ACL (Anterior Cruciate Ligament) Injuries

What Is It?

Ligaments are tough bands of fibrous tissue that connect two bones. The anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) are inside the knee joint. These ligaments connect the thigh bone (femur) and the large bone of the lower leg (tibia) at the knee joint. The ACL and PCL form an "X" inside the knee that stabilizes the knee against front-to-back or back-to-front forces.

An ACL injury is a sprain, in which the ligament is torn or stretched beyond its normal range. In almost all cases, when the ACL is torn, it's almost always due to at least one of the following patterns of injury:

  • A sudden stop, twist, pivot or change in direction at the knee joint — These knee movements are a routine part of football, basketball, soccer, rugby, gymnastics and skiing. For this reason, athletes who participate in these sports have an especially high risk of ACL tears.

  • Extreme hyperextension of the knee — Sometimes, during athletic jumps and landings, the knee straightens out more than it should and extends beyond its normal range of motion, causing an ACL tear. This type of ACL injury often occurs because of a missed dismount in gymnastics or an awkward landing in basketball.

  • Direct contact — The ACL may be injured during contact sports, usually during direct impact to the outside of the knee or lower leg. Examples are a sideways football tackle, a misdirected soccer kick that strikes the knee or a sliding tackle in soccer.

Like other types of sprains, ACL injuries are classified by the following grading system:

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