Email this article to a friend Open a printer-friendly version of this article  (i.e. no nav bars, excess graphics, etc.)

PAGE 1 OF 5  

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).

FIGURE 1a - 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.

FIGURE 1b - 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.

FIGURE 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 FIGURE 2).

FIGURE 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.


® 2007. The Knee and Shoulder Centers of New Jersey and Pennsylvania.