PennHIP: Misconceptions and Misinformation
A 1998 magazine, newsletter and website article
by Fred Lanting
Reproduced with permission.
Breeders have a very interesting tool these days in the Internet and/or e-mail. Information gets out much faster than print media can disseminate it. Among the accepted characteristics of such transmission are slightly higher "I.Q." (inaccuracy quotients) and emotion levels. It seems that these minor failings are forgiven in the informal tone of this medium.
However, it still behooves anyone doing the work of a journalist, commentator, or editor to be as accurate as possible, for people tend to believe anything they see "in print". Recently, there have been lively discussions on at least one list or website, excerpts of which have been circulated to others. These deal with the latest development in diagnostic-predictive techniques, and unfortunately fanciers have taken sides based on less than complete information. Understandable; I found in 35 years in chemical marketing that people make decisions based not so much on logic or reasoning, as much as on emotional leanings.
Here are some recent website quotes I've been given, and my responses. I encourage you readers to make yourselves available to one of my lectures. Better yet, to schedule one. "Have slides, will travel". E-mail me at Mr.GSD@juno.com for details. First, the quotes and misinformation, then my answers.
Quote 1.: "(Some people) bought a dog and it turned out to be dysplastic at 2 years, when her (OFA) prelim at 10 months looked excellent. This may be an argument for using PennHIP, said to be more predictive. In the PennHIP X-rays, more laxity can be measured." This lister tries to be cautious yet seems to lean toward confidence in the newer (a decade now) method of screening dysplasia at younger ages.
Quote 2.: "... a bummer when that happens. However, studies have not shown PennHIP to be more reliable in predicting HD than OFA. In fact they show the opposite (JAVMA volume 21 #9 Nov 1, 1997); in referring to OFA: 'The study showed that a preliminary evaluation of Excellent was 100% reliable; a preliminary evaluation of Good was 97.9% reliable; a preliminary evaluation of Fair was 76.9% reliable...'.
Also, (Am J Vet Res 1993; 54: pp.1021 - 1042) in referring to PennHIP:
'12% of the dogs evaluated as normal at 4 months of age by the PennHIP method were later determined to have degenerative joint disease. 48% of the dogs evaluated as abnormal at 4 months of age, 57% evaluated as abnormal at 6 months of age and 38% evaluated as abnormal at 12 months of age by the PennHIP method did not have evidence of degenerative joint disease at 24 months of age'. A second study on the PennHIP method (Am J Vet Res; 1993; 54: pp.1990-1999) concludes that while a distraction index less than 0.4 is 88% reliable for predicting normal hips, a distraction index of greater than 0.4 is only 57% reliable for predicting CHD. While no method would be 100% reliable, it would appear that the OFA method is more reliable at early prediction of CHD. What bothers me most about the PennHIP method is the very high percentage (38 - 57%) of those dogs evaluated early where they predict CHD will develop, but it doesn't.
Thus, if you use the PennHIP method for early evaluation, you have a very good chance of eliminating a non-dysplastic dog from your breeding program due to less than accurate results."
By the way, while the veterinary community largely uses the abbreviation CHD for Canine Hip Dysplasia, I use the lay practice of referring to it as HD, since the context is always clear that we are talking about the disease in dogs, not in humans or other animals. Now, as to the quote #2 above: far from concluding that OFA is more reliable and accurate, the cited journal references, which I have on the desk before me, can only logically lead to the realization that the opposite is true, except for the Nov.1st, 1997 reference written by Al Corley and Greg Keller of the OFA. To be fair, one must also read at the same sitting, the letter to the editor on page 487 in JAVMA's vol. 212, #4, Feb. 15, 1998 which effectively refutes those OFA conclusions. OFA statistics are skewed because not all films are sent in; PennHIP requires ALL films, whether showing horrible hips or not, be sent in and entered into the database. The OFA article did not let readers know that apples were being compared to oranges --- that the presumably higher rate of false positives PennHIP reports is a result of different definitions. OFA says a dog is dysplastic if it has loose hip joints, signs of arthritic changes or wear, or both. PennHIP diagnoses frank HD on the basis of DJD (degenerative joint disease... bony changes and remodeling). If they had included the lax-hip dogs not showing such changes, the false-positive rates would be much lower. The PennHIP evaluation not only reports as dysplastic those with obvious bone and cartilage abnormalities, but also gives an idea of the relative risk of the particular dog developing such radiographic signs later. The dog whose hip joint's femoral head looks tight and round on the OFA film but shows considerable laxity on the PennHIP view is said by the latter group to be at risk for later DJD. Which information would you want to have?
Remember, the traditional position advocated by AVMA and OFA (as well as almost all other hip schemes of the past 35 years) is the leg-extended (hip-extended) one in which the ligaments of the hip joint capsule are wound tight like the rubber band in those toy airplanes we old fogies used to play with as kids. This tends to present an artificially tight appearance to all but the worst hips, and is certainly not representative of the forces at work in the standing or walking/trotting dog. The PennHIP scheme utilizes this hip-extended view in order to best see some features that might show DJD; by the way, if you want an OFA reading, the vet need only put two films into the cassette when taking this picture. The view with the dog's legs flexed in a position like standing, but upside-down, mimics actual forces. And when the hip joints are stressed in the distraction view (femurs and femoral heads pushed away from each other and the acetabulums) and then the difference in displacement measured from the picture given when they are pressed into the sockets, why then you have a numerical, objective value: something you can use to compare with others of the same breed, for example.
Now, let's look at the predictive value of OFA's preliminary evaluations, and I won't go at length into why suddenly these are supposed to be so accurate, when in 1972 OFA led the way to a 24-month minimum for certification because of the inaccuracy of early diagnoses. Let's assume that only the very worst hips will show bony changes, and that the youngsters now being "prelim'ed" are judged primarily on laxity. The OFA is happy to predict, on the basis of very tight joints IN THE HIP-EXTENDED VIEW at a young age, that such an excellent appearance will continue to look good at 2 years (minimum age at which to certify). But remember, the view at 2 years is going to be the same type, that of an artificially wound-up joint capsule. Surely, then, one would expect fairly good agreement between the pictures at these two ages, especially if rated "Excellent" in the AVMA position. However, if one applies the more stringent evaluation protocols of the PennHIP method at the older age, one finds a disturbing number of "OFA-normals" are indeed not normal in any sense that you and I would consider so, such as compared with the mean or average in the breed. Even an unacceptably high number of OFA-Excellents at 2 years will show up in the PennHIP view to have worse hips than would otherwise be suspected. If you want to know if a bridge can bear a load of 10 tons, you shouldn't expect to run a meaningful test by driving your half-ton pickup over it. The dog also should be evaluated in the strictest method in order to tell if the reading of "excellent" has any validity. Otherwise, the breeder or the person driving a big truck over a bridge may have a false sense of security.
Those who look into professional journal articles should perhaps ask, "Where are the data showing a link between OFA-defined laxity and later DJD?" and "Where are the data on dogs diagnosed as dysplastic (based on laxity at 24 months) but who never developed the bony changes?"
Since there is no such set of statistics, one must conclude that hip-extended radiographic diagnosis (alone), even at 24 months, is not "reliable" as the OFA's JAVMA article proposes, but highly unreliable, or at best, unknown. Are you as a breeder satisfied with repeatability (unfortunately described as reliability)? Suppose you were William Tell's son, and you knew that your dad could shoot a dozen arrows at the apple balanced on your head, with repeatability as to where each would go --- would you be blithely unaffected by the concept that his reliability (accuracy) might not be as good as his repeatability?
You would not want even the first arrow to fly, would you? Diagnosing HD is not as threatening as that, but I for one would want the most accurate as well as the most reliably repeatable evaluations of my breeding stock's hips.
DJD appears in some individual dogs and in some breeds at later ages than in others. The breeder wants to know as early as possible, what the likelihood is that his dogs might develop DJD, and therefore wants some sort of "marker" or predictive evaluation before he sells or breeds. The panacea of gene markers (looking at DNA and finding all the sequences that cause HD) is not practical in our lifetimes (or at least not in our current dogs' lifetimes). If anybody even were to come up with cheap testing of such polygenic traits, this approach is still decades away.
The OFA study reported in that JAVMA article did not include in its references any longitudinal studies to refer the reader to, even though a year or so earlier such a report by Banfield, Bartels, Hudson, et al showed almost no difference in dogs predicted to develop degenerative changes and those described as having normal hips, using the OFA-style methods. The 40 dogs described as "normal" at 2 years of age all had some "minimal or mild degenerative changes" by 9 years of age, and those 22 dogs diagnosed as dysplastic (lax joints in the hip-extended view) at 2 years had the same mild or minimal changes. Why wasn't this study included? Does it give you a warm feeling that you are using the latest and most accurate diagnostic techniques by banking on the OFA readings?
We shouldn't worry so much about false positives (a red flag that a dog might become dysplastic, but doesn't) as we should about false negatives (the dog is evaluated as normal, and later proves to be chock-full of "bad genes" that his offspring inherit).
We would rather cull from the breed an occasional good dog (there are many ready to take his place) than let some covert fifth-columnist into the ranks to poison or sabotage the gene pool. In the OFA system there are false-negative rates of about 83% in 6-month-old German Shepherds, but in the PennHIP scheme, the rate is only 12% in 4-month-old dogs and 0% at 6 months (as compared to the readings at 24 months). The writer who complains about this 12% fails to acknowledge that using the OFA approach on 4-month pups gave a false-negative rate of 24%, double that of the PennHIP compression-distraction method. Even at 6 and 12 months, the OFA-type predictive tests gave false-negatives of 15% and 12% and the PennHIP stress-radiographic method showed zero false negatives for 6- and 12-month old dogs.
To rest one's case on the above-mentioned "48% of the dogs evaluated as abnormal at 4 months of age, 57% evaluated as abnormal at 6 months of age and 38% evaluated as abnormal at 12 months of age by the PennHIP method" as not having "evidence of degenerative joint disease" is to rest on the false positives. What of those dogs that did not have DJD at 2 years of age, but more laxity than the average in their breed?
Are you satisfied that they have as few bad genes as do the dogs that were identified early as having a very low DI (distraction index) and still do not have DJD in old age? The PennHIP method is a far better revealer of genotype, and thus predictor of eventual DJD, in the individual and its progeny.
It should not be surprising to anyone that the looser the hips, the less accurate that a prediction of a specific grade or severity might be. HD is a developmental (DJD might not show up right away), progressive (it'll eventually be worse), multifactorial (environment has a part to play in the expression of the bad genes) disorder. Some dogs will get worse than others even with the same DI at a young age. A DI of 0.4 is not all that bad, nor all that good. You can pretty much guarantee your buyers that the pup you sell them with a 0.3 will never have DJD (HD), but you lose credibility with such a guarantee as that index creeps higher and higher. You can think of an index of 0.4 as being "40% out of the socket" if you want to oversimplify the picture, since the readings runs from a hypothetical 0 (zero would mean no ability to move in the socket) to a full luxation of 100% out of the socket, or DI of 1.0 (and of course in the worst cases, the number can be over one, but those dogs will have bad bone changes, too, so it doesn't take a rocket scientist to see they are dysplastic). In many breeds, the mean or average laxity is around 0.4 and it is worse in many other breeds. Some breeds can tolerate looser hips than a GSD and have less development of arthritic degeneration in maturity. The person who worried about the "38 - 57% of those dogs evaluated early where they predict CHD will develop, but doesn't..." isn't adding "at two years of age",
and again ignores the much worse situation where at least an equally high percentage of OFA-normal dogs might develop late-onset DJD and/or pass on many bad genes to offspring. There is very good reason to presume that dogs that have no signs of DJD but have an index higher than the mean for their breed "represent a carrier state of the disease", as mentioned in the second Am J Vet Res citation above. In other words, it is a clearer picture of the genotype than the OFA approach gives.
Man has advanced over his prehistoric precedents by using more tools, and we should progress in use of modern tools regarding HD as we have from the Neandertal to the Stone Age to the Bronze Age to the Machine Age, and to the Computer Age. PennHIP is such an advanced tool for the serious breeder. The inescapable conclusions are that:
1. Tighter Is better;
2. Position and technique (better tools) can discover covert laxity;
3. PennHIP is more accurate as well as reliable and repeatable.
Most of the people on these Internet chat rooms and e-mail lists are not "professional people" by which in this context is meant trained veterinarians. Thus, it is natural that few would realize until pointed out to them, that there are significant differences between types of articles they read. Something that appears in a medical journal such as JAVMA are closely examined in a process called "peer review" before being edited and published. Both accuracy and logical reasoning are required, in addition to sound references and proper controls and procedures in the reported experimental work. On the other hand, articles that appear in newsletters, flyers, press or publicity releases, brochures, and the like, are not subject to such hurdles and requirements. The OFA press release that was carried by a number of breed magazines such as the Australian Cattle Dog publication in mid-1998 was a rather strongly biased advertisement for OFA business --- absolutely nothing wrong with that approach, by the way. The AVMA Journal version published Nov. 1, 1997 was what remained after the review process removed all the unsubstantiated claims. Now, such claims may be valid; it's just that in this case, they were not subjected to protocols regarding scientific method and therefore the "commercial version" may have been misinterpreted by some as having the weight of the reviewed version.
The optimist in me sees the day when OFA leadership will have to admit the superiority of new tools, adopt the PennHIP approach, and report such data for the benefit of the breeder. The pessimist in me wonders if the parties can put personalities aside and concentrate on science, and if I'll live long enough to see this happen.
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Lectures & Seminars on Orthopedic Disorders, Gait-&-Structure, Etc.
Copyright Fred Lanting, All rights reserved, but reprinting allowed after permission. Please read his other articles on SiriusDog.com, for example, or e-mail him at Mr.GSD@netscape.com or Mr.GSD@Juno.com for specific articles.
Editors Note: A well-respected and frequent GSD specialty and all-breed judge for many clubs around the world, with KC and other-country credentials, Mr. Lanting since 1966 has lectured on Gait-and-Structure, Canine Orthopedic Disorders, and other topics, and has judged in about 30 countries. He has been described by a former OFA director as the worlds leading non-veterinarian authority on hip dysplasia. He has lectured at numerous veterinary schools in the USA and abroad, and is the author of the following must read books for the dog owner (E-mail for curriculum vitae). Canine HD and Other Orthopedics Disorders : This expanded revision is a comprehensive (nearly 600-page), amply illustrated, annotated, monumental work that is suitable as a coffee-table book, a reference work for breeders and veterinarians, and a study adjunct for veterinary students. It is equally valuable for the owner of any breed. It covers every aspect of HD and other orthopedic, bone, or spinal disorders, and includes genetics, diagnostic methods, treatment options, and the role of environment. Your autographed copy will be mailed from the USA as soon as the appropriate amount is received and is processed. Pricing: US $68 in the U.S., or ask about mail overseas. Combine orders with The Total German Shepherd Dog by the same author ($50 plus $4 postage). 17 of the 20 chapters are suitable for owners of any breed. Order both at once direct from the author, and the postage will be waived.