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The
Stifles
By Fred Lanting
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PATELLA LUXATIONS
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 long bones, 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 centre 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 centre 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 fibre 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 centreline (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 centreline of the
dog, the condition is called medial luxation or displacement. If the bone
moves on the “outside”, or away from the centreline, 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-fibre 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, and 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. 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 greate 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 infinite
similarly 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 GSD’s 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 oestrogen
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 shallower.
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 shallower 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, cow hocked, 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
Its 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 orthopaedic 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
Reprinted with kind permission of Fred Lanting Author of The Total German Shepherd
Dog
This
is the expanded and enlarged second edition, a "must" for every true GSD
lover. It is an excellent alternative to the "genetic history" by Willis,
but less technical and therefore suitable for the novice, yet very detailed
to be indispensable for the reputable GSD breeder. Chapters include: History
and Origins, Modern Bloodlines, The Standard, Anatomy, The German Shepherd
in Motion, Shows, Showing, and Training, The Winners, Nutrition and Feeding,
General Care and Information, Health and First Aid, Parasites and Immunity,
Diseases and Disorders, The Geriatric German Shepherd, Breeding, Basics
of Genetics, Reproduction, Whelping, The First Three Weeks, Four to Twelve
Weeks, Trouble-shooting Guide
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The above information is simply informational.
It's intent is not to replace the advice of a veterinarian nor to assist
you in making a diagnosis of your pet. Please consult with your own veterinarian
for confirmation of any diagnosis. Your pets life may depend on it.