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PennHIP:
Misconceptions and Misinformation |
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(Additional
late breaking news: The AKC has back-pedaled
and decided not to include PennHIP scores, only OFA
data, from their official documents such as the registration papers. This is
not fair, and if you wish to protest, call or write AKC. If you want to know to
whom to direct your protest, or simply ask why this turn of policy, you could
e-mail AKC)
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."
MY RESPONSE:
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,
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 here 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:
- Tighter
Is better;
- Position
and technique (better tools) can discover covert laxity;
- 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.
