Clean Run - September 2015 - 40

The CrCL is made up of two bands of
ligament tissue that twist around each
other, each band stabilizing the joint in
various positions of extension and flexion (and not always at the same time).
The cruciate ligament is not "straight;"
it twists around itself as it travels diagonally through the joint. This helps to
stabilize the tibia from not only moving
forward as the femoral condyles roll
along its surface, but also to keep the
tibia from rotating internally due to the
pull of gravity on the body; this gravitational pull is centered in a dog's chest
near its heart. Only when the knee joint
is in full extension are both bands under full tension. As the joint flexes and
extends, the CrCL spirals around itself,
winding and unwinding its fibers.
It is very important to understand this
latter concept of twisting and spiraling. Coming from an engineering and
biomechanics perspective, I always approach patient evaluation thinking that
function follows form and form follows
function. Additionally in the biomechanics of movement, position dictates
function. So considering all of these
interrelationships, why would the cruciate ligaments of the knee evolve in a
spiraling and twisting shape if the knee
moved simply as a perfect and straight
hinge? It wouldn't. The twisting rota-

tion of the femur on the tibia must be
an important movement, otherwise
why would the cruciate ligaments take
they form that they do? Knee joint
movement generates important forces
that body systems must counter and
balance for supple motion and fluid
gait. In fact, every movement of every
joint in the body rotates thanks to the
legs being offset from the center of
gravity, and the exquisite balance and
timing of muscle lengthening and contracting is what controls excessive rotation during movement.
As the femur moves over the tibia in
flexion and extension, it slides across
the tibial joint surface or the plateau
(front-to-back and side-to-side) and rotates on top of it (internal-to-external).
In between the femur and tibia are two
C-shaped, cartilage plates called menisci (a single meniscus). Because the joint
surfaces in the knee do not lock into
place for stability (like the elbow and
the hip), the menisci assist with stabilizing the sliding and twisting of the femur
as it moves over the tibia. Additionally,
they ease the friction of movement and
absorb the loading forces going up
the leg caused by the foot striking the
ground. When the CrCL starts to weaken, the menisci start to pick up the extra
load of joint stabilization and can actu-

ally tear or fray from unbalanced movement, which is why surgeons and rehab
specialists (both for humans and companion animals) care so much about the
status of a meniscus when a cruciate ligaments have been injured. The menisci
are extremely important to joint function, but what little research has been
done on this structure in the dog is food
for another discussion at another time.
There is just one more aspect that plays
an important role in knee biomechanics, and that is the angle of the tibial
joint surface or plateau. In the human,
the entire plateau of the tibia (and the
menisci) presents a nearly flat surface
for the femur to roll on. In the dog, the
rear or caudal portion of the tibia is not
so flat. It actually has a steep angle of
25-35° compared to the human 4° angle. This sharp slope serves to act as a
lever to enhance power with a smaller
effort, a definite advantage to a predatory carnivore needing to suddenly
spring out of a bush to leap on a bison's
back or run down an elk. This tibial angle gets treated as a bad thing by surgeons looking at CrCL injuries in dogs.
Unrestrained by a ligament, this angle
pushes the tibia forward with a greater
force, thus creating more instability in
a ligament-deficient canine joint than
would be found in the human. But an

More research on the canine knee is badly needed to enhance our knowledge
of keeping and restoring a joint in and to health.

Clean Run | September 15

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Clean Run - September 2015

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