Anterior Cruciate Ligament (ACL) Injury

Anterior Cruciate Ligament (ACL) Injury treatment and information


What is the ACL?

A ligament is a band of tough, resistant and elastic fibrous tissue located around joints, attached from one bone to another. Similar to a tendon or fascia, ligaments are made of connective tissue, which gives its elastic properties. Because of their bone-to-bone attachments, ligaments are designed to limit the range of movement allowed in a joint in order to protect it.

The Anterior Cruciate Ligament (ACL) is one of the two ligaments located inside the knee joint, attaching from the back of the femur (thigh bone) to the front of the tibial plateau (leg bone). It controls the forward movement (or “translation”) of the tibia under the femur, and its rotation. The other ligament inside the knee is the Posterior Cruciate Ligament, which also controls knee rotation, as well as the backwards movement of the tibia. Finally, the Medial and Lateral Collateral Ligaments (MCL and LCL), make up the final two of the four ligaments that play a key role in stabilising the knee in all directions and planes of movement.

What are the common causes of an ACL injury?

Cruciate ligaments are thick structures designed to stabilise the knee and support the “stress” of body weight in various contexts. This is why not all ACL damages cause a full rupture: a ligament can be strained, partially torn or fully ruptured.

The most common type of mechanism leading to an ACL injury is usually sports related. The most common motion leading to an ACL damage is when a brutal rotation or bending of the knee is induced in one direction, whilst the foot is planted in the ground in another direction. This traumatic motion can be induced by an external source (for example a tackle in the context of football or rugby) or simply weight bearing (deciding to brutally change direction mid-run, which could happen in the context of skiing for example).

The ACL can be affected by itself, or be apart of a more complex injury, with “collateral” damages: in the context of what is called an “unhappy triad”, the ACL, MCL and medial meniscus will be ruptured, caused by a lateral (sideways) force applied on the knee with the foot fixed on the ground.

What are the symptoms of an ACL injury?

Whether the mechanism leading to an ACL injury is “self-inflicted” or caused by an external source, the symptoms described by the patient are usually similar. A pop noise or “popping” sensation is usually heard or felt at the time of the incident causing the injury. This will be associated with severe pain, but doesn’t always lead to an immediate inability to continue the activity. This is dependent on the severity of the injury (full or partial rupture) and because of the fact that knee stability can be partially preserved via the muscles around the knee if well developed. A sensation of the knee giving way with weight bearing is also one of the key symptoms reported in the case of an injury affecting the ACL.

A swelling developing rapidly can be observed around the knee joint, associated with heat on palpation. This swelling will usually lead to a loss of range of movement in the knee, especially knee flexion, because of the excess fluid occupying space in the joint.

How is an ACL injury diagnosed?

A confirmed medical diagnosis of an ACL rupture, partial or full, can only be made via imaging. An X-Ray only highlighting the most dense anatomical structures such as the bones, won’t show any ligaments. The preferred way to have a clear view of the “soft tissues” such as the ACL, and the surrounding structures (such as the meniscus for example), will be an MRI or ultrasound.

Ideally a specialist (osteopath, physiotherapist) or GP should be consulted as soon as the first symptoms suggesting an ACL injury are declared, in order to determine as soon as possible the severity of the suspected injury. The healthcare professional will be able to conduct a detailed physical examination and perform orthopedics tests such as Lachman’s test or the anterior “drawer” test, used to assess an excess of movement in the knee, which is a sign of an instability incriminating the integrity of the ACL.

While both Lachman’s and the drawer test are designed to test the ACL, they are performed in different ways and have different specificities, sensitivities (how reliable the test is). Lachman’s test is performed with the patient lying on their back with the knee mildly flexed, whilst the therapist pulls the tibia forward. The drawer test is performed with the patient’s hip flexed at 45 degrees and the knee at 90 degrees, with the therapist performing the same movement. In both cases, the test is considered “positive” when an anterior “translation” (forward movement) of the tibia is observed without resistance, indicative of damage to the ACL. With both tests specific to the ACL, the sensitivity of Lachman’s is superior to the drawer’s (Katz et al., 1986) (Petersen and Tillmann, 2002), meaning its result is more reliable than the latter.

What are the treatment options for ACL injury?

The anterior cruciate ligament benefits from a poor blood supply, which associated with the presence of fibrocartilage around its anterior attachment, can explain the poor self-healing potential of this structure when damaged (Petersen and Tillmann, 2002).

Treatment options vary depending on the nature of the injury affecting the ACL: full or partial tear.

In the case of a partial tear, surgery won’t necessarily be required, and treatment can consist of physical therapy designed to stabilise and strengthen the affected knee. The program will essentially be using physical activity at a moderate intensity and adapted to what the patient can do, such as gym activities (squats to strengthen the quadriceps for example), jogging, running, at moderate pace and on soft, stable grounds.

With a full ACL rupture, a reconstruction surgery of the ligament is recommended, in order to bring full stability back into the knee and avoid further damages into the menisci and cartilage that would absorb a greater impact and possibly tear. The procedure is performed using a tendon graft, replacing the entire ligament. The graft can either be an autograft (coming from the patient’s own body), commonly a part of the hamstrings or the patellar tendon, or an allograft (coming from a cadaver). The use of an autograft is usually preferred, as the tissue used being the patient’s own, the risks of rejection are minimal.

Whether the rupture is partial or full, treatment sessions with an osteopath or physiotherapist are beneficial. It won’t directly affect the recovery and healing time, but the aim will be to minimise all the risks of compensations in the lower body, either at the pelvis, in the gait mechanics, and reinforce the range of movement allowed in the knee (especially following surgery with the build-up of scar tissues). The therapist will also help planning when and how is the best return to full physical activity possible.

How long does an ACL injury rehab process last?

The recovery process and its length will obviously be depending on the type of injury affecting the ligament, the success of the surgery – if required – and the good planification of treatments and return to activity.

In the case of a partial tear, the recovery is estimated at 3 months, but the risk of a relapse especially because of the potential residual knee instability is non negligible. In the case of a full rupture, following the surgery it will take 6 weeks before any manual work can be done on the scar tissues in order to free the affected knee from as many restrictions as possible. It is estimated that 4 months after the surgery the patient may be able to run again, and taking up to 6-8 months before engaging actively in sports with no limitation.

Can an ACL injury be prevented?

As in the vast majority of cases anterior cruciate ligament tears happen as the result of a traumatic injury, or following an accidental weight bearing injury mechanism, you can’t exactly prevent it. Stability through the lower limb joints (hip, knee, ankle) thanks to good muscle tonicity will help minimise the impact of such an injury and will favourise a fast recovery during the rehab. A thorough, meticulous and cautious return to activity program will be key to prevent potential relapses (evaluated at 3 to 6% of cases following a ruptured ACL reconstruction).

What are the best exercises for the rehab of an ACL injury?

You can find a range of various exercises recommended for the rehab of an ACL rupture. Here is a selection of them that will be helpful to improve your balance, and strengthen the muscles protecting the knee after a reconstruction surgery: some exercises can’t be done before the swelling in the knee subsides following the surgery, some are safe to be done in the early stage of the recovery. Always consult your orthopedic consultant or physical therapist before carrying out any of these exercises.

Isometric wall sits: this exercise can be done at any stage of the recovery, and consists of placing your back against a wall, with your knees and hips bent approximately around 120 degrees (mild flexion to avoid putting too much weight on the injured knee too soon). You will then slowly raise the “healthy leg” off the floor to only have your injured side weight bearing, and hold the position for 10-15 seconds. This exercise can be repeated 3 times.

Standing on one leg: this exercise is recommended to be done once the swelling in the knee has subsided, at a later stage of the recovery. Simply, you will stand on the injured leg only without assistance, with a very mild flexion of the knee to unlock it, and maintain your balance for 10-15 seconds. This is a good exercise designed to improve and also test your balance on the injured side.

Half squats: this exercise is recommended to be done once the swelling in the knee has subsided, at a later stage of the recovery. With the assistance of a support to place your hands on during the exercise, you will place your legs distant from each other within your shoulders’ width, and slowly bend your knees and bring your hips and body down, in a controlled manner, into a half squat motion, holding the position for 10 seconds, before returning to a straight position. This exercise can be repeated as much as possible depending on the time of the recovery where you will first do it, with a maximum of 10 repetitions at a time.

It is recommended to get the OK from your osteopath, physical therapist or GP before trying out any of these exercises.


Katz, J.W. and Fingeroth, R.J., 1986. The diagnostic accuracy of ruptures of the anterior cruciate ligament comparing the Lachman test, the anterior drawer sign, and the pivot shift test in acute and chronic knee injuries. The American journal of sports medicine, 14(1), pp.88-91.

Petersen, W. and Tillmann, B., 2002. Anatomy and function of the anterior cruciate ligament. Der Orthopade, 31(8), pp.710-718.

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