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Canine Hip Dysplasia:
Are Breeders Winning the Battle?
Including Comprehensive Sections On
Methods for Diagnosing and Predicting Genetic Predisposition
Canine hip dysplasia (CHD), a disease resulting from a
predisposition to increased muscle laxity involving the hip joint
(see Bone Disorders),
poses a major concern to dog breeders. Over the past several decades,
dog breeders have come to rely on the diagnostic methods of the
Orthopedic Foundation for Animals (OFA) and their rating system to
screen breeding stock and breed only those individuals devoid of CHD in
order to reduce incidence of CHD in future offspring. Despite these
attempts and the OFA's encouraging reports of a significant decline in
occurrence of breed-specific CHD, there still remains a very high
prevalence of CHD not accounted for by the OFA. Additionally, frequency
of CHD in offspring from OFA normal parents still remains
disappointingly high.
Over the past 5 years, some revealing scientific
reports have raised serious questions concerning the reliability of the
OFA method for evaluating phenotypic expression (actual appearance of
hip conformation) of parents as a means of predicting genetic outcome in
offspring as well as the consistency with which these evaluations are
interpreted.
The following takes a closer look at current
diagnostic methods for evaluating hip-joint conformation and their
reliability for predicting predisposition to CHD. Additionally, it
presents an overview of several studies performed which suggest that the
OFA method for breeding stock, though currently the most popular choice,
may not be the best method for minimizing CHD in future generations.
Are breeders winning the battle against Canine Hip
Dysplasia (CHD)?
In a 1992 report on the incidence of CHD in purebred
dogs, the OFA reported a 79% decrease in the occurrence of CHD when
comparing dogs bred between 1972-1980 with dogs bred between 1981-1988.
Though upon first glance, this data suggests a significant advance in
the control of CHD, it was recognized that some variables had not been
controlled for in the study. For example, this data would only be
relevant if all hip radiographs (X-rays) were sent to the OFA for
review. However, a review of records at the Veterinary Hospital of the
University of Pennsylvania (VHUP) indicated that only half of clients
having their dogs x-rayed for the purpose of acquiring OFA certification
actually ended up submitting the radiographs to the OFA for review. It
was determined that clients only submitted radiographs of dogs having
the greatest likelihood of being certified. In comparison to the OFA's
reports for breed-specific incidence of CHD, a clinical survey conducted
at VHUP demonstrated breed-specific incidence of CHD to be 2-3 times
higher than reported by the OFA.
What are the current methods for diagnosis of Canine Hip
Dysplasia (CHD)?
There are 3 methods for diagnosis and prediction of
predisposition to CHD in dogs which can be divided into 2 groups:
1) hip extension radiograph methods and
2) the stress radiographic diagnostic method.
These groups are categorized based on the positioning
of the dog while the radiograph is taken. The first group is the most
common method in which the dog, while lying on its back, has its rear
legs fully extended with the knees rotated inward. The second group
requires that the dog be anesthetized and while lying on its back, the
dog has its legs positioned as they would be if the dog were standing. A
custom-designed device is placed between the legs which forces them
apart, thereby displacing the ball of the femur from the hip socket and
allowing for observation of joint laxity.
The first group can then be divided into 2 methods:
The Norberg Angle method
 This
method is a quantitative method because it is based on measurement of
the angle formed by connecting a point at the center of the femoral head
(ball of the hip) to the upper acetabular rim (hip socket). Hips are
then scored as the number of degrees in the formed angle. For example,
scores range between 55 to 115 degrees with low end scores indicative of
greater hip laxity and higher risk for CHD. Dogs receiving scores above
105 degrees are accepted as having normal hip-joint conformation with
lower predisposition to CHD.
The OFA method
This method is a qualitative method because no form of
measurement is utilized. Instead this method is based on subjective
visual criteria such as degree of joint laxity (subluxation) and the
presence of degenerative joint disease.
The second group has one method:
The Distraction method (or PennHIP method)
 This
is a quantitative method like the Norberg angle. However, this method
measures the separation distance of the femoral head center (ball of
hip) from the acetabular center (socket) while the hip joint is forced
to luxate, then divides this measurement by the radius of the femoral
head (ball of hip). In effect, this method evaluates how far the femoral
head can be displaced from the acetabulum. The resulting number is
termed the distraction index (DI). The DI score can range from 0 to 1
with scores closer to 0 indicating less hip laxity. Dogs receiving a
score of less than 0.3 are considered to have normal hip-joint
conformation with little, if no risk for CHD.
How do the results of these three methods compare to one
another for evaluation of hip joint conformation?
1) hip extension radiograph methods vs. the stress
radiographic diagnostic method for evaluating hip-joint laxity
In 1993, Heyman et al. reported that the conventional
hip extension method for evaluating laxity of hips produced at least a
50% reduction in observable hip laxity compared to the stress
radiographic method. This suggests that dogs evaluated based on the hip
extension method may have a greater degree of hip-joint subluxation than
appears.
2) Norberg angle vs. OFA
In a study conducted at the Veterinary Hospital at the
University of Pennsylvania (VHUP) comparing OFA ratings to Norberg angle
measurements it was determined that the average Norberg angle score for
dogs judged to have "normal" hips by the OFA was 104 degrees and those
judged to have "dysplastic" hips was 96 degrees. Of those dogs judged to
have normal hips, 46% had Norberg angle scores lower than 105 degrees
(the Norberg cut-off for normal) and even dogs with a Norberg rating as
low as 89 degrees had been evaluated as "normal" by the OFA. This
suggests that dogs which would be judged predisposed to CHD by the
Norberg angle method are being certified for breeding by the OFA method.
3) Distraction index vs. OFA
A study conducted by Smith et al. comparing DI scores
to the OFA rating system found that dogs judged to have mild, moderate
or severe CHD by the OFA method also had DI scores above 0.3 with a mean
DI score of 0.55, therefore a high incidence of agreement exists between
these two methods in relation to the dysplastic phenotype. However, when
dogs which had been judged by the OFA method as having "excellent" hips
were evaluated for DI scores, 50% of these dogs had DI scores of greater
than 0.3. Of those judged as having "good" hips, 66% had DI scores
greater than 0.3, and in the "fair" hip group, 100% had DI scores
greater than 0.3. In this study, 71% of the dogs certified for breeding
by the OFA method had increased hip laxity which would predispose them
to CHD according to the DI method.
Why is there so much disparity between the Norberg angle
and DI scoring methods in comparison to the OFA method?
1) Each method tells a different story
The most likely answer to this question is that
although all of the methods are used for the diagnosis of CHD, they are
addressing the issue differently. For example, the OFA bases its
evaluation on the appearance of an individual dog's hip-joint
conformation at a particular point in time, therefore, in the absence of
degenerative joint changes the individual dog will be certified by the
OFA. As such, the OFA method is limited as an indicator of phenotype,
the actual appearance of the hip-joint of that individual dog, which
will not necessarily be any indication of influence on future offspring.
However, the Norberg angle method and the DI score, by quantitating hip
laxity which is believed to be the prime cause of CHD, are more
indicative of genotype. That is to say, for the same dog certified by
the OFA method to be free of CHD, a notable increase in hip-joint laxity
demonstrated by quantitative methods may identify the dog as a genetic
carrier for CHD. There are several additional factors which may account
for the alarming disparity found to exist between these different
methods for diagnosing and predicting predisposition to CHD:
2) Environmental factors play a role in phenotypic
expression of CHD
Although the exact cause for CHD is still unknown,
genetic causes for CHD are believed to be polygenic, that is, CHD is
believed to be attributed to many genes. As such, polygenic traits are
often influenced by environmental factors. For example, in studies
conducted with Labrador Retrievers and German Shepherd Dogs it was found
that restricting calories and limiting food consumption resulted in
lower incidence of CHD compared to dogs who were fed high-calorie diets
or were allowed to eat as often as they chose. Even in groups of dogs
genetically predisposed to CHD (offspring of CHD afflicted parents),
restrictive diets yielded a lower incidence of CHD. Because many
breeders and dog enthusiasts are aware of the environmental variables
which may influence phenotypic expression of hip-joint conformation,
they may take measures to reduce risks for CHD within their own dogs.
For example, breeders may reduce risks of CHD in their dogs by feeding a
maintenance diet to discourage rapid growth in puppies. Other breeders
may restrict a growing dog's activity to moderate levels in order to
reduce strain on developing joints and connective tissue. These methods
may help to reduce or eliminate early detectable evidence of CHD brought
on by increased hip laxity in the individual dog. As a result, such
preventative measures may allow the dog to acquire OFA certification,
yet the same dog would still exhibit hyperlaxity of the hip-joint making
it a poor candidate for breeding.
3) Other external factors influencing hip ratings...What
day of the week the x-rays are reviewed?!?!
Subjective methods of evaluation can lead to
unintentional bias and inconsistency for reproducibility of results. A
study conducted at the University of Pennsylvania revealed a startling
amount of variability for hip interpretations among non-OFA and OFA
board certified radiologists. When these radiologists were asked to
grade hips based on the OFA rating system (excellent, good, fair,
borderline CHD, mild CHD, moderate CHD or severe CHD), non-OFA readers
agreed with an OFA reader in fewer than 50% of the cases. The most
disturbing revelation was that when each radiologist was asked to rate
certain cases a second time, each radiologist gave the same rating that
he had given the first time on less than half the radiographs.
If the OFA method is not the best method for lowering
incidence of CHD, why are breeders still using it?
Like the Norberg angle method which has stringent
requirements for achieving a "passable" score, the DI scoring system has
not become a popular screening method among breeders. This may be
because breeders are looking at these diagnostic methods as "pass/fail".
Many breeds, particularly medium and large breeds, have mean DI scores
ranging between 0.40 to 0.74. If one were to use a 0.3 DI cutoff, no
individuals within these breeds would be considered acceptable breeding
stock. Therefore, using the DI score as a pass/fail criteria is not a
suitable alternative. Rather than using the DI scoring method to choose
whether or not to breed an individual dog, perhaps a better suggestion
is to use the method to progressively select for lower hip laxity in
future generations. For example, if a bitch achieves an OFA normal
rating and a DI score of 0.52, it would be better to breed her to an OFA
certified stud with an equal or lower DI score. Additionally, because
the DI method can be performed as early as 16 weeks of age with
indicative results, breeders can use the DI scores as another criteria
for choosing puppies for future breeding stock.
Recent Medical Publications on Canine Hip Dysplasia
with Reference to PennHIP:
Adams WM, Dueland RT, Daniels R, Fialkowski JP,
Nordheim EV. Comparison of two palpation, four radiographic and three
ultrasound methods for early detection of mild to moderate canine hip
dysplasia. Vet Radiol Ultrasound 2000 Nov;41(6):484-90
Puerto DA, Smith GK, Gregor TP, LaFond E, Conzemius MG,
Cabell LW, McKelvie PJ. Relationships between results of the Ortolani
method of hip joint palpation and distraction index, Norberg angle,
and hip score in dogs. J Am Vet Med Assoc 1999 Feb 15;214(4):497-501
Todhunter RJ, Acland GM, Olivier M, Williams AJ,
Vernier-Singer M, Burton-Wurster N, Farese JP, Grohn YT, Gilbert RO,
Dykes NL, Lust G. An outcrossed canine pedigree for linkage analysis
of hip dysplasia. J Hered 1999 Jan-Feb;90(1):83-92
Adams WM, Dueland RT, Meinen J, O'Brien RT, Giuliano E,
Nordheim EV. Early detection of canine hip dysplasia: comparison of
two palpation and five radiographic methods. J Am Anim Hosp Assoc 1998
Jul-Aug;34(4):339-47
Smith GK, LaFond E, Gregor TP, Lawler DF, Nie RC.
Within- and between-examiner repeatability of distraction indices of
the hip joints in dogs. Am J Vet Res 1997 Oct;58(10):1076-7
For more information or a list of other web pages on
Canine Hip Dysplasia visit:
Contact:
International Canine Genetics, Inc.
271 Great Valley Parkway
Malvern, PA 19355
(Tel) 1-800-248-8099
(FAX) 610-640-5754
- REFERENCES:
- Corley, E.A. Role of the Orthopedic Foundation
for Animals in the control of canine hip dysplasia. Vet. Clin. North
Am. Small Anim. Pract., 22:579-593, 1992.
- Heyman, S.J., Smith, G.K., and Cofone, M.A. A
biomechanical study of the effect of coxofemoral positioning on
passive hip joint laxity in the dog. Am. J. Vet. Res., 54: 210,
1993.
- Kealy, R.D., Olsson, S.E., Monti, K.L.,
Lawler, D.F., Biery, D.N., Helms, R.W., Lust, G., and Smith, G.K.
Effects of limited food consumption on the incidence of hip
dysplasia in growing dogs. J. Am. Vet. Med. Assoc., 201:857-863,
1992.
- Lust, G., Williams, A.J., Burton-Wurster, N.,
Pijanowski, G.J., Beck, K.A., Rubin, G., and Smith, G.K. Joint
laxity and its association with hip dysplasia in Labrador
retrievers. Am. J. Vet. Res., 54:1990-1999, 1993.
- Smith, G.K., Popovitch, C.A., Gregor, T.P.and
Shofer, F.S.. Evaluation of risk factors for degenerative joint
disease associated with hip dysplasia in dogs. J. Am. Vet. Med.
Assoc., 206:642-647, 1995.
- Smith, G.K. and McKelvie, P.J.. Current
concepts in the diagnosis of canine hip dysplasia. In Kirk's Current
Veterinary Therapy (ed. J.D. Bonagura), pp. 1180-1188. W.B. Saunders
Company, Philadelphia, 1995.
- Smith, G.K., Gregor, T.P., Rhodes, W.H., , and
Biery, D.N. Coxofemoral joint laxity from distraction radiography
and its contemporaneous and prospective correlation with laxity,
subjective score, and evidence of degenerative joint disease from
conventional hip-extended radiography in dogs. Am. J. Vet. Res.,
54:102, 1993.
- Smith, G.K., Gregor, T.P., Biery, D.N.,
Rhodes, W.H., and Reid, C.F. Hip dysplasia diagnosis: A comparison
of diagnostic methods and diagnosticians. Proc. 1992 Ann. Scientific
Meeting of the Veterinary Orthopedic Society, Keystone, CO, p. 20,
1992.
- Smith, G.K., Biery, D.N., and Gregor, T.P. New
concepts of coxofemoral joint stability and the development of a
clinical stress-radiographic method for quantitating hip joint
laxity in the dog. J. Am. Vet. Med. Assoc., 196:59-70, 1990.
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