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PennHip vs OFA, A Personal Story
By F. Lynann Stuby
Let me
begin by saying I am a member of the Health Committee of the
GPCA. I spent most of my life in nursing, in specialty units in
a major metropolitan hospital. I have always had an intellectual
curiosity about most things, and in particular medical things.
I had heard of this new hip testing procedure for Dysplasia,
and after reading their web site felt it to be potentially more
beneficial for breeders. My beliefs were based on several
things. First was that this procedure used several views,
rather than one, of the dogs hips. Second was that it was read
by a computer thus removing any “opinion” from the mix as is
seen in the OFA readings. And third it could be done at a young
age, long before you would have a lot of money tied up in a dog
whether it was for conformation, obedience or service work. Wow,
what a great thing for the dog world. I believed in this so
strongly I encouraged my vet of 25 years to become certified and
he did attend the seminar, after a year or so of my prodding.
My vet has remained on the cutting edge of new techniques and
equipment as they have come along over the years, and has a
thriving practice with many “breeders” like myself who travel a
distance to go to his clinic.
I had seven bitches of all ages that I wanted to PennHip.
He attended the seminar in January and I had high hopes of
having all of my dogs PennHip evaluated by the end of January no
later than February.
As it turned out, the certification procedure involved five
“test” exams be perform by each new practitioner, each exam
taking five views. These are then evaluated for their technique
before a certification is awarded.
My vet had
my dogs ready and waiting to be used as his first “subjects.”
All five had to be completed before any of them are “officially”
read and numbers issued to either the vet, or the owner. I
could live with this even if it might drag things out a little.
I could not have foreseen that the last result would be arriving
in June. Due to a long term patient/ client relationship, I
was allowed to witness every single examination performed. A
great learning experience for me as well.
The premise from the PennHip research/method which has been
ongoing for many years and now includes 63,000 dogs in their
data base, is the laxity or “looseness” of the joint being a
very accurate prognostication of potential Hip Dysplasia. The
numbers have proven out and this can be done on dogs as young as 16
weeks old with good reliability. The procedure consists of
three views, a extended view, (OFA view), a compression view,
which simulates the dog standing, and a distracted view which
pulls the hips from the socket as much the tendon and joint
capsule will allow showing “passive laxity.” This distracted
view has a device that is used to leverage the hips from the
sockets. The dog must be totally sedated. Unlike OFA, once an
exam is done and it ‘”appears” like a potential poor reading
might ensue, the practitioner is mandated to send the exam in.
Thus all exams are included in their database, and the potential
for the “bad” results to be removed from the figures does not
occur.
Let me take this opportunity to explain a phenomenon that can
occur with this examination. That is cavitation. The very best
explanation of this I could find was in the following article.
Here is the excerpt on cavitation from
New Words in the HD Dictionary by Fred
Lanting.
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.
So now you know as much as I do about cavitation. Once this
occurs there is not a number score given as this can drastically
change the reading toward the negative. RECOGNITION of
cavitation is essential for an accurate reading. Even “partial
cavitation” can cause a very different reading to be given.
The decision of whether to read or not to read, is done at
PennHip. A human decision, whether the computer should read the
film. There is not any other circumstance that can occur during the
exam that can result in an inaccurate result other than inadequate
distraction, that I am aware of. This is also a judgment call from those at
PennHip.
I have PennHip exams on eight of my dogs. I as well have done
OFA certifications or prelims on five of them at the same time.
The OFA view (extended) is done in the PennHip exam, so I originally
thought why not. I had also seen some websites of other breeds
where both were done and the results were very inconsistent,
high positive outcome using one method while negative outcome is
reported with the other. I assumed it
was just a bad day for the OFA viewers on those occasions, as
PennHip is read by a computer, and not subject to “human
opinion.” I have learned this is not necessarily the case.
I received several scores that were very discouraging, like half
of my bitches had hips scores in the area that I did not really
want to breed them. The worst, was one of my imports with quite
a sizable investment in her already. I had decided to choose one
of these bitches and let the others go to pet homes, no matter
what I had invested in them. There were a couple that the
PennHip score coincided with OFA , as well as siblings in the
litter. There were several where they did not coincide with the
OFA grade, and pedigree would not have suggested poor hips. I
have been very cautious to purchase good breeding stock, with
hips and eyes well documented in the pedigrees. I made the
decision to take two, with unexpected poor results, to another
vet who was very experienced in the PennHip method and
technique.
The two that I had re PennHiped both came back better than the
first exam. On one exam, PennHip phoned the vet who did the
re-exam, and discussed this very different reading with her, and they
felt after re-examination and comparison of the first set of
films with the second, that one of the hips were at least
partially cavitated. This same bitch had a Good OFA prelim. At
the same time, a pet that I have, returned with a Good OFA
reading, and a reading of .30 and .50, with a note attached
stating that partial cavitation of the .50 hip was strongly
suspected. I wondered why a reading was given at all, if it was
noted to be partially cavitated. At this point I had gone from
the depths of doom, looking at placing thousands of dollars
worth of breeding stock, to at least seeing some light at the
end of this tunnel. One bitch at this point who had two Fair OFA
evaluations, and PennHip scores indicating that she was a
moderate to low fair was placed.
Annie's Story. On the initial
five certifying tests, she was the only one that PennHip asked
my vet to do over, as “there was no difference between her
compression and distracted view.” To toss another wrench into
the mix, my vet had installed new digital system for his x-ray
equipment. We did the second procedure, and fell victim to new computer
equipment syndrome, they accidentally were deleted. So back
again for a third set. By this time, Annie was beginning to glow
in the dark. We did these and in looking at them in the
office, there appeared to be a large amount of distraction. Her
scores came back, . 67 .65. These readings are indicative of a
dog that would likely be affected at a young age with Hip
Dysplasia. Again doom has struck my kennel. I had actually
talked with possible new pet owners for her.
I decided to have her reexamined when the OFA prelim reading
came back GOOD. How could this be? My vet was kept aware of all
of the numbers on the others I had done elsewhere. He
volunteered to do yet one more set of films. When we arrived it
was decided that we would do five views, the extended, the
compression, and three distracted, one with a slight pressure used,
a “normal” pressure used, and the last would be with enough
pressure to cavitate the hips. He would contact PennHip and
discuss this with them, and let them know why the extra views.
This last set returned .42 .42. A huge difference from the
previous set which returned so horrible. Considering the
quality of this particular bitch, this reading was acceptable
to keep her in my breeding program.
My conclusions to this entire experience.
Technique is of the utmost importance, as it seems in novice
practitioners, cavitation is slightly more likely to occur.
There is a human judgment element involved with PennHip, as the
detection of cavitation, complete or partial, must be accurately
determined before the computer is asked to read the distraction
score.
I will always send in an OFA with the PennHip, if they are
drastically different, there just might be a reason for it.
Will I continue to PennHip? Yes, because within the three
grading levels of OFA, there is a range, and PennHip gives such
an exact number. It will tell you whether you have a low or high
excellent, low or high good, a low or high fair. The
more information available to use in our breeding decisions, the
better. A graph is presented of all of these readings.
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