Anterior Cruciate Ligament Injury

ACL injury - one of the most common knee injuries

"With over 200,000 incidents per year in the United States, anterior cruciate ligament (ACL) tears is one of the most common knee injuries, particularly among active teens and young adults."

Anatomy of the ACL

Three bones meet to form knee joints (thighbone (femur), shinbone (tibia), and kneecap (patella). The kneecap sits in front of the joint to provide protection. The bones are connected to each other by ligaments; ligaments hold the bones together and keep the knee stable. Collateral ligaments, found on the sides of knees, control the sideways motion of knees. Cruciate ligaments are found on the inside knee joint, and they control the back and forth motion of knees. The ACL runs diagonally in the middle of the knee, preventing the tibia from sliding out in front of the femur. It also provides rotational stability to the knee.


Injured ligaments are considered sprains and are graded on a severity scale.

- Grade 1 Sprains: The ligament is mildly damaged. It has been slightly stretched, but is still able to keep the knee joint stable.

- Grade 2 Sprains: The ligament is stretched to the point where it becomes loose. This is referred to as a partial tear of the ligament.

- Grade 3 Sprains: This is a complete tear of the ligament; the ligament has been split into two pieces, and the knee joint is unstable.

Most ACL injuries are complete or near complete tears.


An ACL can be injured in several ways:

  • Stopping suddenly
  • Slowing down while running
  • Changing direction rapidly
  • Direct contact or collision, such as a football tackle
  • Landing from a jump incorrectly

Athletes participating in high demand sports like soccer, gymnastics, football, and basketball are more likely to injure their ACL.


When an athlete injures their ACL, they may hear a "popping" noise and their knee may give out from under them. Typical symptoms include:
  • Loss of full range motion
  • Discomfort while walking
  • Tenderness along the joint line
  • Pain with swelling

Most ligament injuries can be diagnosed through physical examination of the knee. A doctor compares the structures of an injured knee with the non-injured knee.

"Young women are two to eight times more likely than young men to injure the ACL."

Female athletes have a higher incidence of ACL injury than male athletes due to differences in physical conditioning, muscular strength, and neuromuscular control. Other suggested causes include differences in pelvis and lower extremity alignment, increased looseness in ligaments, and the effects of estrogen on ligament properties.


Treatment for an ACL tear will vary depending on a patient's individual needs.

Nonsurgical Treatment

A torn ACL will not heal without surgery. Nonsurgical treatment may be effective for elderly patients with a low activity level. If the knee is overall stable, nonsurgical options might be recommended.

  • Bracing - a brace may be worn to protect the knee from instability. Also, crutches may be used to further protect the knee.
  • Physical therapy - a rehabilitation program can be started to restore function to the knee and strengthen the leg muscles that support it.

Surgical Treatment

  • Rebuilding the ligament - To surgically repair the ACL and restore knee stability, the ligament must be reconstructed. A torn ACL will be replaced with a tissue graft; this graft acts as a scaffolding for a new ligament to grow on. Grafts can be obtained from several sources (the patellar tendon, which runs between the kneecap and the shinbone, hamstring tendons at the back of the thigh, sometimes a quadriceps tendon, which runs from the kneecap into the thigh, or a cadaver graft). The regrowth takes time, and it may be six months or more before an athlete can return to sports after surgery.
  • Procedure - Surgery to rebuild the ACL is done with an arthroscope using small incisions. The surgery is less invasive, and its benefits include less pain from surgery, less time spent in the hospital, and a quicker recovery time.
ACL Reconstruction

"This surgery has a long-term success rate of 82 to 95 percent."


Rehabilitation plays a vital role in getting a patient back to daily activities. A physical therapy program helps patients regain knee strength and motion. Physical therapy focuses on returning motion to the joint and surrounding muscles, followed by a strengthening program designed to protect the new ligament.
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