Canine Hip Dysplasia Part I
by John C. Cargill MA MBA MS
|
|
© Copyright May, 2000 -
2005 |
To understand this genetically transmitted disease, we must
first understand the workings of the normal canine hip.
By John C. Cargill, MA MBA, MS and Susan Thorpe-Vargas, MS
This is the first in a series of articles addressing
canine hip dysplasia. What follows is written from the perspective that
the readers of the series are conscientious breeders who are the
guardians of the genetic pools that constitute their breeds. While this
series of articles will not replace a stack of veterinary medical texts,
it is a relatively in-depth look at the whole problem of canine hip
dysplasia. Furthermore, the series is designed to be retained as a
reference. When you finish reading it you will have a sufficient
background to make rational breeding choices and will be able to discuss
the subject from an informed basis with your veterinarian. You may not
like what you read, but you will be more competent to deal with the
problem.
Hip dysplasia is one of the most controversial and
widespread problems in the dog fancy. So many old-wives tales,
anecdotes, misconceptions and even lies abound that one of the goals of
this series of articles must be to lay things out to the reader as they
are, supported with some scientific basis.
Let's start with a hypothetical scenario, but one
which too many of us have faced:
He's major-pointed; he moves like a dream; that head
piece may just be the best you have ever bred. In short, this boy
typifies everything that is good about your breed and is the culmination
of many years of hard work, hopes, tears, frustration and all the ups
and downs, joys and heartaches common to the fancy. Now it is time to
X-ray his hips so that you can not only use him in your breeding
program, but advertise him at stud. This is one boy that is going to
make it, and we are talking national specialty here.
Problem - the radiographic results come back with a
diagnosis of canine hip dysplasia-severe. What should you do?
More among us than will admit have had this
experience, and most of those who haven't have seen it happen to other
breeders concentrating on similar bloodlines. Now back to our
hypothetical scenario:
You never suspected a thing. The dog never appeared
to be in pain and his gait was what won him his major points. You have
invested time, money and your hopes on this animal, and it all has been
for naught! Now is the time for hysteria and self-blame:
- What went wrong?
- Could this have been prevented?
- Was he not fed correctly?
- Was he kept on an improper surface while growing?
- What is this disease that keeps reappearing in the most
conscientious of breeding programs, and which frustrates our
attempts to eradicate it?
The first step in understanding canine hip dysplasia
is to recognize it as not just one disease but many diseases, which
together result in degenerative effects on the hip joint. An extremely
complex disorder, hip dysplasia is now thought by some to be the most
noticeable manifestation of a systemic condition that can affect not
only the hip joints but also those of the elbow, shoulder and event the
joints between the vertebrae1. Whatever else might result
from the systemic conditions of this polygenic and multifactorial
disease, hip dysplasia remains a common, usually painful and often
debilitating disease. "Efforts by dog breeders and veterinarians to
reduce the prevalence of the disorder have proven marginally effective."
2
While there is much that we do not know we do know
that canine hip dysplasia is a genetically transmitted disease. If you
need to, or if you disagree at this point, please re-read that
statement. We will be repeating it throughout this series of articles,
and this concept is the basis for determination of fitness for breeding.
The genetic concept of heritability is a complicating factor and is one
reason why hip dysplasia remains so controversial. So often when you
breed you get more than you see. Without resorting to too much math,
heritability is equal to the statistical variance due to genetic
influence divided by the sum of the statistical variance due to the
genetic influence plus the variance due to the environmental influence.
It is easier to comprehend the mathematical notation than the statement
of the equation:

H2 = heritability index
Vgenetics = variance due to genetics
Venvironment = variance due to environmental influences
Thus, heritability is defined as an estimate of how
much environmental factors play in the expression of the
inherited genes. A high heritability index means that environmental
considerations are not as important as genetic elements. The numerical
value or heritability index is a function not only of breed type but of
the population from which the data is extracted. "Studies of hip
dysplasia genetics have indicated that the disease is polygenic and
multifactorial, with estimates of heritability index in the range of 0.2
to 0.3"3
For instance, in a 1986 Swedish study, the
heritability of hip dysplasia in German Shepherds was 0.40 in Sweden,
but only 0.25 in the British Isles during the same time period. The
difference between breeds may also reflect their levels of inbreeding.
The more inbreeding, the lower the heritability index because inbreeding
reduces the total genetic variability-that is, the gene pool is smaller.
Inbreeding is not a bad word. It only becomes problematic when
undesirable genetic traits are concentrated within the gene pool. By
definition, every purebred dog of any given breed is highly inbred, or
else it would look like a feral dog. We frequently hear that the problem
with the American Kennel Club purebred dogs is that they are inbred. We
should hope so, otherwise we could never fix type to the point where
there were discernible differences between breeds. On the other hand, we
would hope that the concentrated gene pools for the various breeds would
have been concentrated from stock exhibiting only desirable genetic
traits. We would hope that our field, bench and obedience champions
would be fit to contribute to the gene pool. Of course, we know that is
not true, or there would be no purpose in writing this article.
4,5,6
(diagram based on reprint from the Journal of the
American Veterinary Association, Vol.196, No.1,pp.59-70. "New concepts
of coxofemoral joint stability and the development of a clinical
stress-radiographic method for quantitating hip joint laxity in the
dog," by Gail K. Smith, V.M.D., Ph.D.; Darryl N. Biery, D.V.M.; and
Thomas P. Gregor, B.S.)
To further complicate matters is the fact that the
pattern of inheritance indicates that more than one gene is involved.
Hip dysphasia is polygenic (involves many different genes) and
multifactorial (influenced by many non-genetic factors). This makes
sense when you think of the complexity of the various structures
involved. Every cell in the body, except for sex cells, carries two
copies of each gene and each gene codes for a specific characteristic.
One very simple example is eye colour:
If the cell's two sets of genes for a specific
characteristic are exactly alike, then the animal is homozygous for that
characteristic.
If the two genes are different, i.e., heterozygous,
then one copy of the genes could code for blue eyes and the other could
code for brown eyes.
Let's complicate the matter even further. If the
animal carries two different copies of the same gene for eye color, only
one copy can be expressed in any given eye. Closer to home, in humans
for example, a child born to parents heterozygous for eye color (both
parents have a blue-eyed gene and brown-eyed gene) will have a
one-in-four chance of having blue eyes. This is because the gene for
blue eyes is recessive and both copies for that code for blue eyes must
be present before that characteristic can be expressed. On the other
hand, if the child has brown eyes, we don't know what type of genes for
eye color he or she has. This is because the gene for brown eyes is
dominant and is able to "mask" the physical expression of the blue-eyed
gene. Alternatively, the child could have only the genes that code for
brown eyes. It is beyond the scope of this article to address the
various "odd" eye color combinations, but co-dominance and variable
penetrance may be what we are dealing with in canine hip dysplasia.
What you have just read is an example of phenotype
vs. Genotype. Phenotype is the physical expression of a genetic
characteristic. Genotype is genetic composition of the organism.
Using our eye-color example, the child with two different copies of the
gene will express the brown-eyed phenotype, but his or her genotype will
be heterozygous.
Let's add to the complexity once again. Co-dominance
of genes is a situation where neither gene is dominant. A clear example
illustrating the concept of genetic co-dominance is flower color. A snap
dragon homozygous (both copies of color genes exactly alike) for white
petals crossed with a snap dragon homozygous for red petals will produce
a flower with pink petals, not a flower with either white or red petals
or a mixture of red and white petals. Many researchers feel that hip
dysplasia may be a mixture of dominant, recessive and co-dominant genes.
Quite probably, this is one of the reasons why isolation of the
causative genetic factors of canine hip dysplasia has been so elusive.
The concepts that you need to be clear on as we leave
this mini-course on genetics are: heritability index; genetic and
environmental variability; dominant vs. Recessive genes; homozygous vs.
heterozygous; genetic co-dominance; and most importantly that hip
dysplasia is genetically inheritable and is polygenic and
multifactorial. In short, you can get it in your breeding program when
you bred from animals that did not show it.
Before we can discuss an abnormal process (disease),
we need to first understand the normal process. In this case, we must be
able to answer the question, "What is a normal hip, what makes it
normal, and how does it get that way?"
First, what is the hip? The hip joint is a main
weight-bearing joint consisting principally of a ball and socket. This
joint connects the pelvis to the lower extremities. The ball is on the
end of the femur (thigh bone) and the socket (acetabulum) is part of the
pelvis. Note from figure 1 how the femoral head fits into the acetabulum
in the normal hip joint. This will be key to all our discussions from
this point forth. A true ball-and-socket joint has three degrees of
freedom, that is, it supports rotation about three different axes. The
canine hip joint is unusual as a ball-and-socket joint in that it has a
fourth degree of freedom. The femoral head may be displaced laterally
from the acetabulum. While this is the genius of this joint, allowing
the attached appendage a full range of motion, it can also create a
problem if there becomes too much laxity in the joint. Note the fourth
degree of freedom in Figure 2, which provides for the femoral head
(ball) to move directly away from the acetabulum (socket). From Figures
1 and 2, it should be obvious that much lateral displacement of the
femoral head from its seat in the acetabulum will produce high joint
stresses during weight bearing. This joint laxity will be a major
consideration for the changes it causes in the joint mechanics as we
progress through this series of articles.
The acetabulum is formed from the embryonic process
of fusion of the ilium (top of the hip), the ischium (lowest part of the
hip) and the pubis (below the ilium but above the pubis) and the
acetabular bone. Most researchers feel that normal development requires
close conformity (close, tight fit) between the acetabulum and the
femoral head throughout their growth period. In other words, the joint
must fit tightly, deeply and snugly. This is how a puppy's hip starts
out-dysplastic and non-dysplastic puppies' hips are indistinguishable.
The first six months of life seem to be the most critical growth period
when the depth of the socket must be maintained. It is believed that the
depth of the socket in the growing puppy may be in part a function of
the amount of stress the femoral head can produce on the immature
acetabulum. Think of it as a thumb pushing into a ball of clay. The
harder the thumb pushes, the deeper the indentation in the clay. Much as
a knife edge concentrates force onto a relatively small surafce area
(and a pin of a diameter equal to the width of the knife edge even
more), the two phenotypic traits that maximize the forces between these
two developing bony structures are a small femoral head and a long
femoral neck. Note, however, that the normal acetabulum is well-formed
in utero, thus the stress may only serve to maintain that socket depth.
To cushion the force between these two bony surfaces,
there is a truly remarkable substance called articular cartilage. This
cartilage is similar to a hard sponge with a slick hard surface facing
the interior of the joint. In the normal joint, articular cartilage is
able to change its shape slightly when force is applied to it, thus
spreading and distributing force more evenly into the subchrondal bone
directly beneath the articular cartilage. This is of major importance to
the long-term integrity of the joint.
Holding everything in place is another structure that
does more than just enhance the stability of the joint. The joint
capsule is a fibrous structure filled with synovial fluid that
surrounds, isolates and protects the joint. This joint capsule is
essential to proper development and functioning of the joint. This
structure is similar to the rubber grease bladder around a ball joint in
the front suspension of your car. The cushioning effect of the grease
with the fluid pressure of the grease and the elasticity of the bladder
helps to stabilize the joint. The bladder helps keep out contaminants.
This function becomes even more important as the joint ages and surfaces
become worn. The joint capsule contains the all-important synovial
fluid, the most important ingredients of which are nutrients, which
diffuse into the joint from the blood supply, and hyaluronic acid (HA).
The tissues within the joint extract nutrients from the synovial fluid
in which they are bathed.
Hyaluronic acid has a critical function: to provide
lubrication. This slippery and viscous substance prevents rapid erosion
of the articular cartilage and the surfaces of the femoral head and the
acetabulum. A membrane called the synovial membrane lines the inside of
the joint capsule, providing further isolation of the joint space.
Should the synovial membrane become injured or ruptured, white blood
cells release enzymes and oxygen radials (free radicals) that attack and
destroy hyaluronic acid. When this occurs, the loss of HA reduces the
lubrication that prevents friction and limits erosion of the articular
cartilage. Even worse, loss of HA allows the enzymes from white blood
cells to join forces with oxygen free radicals and attack the articular
cartilage. Free radicals play a major role in degenerative arthritis.
The ball-and-socket (coxofemoral) joints of an
affected puppy radio graphically appear to be structurally and
functionally normal at birth. The hips of an affected puppy are
indistinguishable from a normal puppy at birth. This is an important
point to remember. As an affected puppy grows, the hip joint undergoes
severe structural alterations. The changes result from joint laxity and
adulteration/destruction of the constituents of the synovial fluid and
subsequent loss of lubrication and nourishment, which serve to reduce
the regenerative and elastic (force-absorbing and distributing)
properties of the articular cartilage. The normal joint retains its
tightness and close fit. Whereas in the genetically dysphasic-to-be
puppy, the acetabular rim and femoral head become eroded.
Remember that the acetabular depth is partially a
function of the small "footprint" of the femoral head which concentrates
force into a small surface area. As the femoral head is flattened, the
coxofemoral joint no longer fits snugly. Excessive force is applied
unevenly, especially at the edges of the flattened femoral head.
Visualize this joint looseness as the difference between the impact of a
boxer's fist when the punch is thrown with the glove already in contact
with the opponent's jaw as contrasted with an initial stand-off distance
of say 20 inches. In the first case, little impact force is transmitted
and no damage is done; in the second, there may be a knock-out. In the
joint, the increase in stress results not only in abnormal wear of the
articular cartilage, but causes tiny micro-stress fractures to appear in
the subchondral bone. The body attempts to heal these fissures, causing
the acetabulum to become filled in, i.e., made shallower. It is this
cycle of damage and repair (osteophyte formation) that leads to
deformation of the joint, and degenerative hip disease.
Conclusions: Hip dysphasia is not something a dog
acquires; a dog either is genetically dysphasic or it is not.
Initially, the hips of affected and normal puppies are
indistinguishable. Later in life, an affected animal can exhibit a wide
range of phenotypes, all the way from normal to severely dysphasic and
functionally crippled. You should take away from this article the idea
that hip dysphasia is genetically inherited. Never believe a fellow
breeder or fancier who claims there is no hip dysphasia in his or her
line. Never believe breeders who claim that if their breeding lines
carried the genes for hip dysphasia they would be able to see it in
their animals' gaits. This just is not true.
Although work has been started to find the genetic
markers for the disease, we have as yet no method of genetic analysis
that can tell breeders whether their dogs are dysphasic or not. We only
have physical expression of the disease, and an effort to "back door"
into clear stock for breeding purposes. Breeders must come to understand
that the only way to reduce the incidence of hip dysphasia is by trying
to breed from as few animals that have progenitors, siblings, get, or
get of siblings that had clinical manifestations of hip dysphasia.
Obviously, a great amount of information is lacking to make a rational
breeding choice. These are hard words to have read, but much of our
problem has come from thousands of years of less than natural selection
resulting from the domestication of the dog.
In our second article in this series we will address
in greater detail the parts nutritional, environmental and other factors
play in mitigating or increasing the physical expression of canine hip
dysphasia.
CREDITS
References
1. Olsewski J.M., Lust G., Rendano B.T., et al.
"Degenerative joint disease: Multiple joint involvement in young and
mature dogs." Am J Vet Res. 1983; vol 44:1300-1308.
2. Smith G.K., Biery D.N. "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.
1990;196:59-70.
3. Ibid., p. 59.
4. Cargill J. "Truth in advertising: breeder self-regulation I." Dog
World. 1990(Jul);75 No.7:38-82.
5. Cargill J. "Truth in advertising: breeder self-regulation II." Dog
World. 1990(Aug);75 No.8:111-116.
6. Cargill J. "What should 'champion' mean?" Dog World.
1993(Feb);78 No.2:34-35.
|