The knee cap (patella)
normally fits into a groove in the thigh bone (femur). The patella
slides up and down in this groove as the leg bends and straightens.
Patellar luxation means that the knee cap has slipped out of the
groove. There are several
reasons why this happens, including malformation of the groove.
Luxation may
happen only occasionally, or may happen continuously. The knee cap may
pop
back into the groove on its own, or your veterinarian may need to push
it
back into place. Your dog will be lame when the patella is out of
place. Over
time your dog may develop other degenerative joint changes, such as
osteoarthritis.
How is patellar luxation inherited?
The mode of inheritance is not yet known. Some researchers think that
this disease may be polygenic.
What breeds are affected by patellar luxation?
This disease is inherited in the following breeds: miniature and toy
poodle, Yorkshire terrier, pomeranian, Pekingese, chihuahua, Boston
terrier, basset hound, shih tzu, silky terrier, and lhasa apso.
(Patellar luxation may also occur in any breed as a reult of trauma.)
For many breeds and many disorders, the studies to determine the mode
of inheritance or the frequency in the breed have not been carried out,
or are inconclusive. We have listed breeds for which there is a
consensus among those investigating in this field and among veterinary
practitioners, that the condition is significant in this breed.
What does patellar luxation mean to your dog & you?
When present, the condition is usually evident in young dogs by
around 6 months of age, but if mild it may go unnoticed until the dog
is older. When the knee cap is out of place, your dog will be lame and
may refuse to bear weight, or his/her knee may be "locked". The
severity
of the condition varies widely. In mild cases, the knee cap may only
slip out of place occasionally, causing your dog to "hop" for a few
steps,
and then it may slide back into the groove on its own. In severe cases,
the knee cap slips out of place more often, or is never in a normal
position.
It may not go back into the groove on its own and your veterinarian may
need to push it back into place.
As a result of patellar luxation, your dog may develop other
degenerative joint changes, such as osteoarthritis. If your dog has a
mild case of this disease, you may not notice the actual luxation, but
your dog may eventually develop pain due to osteoarthritis.
How is patellar luxation diagnosed?
Your veterinarian will diagnose this disease based on physical
examination and palpation. He/she may take radiographs to see if your
dog has other problems (such as osteoarthritis) as a result of this
disease.
How is patellar luxation treated?
The treatment and long term outcome (prognosis) depend on the severity
of disease (how often the knee cap slips out of place, and how easily
it slips back into the normal position), and whether there are other
problems such as osteoarthritis. Moderate or severe cases often require
surgery to make sure that the knee cap stays in the groove in the
femur, and to prevent painful osteoarthritis. Exercise restriction is
important for a period after surgery, and the results are usually very
good.
For the veterinarian:
Patellar luxation may be classified in four grades, with grade I being
the most mild. Mild patellar luxation may be discovered as an
incidental finding. Severe cases in growing dogs may result in limb
deformity. Surgery is usually recommended in moderate or severe cases
to stabilize the patella and correct the underlying anatomic deformity.
Surgical correction may or may not stop the progression of degenerative
joint disease, and reluxation is a possible complication of surgery.
Breeding advice
Affected dogs, as well as their parents, their litter-mates, and any
dog which has had surgery for patellar luxation, should not be bred.
The Orthopedic Foundation of America - www.offa.org
- and Institute for Genetic Disease Control in Animals - www.vetmed.ucdavis.edu/gdc/gdc.html
- have registries for this condition.
FOR MORE
INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
Resources
Martinez SA. 1997. Congenital conditions that lead to osteoarthritis in
the dog. Veterinary Clinics of North America Small Animal Practice. pp.
261-290.
Schrader SC . 1995. Differential diagnosis of nontraumatic causes of
lameness in young growing dogs. In JD Bonagura and RW Kirk(eds.) Kirk's
Current Veterinary Therapy XII Small Animal Practice p. 1171-1180. WB
Saunders Co., Toronto.
Copyright
© 1998 Canine Inherited Disorders Database. All rights reserved. Revised: April
23, 2002.
This database
is a joint initiative of the Sir James Dunn Animal Welfare Centre at
the Atlantic Veterinary College, University of Prince Edward Island,
and the Canadian Veterinary Medical Association.
reprinted
with kind permission from:-
Alice
Crook, DVM,Coordinator, Sir James Dunn Animal Welfare Centre, Atlantic
Veterinary College,University of Prince Edward Island, 550 University
Ave.Charlottetown, PEI C1A 4P3
***************************
back to top
by Fred Lanting
Copyright June, 1999 - 2009
The knee is an engineering
marvel, extremely complicated and concise, able to move in multiple
combinations of direction, and normally able to withstand considerable
torque forces. The structure is encased in an exquisite joint capsule
with ligaments connecting three large longbones, tendons running into
and over it and connecting bone with muscle, as well as muscle to
muscle via the sesamoids and patella. The capsule is lubricated by
synovial fluid.
The most common joint disorder in the stifles, other than ruptured
cruciate ligaments, is luxated patellas. The patella, or “knee cap” is
a small bone (the body’s largest sesamoid bone) that seems to “float”
outside
of a joint. In actuality, it normally is securely attached to the area
and the muscles by means of ligaments and tendons. It can be considered
a localized ossification center in the tendon connecting the quadriceps
muscle in the upper thigh to the tibia in the lower leg. You should be
able to feel it somewhat above the slight indentation along the front
of
your dog’s rear leg, where the breed standards like to refer to the
“bend
of stifle”; this is where the upper and the lower thighs meet. In
motion,
the patella will normally slide down into a groove between the condyles
of
the femur. The quadriceps muscle group acts to extend (straighten) the
limb;
because the patella is enclosed in the lower tendon of that muscle
group,
the knee cap is more in front of the joint when the muscle is relaxed,
and
slides upward in front of the lower femur when contracted. That’s when
the major discomfort occurs in dogs with defective stifles, with the
patella
missing the center of the groove in which case it is referred to as
being
luxated. Almost all dogs with patellar luxation have some other
structural
abnormality related to it; thus, in order to improve the individual’s
life
and prevent passing the defect to others, a regime of early evaluation
and
detection is important.
The alignment of muscle and tendon insertions or the relative positions
of the leg bones may be such that the patella is pulled more to one
side and hence misses that groove. The biomechanics are easily
described by
what we call the “bowstring effect”, in which the muscle fiber group on
one side works better than the other side, and puts tension on the
patellar
tendon and associated bone structures. In so doing, the kneecap is
pulled
toward the dog’s centerline (medial displacement) and if the force is
strong enough it will slide out of the groove running between the
femoral
condyles. This wears away the ridge of that “trochlea” so the effect is
to slip out of the groove more easily as time goes on. Meanwhile, the
underside of the patella also abrades, becoming less V-shaped and less
likely to stay in the track where it belongs. In the more severe cases,
the abnormal strain on the muscles and joint caused by forces being
misdirected in this manner results in pain. It is usually considerable
and almost always leads to a very noticeable change in gait, such as
skipping or perhaps even a refusal to walk at times. Mild cases can go
undiagnosed by a vet and, to some extent, unnoticed by the owner. Most
pups can be diagnosed only after 4 to 6 months of age. If the luxated
patellar bone is offset toward the centerline of the
dog, the condition is called medial luxation or displacement. If the
bone
moves on the “outside”, or away from the centerline, it is called
lateral luxation. A method you can use to keep these terms straight
might be to think of the “median strip” in the middle of a divided
highway. Medial patellar luxation is almost undoubtedly inherited.
We know that in limbs with two bones, such as the lower thigh and
the lower foreleg, there are frequently small differences in the growth
rate or eventual length ratios between tibia and fibula or between ulna
and radius. It may well be that a disruption in growth rate of one of
such
a pair causes a twist or misalignment of both hard and soft structures
in joints such as the wrist, elbow, knee, and hock. On the other hand,
slightly
abnormal muscle-fiber type or ratios might cause more tension on one
side
than on another. Some have tried to solve to solve the puzzle of the
chicken-or-the-egg with explanations of one sort, and others have
proposed the other. Whichever happens, and it could be that either is
involved in any specific case,
we are more concerned here with the “after-the-fact” condition, and how
it relates to a dog’s gait, health, and value as a show or breeding
animal.
Whatever is the reason for a specific abnormality, sometimes all I can
do
in this book is describe it. Where possible, I shall try to explain it.
In some individuals of achondroplastic breeds such as Bassets and
Dachshunds, luxation might be more a result of “breeder-selected”
abnormal
shape and position (curvature) of tibia and femur. The lateral
deviation
of the distal femur and the medial deviation of the proximal tibia seem
to have similar results as found with luxation caused by muscle
abnormalities.
Medial Luxation
This is the most common form. Most cases of medial patellar luxation
involve toys and other small breeds, many of which are rather “straight
in the stifle” such as the Shar Pei, but there may not be more than
coincidence in that. Small (Toy and Miniature) Poodles, perhaps because
there are so many of them indiscriminately bred, seem to be the most
“visible” to the casual observer, but there is evidence that indeed,
there is a 4 to 8 times greater predisposition in the miniature and toy
breeds in general, and
“the incidence of medial luxations is high in toy poodles”. Dr.
Priester, the same man who did a study on breed, sex, and other factors
in relation to HD, also studied these factors and their connection to
canine patellar dislocation. Medial luxation in larger dogs is far less
common, but does occur.
Diagnosis and Severity
The experienced dog show observer can often spot the more severely
afflicted dogs, usually while watching the Pomeranian, Lhasa Apso,
Peke, Boston, and other small breed competition. The worst will
ambulate on three legs, or even appear to be trying to put all their
weight on the front limbs. Frequently, dogs with the mild to moderate
forms of medial patellar luxation will “skip” every once in a while,
perhaps during a requested change of speed or making a corner, perhaps
without other stimulus except the discomfort. So momentary is it, that
many a judge has missed it. Some of those that do not make it to the
show ring because the severity makes symptoms too obvious might
evidence some pain, run on three legs for a while, then put weight on
the leg again in an apparently normal manner. Some dogs have such
a mild case that they can fool you until they are palpated by an
expert, or are subjected to force-plate analysis, in which
instrumentation records how much weight is applied to each of the four
legs.
Four grades of medial luxation are used to describe the disorder.
Grade 1 is for the dog that usually has a normal condition, but which
can be luxated by minor trauma or pushing firmly with the thumb and
fingers.
In Grade 2, the patella can be manually displaced by adequate finger
pressure
or can slip out when the leg is fully extended, though it can be pushed
back by the owner or a vet. With the next two grades it is difficult
(grade
3) or impossible (grade 4) to put the patella back in place. Other
signs
are progressively worse, too, of course. Older dogs with Grades 1 or 2
may seem O.K. until a sudden onset that may be triggered by trauma or
arthritic change that the dogs no longer fully tolerate; nevertheless,
it is a genetic problem.
Grade 1 dogs occasionally skip or carry a leg, sometimes only for
one stride in a twenty- or thirty-foot stretch of gaiting. Pain might
be barely or not noticed, even when the cap is luxated slightly with
finger
pressure. Grade 2 dogs will more frequently to usually “carry” a leg,
and occasionally bear weight on it; crepitus might be felt and heard,
depending on the dog’s age. Surgery is recommended for Grades 2 to 4,
earlier for the worse degrees. In Grade 3, a little weight bearing
might
be possible, but there is permanent dislocation; you can push the
patella
back into the shallow groove, but it will ride out as soon as you ease
up on the pressure. Most dogs will stand bowlegged or crouched. In
Grade
4, the affected leg will always be carried so that weight is not
transmitted
to the ground. The luxation is permanent and surgery will have to
address
other surfaces in the joint as well.
Lateral Luxation
When the patella is displaced toward the “outside”, it is said to
be luxated laterally. This condition is not as common, accounting for
less than 24% of patellar dislocations. While it can be found in any
size
dog, it is more commonly found in large breeds. I recently was informed
of a couple of Finnish Spitz littermates with lateral luxation, bad
enough
to require surgery; this is not a large breed. Any misalignment in the
quadriceps muscles, the trochlear groove between the condyles of the
femur, the tibia (larger of the two lower leg bones), and the ligaments
early in life will abnormally shape the cartilage and, as it ossifies
into bone, this legacy will solidify. Other puns aside, though, it has
been seen that changing the directions or vectors of the muscle
contraction forces will give unbalanced strains on the growth plates
and other cartilaginous tissues in the young pup, and this means
permanent deformities in the bone structures they will have developed
into. It only takes a couple of weeks of misdirected stresses to
produce this permanent deformation.
Coxa valgum, the condition in which the femoral head is inserted into
the acetabulum at a greater-than-normal angle, tends to force the lower
part of the femur toward the midline, which changes the relative
position of the trochlear groove in which the patella should slide
without pressure on one side of the groove or the other. An increase in
the anteversion angle between the femur and the acetabular portion of
the pelvis carries with
it a corresponding torsional force on the whole femur, and another
tendency for medial displacement of the distal femur as well. A vet
school professor I consulted years ago felt that there was a
cause-and-effect relationship between patellar luxation and hip
dysplasia, but admitted there was disagreement on which caused what.
Others have mentioned that lateral luxation can cause deformation of
the femoral head in large, fast-growing breeds.
What happens when the stifles are brought closer together in this
“knock-kneed” condition known as genu valgum? The dog’s weight is
applied
unevenly to one of the femoral condyles, and as the puppy is subjected
to this, the development of the growth plate on that (lateral) side is
inhibited
while the medial condyle continues to grow and ossify normally. The
change
in the height of the lateral trochlear ridge, plus the fact that the
patella
rides atop it instead of next to it in the groove, makes for a shallow
groove. In addition, if luxation started early, there were little or no
developmental forces to even create a normal groove in the first place.
All this presents an easy escape for the patella into an increasingly
bad position. With every step the dog takes, the abrasion continues,
cartilage is destroyed, ligaments are stretched, and pain probably
increases. At almost infinitesimally small increments, to be sure, but
progressively nonetheless.
The same sort of “bowstring effect” exists as in medial luxation,
but bowing the femur inward instead, and rotating the tibia laterally
(outward) while displacing the developing tuberosity on the tibia where
the patellar tendon is inserted. The vastus medialis part of the
quadriceps
muscle is supposed to act as half of the couple (an engineering term
meaning
balanced rotational forces) and insure the patella rides in the groove
and transfers the quadriceps’ force to extend the lower leg in a
straight
fashion.
Lateral luxation of the patella can begin with a hypoplastic muscle
called the vastus medialis. It is the part of the quadriceps muscle
group that is the last to develop and is the first muscle to undergo
atrophy
following injury or disuse. Hypoplasia means underdevelopment, with
fewer
cells and myofibers, so the muscle doesn’t stretch properly. It is
probably
not clinically identifiable in many or most cases. If it is not working
properly, it does not balance the force of the vastus lateralis, which
is then able to exert too much pull toward the “outside” (laterally).
The
vastus lateralis may also cause or contribute to the femur and tibia
being
twisted out of alignment with each other. This compounds the problem,
sending
the forces of the contracting quadriceps into a slightly (but
importantly)
different direction. The whole joint capsule is abnormally stressed and
there is a possibility that a ruptured cruciate ligament so often seen
in sporting and working breeds might be genetically related. Some 15 to
20% of luxated patella cases have accompanying (or resultant?) cranial
cruciate ligament rupture. It could be that the genes for one muscle’s
hypoplasia give cruciate ligament damage if combined with genes for
certain characteristics, and patella luxation if combined with those
for other
characteristics.
Causes for lateral patellar luxation are often unproven, partly because
there are so few cases. Willis in his excellent work on the German
Shepherd Dog, says that he has only seen two cases in the breed, and
Priester reported Saints but no GSDs in his 1972 study, despite the
fact that the latter breed is the most populous in the world and was
one of the five or six most
popular breeds in the USA at the time. I hold the premise that almost
everything has a genetic base, and that the reason some dogs are
afflicted by an apparently environmental occurrence is that they are
genetically more susceptible
than others, be it rupturing a cruciate ligament, having gastric
volvulus,
heat stroke, or whatever. However, the low incidence makes it hard to
convince some people. Just how such a genetic weakness is expressed,
that is, its etiology and pathogenesis, can easily and understandably
be a point of disagreement. Thus, such “causes” have been proposed as
estrogen imbalance or trauma
while in the uterus; both have been pretty much discounted as
possibilities.
In lateral luxation, the lateral condyle of the femur, because its
growth plate has been subjected to excess pressure and growth has been
altered, is misshapen. The trochanteric ridge is lower, sort of like
riding on one of those compact spare tires, and the trochlear groove is
more shallow. Thus the patella does not have a nice, deep, secure
groove to ride in, and
it is easy to pull it out of normal direction, especially since the
force
to one side is so persistent (every time the dog flexes or extends the
knee). The groove becomes even more shallow due to wearing away faster
than the
dog can rebuild with new cells. Still, symptoms of lateral luxation
usually
are slower to develop than medial luxation signs. The dog may be over 5
years old by the time clinical signs of Grade 1 or 2 appear. The
affected dog
may be knock-kneed, cowhocked, or toed-out, but then, many non-affected
dogs may show these traits, so do not base a diagnosis on those alone.
In
sudden-onset cases involving both rear limbs at once (perhaps brought
on
by strenuous exercise), the dog might not be able to stand and the
condition
may appear to be a neurological disorder.
Treatment
Surgical correction (not cure) generally is aimed at re-establishing or
creating a deeper groove and repositioning the muscle vectors. If
surgery is postponed, osteoarthritis may be a problem of varying
severity, with osteophytes and a thickened, swollen knee joint capsule
making correction more difficult. Steroids are often prescribed (they
are as commonly resorted to as Valium for adult humans or Ritalin for
kids) but given the dangerous and unpleasant side-effects of such
medication, your best recourse is probably surgery, which sometimes is
very extensive. Since patellar luxations are
progressive, the earlier you detect and treat, the greater the success
will
be.
Genetics
It’s part of my usual soapbox to point toward genetic origins for
almost everything, and this is no exception. Noted geneticist Frederick
Hutt flatly said of patellar luxation, “It is inherited and polygenic”.
However, as Olmstead says, the heredity of lateral patellar luxation is
difficult to substantiate because the condition does not occur as often
as, say, HD or many other more frequently seen orthopedic disorders.
While
the role of trauma is easy to see, the “primary pathophysiology” is
not.
Just how and why the genes express themselves in luxated patellas,
whether
medially or laterally, is not all that clear. But perhaps it need not
be,
as long as we follow the wise breeder’s rule: don’t breed defective
dogs,
whatever the defect or supposed cause.
Copyright
Fred Lanting, All rights reserved, but reprinting allowed after
permission. Please read his other articles on http://siriusdog.com/sphider/search.php?query=lanting&search=1
, for example, or e-mail him at: Mr.GSD@netscape.com
or Mr.GSD@Juno.com for specific
articles.
Editor’s Note: A
well-respected and frequent GSD specialty and all-breed judge for many
clubs around the world, with KC and other-country credentials, Mr.
Lanting since 1966 has lectured on Gait-and-Structure, Canine
Orthopedic Disorders, and other topics, and has judged in about 30
countries. He has been described by a former OFA director as the
world’s leading non-veterinarian authority on hip dysplasia. He has
lectured at numerous veterinary schools in the USA and abroad, and is
the author of the following “must read” books for the dog owner (E-mail
for curriculum vitae). “Canine HD and Other Orthopedics Disorders” :
This expanded revision is a comprehensive (nearly 600-page), amply
illustrated, annotated, monumental work that is suitable as a
coffee-table book, a reference work for breeders and veterinarians, and
a study adjunct for veterinary students. It is equally valuable for the
owner of any breed. It covers every aspect of HD and other orthopedic,
bone, or spinal disorders, and includes genetics, diagnostic methods,
treatment options, and the role of environment. Your autographed copy
will be mailed from the USA as soon as the appropriate amount is
received and is processed. Pricing: US $68 in the U.S., or ask about
mail overseas. Combine orders with “The Total German Shepherd Dog” by
the same author ($50 plus $4 postage). 17 of the 20 chapters are
suitable for owners of any breed. Order both at once direct from the
author, and the postage will be waived.
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