Anatomy of the Knee
A healthy knee allows you to move your body freely. The most basic movement, such as walking effortlessly across a room, climbing stairs with confidence, even crossing one leg over another without pain, requires knees that are basically sound. A complicated athletic feat—one that involves jumping, twisting, and cutting—demands knees that are functioning exceptionally well.
The knee can be thought of as a magnificent hinge connecting the femur (thighbone) and the tibia (shinbone). When healthy and whole, the knee enables the tibia and femur to move together in perfect harmony.
The anterior cruciate ligament (ACL) runs through the center of the knee—from the back of the femur to the front of the tibia. The job of the ACL is to keep the femur and the tibia properly aligned. It is composed of tissue that is tough and elastic. But, as you have learned all too well, the ACL can be stressed beyond its tolerance. Any action that twists the knee too far can cause the knee to hyperextend and result in an ACL tear.
ACL injuries are common in sports such as skiing or basketball that place excessive demands on the knee. Contact sports such as football and soccer also produce their share of ACL casualties, for knee ligaments are vulnerable to injury whenever the leg is hit after the foot is planted firmly on the ground.
Of course, an ACL injury can occur in any number of ways. Many people rupture this ligament on the job or at home when the knee is overly stressed—by anything from a simple misstep to a major accident.