An anterior cruciate ligament (ACL) injury often is called a sprain. A sprain occurs when the threads or fibers of the ligament stretch or are torn. Most ACL injuries occur in the middle of the ligament.

See an illustration of the knee and the ACL.

An ACL injury is classified as a grade I, II, or III sprain.

Grade I sprain

• The fibers of the ligament are stretched, but there is no tear.
• There is a little tenderness and swelling.
• The knee does not feel unstable or give out during activity.

Grade II sprain

• The fibers of the ligament are partially torn.
• There is a little tenderness and moderate swelling.
• The joint may feel unstable or give out during activity.

Grade III sprain

• The fibers of the ligament are completely torn (ruptured); the ligament itself has torn completely into two parts.
• There is tenderness (but not a lot of pain, especially when compared to the seriousness of the injury). There may be a little swelling or a lot of swelling.
• The ligament cannot control knee movements. The knee feels unstable or gives out at certain times.

An ACL avulsion occurs when the ACL is torn away from either the upper leg bone or lower leg bone. This type of injury is more common in children than adults.

An avulsion fracture occurs when the ACL is torn away from the leg bone with a piece of the bone.