A Veterinarian- Breeder’s View–Use of OFA & PennHIP
Ray Calkins, DVM
(The following article, reprinted with permission, was taken from the 2008 Canine Breeder’s Symposium, sponsored by the Society for Theriogenology and the American Kennel Club Canine Health Foundation. The Society for Therinogenology will be holding another Breeder Seminar on August 29, 2009 in Albuquerque, New Mexico. It will be a one day seminar and you can find more information and registration requirements posted on their website at http://www.therio.org in the near future.)
Canine hip dysplasia (CHD) was first described in 1937 and is the most common degenerative joint disease (DJD) in dogs. It is present in almost all breeds and has a world wide distribution. CHD is considered to be a disease of complex inheritance, and thus the clinical signs may vary from mild to severe. In the United States, two nonprofit organizations have been formed to diagnose and grade CHD. Through the use of the results from these organizations, responsible breeders can reduce the incidence of this costly and debilitating disease.
The Orthopedic Foundation for Animals (OFA) was founded in 1966. John M. Olin, an industrialist, sportsman and field trailer is generally credited for its foundation. He organized the first meeting of veterinarians and breeders to find ways to limit this disease. This meeting led to the formation of the nonprofit organization.
The OFA evaluates hip genetics (genotypes) by radiographs of the pelvis in the “hip extended view”. Three board certified radiologists are randomly picked from a pool of 20 – 25 specialists. Their subjective opinions as to the phenotype (the observable physical characteristic that reflects the genetics) of the hip joint is based on a 9 point evaluation of each hip. The hips are classified as Normal (Excellent, Good or Fair), Borderline (needing re-evaluation) or Dysplastic (Mild, Moderate or Severe). OFA will evaluate young animals but will not certify them as Normal until they are two years of age or older. The OFA strongly recommends sedation or anesthesia for hip radiographic evaluations. Submission of hip studies is optional with OFA. Owners may elect not to submit radiographs of dogs with obvious DJD. Therefore, the OFA data is biased by evaluating primarily dogs that have the possibility of receiving a normal rating.
At the University of Pennsylvania in 1983, Gail Smith started researching a technique for the early diagnosis of CHD. This research led to the establishment of the PennHIP method of predicting the likelihood of a dog developing DJD of the hip or CHD. In 1994 this technique was licensed to an outside company and became commercially available. The University of Pennsylvania regained PennHIP and established nonprofit status for the organization in November 2002.
PennHIP utilizes three radiographic views of the pelvis to scientifically evaluate the hip’s phenotype. The hip extended view (the first radiograph and the same view used by OFA) is evaluated subjectively for degenerative changes (DJD). Then a “compression film” is taken utililizing a lateral fulcrum to seat the femoral head as deeply as possible into the socket (acetabulum). The third radiograph called the “distraction radiograph” requires a special adjustable device that acts as an interior fulcrum to expand the hip joint to the extent allowed by the round ligament and joint capsule. These two radiographs are used to calculate the distraction index (DI) of each hip. The DI is a measurement of joint laxity. The DI is calculated by measuring the radius of the femoral head (r) and the magnitude of separation, or the distance between the acetabular and femoral head centers under distraction (d). DI=d/r . The DI ranges from 0 to >1.
This procedure can be performed as early as 16 weeks of age and the DI is considered to be stable throughout the animal’s life. PennHIP utilizes a network of trained veterinarians and anesthesia is required. All studies are required to be submitted to PennHIP for evaluation. A database that contains all studies, tight and lax, gives a truer picture of the hip status for that breed.
Performance Borzois and Greyhounds are two breeds that are recognized to have an extremely low prevalence of CHD and have uniformly tight hips (DI<0.3). The mean DI (hip laxity) from canine breeds known to have a high prevalence of CHD had significantly greater mean DI than Borzois and Greyhounds. Importantly, tight hipped dogs (DI<0.3) within breeds of dogs predisposed to DJD had a similarly low (near 0) risk of developing DJD .
PennHIP issues a Hip Evaluation Report on each animal giving a subjective evaluation for DJD of each hip. The objective DI is calculated for each hip and a laxity profile ranking for the dog is shown on a sliding scale for that particular breed.
An ideal test for a genetic disease would be one that is 100% at differentiating genetic normal from abnormal. At this time there is no genetically based test for the polygenetic disease of CHD. Both OFA and PennHIP evaluate the phenotypic appearance of the hips on a radiograph to predict the genotype. These evaluation methods identify true negative individuals (genetically and phenotypically normal) and true positive individuals (genetically and phenotypically abnormal). In addition, there are a few false positive individuals; these are animals with good genes, but who have DJD due to hip trauma. These animals will fail with both OFA and PennHIP. A more disturbing problem is the false negative individual with the hip extended view (this is the sole view of OFA, but only 1 of 3 views obtained for PennHIP evaluation). It has been shown that this view actually “tightens” the joint capsule which can disguise laxity. In addition, manipulation of the hip can reduce the observed laxity on the x-ray. False negatives allow bad genes to remain in the gene pool. Hips that are rated as normal at two years of age with the hip extended view may develop DJD later in life.
It has been shown by PennHIP that a “direct comparison of the official OFA score with DI in a pool of 260 large breed dogs showed that a large proportion of dogs that had officially passed as excellent, good or fair had DI scores in excess of 0.3” . Dogs that have passed OFA as excellent or good may have occult lax hips. Not all lax hips will develop DJD, but laxity is a predictor of susceptibility to CHD. Knowing the DI of your breeding stock may answer the question why your “OFA Good” stud dog develops DJD later in life or why he occasionally produces dysplastic offspring.
The OFA recommends breeding only dogs that received a ‘Normal’ rating to bitches who also received a ‘Normal’ rating. To get better results in reducing the incidence of CHD, the OFA recommends breeding ‘Normal’ dogs that come from ‘Normal’ parents and grandparents, and that breeding ‘Normal’ dogs that have more than 75% ‘Normal’ siblings will result in more rapid improvement. In addition, the OFA recommends choosing replacement animals that exceed the breed average.
PennHip states that to get maximum genetic change, one must breed ONLY dogs with a DI of less than 0.3. These animals have almost zero chance of developing CHD. This approach dramatically limits breeder choices and genetic diversity. Because of this, PennHIP advocates a more moderate approach—use only animals with hips tighter than the breed’s average as breeding stock. In this way the breed moves to tighter hips with less chance of developing CHD over time while maintaining the other desired traits of the breed.
All breeding stock should be evaluated for CHD. Early elimination of high risk individuals reduces the economic and emotional loss caused by the disease. Both OFA & PennHIP agree that hip laxity is the major contributing factor leading to the formation of CHD. PennHIP evaluation at a young age (4 mo – 1 yr) will allow breeders to concentrate their time and energy on a pup that has a better than average hip for their breed. Prior to making the decision to breed an individual dog, a radiograph at two years of age should be taken to detect if DJD is present. Both OFA and PennHIP make this evaluation. The combination of early evaluation for laxity by PennHIP and OFA certification at two years of age will over time improve the hip status of your kennel and breed.
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