The Knee and Patellofemoral Joints 241ment (LCL) or fibular collateral ligament. The lateral
collateral ligament is cordlike in shape and joins the
lateral condyle of the femur with the head of the fibula.
It becomes taut with knee extension. This ligament
helps provide lateral stability to the knee and is the
primary constraint to forces that tend to open up the
lateral aspect of the knee, termed varus stress (L. varus,
bent inward). It has been estimated that the lateral
collateral ligament supports close to 70% of the varus
stress applied to the knee (Besier, Lloyd, Cochrane,
and Ackland, 2001). An example of this varus stress
would occur if the dancer were to sit with the lower legs
crossed with the feet resting on the inside of the knees,
such as is done in the yoga lotus position. Dynamically,
varus stress would occur in lateral movements and
crossover movements like a grapevine.
The lateral collateral ligament may also assist the
medial collateral ligament and other structures in
limiting external rotation of the tibia, particularly
at about 35° of knee flexion (Levangie and Norkin,
2001). Both of the collateral ligaments slacken with
knee flexion, and lessening of these constraints is
vital for allowing functional rotation of the tibia used
in movements such as pivoting.
Anterior Cruciate Ligament
The cruciate ligaments (L. cruciatus, shaped like a
cross) are strong, cordlike ligaments that internally
join the tibia and femur. These ligaments derive
their name from the fact that they cross within the
knee joint. The anterior cruciate ligament (ACL)
runs from the anterior intercondylar area of the
tibia upward and backward and outward to insert
onto the inner and back part of the lateral femoral
condyle. As could be postulated from its attach-
ments, this ligament is important for preventing
anterior displacement of the tibia relative to the
femur, or posterior displacement of the femur
relative to the tibia; and it has been estimated
that the ACL is responsible for 85% of the force
that restrains anterior displacement of the tibia
(Irrgang, 1993). The ACL also has secondary func-
tions of helping to control rotation of the knee
(Diduch, Scuderi, and Scott, 1997), varus and valgus
stresses, and hyperextension when the knee is fully
extended (Caillet, 1996; Magee, 1997). The rotary
restraints offered by the cruciates are important for
normal functioning of the locking mechanism of the
knee, discussed later in this chapter. Functionally,
the anterior cruciate plays a particularly key role
when large forces or deceleration is involved as with
jumping, lowering the body down to the floor, or
quick changes of direction in dance. This ligament
is essential for joint integrity, and its loss critically
affects stability and alters the normal mechanics of
the knee.Posterior Cruciate LigamentThe posterior cruciate ligament (PCL) runs from
the posterior intercondylar area of the tibia upward,
forward, and inward to attach to the outer and front
part of the medial femoral condyle. The posterior
cruciate is key in preventing posterior displacement
of the tibia relative to the femur or anterior displace-
ment of the femur relative to the tibia. It is easiest
to remember the functions of the cruciate ligaments
relative to the tibia; that is, the anterior cruciate
prevents anterior displacement of the tibia while
the posterior cruciate prevents posterior displace-
ment of the tibia. The posterior cruciate has been
estimated to provide 95% of the total restraining
force to posterior movement of the tibia, termed
“posterior drawer” (Butler, Noyes, and Grood, 1980).
Unlike what occurs with the knee ligaments previ-
ously discussed, a majority of the posterior cruciate
ligament appears to become taut with knee flexion
versus extension, and some authors hold that it is
the key stabilizer of the knee when the knee is not
extended. During early knee flexion, the posterior
cruciate ligament becomes taut when the tibia dis-
places posteriorly and then becomes the fulcrum
about which further knee flexion occurs (Caillet,
1996). Large forces have been shown to be borne by
this ligament when deep knee flexion is performed,
such as in a parallel squat (Escamilla, 2001). How-
ever, the posterior cruciate ligament is 20% to 50%
greater in cross-sectional area, up to 50% stronger
(Diduch, Scuderi, and Scott, 1997), and less com-
monly injured than the anterior cruciate.Ligamental Stress TestsSimple ligamental stress tests that are performed
by physicians are shown in Tests and Measurements
5.1 to illustrate the key function of the primary liga-
ments just discussed. The presence of abnormal or
pathologic motion suggests that the ligament serving
as a primary constraint to that motion is injured. Many
more complex tests are also classically performed that
utilize multiplane motions and incorporate rotation
to further evaluate functional stability of the knee.Specialized Structures of the Knee
Various specialized structures are associated with
the knee that provide additional joint stability and
aid with knee function. These structures include the
menisci, bursae, and iliotibial band.