The PennHIP Radiograph
Rationale, Technique, Differences, and Value
by Fred Lanting

[This paper is � copyrighted and may only be used for information
purposes. It is the sole property of the author and any commercial
use or reproduction without written permission is prohibited.]

As the author of "Canine Hip Dysplasia", and an international lecturer on orthopedic disorders, as well as a dog show judge, I am frequently asked to comment on similarities and differences in the procedures used and information obtained when radiographs are taken for OFA and other leg-extended positions as compared to the PennHIP evaluation, which you will see is an improved diagnostic technique.

The Methods
By now, you know that the acronym stands for (University of) Pennsylvania Hip Improvement Program. This program arose from scientific inquiry, which in turn had its roots in the two related parents of invention: need and curiosity. The need was the desire of breeders and buyers for an earlier idea of how good were the hips of their canine "products". For several years in the beginning of OFA, breeders who got into the program in earnest made some progress, and many were able to avoid high incidence of severe HD in their lines. But a plateau was reached before total satisfaction could be attained, and they started to look for a means to progress beyond where they were, especially regarding early identification of the most likely carriers of the most "bad genes". Curiosity is the very heart of science, the "need to know"; in this case the question was "What must we learn to do in order to provide that early information in a valid and reliable manner?" The 30-plus years' history of the older hip dysplasia control programs had not resulted in satisfactory progress, so by the early 1990s researchers at that veterinary college in Philadelphia developed equipment and techniques to satisfy both breeder and scientist needs. Times change: what was acceptable in the past is not enough now; the bar has been raised, and to perform today we must jump higher, do better.

The Orthopedic Foundation for Animals was established in the mid 1960s to collect radiographic data on hip dysplasia (abnormal hip joint development) and to register and publicize those dogs with more normal joint appearance so breeders could avoid the worst ones, which also might be the worst "carriers". The American Veterinary Medical Association (AVMA) developed guidelines for positioning the dog for its radiograph in order to show the maximum number and extent of bony growths and remodeling of bone contours. In doing so, vets discovered an important principal: there was a correlation between those abnormalities and laxity (loose fit). Both for the individual's risk of affliction and the risk of bestowing the causative genes upon future descendants, the phrase "Tighter Is Better" became an obvious truth.

The AVMA position, adopted by OFA and foreign breed clubs, is that of a dog lying on its back in a similar way that we bipedal humans stretch out in our beds or coffins. It is certainly not a "natural" position for a quadripedal animal - one that travels on all four limbs of approximately equal lengths. In order to make a dog assume this supine humanoid position, the legs must be pulled (extended) with some force and restraint, or the dog would pull the knees up (flex them forward toward the chest and head). Conversely, the "neutral/natural" position for the standing or moving dog is with the vertical femurs (nearly 90 degrees from horizontal) making an angle with the pelvis of somewhere near 120 degrees. For Homo sapiens it is the erect position when standing or moving. Neutral means that position in which there is the greatest state of relaxation in the muscles used to extend or flex the limb. Not only are the muscles and ligaments most relaxed, but also the joints are then the loosest they will ever be. When the quadripedal dog or bipedal man is standing at ease, a very few nerve impulses are all that are needed to maintain balance by triggering a very few muscle fibers on all sides of the joint. The contractions in the rear parts of our legs keep us from falling forward, for example, while at the same time the momentary contraction of a few "front" muscle fibers counteract their effect.

It is very important to understand this stasis or position of most neutrality, this balance of forces, in order to understand one of the significant differences in AVMA's current protocol and the position used by PennHIP. The AVMA-OFA position stretches (tightens) the muscles on the belly side and front of thigh while not letting those on the back side operate in contraction and balance. Using this view with legs extended unnaturally, we "wind up" the muscles, tendons, and ligaments in and around the hip joint and tighten the joint capsule. The soft tissues closest to the joint are primarily the white-tissue, high-collagen types such as tendons and ligaments, and these do not extend (change length) to the degree that muscle fiber can. Thus, the twisting of white-tissue fibers is like twisting a nylon rope with two sticks turning in opposite directions, but in this case it tends to cause bones to be pushed closer together - the femoral head deeper into the socket than it would otherwise be. This artificially tighter-than-natural aspect contributes to the high false-negative rates in the OFA-certified dogs, as pertaining to laxity. Remember, both degenerative joint disease (DJD) and joint space are grounds for diagnosing HD in this method. Penn makes a semantic distinction between DJD as the definition of HD, and laxity as being a risk factor for eventual DJD.

While the hip-extended position is best for discovering DJD, it is not best for uncovering latent laxity, or what I call "covert laxity". False-negative means that a passing grade is given because the true laxity was not observed, and that is the biggest drawback of the hip-extended methods worldwide. There are some individuals (usually of certain giant mastiff-family breeds) that do not develop DJD but are OFA-assessed as dysplastic because of laxity at two years' age. But even more importantly, there are a greater number of dogs of other breeds that are adjudged "normal" at one or two years but later develop DJD or produce an unacceptably high percentage of dysplastic descendants. Thus, the accuracy of the hip-extended methods is gravely flawed. The gene pool is hurt most by these false negative diagnoses.

Latest Improvements
Two movements in America arose in the past decade or two that promise better progress than does adherence to OFA numbers as the way to coxofemoral nirvana. One is the proposal to use a voluntary "open registry", promulgated by the Institute for Genetic Disease Control (GDC). The other is PennHIP (University of Pennsylvania Veterinary School Hip Improvement Program). I had the pleasure of working with the OFA's first "program director", Penn's Dr. Wayne Riser, when I was researching and preparing my book, Canine Hip Dysplasia, and I also have had the good fortune to visit Dr. Gail Smith (PennHIP) in Philadelphia in the late 1980s. I reviewed his methods, philosophy, and results, and am increasingly a supporter of this protocol. At present, only PennHIP has the accuracy, repeatability, precision, and scientific foundation for real and rapid progress in producing better hips. The Seeing Eye, Inc. has turned to the distraction index (PennHIP) as a means of assessing hip quality.

You can learn more about the procedure if you are on Internet, by "tuning in" to <www.Synbiotics.com>, <http://realgsd.net/GSDinfo/Care/HD>, and <www.vet.upenn.edu/researchcenters/pennhip/>.
In the PennHIP technique, the dog is placed in a position that is even more neutral than standing naturally because the small effect of gravity is diminished. While under chemical relaxants sufficient to prevent resistance to manipulation, the dog's femurs are spread apart (distracted) with the force applied as close to the hip joints as possible. One of three radiographic exposures is made at that time, and the actual displacement is measured. An index is calculated in order to take into account the various sizes of dogs and their femoral heads/acetabulums. Any dog with an index of lower than 0.3 is practically guaranteed to never get HD. So far there have only been a few "semi-exceptions" in the many thousands of dogs evaluated. PennHIP does not make breeding recommendations, only evaluations; it leaves the decisions up to you, and counseling up to your veterinarian and peers.

It should not be surprising to anyone that the looser the hips, the less accurate a prediction of a specific grade or severity might be, especially in the hip-extended method. HD is developmental (DJD might not show up right away), progressive (it'll eventually be worse), and multifactorial (environment has a part to play in the expression of the bad genes). Some young dogs will get worse than others even with the same DI.

Other Differences
Other differences exist. There are three radiographs used in the PennHIP procedure, and only PennHIP-certified vets may submit them. Every dog's films enter the database, so there is not the skew or bias as found with the OFA-type registries. The "first" film (actually, it doesn't much matter in which order they are made) is identical to that used by the older method: the traditional extended-leg picture for the study of bone abnormalities - in some cases, especially the worst ones, laxity is also apparent here. The second film is of the knees-up neutral position with a very small compressive force pushing the femoral heads into the sockets. While not as important as the other two, this view allows an evaluation of congruity, how neatly the round head fits into the curve of the socket. It is the third view that really makes all the difference. While the dog is deeply "under", the patented distractor unit is placed between the legs at the groin, roughly parallel to the pelvis. Twin bars in this device that is shaped like the Roman numeral II act as the fulcrum, and when the lower legs are held near the hocks and pressed together, the vet leverages the femoral heads away from each other and outward (laterally) from the sockets. No covert laxity escapes this view.

The films are sent to the PennHIP Analysis Center, where a handful of people evaluate them (OFA uses a panel of radiologists that rotates or varies constantly). DJD presence or absence is noted on the first film, and circle gauges are laid on the third radiograph for use in objectively measuring the displacement. It is here where the paths diverge markedly: OFA, AVMA, SV, and most foreign hip registries or breed clubs use only the subjective hip-extended view, while PennHIP adds the objective view. At Penn, the results are added to those already in the database and compared. A report is issued that gives the Distraction Index, which can be thought of as expressing the percentage that the head is out of the socket. Another part states where this particular dog stands in relation to the average (mean) for its breed, expressed as "percentile". For example, if the mean DI for GSDs is 0.41, your Shepherd with a DI of 0.53 will be in a percentile between 50 and zero (worse than half of the breed). A percentile of 80 means that your dog has tighter (better) hips than about 80% of those in the breed. The mean can vary a little with time, especially when there is a low initial number of dogs in the database. However, there is no escaping the facts that "tighter is better" and that a relative threshold of safety of 0.3 exists.

PennHIP-certified vets have to pass a training and subsequent testing regimen. For OFA, any local practitioner may submit films, even if all she or he has ever X-rayed for in the past has been fractures. Some clubs, such as the SV (GSD club in Germany) have a list of approved vets who may submit films.

PennHIP researchers and method do not show estrus to be a factor in the distraction view. In fact, there appears to be no veterinary literature yet, to support the idea that it is so, even in the leg-extended view. Furthermore, a study performed at the veterinary school at U of PA definitively showed that hip laxity, whether on the distraction view or the hip extended view, was not affected by estrus. Their conclusion is that that scientific evidence refutes the purported relationship of estrus to hip laxity.

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