ACL Sprain

ACL Sprain

An ACL sprain or anterior cruciate ligament sprain is a tear of the anterior cruciate ligament in the knee joint. Symptoms include sudden knee pain with swelling and is common in contact sports. Often ACL tears do not occur in isolation and are often associated with damage to other structures within the knee. Surgery is often indicated followed by a comprehensive rehabilitation program.

On this page:

  • Symptoms
  • Causes & anatomy
  • Treatment
  • ACL taping
  • Exercises
  • Surgery

Torn ACL symptoms

ACL tears usually occur as a result of a twisting action which involves the foot being planted in the ground whilst the knee turns excessively inwards. The main symptom on the field of play is usually a sudden pain in the knee and in most cases, the athlete is aware that something serious has happened.

Patients describe an audible pop or crack at the time of injury. Following the injury, the swelling will usually develop rapidly (but not in all cases) and the athlete may have a feeling of instability in the knee. The swelling is caused by bleeding within the joint, due to rupturing of blood vessels around and within the ACL which causes bleeding into the joint cavity. The athlete will know this has happened because the knee will also become warm to touch.

Once the injury has occurred, the athlete will usually seek professional advice at an injury clinic over the next couple of days. By this time, there will probably be a considerable amount of swelling and this will make it very difficult to move the knee, making diagnosis much harder for the physiotherapist or doctor. They will often offer some simple advice to get the swelling down so they can reassess it after a few days when it will be easier to examine the knee joint.

The movements in the knee will be restricted and with particular difficulty in straightening the leg fully and there may be tenderness or pain felt around the knee joint (due to the swelling and inflammation). The physiotherapist will test the stability of the knee using tests such as the Lachman's test and the Anterior Drawer Test and if the ligament is significantly damaged, these will show excessive movement (or laxity) (Magnussen et al., 2016)

Anterior Draw Test

Anterior Drawer Test

The anterior drawer test (play video) - patient lies on their back and bends the injured knee to 90 degrees with the foot flat on the treatment bed (the practitioner may stabilize the foot by sitting on it). The practitioner grasps the upper tibia (shin bone) with both hands, just below the knee joint and then pulls the tibia forwards to assess how much movement there is and if there is any pain. The degree of movement will be assessed in conjunction with the end feel of the test and a positive result is indicated if the tibia moves excessively forwards on the femur without a hard end feel. This indicates that the ligament has been damaged and probably torn completely. Every patient is different, so in order to assess whether the movement is excessive, the injured knee should always be compared to the healthy knee which shows the patient’s normal 'normal' movement. If the ligament is completely torn then this test may be pain-free because there is no ligament to be stretched.

Lachman's Test

Lachmans Test

Lachman's test (play video) - patient lies on their back with the knee flexed between 15 and 30 degrees. The practitioner grasps the outside of the lower femur (thigh bone) with one hand, just above the knee joint, and the inside of the upper tibia with the other hand. The femur is then kept still with the upper hand while the lower hand pulls the tibia forward. Again, similar to the anterior draw test, a positive result is seen if the tibia moves excessively forward compared to the other (healthy) knee without a hard end feel. If the ligament is completely torn then this test may be pain-free because there is no ligament to be stretched.

In addition to the tests described above, the practitioner may also choose to test the range of motion of the knee, the strength of the surrounding muscles and use a number of other tests to assess for possible associated injuries, such as meniscus tears.


An MRI scan can confirm the diagnosis of an ACL sprain whereas an X-ray is only useful to detect a bony injury such as an associated avulsion fracture (where a piece of bone attached to one end of the ligament is pulled away from the main body of the bone) which can sometimes happen the ligament pulls a piece of bone away.

Causes & anatomy - what is an ACL sprain?

acl sprain

An ACL sprain is a damage to the Anterior Cruciate Ligament in the knee and this can either be a partial tear or a full tear (rupture) depending on the force of the injury. The ACL is a ligament that runs diagonally from the back of the femur (thigh bone) upwards and forwards to the front of the tibia (shin bone) and its function is to prevent the shin bone from moving excessively forward in the knee.

Injuries to the ACL usually occur as a result of either a twisting force in the knee after a sudden turning action or landing after a jump causing the foot to plant firmly in the ground whilst the knee twists inwards. They can also occur from contact situations from opponents, as seen in sports such as rugby or football, but these are far less common (Boden et al., 2010). In these situations, a direct blow to the outside of the knee causes it to buckle inwards leading to excessive movement of the tibia and the result is often a torn ACL. With this type of injury, there is usually associated damage to other structures within the knee joint such as the medial collateral ligament (MCL sprain) or the meniscus (cartilage - medial meniscus tear) and these also need to be accurately diagnosed and treated.

Treatment for ACL sprains

If you believe that you have a suspected ACL injury or tear, we strongly advise you seek a professional medical opinion as soon as possible to confirm the diagnosis and if necessary, refer you onto an orthopedic consultant to discuss the option of surgery.  Surgery is not always required for ACL injuries and depending on the level of competition that you play/performs at, you may be advised to follow a non-surgical (conservative) rehabilitation program with the aim of building up the muscles to protect the torn ACL.

What can the athlete do?

Immediate first aid for ACL tears involves applying the P.R.I.C.E. therapy principles (protection, rest, ice, compression, and elevation) to reduce the swelling and encourage healing. One way of reducing swelling is to apply a cold compression wrap for 15 minutes intervals and for at least the first 72 hours or until you have had the knee assessed by a specialist.

What can a sports injury professional do?

Early stage treatment of a torn aclExpert interview (play video): Sports Physiotherapist Neal Reynolds explains early stage treatment for ACL sprains.

A doctor or professional practitioner can make a full diagnosis which may not be possible until swelling has reduced. They are likely to send you for an MRI scan or X-ray to confirm the diagnosis and depending on the result, they may advise you to see an orthopedic surgeon.

ACL sprain taping & braces

ACL tapingThe aim of using a brace or acl taping (play video) is to support the knee following injury and to make the athlete feel more confident to move around. This may be important if the athlete has an unstable knee (laxity in the joint) to ensure other structures such as ligaments or cartilage get damaged. Do not tape if you are unsure of the injury or why the taping is being used as this can sometimes give you a false sense of security and you may damage the knee even more.

Expert interview (play video): Knee braces may be used to help protect the knee joint, especially if it is unstable. It is not possible to brace, tape or support the knee to completely prevent injuries but knee taping can certainly help.

Read more on rehabilitation of ACL injuries >>>

ACL sprain exercises

Once a diagnosis has been made and the initial acute injury management of P.R.I.C.E. has passed, the first stage of ACL injury rehabilitation begins and the aim is to regain normal movement as soon as safely possible back into the injured knee through mobility exercises. In cases where surgery is the treatment of choice, the surgeon will often delay the timing of the surgery to allow time for the swelling to be reduced and for normal knee movement to be regained before reconstructing the ligament, as this has been shown to significantly improve the overall outcome.

If surgery is indicated, then pre-surgical exercises are strongly advised and this is for three reasons; to reduce swelling, to regain normal movement and to build up the strength in the muscles around the joint.

If surgery is not indicated, then mobility and strengthening exercises are also advised with the aim of slowly returning the movements and function of the knee back to normal as soon but as safely as possible, and so the healing process of the ligament and surrounding structures is not compromised.

One of the most important aspects of ACL injury rehabilitation is to restore proprioception in the knee. Proprioception is the body's own ability to detect unwanted movements in the joints and ligaments and involves rapid muscle contractions to protect the structures from being damaged.  After an ACL injury, the proprioceptive ability of the body is diminished and needs retraining. Proprioception exercises are strongly advised to teach the brain to know 'where the knee is in space' and to facilitate surrounding muscles to react quickly to prevent further injury and these should be performed progressively throughout the rehabilitation process.

Read more on ACL injury exercises

ACL reconstruction surgery

Surgery for a torn aclExpert interview (play video): Leading orthopaedic surgeon Mr Richard Villar explains the surgery that is used for a torn acl. Firstly the diagnosis should be confirmed with manual tests and MRI scans.  Years ago, extra-articular (outside of the knee joint) techniques were used for ACL reconstruction operations and this involved tightening a tendon from outside the knee joint to stabilize the joint rather than repairing the ACL itself.   It has since been discovered that it is more advisable to repair the ACL itself and therefore the majority of operations that are now performed are intra-articular (inside the knee joint) reconstructions where another structure is used to replicate/replace the ACL. This can either be a ligament or tendon from the patient (autograft) themselves and this could be part of their patella tendon or semitendinosus (hamstring) tendon and re known as bone-tendon-bone grafts because the piece of tendon is attached to bone 'plugs' at each end. Alternatively, an artificial graft or donated graft might be used in some cases.

Who should have surgery?

The decision on whether to operate on the knee or not for an ACL sprain is a controversial one. It often depends on a number of factors such as the athlete's age, their occupation, their lifestyle and the degree of instability within the knee, and an orthopedic surgeon will be able to advise on which treatment approach is preferable. If reconstructive surgery is chosen, then the operation usually takes place as soon as possible after the injury has occurred or if the knee is very swollen, however, you may be advised to wait a few weeks to let the knee settle down and the swelling to significantly reduce so the surgery is more successful. A recent study indicated that cartilage loss from the back of the patella in young patients who have early reconstructive surgery was greater.

Recovery rates after an ACL injury, either following reconstructive surgery or not, are extremely variable. It is generally accepted that it will at least 9 months or more to fully recover although within 4-5 months the knee should be starting to function normally and the athlete may be able to start to introduce changes of direction movements into their rehabilitation program. It should be noted however that it will take several more months for the athlete to fully recover and regain full confidence in the knee to be able to play sport again. It is not unusual that even 1-year post surgery/injury, the athlete may still feel differences in the knee and may lack full confidence in it with the occasional feeling of minor instability, and this can continue for several years afterward and this is why you sometimes see some even professional athletes never fully return back to their previous level of performance or skill.


Expert interview (play video): Sports Physiotherapist Neal Reynolds gives tips on preventing torn anterior cruciate ligament injuries.


  • Boden B.P., Sheehan F.T., Torg J.S. and Hewett T.E. (2010) Non-contact ACL Injuries: Mechanisms and Risk Factors Journal of the American Academy of Orthopaedic Surgeons 18(9) pp 520-527
  • Magnussen R.A., Reinke E.K., Huston L.H., Hewett T.E. and Spindler K.P. (2016) Factors Associated with High-Grade Lachman, Pivot Shift and Anterior Drawer at the Time of Anterior Cruciate Ligament Reconstruction The Journal of Arthroscopic and Related Surgery 32(6) pp 1080-1085
  • Culvenor AG, Eckstein F, Wirth W, et al. Loss of patellofemoral cartilage thickness over 5 years following ACL injury depends on the initial treatment strategy: results from the KANON trial Br J Sports Med Published Online First: 08 February 2019.

This article has been written with reference to the bibliography.