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What
Happens to the PennHIP Radiograph? |
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Technique and Reporting Process by Fred Lanting |
Early
in many breed magazines featured an OFA public relations release entitled
"What Happens to a Radiograph at the OFA: The Submission and Evaluation
Process”. As the author of “Canine Hip Dysplasia”, and an international lecturer on orthopedic disorders, as well as a long-time specialty
judge of the Anatolian Shepherd and related breeds and varieties such as the Kangal and Akbash, I was asked by
many readers to comment. Specifically, on similarities and differences in the
procedures used when radiographs are taken for the PennHIP
evaluation.
As
you probably know, the acronym stands for (University of)
Despite
years of data availability, many breeders and other dog fanciers appear either
totally unaware, or to have various bits of fact and misconception mixed
together like a tossed salad, regarding PennHIP and
its relationship to OFA. Almost everyone now knows that 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 might also 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. To this day, it is one of the two definitions of HD that OFA
uses, the other of course being DJD --- degenerative joint disease.
The
AVMA position, popularized by OFA, 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). 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 (60 degrees if you start from the tail end). The “neutral/natural”
position for Homo sapiens (or Homo erectus as we perhaps should have
called ourselves) is the erect one when standing or moving. By neutral, I mean
that position in which there is the greatest state of relaxation of muscles
used to extend or flex the limb. Not
only are the muscles and ligaments most relaxed, but also the joints are 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. In other words, the leg-extended view
unnaturally and artificially “winds up” the muscles, tendons, and ligaments in
and around the hip joint. These 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 rubber band with two sticks turning in opposite directions.
Actually, the extensibility of joint capsule tissue makes it more analogous to
use nylon rope instead of an elastic band for the illustration. This
artificially tighter-than-natural position contributes to the high
false-negative rates in the OFA-certified dogs, as pertaining to laxity. While
it is the best for discovering DJD (degenerative joint disease) it is the worst
for uncovering latent laxity, or what I have taken to calling “covert laxity”.
False-negative means that a passing grade is given because the true laxity was
not observed in spite of, or due to, the method used to look for it.
In
the most current and nearly the only stress-radiographic analysis method used,
the PennHIP technique, the dog is placed in its most
neutral position, 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 these
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, whether as defined by Penn as DJD, or by OFA as either DJD or laxity. So far there has been only one
“semi-exception” in more than 25,000 dogs, a dog with 0.29 in one hip when
evaluated at a very young age and later discovered to have very minimal joint
quality change. There is a mention in a 1993
AJVR publication of this exception. Few dogs will show later evidence of DJD,
and then it is minimal. The threshold of
0.3 is not absolute, but such findings do not at all diminish the value of PennHIP. Exceptions do not make or negate the rule.
All
of the above is meant to be as concise an introduction to the question of what
happens to a PennHIP radiograph as is feasible, while
not leaving other important questions unaddressed. There are actually three
radiographs used in the PennHIP procedure, and only PennHIP-certified vets may submit them. Additionally, every
dog’s films enter the database, so there is not the skew or bias as found with
OFA. The “first” film (actually, it doesn’t much matter in which order they are
made) is identical to that used by OFA and so many countries: the traditional
AVMA extended-leg picture for the study of bone abnormalities — in the worst
cases, 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 should be emphasized that PennHIP consists of the OFA view plus two additional views. To those who have been confused by rumor or news releases, we must ask: “How could more information be less useful?”
It
is the third view that really makes all the difference. While the dog is deeply
“under” and does not feel anything even on a nearly subconscious level, 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, though much goes unobserved in the leg-extended and
non-stressed views.
The
films are sent to the
Contrary to the publicity
release OFA put out in the spring of 1999, in which it was again intimated that
estrus affects or may increase joint laxity, the PennHIP researchers and method do not show it 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 OFA's
purported relationship of estrus to hip laxity, and
that not only is there "no support”, there is strong enough scientific
documentation to refute it.
Strict standards are imposed at the
The
great value of PennHIP is the accuracy and
reliability of evaluations done at an early age, so owners don’t spend more
money than necessary in training for more demanding work, or even breed a dog
that has a relatively high risk of later transmitting many bad genes to
progeny, or itself developing DJD. The accuracy and repeatability of the D.I.
is as valid at six months age as throughout life; in fact very high reliability
(96%) is seen in pups even as young as four months. The report by OFA that they
too, now have equal predictive value (JAVMA, 1997) was refuted by a
During
the seminar on HD and other orthopedic disorders that
I have presented in many countries, I am often asked for advice in a number of
areas. One question that has come up since PennHIP’s
advantages have become more widely known is “Should I chuck OFA out the
window?” No. At least
not yet. In fact, I recommend to those who want to compare and to
communicate info to their OFA-only colleagues, that they ask the PennHIP vet to slip an extra film into the cassette for the
first shot, and send that one to OFA. For advertising
purposes, there is still an awareness advantage to also mentioning OFA. Those readers of the ad who know nothing of PennHIP may be more set at ease with an “OFA-excellent”
than with an as-yet unfamiliar rating, and certainly more than no rating at
all. This advantage is slowly receding, though, as increasingly people become
knowledgeable about PennHIP and discouraged with the
relatively poor reliability of OFA. There
are a few points that have to be clear, basic things to keep in mind:
- PennHIP is the OFA's hip-extended plus
two more radiographs that show different things,
- PennHIP has performed
biomechanical studies on its radiographic positioning while the OFA has
not,
- PennHIP has performed much
research in general and these have been published in refereed journals to
prove the science is valid. OFA has
not.
An even
better idea is to send that extra film to GDC, along with pedigree and info on
siblings. OFA (and unfortunately PennHIP at this
date) are “closed” registries, which means you cannot
get info on OFA’s failed (dysplastic)
dogs such as relatives of the ones you want to breed. GDC is an “open”
registry, with such valuable breeders’ information being available. Institute
for Genetic Disease Control (GDC) and its international corollary, the IEWG
(Int'l Elbow Working Group): fax 650-941-7848, or
I
used to suggest a program of Bardens palpation, wedge
X-ray, and OFA-Good or Excellent. Times have indeed changed, technology has
improved, and today I recommend PennHIP at 4-6 months
(or any time before breeding) as a viable and more accurate evaluation than all
three of those. For breeding, I tell my audiences to breed only to a partner
with higher than 50th percentile and lower DI than the mean, or a lower
DI than their own dog has, and if they really
want to accelerate progress, to breed dogs with 0.3 or better. At least, get as
close to that threshold as possible, consistent with preservation of breed type
and character. For the sake of the unbelievers or unreached,
I also suggest thinking about getting an OFA reading as an adjunct to the
better method. When you can easily sell all you produce without that crutch,
you can then forget about OFA. Or perhaps OFA by then will have adopted this
improved technique for the sake of faster progress in reducing HD incidence.
