The 2009 AVMA convention was last weekend and I was fortunate to be able to attend several lectures given by Dr. Randall Fitch. These lectures were focused on localizing lameness in primarily the rear limb. I wish that they would have focused more on sports medicine as originally advertised, but we can’t have everything we wish for. During one of the lectures Dr. Fitch showed several pelvic radiographs from a bitch that was evaluated as OFA Excellent but turned out to be severally dysplastic.
This caught my attention since Indy, our gold border collie, has recently turned 2 years old and is now eligible to have his OFA radiographs submitted. There are two most popular screening organizations in the U.S. is the OFA and PennHIP. There is a lot of animosity and debate between the two groups as to which is the superior method and this doesn’t appear to be ending any time soon.
OFA has been around for a longer time period and is the more popular method of screening. Dogs must be at least 24 months of age to be certified and it is recommended that they be sedated for the procedure but it is not required. Radiographs are taken with the dogs lying on their backs with their rear legs extended and rotated slightly inward. When submitted the radiographs are subjectively measured on 7 points that takes into account evidence of hip laxity, osteoarthritis, or both. From this subjective evaluation the hips are rated into the seven different phenotypic classifications: excellent, good, fair, borderline, or mild, moderate, and severely dysplastic. These seven levels are used to help owners make informed decisions for breeding purposes. An advantage of OFA screening is that any veterinarian can take the radiographs and submit them as long as they are good quality. Also if the veterinarian is willing to take the radiographs without anesthesia they can be significantly cheaper then PennHIP. This is not advisable since tense muscles can make the radiographs appear better then they actually are. The OFA also maintains easily accessible records of all the submitted radiographs, which can be helpful when potential buyers are researching breeders.
PennHIP was developed to quantitatively assign a measure to hip joint laxity, the Distraction Index (DI). Since it has been recognized that there is a correlation between joint laxity and the development of osteoarthritis. PennHIP radiographs can only be taken by a certified veterinarian, which limits the availability of the procedure. Three views are taken and the dog must be under anesthesia or heavy sedation. The first view is the traditional rear limb extended view just like for OFA screening. The next two views, the compression and the distraction are done with the legs in a neutral or normal stance position while the dog is laying on it’s back. These are used in the measurement of the DI. The compression view is obtained by placing downward and inward pressure on the femur in order to place the femoral head into the acetabulum (hip socket). This allows for a visual evaluation of how well the femoral head fits into the acetabulum basically showing ideal congruency if no OA is present in the joint. The third view is the distraction view. A distractor is used to act as a fulcrum to displace the femoral heads from the acetabulums. The distractor is placed between the femurs with the rods placed over the femoral heads. Force causes the femur to distract from the acetabulum based on the inherent laxity of the joint. The DI is calculated and is given as a unit-less number between 0 and 1. As this number increases the joint is considered to have more laxity, and subsequently is more susceptible to arthritis. The laxity of the joint is the amount of displacement of the femoral head. Studies have shown that dogs that have DI <0.30 are virtually at no risk to develop arthritis. DI’s >0.30 are susceptible to arthritis. PennHIP screening can be performed as early as 4 months of age. Each report also contains a percentile ranking of the dog amongst others of the same breed. PennHIP advises that dogs below the 50th percentile of their breed not be used for breeding.
Canine hip dysplasia continues to be a very prevalent disease in the canine population despite many years of working to decrease its occurrence. This is evidence of a screening method that is not accurate enough and has a low heritability. Which is the case with the OFA technique, which has been the most commonly used means of evaluating the dog for hip dysplasia for over 40 years. Despite this time, the prevalence of the disease has only decreased slightly. False negative results in the test are likely the main cause of the slow progress; i.e. dogs been graded acceptable when they are in fact dysplastic. The PennHIP technique has been found to have a negligible number of false negatives thereby minimizing the number of dogs with the genotype for hip dysplasia to be used in selective breeding programs. This is the main factor in decreasing the prevalence of the disease in the canine population. I believe, as more veterinarians are able to perform this screening technique the more popular it will become, and real progress can be made in eliminating hip dysplasia.
