PennHIP: Misconceptions and Misinformation
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 mrgsd@hiwaay.net
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, 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:
- 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.
Fred Lanting, Canine Consulting.
mrgsd@hiwaay.net
; 3565 Parches Cove Rd, Union Grove AL
35175-8422; Lectures & Seminars on Orthopedic
Disorders, Gait-&-Structure, Etc.
Copyright 1998 by Fred Lanting. For
reprint rights, contact the
author.
|
OFA Update: The Issue of Joint Laxity and Stress
Radiography
By G.G. Keller, D.V.M., MS, Diplomate of
A.V.C.R., Executive Director Orthopedic Foundation for
Animals, Inc. and E.A. Corley, D.V.M., Ph.D., Diplomate of
A.V.C.R.
OFA does not normally respond to the
various opinions expressed by individuals on Internet web
sites and/or chat lines. Instead OFA maintains a web site
(http://www.offa.org) to provide information that may be
of value to breeders and veterinarians. However, a
response to the opinions expressed by many people is
prompted, as the opinions appear to have deteriorated to
the point of becoming non-productive. OFA stated its
position on any testing method, including PennHIP, that
involved stress radiography to the breed clubs in 1994.
This posting is a review of that position.
Contrary to some Internet postings,
OFA, a not-for-profit organization, does support and
encourage research on joint laxity and its meaning. The
fact that joint laxity plays a role, but is not the only
factor to be considered in development of hip dysplasia
and its secondary changes of degenerative joint disease,
has been recognized for over 30 years. This fact is not in
dispute. The issue has been, and remains to be, the
relationship of laxity that is demonstrated by forcing the
heads of the femurs away from the acetabula by palpation
or a fulcrum/stress device (i.e., a distraction device) to
abnormal laxity (functional laxity that occurs in hip
dysplasia.) Since 1972, when an independent panel of
scientists classified the techniques for demonstration of
joint laxity by use of an externally applied force as
experimental, OFA has financially supported three research
projects, recommended by external review, to answer the
basic question. Dr. Belkoff, et.al. (VCOT 1: 31-36 1989)
developed a device that measured the amount of force
applied to the hips and noted that some dogs that
demonstrated abnormal amounts of laxity were free of hip
dysplasia at necropsy. These authors questioned the
meaning of joint laxity as demonstrated by force. The
other two projects supported by OFA are ongoing.
PennHIP is another technique for
demonstration of forced (passive) laxity that is also
attempting to answer the basic question of the
relationship of passive laxity to functional laxity. OFA
encourages their research efforts; however, OFA takes
exception to the marketing techniques and claims used to
promote the PennHIP testing method for clinical use, as
the use of this method appears to be premature. In other
words, commercialization (marketing) of the method has
outreached the science.
OFA feels that general use of PennHIP
as a mass screening test method for hip dysplasia is
premature because:
- The primary basis for marketing PennHIP was reported
by Dr. Smith, et.al. (Am J Vet Res, July 1993) using a
modification of a previously described positioning,
stress/fulcrum technique. The study was a survey type
involving 142 dogs (105 of which were German Shepherd
Dogs). The results of the study were questioned by Dr.
Susan Shott of the Biostatistical Unit, Rusk Cancer
Institute (Am J Vet Res, December 1993) who challenged
the analysis of the data and stated: “The data
does not support the author’s conclusion that the DI
was the most important and reliable phenotypic factor
for determining susceptibility of hips to degenerative
joint disease … and that this determination could be
made with an acceptable degree of accuracy as early as
4 months of age.”
- Dr. Lust, et.al. (Dr. Smith was a coauthor) in a
report involving 42 Labrador Retrievers (Am J Vet Res,
December 1993) concluded that a DI of <0.4 at 4
months of age correctly predicted normal hips in 88%
of the cases and a DI of >0.4 correctly
predicted hip dysplasia in 57% of the cases. The
authors further concluded that: “Distraction
indices between 0.4 and 0.7 and at either 4 or 8
months of age were not associated strongly enough with
evidence of disease to be clinically reliable in
predicting, on an individual basis, the outcome for
dysplastic hip conformation when the dogs were
older.”
-
No breeding data based on PennHIP has been
reported. Dr. E.A. Leighton (JAVMA, May 13, 1997)
reported on genetic progress in improving the hip
quality in German Shepherd Dogs and Labrador
Retrievers in the Seeing Eye closed colony of dogs.
In less than 5 generations the percentage of hip
dysplasia was decreased from 55 to 24% in the German
Shepherd Dogs and from 30 to 10% in the Labrador
Retrievers using the hip extended position and a
modified OFA evaluation procedure. PennHIP DI
measurements were also made but the mean DI across
generations did not change. It should be pointed out
that DI was considered experimental and breeding
selection criteria did not include the DI. It will
be interesting to see the results when DI is
included as a selection criterion.
With the above reservations, plus
experience with the issue of joint laxity, OFA would be
remiss in its responsibility to either endorse or reject
the PennHIP testing method. In other words, the jury is
still out! This leaves the breeder in a dilemma as to
which testing method to use, OFA or PennHIP or both, as
they are entirely different test methods for the same
disease.
There is a great economic advantage
to breeders for determination of the hip status at a
young age and to assess the risk for development of hip
dysplasia at a later age. OFA reported (Vet Clinics of
No Am, May 1992) on a study of 3,369 dogs from 25
breeds. Reliability of the preliminary evaluations
ranged from 71.4% in the Chesapeake Bay Retriever to
100% in the Welsh Springer Spaniel. The preliminary
evaluation appeared to be breed dependent and dependent
on the evaluator’s experience with the skeletal
development of that breed at the age of evaluation.
When faced with the problem of
comparing entirely different test methods for
determining dysplasia, scientists evaluate the results
of reported values for false negative (probability of
diagnosing a dysplastic dog as normal), false positive
(probability of diagnosing a normal dog as dysplastic),
specificity (probability of a normal dog receiving a
normal evaluation), and sensitivity (probability of a
dysplastic dog receiving a dysplastic evaluation). These
values for OFA preliminary evaluations by age and hip
ratings, in a different population of dogs than
previously reported (Vet Clinics of No Am., May 1992)
have been reported (JAVMA, November 1, 1997). The false
negative and false positive values for PennHIP were
reported by Dr. Smith et.al. (Am J Vet Res, July 1993).
No report of selectivity or sensitivity values for
PennHIP were given. There were 2,332 dogs in this OFA
study and 142 dogs in the PennHIP study. The limited
number of dogs resulted in a larger confidence interval
for the PennHIP values. Confidence intervals (CI) are
determined so that one can be 95% confident that the
true value lies within the calculated range. The false
negative values for OFA evaluations were 8.9% (CI=5.9 to
12.9%) at 3-6 months, 6.0% (CI=4.4 to 8.0%) at 7-12
months and 3.8% (CI=2.6 to 5.4%) at 13-18 months of age.
The false negative values for PennHIP evaluations were
12% (CI=1.5 to 38.3%) at 4 months and 0% (CI=0.0 to
15.4%) at 12 months of age. It appears that the
probability of retaining a dysplastic dog in the
breeding pool is the same for either test method.
However, the false positive values
for PennHIP were significantly greater (48% at 4 months,
57% at 6 months and 38% at 12 months) than those for OFA
evaluations 17.6% at 3-6 months (CI 10.8 to 26.4%),
10.0% at 7-12 months (CI 5.7 to 15.9%) and 8.5% at 13-18
months (CI 4.8 to 13.6%). It appears that the
probability for removing a normal dog from the breeding
pool is less with the OFA evaluations.
Dr. Adams, et.al. (JAAHA, 1998, 34:
339-47) reported (using palpation, OFA method, PennHIP,
and Norberg angle measurements) on results of a study of
hip laxity, in 32 dogs from 4 breeds (12 Greyhounds, 4
Labrador Retrievers, 12 Irish Setters, and 4 hound-mix)
at 6-10 weeks and 16 to 18 weeks that were compared to
detection of degenerative joint disease at 52 weeks of
age. Five hips with evidence of subluxation but no
evidence of degenerative joint disease on the OFA type
evaluation of the hip extended view were eliminated from
analysis. The authors concluded that DI and Norberg
Angle measurements at 6-10 and 16-18 weeks were the most
reliable predictors of hip dysplasia, at 52 weeks of
age, with DI being more reliable than Norberg. The OFA
and palpation methods at 6-10 or 16-18 weeks were not
reliable predictors. This is not surprising as
reliability of OFA preliminary evaluations has been
shown to increase with age of evaluation. The OFA report
(JAVMA, Nov. 1997) included 380 dogs evaluated at 3 to 6
months of age. The reliability was 89.6% (CI=85.4 to
92.9%) for normal evaluations and 80.4% (CI=71.4 to
87.6%) for dysplastic evaluations. The mean age was 4.8
months (19.2 weeks) and the median age was 5 months (20
weeks) which means that over half of the dogs in the OFA
study were older than in the study reported by Dr.
Adams.
OFA data and PennHIP data are not
representative of the general population of dogs because
the programs are voluntary, most dogs are in pet homes
and are not radiographed, and not all radiographs of
dogs radiographed are submitted for evaluation by either
program. For example; if an attending veterinarian
determines a dog to be dysplastic, by either method, the
radiograph(s) may not be submitted to save the owner
money. PennHIP collaborators may take the hip extended
view first and if the radiograph shows evidence of
dysplasia the DI views may not be taken or the owner may
not allow submission of an obviously large DI
measurement.
Breeders are aware of the economic
value of early screening of dogs for determination of
the hip status. They should also be aware that both OFA
and PennHIP use the radiographic evaluation of the same
hip extended projection as the standard for comparing
with the results of the early evaluations. The OFA
standard represents the consensus of 3 independent
evaluations at >24 months of age by board certified
veterinary radiologists. It is not clear who evaluates a
radiograph submitted for PennHIP determination, but the
original study reported the standard to be Dr. Smith’s
evaluation. This evaluation at >24 months of age has
approximately 5% false negative finding as reported by
Dr. Jessen (Proceedings of a 1972 symposium on hip
dysplasia) and by an internal OFA study of dogs
evaluated at 24 months that were re-evaluated at an
older age. This is why OFA requires the 24 month
certification age. Voluntary submissions to PennHIP will
provide information on the range, mean and median of the
DI measurements for the various breeds. The meaning of
the measurements remains unclear and will require repeat
studies, on the same dogs, at >24 months of age.
Breeders must be aware of the cost,
strengths, and weaknesses of the test methods available
for evaluation of the hip status before making the
choice of a specific testing method. Once the choice is
made, it must be followed for generations before
progress in improving the hip status can be evaluated.
OFA data has demonstrated marked improvement of the hip
status in the Portuguese Water Dog (AKC Gazette, Nov
1991) and the Chinese Shar Pei (Barker, Mar/Apr 1995).
OFA data on all breeds was independently evaluated and
reported by Dr. Kaneene (JAVMA, Dec 1997) an
epidemiologists from the Population Medicine Center at
Michigan State University. The study compared OFA
evaluations on dogs born between 1972 and 1980 with dogs
born between 1989 and 1992. The population consisted of
270,978 dogs. The authors, having acknowledged the fact
that submissions are voluntary and that there is bias
due to prior screening, concluded:
We do not believe that this is the
most likely explanation, because the increase in the
percentage of dogs classified as having excellent hip
joint phenotype (+36% [7.82 vs 10.64%]) was
substantially larger than the decrease in the
percentage of dogs classified as having canine hip
dysplasia (-21.% [17.39 vs 13.82%]). If better
screening of radiographs prior to submission to the
OFA was the cause of the increase in percentage of
dogs classified as having an excellent hip joint
phenotype, then because it is easier to differentiate
dysplastic hips from hips with normal phenotypes than
it is to differentiate hips with excellent, good and
fair phenotypes, we would have expected that the
decrease in percentage of dogs classified as having
canine hip dysplasia would have been larger than the
increase in percentage of dogs classified as having an
excellent hip joint phenotype.
Unfortunately, PennHIP has not been
available long enough to accumulate the data necessary
to evaluate the effect of this test method over time.
G.G. Keller, D.V.M., MS, Diplomate of
A.V.C.R., is the Executive Director of Orthopedic
Foundation for Animals, Inc. Dr. Keller received his
Doctorate in Veterinary Medicine in 1973 and was in a
small animal private practice until 1987 at which time
he accepted the Associate Director position for the
Orthopedic Foundation for Animals. He received the
Masters degree in Veterinary Medicine and Surgery in
1990 and Diplomate status in the American College of
Veterinary Radiology in 1994. He assumed the role of
Executive Director for the Orthopedic Foundation for
Animals in January, 1997.
Copyright 1998 by Orthopedic
Foundation for Animals. This article may be reprinted
for educational purposes only, with the copyright notice
intact.
To contact the OFA:
2300 E. Nifong Blvd.
Columbia, MO 65201
Tel: 573-442-0418
Fax: 573-875-5073
Email: ofa@offa.org
Web site: http://www.offa.org
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