PennHIP Radiographic Evaluation for Canine Hip Dysplasia
by Dr. Peter Haase, D.V.M.
Canine Hip Dysplasia (CHD) is the most common, heritable orthopedic problem seen in dogs. It affects virtually all breeds of dogs but is especially problematic in large and giant breeds.
Breeders and veterinarians have long sought a reliable method to determine the likelihood of a dog having CHD and passing that genetic trait to any offspring. Despite years of research and the combined effort of the Orthopedic Foundation for Animals and responsible dog breeders, it has been impossible to eliminate hip dysplasia from breeds of dogs like the Labrador Retriever, Golden Retriever, and German Shepherd Dog. Unfortunately, there is still a 20-25 percent chance of producing a puppy with CHD even when using the OFA method of hip evaluation.
In 1983, Dr.Gail Smith, a veterinary orthopedic surgeon and bioengineer from the University of Pennsylvania School of Veterinary Medicine, began to actively research and develop a new scientific method for the early diagnosis of Canine Hip Dysplasia. Research in his laboratory resulted in a diagnostic method capable of estimating the susceptibility for CHD in populations of dogs as young as sixteen weeks. The method has shown distinct advantages over the OFA method that evaluates dogs at two years or older.
PennHIP stands for the University of Pennsylvania Hip Improvement Program. The technique evaluates both the shape of the hip joint as well as measures hip joint laxity or looseness, which is the primary cause of osteoarthritis. Osteoarthritis, also known as degenerative joint disease (DJD), is the hallmark of canine hip dysplasia. The PennHIP method of evaluation is very accurate in its ability to predict the onset of osteoarthritis.
The x-rays are taken by trained and certified PennHIP veterinarians worldwide and are sent to the PennHIP analysis center for evaluation. The resulting data is stored in the database, which is continually monitored as it expands. As more information becomes available, the PennHIP laboratory is able to obtain more precise answers to questions about the cause, prediction and genetic basis of CHD.
PennHIP Radiographic Evaluation at a Glance
To obtain diagnostic radiographs, it is important that the patient and the surrounding hip musculature be completely relaxed. For the comfort and safety of the animal, this requires heavy sedation. Typically a reversible drug is used so that the dog can be bright and alert shortly after the evaluation and can go home right away. Three separate radiographs are taken: the compression view, the distraction view and the hip-extended view (see below). The distraction view and compression view are used to obtain an accurate and precise measurement called the distraction index. The hip-extended view is used to evaluate the shape of the hips and check for existing osteoarthritis.
The radiographs above are of the same dog, yet the hip joint laxities (looseness) in each view look very different. Notice that the hips in the distraction view appear to be much looser than they do in the hip-extended view. On average the distraction view has been shown to reveal 2.5 - 11 times more hip laxity (depending on breed) than the hip-extended view. Also the PennHIP method can measure the laxity of a hip joint with greater precision than the hip-extended method.
The goal of PennHip Evaluation is to obtain a precise measurement called the distraction index (DI). The DI is the ratio of the joint space present on the compression view compared to the joint space on the distraction view. The DI is an exact measurement that correlates directly to the degree of laxity of the hip joint and therefore the chance of developing DJD from hip dysplasia. The degree of hip joint laxity, as measured by the PennHIP method, has been shown to be the most important risk factor in determining whether a dog is prone to developing CHD and DJD.
A lower distraction index, especially if near 0.3, means that the hips are very tight and at very low risk of developing DJD. A higher distraction index, especially if near 0.7, means that a patient has dysplastic hips with a great chance of developing DJD.
The distraction index not only directly correlates to hip dysplasia but has also been shown to be a highly heritable trait. Dogs with a higher DI bred to dogs with a lower DI will have puppies with a DI averaged between the two. In this way, a breeder can use the DI to selectively improve the joint health of their puppies by selecting parents with better distraction indices.
A breeder can also use the DI of a dog at a very young age, as young as sixteen weeks, to select potential sires and dams. The distraction index has been shown to remain constant thru a dog’s life. The DI measured at 16-20 weeks will be the same when measured at two or three years of age or older. Therefore, young dogs that are candidates for a breeding program can be selected or removed from that program before there has been years invested in them waiting for OFA ratings.
Furthermore, PennHip can be used to diagnose dogs with suspected CHD at a very young age. Puppies as young as sixteen weeks old can have their DI measured. Because the DI directly correlates to hip dysplasia and because the DI remains constant thru a dog’s life, puppies of at-risk breeds can be screened for hip dysplasia early in life. If it turns out that a puppy has loose hips, then lifestyle changes can made or preventative surgeries such as triple pelvic osteotomy or juvenile pubic symphiodesis can be performed to prevent the formation of debilitating arthritis.
PennHip vs. OFA
PennHip differs from OFA hip evaluation in some very fundamental and important ways which make PennHip evaluation superior to OFA evaluation.
PennHIP's evaluation protocol quantitatively measures passive joint laxity. Based on the degree of laxity, the individual dog is ranked relative to other members of the same breed. This allows breeders to easily identify animals with tighter hips within each breed. Dogs with tighter hips are less likely to develop osteoarthritis. Note that the hip evaluation report is not issued in a subjective pass/fail framework. This approach allows genetic progress to be made by simply by breeding dogs in the tighter half of the population.
OFA ratings are the subjective opinions of veterinary radiologists about the conformation of the hips on a single hip-extended view. The hip-extended position has been used for more than 37 years to screen hips for DJD, laxity or both. However, studies have indicated wide diagnostic variability among radiologists in interpreting this view. There is also no objective measurement of anything about the hip. It is simply an opinion about how they look. Further, through biomechanical testing, the hip-extended view was found to mask the underlying true joint laxity.
PennHIP was developed and tested following strict scientific protocol and the results of these studies have been, and continue to be, published in peer-reviewed scientific journals. More than a decade of research and analysis has produced a body of information in support of PennHIP's effectiveness. (As with all diagnostic tests, PennHIP's accuracy is not 100%, but in direct comparisons it is far superior to any other available diagnostic methods.) No other method, published or practiced, has similar compelling scientific support.
The PennHIP method is based on strict quality control. To take PennHIP radiographs, veterinarians must undergo a training and certification process to demonstrate competency. PennHIP films are critically evaluated and the veterinarians are asked to repeat the procedure if the films do not meet PennHIP's stringent standards. The data generated from PennHIP undergoes regular review and statistical analysis so that useful information, by breed, is available to judge progress toward reducing CHD.
Sample PennHIP Report
