists who grade the hips into one of 7 categories: excellent, good, fair, borderline, or mildly, moderately, or severely dysplastic. The former 3 categories are eligible for OFA certification (2).

The PennHIP method (www.pennhip.org) was developed by Dr. Gail Smith at the University of Pennsylvania in 1983 with the current radiographic submission system dating back to 1993. PennHIP is an objective measurement of hip laxity, which is considered to be the root of degenerative hip changes in later life. The method has several key differences from the OFA approach. 1) puppies as young as 16 wk of age can be assessed through this method with a high degree of correlation to results obtained at a later age. This is of critical importance to those wishing to breed or begin a dog’s working career before 2 y of age. 2) the PennHIP radiographs must be taken with the animal under anesthesia or heavy sedation to overcome the effects of muscle tension on the assessment of hip laxity. 3) the hips are positioned in a standing angle. The hip-extended position used for OFA radiographs tightens up the joint capsule which has the effect of bringing the hip into closer alignment. 4) a “distracted” radiographic view (Figure 1) is taken using the PennHIP distractor. This device acts as a fulcrum against which the proximal femurs are levered to demonstrate the degree of laxity in the hip (Figure 2). 5) PennHIP radiographs can only be taken and submitted by veterinarians or technicians certified to do so after having taken a training course and submitting trial radiographs demonstrating their proficiency with the technique. 6) all radiographs taken with the PennHIP method must be submitted for inclusion in the database regardless of how abnormal they may appear. 7) the PennHIP analysis does not confer “certification” but rather a “distraction index” (DI), which is an objective measure of the degree of hip laxity, is calculated.

Figure 1.

A radiograph of the coxofemoral joints is taken using the PennHip distractor as a fulcrum against which the proximal femurs are levered to demonstrate hip laxity.

Figure 2.

The distracted radiographic view demonstrating hip joint laxity. From this view the distraction index (DI) is calculated and compared to other DI’s for the breed.

The DI correlates with the likelihood of the animal developing future degenerative changes in its hips. The PennHIP report also compares the dog to others of the same breed in the database, since DI’s tend to be somewhat variable and breed- specific. PennHIP suggests that breeding stock be taken from dogs that are at least in the top 50% of favorable DI readings for the breed. Given this aspect of the report, the requirement for mandatory submission of films becomes self-evident. It is estimated that perhaps as much as 50% of the OFA films taken are never submitted if there is obvious evidence of dysplasia and little chance of certification (3).

As indicated earlier, these 2 organizations have frequently clashed throughout their histories. The OFA claims that their efforts have resulted in a steady decline in the incidence and severity of CHD and an increase in “good” to “excellent” hips from 1972–2000 (2,4). PennHIP credits the OFA with some impact on the severity of the problem, but points out that due to the lack of mandatory submission of OFA films, any change in the incidence of CHD is unknown (3,5). The OFA radiologists agree that a dog is “normal,” “borderline,” or “dysplastic” almost 95% of the time, and agree on the specific rating almost 75% of the time (2). PennHIP disputes this level of agreement and points out that other “non-OFA” board certified radiologists don’t show anywhere close to this level of agreement in reading hip films. The conclusion they draw is that the OFA method is handicapped by being subjective (3). While the OFA agrees that the PennHIP method is an objective measure, they do not accept the significance of this measurement. In the OFA guide they state, “The degree of joint laxity as demonstrated by… using a fulcrum/stress device — that can be normal, and what degree is abnormal is unknown.” (2) While both sides agree that “tighter” hips seem to be better hips, the OFA points out that not all dogs with demonstrated loose hips go on to develop osteoarthritis (2). PennHIP counters that a Nestlé Purina study demonstrated 55% of dogs certified “normal” by the OFA went on to develop hip osteoarthritis and breedings of OFA normal German shepherds have been able to produce no better than 19% dysplastic pups (3)…and on and on it goes!

What does all this mean for the small animal practitioner and his or her clients? First, it means that the laws of sensitivity and specificity of scientific tests apply very well to OFA and PennHIP radiographs; neither is perfect! In general terms, because of the nature of OFA radiographic positioning and the subjectivity of the test, some false negative results will be produced allowing dysplastic animals into the gene pool. On the other hand, the fact that not every dog with loose hips develops osteoarthritis means that PennHIP assessment will produce some false positives removing some animals unnecessarily from breeding lines. The OFA provides a well-respected, widely recognized standard for diagnosing CHD. PennHip provides a method that can be used much earlier in the animal’s life and implies some conclusions about the dog’s coxofemoral genotype since DIs have been shown to be heritable. There would appear to be room for both methods in the evaluation of dysplastic hips.