There are four knee ligaments, the anterior cruciate ligament, posterior cruciate ligament, medial collateral ligament and lateral collateral ligament. These ligaments connect the upper leg bone with the large lower leg bone and provide for stability.
The two cruciate ligaments form a cross, hence the term cruciate, within the knee. The ACL is the smallest and strongest but least flexible ligament of the four ligaments of the knee. It is also the primary stabilizer. In conjunction with the posterior cruciate ligament, it prevents the lower leg bone from sliding forward or turning inward when the leg is straight, the knee from being stretched or straightened beyond its normal limits and supports the knee ligaments that keep the knee from bending sideways.
Injuries to the ACL are one of the most common sports-related knee injuries. It's estimated that each year in the United States between 95,000 and 250,000 people sustain a torn or ruptured ACL. However, ACL injuries can occur nearly any time the foot is firmly planted and the knee is twisted. Remarkably, women are more likely to develop ACL injuries than men. It is unclear whether this is a function of bone structure, muscle development or hormones.