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Anterior Cruciate Ligament (ACL) Tears


Diagram of a Normal Knee Joint

The anterior cruciate ligament (ACL) is a thick band of tissue which has two major strands that extend from the lower leg bone (tibia) to the thigh bone (femur). This ligament is very important for maintaining stability of the knee. When it is injured or torn, the patient feels the instability of the knee when they turn or pivot. This instability is particularly problematic when participating in pivoting sports such as soccer and football. The ligament sits just in front of its counterpart, the posterior cruciate ligament, directly in the middle of the knee joint.

Mechanism of Injury

Most anterior cruciate ligament tears occur during a sporting activity, usually in younger patients. When you consider the number of sport hours played, they are more common in women. There have been a variety of reasons proposed for this, such as muscle imbalance and slight variations in the anatomy of the knee joint in women compared to men. The most common sports for this type of injury are football and basketball in younger patients, and skiing in older patients. It is, however, possible to injure the anterior cruciate doing a variety of activities. We’ve seen bilateral ACL tears in a weight lifter who was doing an incline bench and popped both his knees at the same time when bench-pressing 350 pounds. It can also be a work-related injury. Interestingly, most people would expect that it is due to contact, but this is not true. Mostly, it is caused by non-contact deceleration where the athlete suddenly turns to the opposite side of the planted knee. As the patient turns and pivots, the ligament tears. In basketball, it is usually the result of a hyperextension and internal rotation of the tibia on the femur, associated with deceleration.

Usually the patient will feel a sudden “pop” in their knee immediately after injury. Surprisingly, sometimes the knee will not get very swollen, although it certainly can. The injury is often missed because the physical examination requires some experience and training. It may be easily over-looked in the initial stages.

Diagram of a Torn ACL

Natural History of the Torn Anterior Cruciate Ligament

If left untreated, the laxity which is immediately present only becomes worse. The other structures of the knee try, in vain, to provide some stability to the knee. Over time and with more usage, these other structures stretch out as well, resulting in increased instability and associated meniscal (cartilage) tears. There is an incidence of approximately 1 in 3 patients, who at the time of the anterior cruciate ligament tear, will tear their cartilage as well. This progresses with time because in an untreated knee, the knee is unstable and produces greater stress on the cartilage. Up to 80% of affected knees will eventually develop a cartilage tear. The smooth Teflon lining of the knee, which is known as articular cartilage, is often damaged at the time of the ACL tear. If left untreated, this will again progressively wear at the knee, causing an increased rate of osteoarthritis development. Patients will alter their gait and develop a rather specific quadriceps avoidance gait, because when they contract their quads during normal walking the tibia slides forward (which is usually stopped by the anterior crucial ligament). The patient will naturally and unconsciously try to prevent this. These problems mean that the knee will progress to late degenerative changes, and osteoarthritis will develop much earlier than in a normal knee. There is not sufficient evidence to suggest that wearing a brace alone will decrease the rate of re-injury to the knee. However, in older and non-active patients, there is definitely a role for non-operative treatment by simply modifying their activities and avoiding all situations where they may pivot and damage their knee further.


The anterior cruciate ligament is the main factor causing resistance to the anterior displacement of the tibia on the femur. This is demonstrated when the orthopedic surgeon pulls the tibia forward on the femur performing a test of the anterior cruciate ligament. The tibia will displace much further forward than it should when the ACL is torn. The ligament is tight when the knee is in full extension and has the least amount of tension at approximately 45’ of flexion. Because there are different bands to the anterior cruciate ligament, different areas of the anterior cruciate tighten at different angles of the knee.

Physical Examination

Diagram of O'Donohue's Terrible Triad Showing a Torn ACL, Torn MCL and Torn Medial Meniscus

Examination immediately at the time of injury will usually reveal at least mild swelling of the knee, but not necessarily. The best test is called a “Lachman Test”, where each of the examiner’s hands are placed just above and just below the knee joint. The lower bone is brought forward with the knee angled at approximately 15’ and the examiner assess the end point. Usually, there is a firm endpoint with an intact ACL when the tibia is pulled forward. When the ligament is torn, that endpoint is no longer present. The examiner will also look for increased excursion of the tibia forward on the femur. A “Drawer Test” is when the knee is flexed to 90’. Essentially, the same test is performed. It is more difficult, in an acute situation, to perform this test because usually the athlete’s knee is too sore to allow the knee to bend to 90’. A “Pivot Shift” is a test where the knee is brought from an extended position into flexion. Usually the knee will show a slight and subtle shift as the tibia rotates on the femur and shifts back into proper position. It is actually sub-flexed in the full extended knee position and returns to its natural position as the knee is flexed. As it returns to its natural position there is a “pivot shift” which takes experience to detect.

Associated injuries are always assessed for at the same time. Joint line tenderness representing torn cartilage and tenderness over the lateral knee may reflect tearing of the collateral ligaments. O’Donohue’s “terrible triad” injury involves not only the ACL, but also the medial meniscus and the medial collateral ligament. It is unfortunately fairly common.


Originally, it was felt that the knee should be repaired surgically as soon as possible. Now, most orthopedic surgeons feel that the swelling should subside and the patient should work to improve range of motion with physiotherapy for 2-3 weeks. Once this is accomplished, the patient can then proceed to an anterior cruciate ligament reconstruction. As stated earlier, surgery does not have to be performed on a sedentary older patient, but it is almost always recommended to a younger, active athlete. With modern techniques, surgery is performed on an outpatient basis – meaning the patient is discharged from the hospital the same day. The patient will leave the hospital on crutches, wearing a knee immobilizer for approximately 10 days while they are up and getting around. When the immobilizer comes off, the patient usually will use a passive motion machine that moves the knee through flexion and extension. Physical therapy is started immediately post-operatively. Treatment of a torn anterior crucial ligament in the older patient usually consists of physical therapy and exercise training, as well as potentially brace-wear for some activities.

Surgical Treatment Options

Diagram of ACL Reconstruction

There have been many options described for the surgical treatment of tears of the anterior cruciate ligament. The most popular option, currently recognized as the gold standard, is an operation where the middle third of the patella tendon is used as a graft. It is virtually impossible to repair the ligament that is torn. The torn ACL is simply removed and the replaced with the patella tendon graft. Two thirds of the patella tendon is left behind and will repair themselves, not compromising the function of the knee. At each end of the patella tendon, a bone block is also taken; one piece from the tibia, and the other from the patella (kneecap). These two bony blocks are inserted into holes that are drilled into the tibia and femur and held into place with screws, which provide stabilization of the ligament graft.

There are other tissues that can be used to substitute for the anterior crucial ligament. Most commonly, the second choice are hamstring tendons which are weaved into a graft close to the size of the anterior crucial ligament. We have also used quadriceps tendons and allografting. An allograft is donated cadaver tissue which is freeze dried until the time of usage, upon which time it is thawed out and trimmed to size to be used as an ACL substitute. The advantage of an allograft operation is that there is a smaller incision required, and the rehab is often shorter and less painful. The disadvantage is that it is not quite as strong as a graft formed from the patient’s own tissue.

Risks, Complications and Alternatives to Surgery

Any time an operation is performed, no matter how small or major, there are going to be a risks. With anterior cruciate ligament reconstruction surgery, the most common risks are infection, blood clots in the legs, failure of the graft, stiffness of the knee, and persistent pain and instability. There are other rare complications such as neurovascular injury and medical complications both general and related to the anesthetic. All would have to be understood and accepted by the patient prior to the surgery. In particular, all of these should be discussed with your surgeon pre-operatively. Unfortunately, there is no way to perform surgery that is completely devoid of all risks, but anterior cruciate reconstruction surgery is still effective in 95% of cases. Even if a complication does occur, it can usually be treated and resolved.

Long Term Prognosis

With an anterior cruciate ligament repair, the patient’s long-term prognosis without any other associated significant injury is excellent. It certainly carries a much better prognosis than when the knee if left untreated. The patient can usually return to any activity that they were doing pre-operatively, and many athletes have gone on to excel again at their chosen sport.


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