| "These authors questioned the meaning of joint laxity as demonstrated by force." | 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. |
| "The results of the study were questioned by Dr. Susan Shott of the Biostatistical Unit, Rusk Cancer Institute (Am J Vet Res, December 1993)" |
OFA feels that general use of PennHIP as a mass screening test method
for hip dysplasia is premature because:
|
| " It appears that the probability of retaining a dysplastic dog in the breeding pool is the same for either test method. " |
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. |
| "the probability for removing a normal dog from the breeding pool is less with the OFA evaluations" |
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. |
| "PennHIP . . . . The meaning of the measurements remains unclear and will require repeat studies, on the same dogs, at >24 months of age. " |
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.
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