newwordsfredlanting

New Words in the HD Dictionary

by Fred Lanting

Introduction
Lisa Ford requested a short article on Cavitation but I decided to expand on the subject and offer it more broadly, starting with something just for Fila folk. She had dogs evaluated by PennHIP and got a fantastic DI (distraction index) reading of 0.30 on one side, but her other hip was unreadable due to cavitation. She asks: "What exactly does this mean? I had a bitch that got a .30/.33 and then was rated with the OFA as 'Fair'.

It is my plan to post this article to my webpage as an explanation/validation of my dog's lack of a left hip score." First, let me assure Lisa that she should celebrate those wonderfully low readings; despite what OFA said, the odds of that bitch ever getting degenerative joint disease (DJD) are extremely low... the bitch has hips like a performance Sighthound! I would recommend that she find out if the bitch can be done again free for the purpose of PennHIP database increase, just as a matter of additional proof to any doubting Thomases. But if not, I would be very happy to own dogs with such an evaluation. Especially in this high-HD-risk breed.

Every so often, as the author of the book, "Canine Hip Dysplasia", I am asked if there is anything new in the field of orthopedic disorders. It always reminds me of the eccentric owner-editor of a Penns Grove NJ newspaper in the 1950s who was criticized often for putting "old news" into his paper; his reply was always the same: "If they haven't heard it yet, it's still news!" Sometimes I relate that anecdote as I look over a seminar audience obviously composed in part of people who "haven't heard it before". But there really are new things going on all the time, which is why I keep getting lecture assignments and why I am bringing out a new edition of the book soon. They may not all be quantum leaps, but people who went to my slide shows a couple years ago will find it well worth while to go again now.

PennHIP
There are a number of terms in the new knowledge of HD and the continuing battle to improve hips that will be unfamiliar to many of you. Others may have heard them on e-mail "lists", the speedy news and gossip line of the late '90s. The most well-known of recent years is "PennHIP" This is a clever acronym for "University of Pennsylvania Hip Improvement Program", and was developed by the surgery and engineering professionals at the Vet School in Philadelphia, most notably Drs. Smith and Biery. In the early 1980s, older concepts were refined and perfected so that laxity (looseness of the ball-and-socket joint in the hip) could be given a mathematically-precise quantitative value, something never really done before. In the succeeding decade and more, steady investigation into this technique has resulted in numerous studies reported in the veterinary journals, and replication of results at various schools. The dog fancier must first understand that there is a distinction drawn between true diagnosis and prediction of HD. Diagnosis is generally defined as the presence of abnormal changes such as shape remodeling and/or "deposits"
and arthritis. Laxity, used as a definitive diagnosis of HD by OFA, is perhaps really a prediction that the degenerative joint disease (DJD) will sooner or later occur. The PennHIP people prefer to keep the distinction, saying that their measurement of laxity is actually a very valid predictor of relative risk for later DJD. In my experience, degeneration happens eventually and so regularly that for practical purposes, the dog owner can safely say that laxity is dysplasia. The science is sound, the data is voluminous, the evidence is clear: it is not worth it, in most cases, to breed dogs with excess laxity.

Distraction Index and Covert Laxity
The big difference that enables the PennHIP method to find what I have coined "covert laxity" which may not be apparent in the OFA view, is that the latter is an extended-leg, unnatural position that actually winds up the soft tissues (ligaments, tendons, muscles) in and surrounding the joint capsule. This is like putting two sticks into an elastic band and turning them in opposite directions: a few turns and the band is very tight and the sticks not loose. The extended leg view, adopted a long time ago by the AVMA as a standard procedure, tightens the capsule in a similar way, forcing the ball tighter into the socket than it would normally be if the dog were in a more neutral position, such as standing with femurs vertical. PennHIP starts with an AVMA view to see any DJD, then this loosest, most neutral position, only the dog is anesthetized and upside-down on the table instead of standing and awake. Now, if the joints were so horribly loose that the balls nearly fall out of the sockets in that position, you would not get a meaningful reading or comparison between dogs. Therefore, this variable is removed by pressing the femoral heads into the sockets so that all dogs' hip joints look pretty much alike in this X-ray picture. The third picture is taken when the heads are distracted (levered) out of the acetabular sockets as far as they will go. A specially made instrument is used to separate (distract) the femurs and of course their heads, and the shot is made.

One dog's hips will be looser than another's, and by a specific, measurable amount. Because some heads are larger than others, a simple formula is used that divides the distance distracted by the radius of the ball; this resultant number is called the distraction index or DI. The less the difference between fully seated and fully distracted, the lower this number and the lower the risk that the dog will get as many (if any) of those degenerative signs of HD. After 20,000 dogs, the theory has been unquestionably proven. There seems to be a threshold DI of 0.3 (equivalent to a maximum distraction of 30% out of and 70% in the socket), below which the risk for all practical purposes is zero. Look for "DI" in more breeders' future ads.

Cavitation
A word very few will come across when using the advanced PennHIP evaluation is "cavitation". The space between the acetabulum and the femoral head (the socket and the ball) has a small amount of lubricating liquid called synovial fluid that performs a function similar to that of the grease in your car's ball joints. However, it also apparently has another critical function, and that is related to the fact that it is permanently sealed in, and if you were to try pulling the ball out of its socket, there would be a vacuum-like effect. The harder you pull, the harder it resists. Of course, if you puncture that joint capsule, you would let in some air or fluid, and the "vacuum" would be lost. Likewise, if the dog had a genetic defect that allowed production of excess synovial fluid, the negative pressure would not be as significant, and you'd be able to pull the ball further out before resistance built up. In order to get a good reading of the difference between compression and distraction views, and measure the displacement to give that index, the vet has to exert a considerable force, up to the point where even more force no longer produces any more displacement. The limit to which he or she can pry the bones apart illustrates a principle of physics and mechanics known as Hook's Law. A spring or elastic material can be stretched with relative ease up to the point of its elastic limit, and will stretch no further, no matter if you add one ounce to the weight or 100 pounds. Of course, if you put a ton or so on the steel cable or spring, you might exceed the elastic limit and the steel will fail, and not regain its length the way it did when you stayed within the limit and relaxed the tension.

In distracting a dog's hip joint, a slightly similar thing happens, but you do not go past the yield point the way you can with an enormous weight on the steel test piece. Instead, something else "gives", a temporary and harmless phenomenon that is quite rare (about 4%) in the distraction procedure: voids appear in the synovial fluid and give a sudden decrease of pressure in the articulation area. Nitrogen and/or other normally-dissolved gases in the fluid are subjected to enough suction force that they vaporize, and can show up as radiolucent areas on the radiograph. When it happens, it is almost always only one hip that is involved. Within minutes to an hour, they re-dissolve and the dog can be re-tested. If it takes longer than the vet wants the dog to be under anesthesia, the PennHIP evaluators in Philadelphia will just take the DI reading of the other hip. In the even less frequent instance of a vet causing cavitation in both sockets, that film is rejected or the vet has already realized what he did, and repeats that part a little later. A similar pop or snap happens when one cracks his knuckles, and it takes a while before it can be done again. I've been annoying family members that way for fifty years with no damage, and there is likewise no damage to the dog when cavitation occurs.

Breed Value
Another new term that is just now catching on in the dog world in one respect is "breed value" or, as the Germans call it, "Zuchtwert". The way the magazines and e-chatter play it up, you might think it is new, but it has been at the core of food animal breeding for a long time. Moreover, the geneticist in charge of the Seeing Eye breeding program has employed it for more that twenty years. So why is "ZW" suddenly in the news and being implemented by breed clubs? Why had they not used it long ago? Good question. Some people, especially some dog people on some countries, are notoriously slow to put good ideas into practice. They are all protecting their own sacred oxen from being gored. In the U.S., for example, we showed as far back as the early 1970s that the GSD Club of America had the responsibility of requiring hip certification for the dogs they officially promoted as their best breeding stock (the "Selects"). However, the leaders have consistently turned down the membership when the proposal was brought up again and again. Perhaps there was something of a head-in-the-sand attitude; perhaps there were just not enough geneticists involved with dogs and dog clubs. In any case, a big boost was recently given the concept when the German Shepherd Dog Club in Germany (SV) adopted ZW as a means of informing its membership of the potentially faster progress that could be had. The practice of simply excluding the severely dysplastic dogs from breeding and registering had reached a plateau, and breeders obviously needed another rung to step on. Smaller clubs such as the Bernese Mt. Dog and Jagdterrier associations in Germany and elsewhere in Europe had shown real progress in reducing HD incidence, so the SV decided to use it, too.

Dogs are given rather arbitrary ZW or BV index numbers reflecting how far from average their hip production is or has potential to be. If a stud dog is carefully bred only to bitches with exceptional hips or a good-hip production record, that stud's ZW number falls. Supposedly everybody starts off with 100 "points", and if the hips of the progeny are worse than the parent, his or her number goes up; if they are better, his/her number goes down, reflecting a better value to the breed. Even if a dog is not bred, his number can change a little based on what his parents are or produce, and even what his siblings do in this scenario.

One of the drawbacks is that you could have a litter of dogs whose ZW numbers all are 100 at birth because that is the average of their parents (e.g. 105 and 95). Of five dogs that go on to be recorded (registered, breed-surveyed, and X-rayed in order to be eligible for breeding), one may "earn" a ZW of 82 by being judiciously bred, while the others retain a 99 or 98 without ever having as much as wagged their tails at another dog. Yet, all may have the same quality hips and the same load of bad vs. good genes. It just that the one that was bred has had a chance to more or less "prove" its value, but its littermates have not. Still, the peer pressure is on, as the SV has declared that any pairing of dogs must average 100 or less, or the breeding will not be allowed. So a Sieger (world specialty show winner) with a ZW of 118 will suddenly find himself with far fewer bitches to breed to (each must have no more than 82) than will a lower-rated but excellent dog with a ZW of 85, 90, or 79, for examples. The breed value concept for dogs is a good one, and though not as accurate as it could be, I believe that it may slightly accelerate progress.

Will They Help?
Do these "new" words add anything to our knowledge of hip dysplasia? Do they offer any new tools or at least better handles on old tools, so that we can dig deeper and get to the bottom of the problem? Can we expect faster progress with such improved approaches and methods? Yes and no. There is no doubt that the more you know about the problem, the more rapid and even possible the improvement. But to take advantage of a tool, you must use it. So there are two steps to progress: learning and doing.

The PennHIP technique offers so much more knowledge about hip dysplasia than previous methods, that it is at first surprising that the country's leading orthopedic foundation had not latched onto it and absorbed it into its protocols. But there are humans involved with science and business, and that means potential personality conflicts. This is the primary if not the only sound reason (or excuse) for not accepting or copying the new knowledge. What is now available is the ability to accurately predict at 6 months or even 4 months of age the likelihood of a dog developing DJD later in life. PennHIP and independent researchers have found that there is virtually no change in the DI at such a young age compared to that determined in adulthood. This should not be confused with the OFA's recent claims that today their own 6-month predictions are better than 90% accurate. For one thing, the extended-leg view used at both ages does not give the best picture of laxity; for another, it doesn't quite ring true that after a quarter-century of maintaining that accuracy at 12 months was about 70% accurate (in that view), suddenly the
4 or 6 month evaluation is so good. Neither the dogs nor the procedure has changed, and the vets aren't using different standards now. It appears more likely to be a reaction to the validity of PennHIP's early-predictability and accuracy with the new position and technique.

The new technique is here, and those who ignore it will be left behind. Should breeders replace OFA or OVC with PennHIP? I do not think I would recommend it at the present time. There is so much history and name recognition that the OFA number helps breeders indicate to potential customers that they care, and that their dogs are not severely dysplastic. The same thing can be said about the German "a"-stamp, which is given to any except what the SV (GSD club in Germany) considers moderately or severely dysplastic. These old schemes are still fairly useful for a rough comparison with dogs that have not net been evaluated with the newer method, and are still a bit more recognizable by novices.

The greatest benefit of the OFA reading is that it presents us with the best position for seeing any calcium deposits or remodeling (DJD). On the other hand, since the PennHIP scheme includes an identical AVMA extended-leg view, perhaps the only reason to get an OFA reading is for advertising. Gradually, as PennHIP attains the widespread public identity that OFA has earned over more than 30 years, this will change. Or else, OFA will adopt the better science of the compression-distraction procedure - it could happen! For now, the best course of action for the breeder who wants to do better than he had been able to before, is to have the PennHIP vet put an extra film into the cassette when taking the first radiograph, the one of the extended-leg position, and send it to OFA for evaluation. This way, breeders can compare DI and OFA rating for themselves and make up their own minds as to what the future procedures should be.

Percentiles, Medians, Means, and Decisions
Should breeders add the newer knowledge PennHIP offers to their decision-making processes? Without a doubt! To purposefully ignore the chain saw sitting on the tailgate of your pick-up truck while you try to cut down a tree with a small hatchet could reasonably be termed foolishness, and to ignore the newer and better tools in your approach to progress in decreasing genetic disease is in the same category. After you have learned enough of the information, the action steps are necessary, as I mentioned a couple of paragraphs earlier. PennHIP does not make breeding recommendations, only evaluations; it leave the counseling up to your veterinarian and peers. If you get the PennHIP report and it says your dog is in the 50th percentile, that means that 50% of its breed have tighter hips than it does, and 50% are looser. In the 80th percentile means that your dog's hips are tighter than about 80% of the dogs in its breed, and as you have guessed by now, the important thing you learn from both OFA and PennHIP is that "tighter is better". You are perfectly able to decide which dog is better for breeding from the standpoint of hip joint quality.

Another reading you will find on your report is how your dog's hips compare to the median DI (middle of the range) of the breed. Naturally, you will want to breed only those dogs that are in the higher percentiles and have a DI lower than the mean (since the index increases in proportion to the laxity, smaller is tighter). In addition, you will be informed by your vet what the current mean (average) DI is for your breed; some of you may have breeds with small numbers of dogs evaluated or a rigorous program stimulated by your club's code of ethics, in which cases the DI may have changed since your fellow Hungarian Trufflehound breeder's dog was tested a couple of years ago. The best progress will be seen by selecting the lowest numbers, or at least those below the mean and breeding to a dog with a lower DI than your own dog's. For all practical purposes, a DI of 0.3 will guarantee your dog will not develop HD (DJD). The higher the number, the greater the risk. If, as is certainly indicated, the DI gives a better picture of future hip quality in your dog, then deductive reasoning would lead you to think of it as a reasonably accurate indicator of the genotype of your dog. That means a better idea of the proportion of bad hip genes to good hip genes, which in turn means relatively how many bad genes are likely to be transmitted to the next generation Now that, dear friends, is really revolutionary. The poor progress we have seen in modern times, with ratings by BVA, OFA, SV, ADRK, OVC, and other breed and veterinary organizations is a direct result of their inability to indicate those hidden genes. A dog that has a good picture in the extended-leg view yet still produces an unacceptably high number of dysplastic offspring has too many of those hidden genes. Since OFA would be the first to tell you of the link between laxity and HD (they actually use that as a definition), the only reason for the poor progress is that hidden, covert laxity I earlier mentioned. Therefore, using the logical process of thought, if PennHIP shows more of this laxity than shows up in the AVMA type view, it in effect shows us the effects of more "hip genes". Since mapping the dog's genome (at least finding markers for enough of the perpetrators) is decades away, the DI evaluation as promoted by PennHIP is by far the best tool in our tool chest.

Fred Lanting is the principal of "All Things Canine" Consulting, behaviour analysis, evaluations, seminars, and lectures. He is the author of "The Total GSD" and "Canine Hip Dysplasia."

reprinted with kind permission from Fred lanting