In Brief: The anterior
cruciate ligament (ACL) is an important stabilizing structure
in the knee, which when torn, may result in a "trick knee"
syndrome. The more physically or athletically active an individual
with a non-functioning ACL is, the more likely their unstable
knee will repeatedly buckle or "go out", causing additional
knee joint injury. Torn ACLs rarely if ever heal on their own
and cannot simply be stitched back together. While non-operative
treatment is an option, effective surgical treatment requires
that a ruptured or severely stretched out ACL be rebuilt (reconstructed).
Multiple methods for surgically rebuilding a torn ACL exist, some
of which are better suited for a particular patient than others.
Each patient's individual circumstances and knee injury pattern
must be taken into account when deciding whether or not to undergo
surgery and which surgical method to employ.
The anterior (frontal) cruciate (crossed) ligament, or "ACL"
is an integral part of the knee's hinge mechanism. Working side
by side with its immediate neighbor in the knee joint, the posterior
cruciate ligament, the ACL holds the two main knee bones (femur
& tibia) together and helps keep your knee bending on its
proper axis, somewhat similar to a door hinge
(see FIGURES 1a - 1c).
- Diagrammatic view of a flexed (bent) knee viewed from the
front, with the patella removed. The anterior and posterior
cruciate ligaments are seen to cross each other within the
open, center region of the joint (inter-condylar notch), creating
an internal hinge mechanism that controls the knee's axis
of rotation as it bends and straightens.
- Anatomy laboratory dissection of a knee joint by the author
(cut-away, side view), demonstrating that when the knee is
extended (straight leg position), the ACL is a relatively
flat, ribbon-like ligament composed of many parallel fibers.
It connects the tibia (very bottom of picture) to the femur
(above). The front of the knee is to the left in this picture.
1c - Diagrammatic illustration by the author, of the
ACL's internal fiber anatomy. While the ACL is not naturally
divided into physically separate sections as drawn here, some
"fiber-regions" of the ACL are more important as
"first line" stabilizers of the knee than others.
In general, most of the ACL's fibers are relatively tight
when the knee is either fully extended (straight, as in FIGURE
1-b and in this diagram) or fully flexed (as in FIGURE 1a).
The ACL as a whole is most relaxed when the knee is partially
flexed to a mid-range angle of 30-60 degrees. Since it is
not possible for surgeons to precisely reconstruct all of
the ACL's natural fiber-architecture with currently available
tendon grafts, they normally rebuild the most functionally
important of the ACL's fiber-regions, these being the anterior
(frontal) and central regions. (Reference: Sapega, A., et
al: Isometry Testing During Reconstruction of the Anterior
Cruciate Ligament. American Journal of Bone and Joint Surgery,
Volume 72A:259-267, 1990).
Specifically, the ACL prevents the upper tibia, or "shin-bone",
from slipping forward, out from under the lower end of the femur
(thigh-bone) during knee twisting movements and contraction of
the quadriceps (frontal thigh) muscles (see
2 - This photo shows a rugby player with a combination
of weight-bearing and twisting stress, plus quadriceps (frontal
thigh) muscle contraction stress, being placed on his left
knee (see arrow). While such forces can easily cause a knee
with a loose or torn ACL to "go out" or sublux (shift
out of place), they can also occasionally cause a healthy
ACL to tear suddenly. That is exactly what happened to this
athlete at the very instant this photo was taken! He went
on to have his ACL reconstructed with a hamstring tendon graft
and had an excellent result.
In a knee with a torn or stretched-out ACL, a sudden, unexpected
shifting forward of the tibia relative to the femur (i.e., a partial
dislocation, or "subluxation") may occur during the
weight-bearing phase of a physical activity such as pivoting or
changing direction, causing the knee to feel as if it has buckled
or given way. Patients often say that their knee suddenly just
"went out" on them. Such unanticipated joint subluxation
episodes gave rise to the term "trick knee" syndrome,
long before orthopedic surgeons recognized that a torn or loose
anterior cruciate ligament was usually responsible for this rather
common problem. As recently as just three decades ago, the ACL
was considered by many surgeons to be a vestigial structure in
the human knee that served no useful purpose! People who suffered
from a "trick knee" syndrome usually had their problem
blamed on cartilage troubles or other knee maladies. Over the
course of the past three decades, the ACL has metamorphosed from
the most ignored structure in the knee to the most frequently
repaired or rebuilt structure! While its importance is now widely
recognized amongst physicians, to some extent the prevailing surgical
opinion "pendulum" has swung so far back from where
it was 30 years ago that perhaps too many torn anterior cruciate
ligaments are now being reconstructed. Many knee injury patients
today somehow develop the notion that without a functioning anterior
cruciate ligament, they have no hope of leading an active life,
which is clearly not the case.