PATELLA
LUXATIONS
by Fred Lanting
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,
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.
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Copyright 2001 Fred Lanting, Canine Consulting. Mr.GSD@juno.com. All rights reserved. Please
view his site Real GSD.
NOTE: A well-respected AKC and
Schaferhund Verein judge,
Mr. Lanting has judged in more than a dozen
countries, including the prestigious FCI Asian Show hosted by Japan Kennel
Club, the Scottish Kennel Club, a Greyhound specialty in