The anterior cruciate ligament is an important, internal, stabilizer of the knee joint, restraining hyperextension. It is injured when its biomechanical limits are exceeded (over stretched), often with a hyperextension mechanism. Formerly, this occurred most often in a sports contact injury, when other structures were frequently involved. A particularly severe form of the contact injury is called the “unhappy triad” or “O’Donaghue’s triad”, and involves the anterior cruciate ligament, the medial collateral ligament, and the medial meniscus. Presently, ACL injury is more commonly a non-contact injury, such as a dismount from a layup in basketball. Both forms occur more frequently in athletes than in the general population and are prevalent in alpine skiing, Association football, American football, Australian rules football, basketball, rugby, professional wrestling, martial arts, and artistic gymnastics. It is also known to be about three times more common in women than men.
The consequences of the injury depend on how much the stability of the knee is affected, and the extent to which other structures have been involved, and this can vary on a case-by-case basis. If instability is evident, particularly rotatory instability, then the menisci will get injured, sooner or later, setting the scene for progressive, degenerative, arthritis of the knee.
The combination of “pop” during a twisting movement or rapid deceleration, together with inability to continue participation, and followed by early swelling, is said to indicate a 90% probability of rupture of the anterior cruciate ligament.
The pivot-shift test, anterior drawer test and Lachman test are used during the clinical examination of suspected ACL injury. The Lachman test is recognized by most authorities as the most reliable and sensitive test, and usually superior to the anterior drawer test. The ACL can also be visualized using a magnetic resonance imaging scan (MRI scan).
An ACL tear can present with a popping sound heard after impact, swelling after a couple of hours, severe pain when bending the knee, and buckling or locking of the knee during movement.
Though clinical examination in experienced hands can be accurate, the diagnosis is usually confirmed by MRI, which has greatly lessened the need for diagnostic arthroscopy and which has a higher accuracy than clinical examination. It may also permit visualization of other structures which may have been co-incidentally involved, such as a meniscus, or collateral ligament, or posterolateral corner of the knee joint.
The term for non-surgical treatment for ACL rupture is “conservative management”, and it often includes physical therapy and using a knee brace. Instability associated with ACL deficiency increases the risk of other knee injuries such as a torn meniscus, so sports with cutting and twisting motions are problematic and surgery is often recommended in those circumstances.
If surgery is decided upon, either because obvious instability interferes with activities of daily living, or because the knee is subject to repeated, severe, provocative maneuvers, such as the case of the competitive athlete involved in cutting and rapid deceleration etc, then several issues need to be decided upon.
- Timing. Immediate repair is usually avoided and initial swelling and inflammatory reaction allowed to subside.
- Choice of graft material,
- Autograft or allograft.
- Choice of anterior cruciate ligament augmentation, patellar tendon or hamstring tendon.
Initial physical therapy consists of range of motion (ROM) exercises, often with the guidance of a physical therapist. Range of motion exercises are used to regain the flexibility of the ligament, prevent or break down scar tissue from forming and reduce loss of muscle tone. Range of motion exercise examples include: quadriceps contractions and straight leg raises. In some cases, a continuous passive motion (CPM) device is used immediately after surgery to help with flexibility. The preferred method of preventing muscle loss is isometric exercises that put zero strain on the knee. Knee extension within two weeks is important with many rehab guidelines.
Approximately six weeks is required for the bone to attach to the graft. However, the patient can typically walk on their own and perform simple physical tasks prior to this with caution, relying on the surgical fixation of the graft until true healing (graft attachment to bone) has taken place. At this stage the first round of physical therapy can begin. This usually consists of careful exercises to regain flexibility and small amounts of strength. One of the more important benchmarks in recovery is the twelve weeks post-surgery period. After this, the patient can typically begin a more aggressive regimen of exercises involving stress on the knee, and increasing resistance. Jogging may be incorporated as well.
After four months, more intense activities such as running are possible without risk. After five months, light ball work may commence as the ligament is nearly regenerated. After six months, the reconstructed ACL is generally at full strength (ligament tissue has fully regrown), and the patient may return to activities involving cutting and twisting if a brace is worn. Recovery varies highly from case to case, and sometimes resumption of stressful activities may take a year or longer.
The reconstructed ACL has a high success rate. Studies show that cases in which the ACL retears are generally caused by a traumatic impact. Some studies indicate that wearing a brace during athletic activity does not reduce probability of re-injury to the ACL, but a study of active post-ACL replacement skiers shows a 64% reduction in re-injury likelihood by using a knee brace after recovery. A sufficiently traumatic impact to retear the ACL is unlikely to be mitigated by the use of a brace