The anterior cruciate ligament (ACL) is one of four major ligaments that stabilize the knee joint. A ligament is a tough band of tissue, similar to a rope, which connects the bones together at a joint.
The ACL prevents the shin bone (tibia) from sliding forward too much and stabilizes the knee to allow cutting, twisting and jumping sports.
The most common mechanism that tears the ACL is the combination of a sudden stopping motion on the leg while quickly twisting on the knee. This can happen in a sport such as basketball, football, soccer, or lacrosse. A contact injury, such as when the player is hit in football, forces the knee into an abnormal position. This may tear the ACL, MCL and other structures.
If only the ACL is injured, the knee usually regains its range of motion and is painless after six or eight weeks. The knee will typically feel completely normal. However, it can be a “trick knee”. If a knee does not have an ACL it can give way or be unstable when the person pivots or changes direction. This abnormal motion can damage the menisci or articular cartilage and cause further knee problems.
If a person does not do sports and is relatively inactive, the knee can feel quite normal even if the ACL is torn. Thus, some patients especially over the age of forty may not need to have the ACL reconstructed, especially if they do not participate in sports that require quick changes in direction. In young, athletic patients, however, the knee will tend to re-injure frequently and give way during activities in which the person quickly changes direction. Therefore, it is usually best to reconstruct the torn ACL.
Symptoms / Diagnosis
When the ACL tears, the person often feels the knee go out of joint and often hears or feels a “pop”. If he or she tries to stand on the leg, the knee may feel unstable and give out. The knee usually swells a great deal (within two hours). Over the next several hours, pain becomes more severe and it becomes difficult to walk. You may notice the following symptoms:
When the ACL tears other structures in the knee are frequently injured, most commonly the meniscus and the articular cartilage. The meniscus is cartilage that functions as a shock absorber between the femur and tibia, and the articular cartilage is a smooth, white glistening surface that covers the ends of the bones. The articular cartilage provides lubrication and as a result, there is very little friction when the joint moves.
If this articular cartilage is injured, the joint no longer moves smoothly.
The initial treatment of the injured joint is to apply ice and gentle compression to control swelling. A knee brace and crutches are used. The best treatment following acute ACL injury is to protect the joint and apply ice and use crutches for several weeks. As the swelling and pain subside, one can put weight on the leg; then the brace and crutches are discontinued. The emphasis is on regaining knee motion. Resistive exercises to build up strength should not be done during this time to prevent damaging the knee cap.
It is best to wait for the pain and swelling to subside and to allow associated injuries to heal before performing surgery for the ACL. If surgery is done soon after injury, rehabilitation is difficult, and the knee may get stiff and have permanent loss of motion. If surgery is delayed until the joint has full painless range of motion, then an accelerated rehabilitation program can be used after surgery allowing a faster return to activity.
If the knee also has an injured medial collateral ligament (MCL), it is best to allow the MCL to heal completely (usually six to eight weeks) before reconstructing the ACL. Then an arthroscopic procedure can be performed to reconstruct the ACL. The torn MCL usually does not need to be repaired surgically.
There may be instances when immediate surgery is indicated following injury. Examples are knee dislocation when multiple ligaments are torn. Tears of the outer knee ligaments (lateral collateral ligament) often do require surgical repair. Individual decisions need to be made on whether or not to reconstruct the ACL soon after injury in these instances where immediate surgery may be required.
Some patients can function well even if the ACL is torn. However, it may be necessary to modify activities and avoid high risk sports (such as basketball, soccer and football). The key in preventing a knee with a torn ACL from giving out is to avoid quick pivoting motions. Wearing a knee brace can help re-injury. The main effect of a knee brace is to be a constant reminder to be careful. However, a brace will not completely stabilize a knee that has a torn ACL. Exercises that restore the muscle strength, power, coordination, and endurance will also improve knee function and help stabilize the knee. However, a fully rehabilitated knee that has a torn ACL can still give way if a quick change in direction is unexpected.
Surgical reconstruction of a torn ACL involves replacing the torn ACL with a tendon (called a graft) from another part of the knee and putting it into a position to take the place of the torn ACL. A commonly used graft is taken from the middle third of the patellar tendon (the tendon connecting the knee cap to the tibia bone). Hamstring tendon grafts taken from the inner thigh are also used. Occasionally, tendon grafts are taken from cadavers (referred to as allograft).
For most of these procedures, the operation is done arthroscopically. The knee is examined arthroscopically and associated injuries such as torn menisci, loose bodies, etc are treated. While viewing the inside of the joint through the arthroscope, guides are used to create bone tunnels in the exact positions to allow proper placement of the graft. The graft is then pulled into the bony tunnels and secured. This graft then replaces the torn ACL.
Within two or three weeks after surgery, you will be able to walk on level surfaces with a brace and with or without crutches (depending on the type of graft used). At about five or six weeks, you will usually be able to go up and down stairs. For the next several months, exercises are done to regain motion in the knee. At six months, the patient is usually running and at nine months, participating in sports.
Results and Risks
The results of ACL reconstruction surgery are good-excellent in more than 90 percent of patients. Approximately 80 percent of patients will return to their pre-injury level of activity after surgery. The goal of surgery is to restore stability to the knee and prevent further knee injury.
- Repetitive injuries may cause further permanent damage and eventually lead to arthritis.
- Inability to participate in sports that require pivoting.
- 90-100% – Permanent numbness in the front of the knee near the incision
- 0.5% – Other nerve injury
- 5% – Patello-femoral pain (kneecap)
- 10% – Flexion contracture (stiffness or reduced motion of the knee)
- 5-10% – Re-injury (knee becomes unstable again)
- 10% – Swelling
- 1% – Superficial infection
- 0.5% – Deep infection
- 0.5% – Deep vein thrombosis (blood clots)
- 5% – Delay in regaining motion
Learn more about a variety of orthopedic conditions in our comprehensive, physician approved patient library.
- Anterior Cruciate Ligament (ACL) Injuries
- ACL: Exercises to do before treatment
- Physical rehabilitation for ACL injuries
- Surgery for ACL injury