Anterior cruciate ligament (ACL) sprains are one of the most common sports injuries of the knee, especially in “stop and go” sports. Athletes participating in high load activities like soccer, rugby, hockey or sprinting are more likely to injure their ACL, compared to the general population. The ACL is the strongest and most important ligament in the knee, yet tears of this ligament are quite common, traumatic and need attention straight away.

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What does the ACL do?

ACL stands for anterior cruciate ligament. “Anterior” means front, “cruciate” means shaped like a cross and a “ligament” is a band of tissue connecting bones. It is one of the four ligaments in the knee that assist in stabilising the joint.

The ACL has two main functions: it is mainly responsible for providing stability in the knee joint by preventing forward movement of the shin bone (tibia) on the thigh bone (femur), thus hyperextension. It also helps to prevent excessive twisting.

The ACL originates on the back of the thigh bone and runs forwards and downwards to insert on the front of the shin bone. It is approximately 2 cm long and as thick as a pencil. Together with the posterior cruciate ligament, it forms an “X” inside the knee joint, thus preventing excessive movement.

Causes of an ACL tear or sprain

Although ACL knee injuries are relatively common, most ACL tears happen when there is no impact or contact with another player. It usually occurs if the knee twists when landing from a jump. The pressure from your body weight and the angle of pull on the ligament pushes it beyond its normal limits causing a ligament sprain or tear.

For example, you play a rugby match and your teammate pass you the ball. The defender from the opposing team is approaching you from the front with extreme speed and within milliseconds you have to decide which direction you are going to turn in order to avoid the tackle. At the last moment you suddenly shift to the opposite side. Your foot is still on the ground while your body is already moving sideways with speed. You hear a pop and immediately feel pain in the knee. This is an example of a non-contact injury.

Non-contact injuries

Non-contact injuries account for about 70% of all ACL sprains. Twisting your knee, pivoting with your foot planted or landing awkwardly from a jump are the prime mechanism of injuries. Hyperextension of the knee (bending the knee the wrong way by more than 10 degrees causing it to gap at the back) or sudden deceleration i.e. coming to an abrupt stop whilst the foot is firmly planted, are also possible mechanisms of an ACL sprain.

Contact injuries

Contact Injuries account for about 30% of ACL injuries. Usually a blow to the side of the knee when the foot is firmly planted on the ground e.g. with studs or spikes which forces the shin bone too far inwards or backwards.

Women are more likely to sustain knee ligament injuries
Women are three times more likely to suffer an ACL injury or knee sprain. This is thought to be due to a combination of factors including hormone levels, ligament structure, pelvis angle, hamstring flexibility and size of the ligament itself.

The ACL has two main functions

It provides stability in the knee and prevents excessive twisting

ACL injury,

What happens to the ACL on a cellular level

A ligament sprain happens when the ligament is overstretched.  There are several classifications for ligament sprains depending on the severity (grade 1, 2 or 3). The fibrous band is like a pulley, which prevents the joint from moving into extreme ranges. When the force is too big for the ligament to resist, the fibers will tear. Overstretching the ACL may result in tears or a sprain of the ligament.  It is important to know and understand the difference and implications. In the case of a tear, you may hear a snap or tearing sound.

The collagen fibers tear like a tension cable when overloaded and the fibers unravel. On a cellular level, injury will cause a split along the sheath of the ligament (like a sleeve around the ligament). Inflammation develops in response to injury which causes swelling, pain and loss of function, this causes the knee to buckle forwards. The swelling contributes to loss of space around the ligament that restricts the normal movement of the ligament in its sheath. Mucoid degeneration and fibrinoid necrosis (a type of cell death) indicate ligament tears.

How bad is it?

Grade 1 ACL sprain (mild)

A small amount of fibers torn, you will have some discomfort and pain. The ligament only sustained a mild stretch injury. No swelling and minimal pain, but you may run into problems if the cycle repeats itself. Recurrence of micro tears in the ligament could lead to more severe tears. You will have no loss of function or range of movement of your knee, but may experience pain at some point during the movement. You will be able to continue playing.

Recovery time for a grade 1 ACL injury is anything between 2-4 weeks. However, the collagen fibers only align after 6 weeks (maximal ligament strength), so to limit the possibility of re-injury it is advisable not to return to sport too soon.

Grade II ACL Sprain (moderate)

A partial ACL tear with some swelling and bruising. Pain level increases quite a bit from a grade I and moving your knee will cause pain. It may feel uncomfortable to bend or straighten your knee and it may feel unstable, as if the knee wants to “give way”.

In a grade 2 ACL injury between 20% to 80% of the fibers are torn. You will not be able to put weight on the leg, in other words; take a normal step. Even if you try to continue playing, you will have a considerable amount of pain and limping. The pain will not allow you to straighten your knee or lift your leg forward (knee to chest).

Recovery Time will be 6 to 8 weeks (With early treatment, we try to get you jogging at 6 weeks).  Expect return to activity to be no sooner than 6 weeks.

Grade III ACL sprain (severe)

A complete ACL tear. Swelling and discoloration (bleeding under the skin) are visible. The knee feels as if it wants to give way completely. Putting your full weight on the leg is nearly impossible. Bleeding and swelling will definitely be present.

The main concern with a grade 3 tear is the ligament’s ability to stabilize the knee joint – if the ligament tears into two separate flaps, there is no way the ligament can ‘heal’ or reattach without surgery. A Sonar or Diagnostic Ultrasound will show the full extent of the damage. Surgery is the best option to reconnect the separated segments of the torn Anterior Cruciate Ligament.

Often there will be no pain, but the disfigurement is quite obvious, since all the fibers are torn. It is important to know that there could be other structures involved with such an injury, but we will discuss this in more detail with our differential diagnosis later.

Recovery Time will depend on whether the conservative or surgery option is chosen. Conservative treatment may involve more than 8 weeks of intensive rehabilitation.  Surgery is indicated if a complete rupture of the medial collateral ligament is present with ACL injury and full recovery may take 4-6 months and return to sport only from 9 months.

Symptoms of an ACL injury

Test your ACL

  • Do a small squat (bend the knee slowly).
  • You will either feel pain at the front of your knee, or you won’t want to stand on one leg, depending on the severity of your sprain.
  • With a complete rupture, you won’t be able to put all your weight on your injured knee without feeling like it wants to give way completely.
  • Stand on a step.
  • Step down with the uninjured leg – i.e. the injured leg stays on the step.
  • Your knee will feel as if it wants to give way forwards and down.
  • Pain inside the knee joint is an indication of a sprain during these tests.
  • Stand on your injured leg.
  • Bend it slightly.
  • Twist your body towards your knee.
  • Then twist your body away from the knee.
  • Again, your knee would want to give way forwards with a sprain (buckle).
  • Stand with your injured leg on a small step.
  • Draw your toes up, so only your heel is touching the step.
  • Let your knee straighten as much as it can.
  • Pulse your knee.
  • If there is a sprain it will feel as if the knee is moving too far backwards.
  • Sit on a chair with your knee facing directly forward, bent at a 90 degree angle. Relax your leg in this position.
  • Place both your thumbs at the back of your knee (over your calf muscles) and hold your shin bone with your other fingers.
  • Gently and slowly slide your tibia (shin bone) forward.
  • If you feel that same pain, then you may have sprained your ACL.
  • If there is no resistance and your tibia just slides off without the movement being stopped, you may have a torn ACL.

What NOT to do

  • Anti-inflammatory medications are not recommended, especially in the first 48 hours as they are thought to delay healing.

  • Walk, run or jog through the pain.

  • Leave it untreated if you are uncertain of the diagnosis, rather call us and be safe.

What you should do

  • Follow a POLICE or PRICE protocol.

  • Make an appointment to confirm the diagnosis and determine the severity of your problem.

  • For a grade 2 or 3 ACL sprain, use a weight-bearing brace or supportive strapping to decrease the load on the ligament.

  • Use crutches when walking if you experience pain with weight-bearing.

Making the injury worse

  • Squats.

  • Jumping lunges.

  • Downhill running.

  • Kicking.

  • Climbing down stairs.

  • Driving.

  • Kneeling.

  • Hopping.

  • Sitting with legs crossed.

  • Standing for long periods of time.


Physical Examination

After taking a thorough look at your history, physiotherapists can determine which parts of your knee may be injured by finding the root cause of the problem.  We do specific tests for each structure in the knee, so we can differentiate between what is injured and what isn’t. If you’re uncertain, let us have a look and give you our opinion.

To test the integrity of the ACL we use the Lachman- , Anterior Drawer- and Pivot Shift tests. Using these tests we can accurately determine if there is an instability or torn ACL in your knee. In our assessment we also test the ligaments on the inside, outside and the menisci (cartilage) in the knee.


X-rays can be helpful to exclude fractures of the patella, femur, tibia or fibula, but will not show an ACL ligament tear. This is usually the test most emergency rooms opt for, but when there are no fractures diagnosed, they conclude that the ‘knee ligaments’ are torn.  However, this doesn’t tell us much. Get to your physiotherapist so that they can test each ligament individually and get a more conclusive diagnosis.

It is important to note that children have softer bones. The anchor of the ligament can tear away from the bone more easily resulting in an avulsion fracture, which may be visible on x-ray. It is essential to make sure there are no fractures if we suspect an ACL injury in a child before proceeding with further tests.


The gold standard (best in the industry) to diagnose an ACL tear would be via MRI, then a CT scan. MRI has the advantage of providing a clear image of all the anatomical structures of the knee. An MRI can diagnose ACL injuries with an accuracy of 95% and will also reveal any related meniscal tears, chondral injuries, or bone bruises. However, this is an expensive test and you will need to consult an orthopedic surgeon to be referred for an MRI.


If you have limited funds and would like to confirm your diagnosis, then we advise you to have a sonar or diagnostic ultrasound done at a reputable musculoskeletal sonographer. In this case, the ACL itself won’t be visible on the scan, but injuries to the structures around the ACL may lead us to assume an injury to the ACL. If this is the case, we may refer you for further testing to confirm, such as MRI or CT scans.

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Physio treatment for an ACL tear – rehabilitation and goals

Physiotherapy treatment of an injured or torn ACL will start with a detailed assessment. This enables us to plan the course and goals of treatment. For example, the treatment plan and goals of a rugby player may differ from a long distance runner.

Physio modalities used to treat knee ligament injuries may include joint mobilisation if stiffness of the knee is present, electrotherapy and strapping for swelling and pain relief, myofascial release for muscle spasm and specific rehabilitation exercises according to the degree of the injury. Exercises will also be progressed as the injury heals with the end goal of returning patients to their previous level of activity. Education is also a major part of physiotherapy and you are encouraged to ask questions and provide feedback during the course of treatment.

1st Phase: Protection & initial Healing



A knee ranger brace may be necessary to protect the ACL and limit movement of the knee joint.  If you experience pain when putting your full weight on the knee, you should use crutches when walking until you can tolerate full weight-bearing.


Do not continue training/sporting activity at this stage.  A physiotherapist will be able to guide you in gentle, pain free exercises to start strengthening muscles around the knee without putting excessive strain on the injured ACL.

It is also important not to return to sports (even if the pain improves) without having the knee assessed.  No pain does not necessarily mean the injury has healed completely.



An ice pack or ice cubes wrapped in a towel over the injured knee may reduce pain and inflammation.  Ice can be applied for about  20 minutes every 2 hours, but always have a towel between the skin and the ice to prevent an ice burn.



Strapping or an elastic compression bandage can be used around the knee joint to help control swelling which may also help with pain relief.



Lying on your back with your foot on a chair (your knee must be higher than your heart).  Raise your leg for 15 minute intervals during the day.

2nd Phase: Regain Full Extension of the knee

One of the most important components of ACL rehabilitation is to regain full range of movement of the knee joint. We use massage, stretches, neurodynamic- and joint mobilizations to achieve full range of movement.

In this phase of rehabilitation physiotherapy will focus on knee extension (straightening) as the knee ranger limits flexion (bending) of the knee to protect the ACL.  We will guide you with static exercises, for example pressing the knee down into a rolled up towel, to start regaining muscle control around the knee.  It is important to decrease swelling if the knee is still swollen at this point.

Once you can tolerate full weight-bearing you will progress to the next phase of rehabilitation.

3rd Phase: Restore Full Muscle Strength

Another crucial part of rehabilitation after an ACL injury is to restore full strength of the muscles around the knee joint.  Dynamic strengthening exercises are introduced during this phase of rehabilitation while treatment will also still aim at restoring and maintaining full range of movement (bending and straightening the knee).

The main goal during this phase of rehabilitation is regaining muscle control during daily activities for instance walking, squats and standing on one leg.  After this phase of rehabilitation you should be able to start light jogging and return to full activities of daily living i.e. walking, driving etc. without pain.

To progress to the next phase of rehabilitation you should be able to stand on the injured leg and bend the knee (single leg squat).

4th Phase: Improve control of rotational forces on the knee

In this phase of rehabilitation we will add a rotational (turning) component to exercises.  The muscles that control rotation of the knee should be strong enough to prevent excessive load on the ACL. Rehabilitation will focus on controlling twisting movements for example when you get into the car or change direction when walking fast.

You can progress to the next phase of rehabilitation when you can perform walking lunges with an upper body twist.

5th Phase: Agility training

Agility training will commence once range of movement and muscle strength have been restored.  The goal of this phase of rehabilitation is to restore your ability to move quickly and easily.  Examples of exercises would be quick steps in different directions, sprints with change of direction, acceleration (going faster) and deceleration (slowing down) exercises.

This phase is important to enable the knee to respond to sudden changes, for instance if you slip on a wet floor and lose your balance.

You will progress to the next phase of rehabilitation once you can perform changes of direction quickly and easily.

6th Phase: High load resistance training

One of the final phases of rehabilitation is strength training with high loads.  All the components of the previous phases will still be maintained, but more resistance will be added to exercises.  This is important for heavy everyday activities you may need to perform for instance carrying heavy furniture up a flight of stairs.

You will progress to the sport-specific phase if range of movement, muscle strength, agility are restored.

7th Phase: Sport specific training (if applicable)

Rehabilitation during this phase will start focusing on return to sport.  Range of movement and strength developed during the previous phases will be maintained and exercises for power and sport-specific agility will be added to the exercise program.  Exercise repetitions and training time will be increased to build endurance.  Plyometrics – like jumping down from a step, jumping onto a box etc – will also be an important part of your rehabilitation during this phase.

At the end of this phase of rehabilitation you should be able to run, change direction and perform jumping activities in different direction without pain, discomfort or apprehension.

Problems we see with the ACL

Getting back to sport too soon: Not completing the rehabilitation process can be detrimental to your ACL. If you load it too much, too soon, the ligament will not be able to withstand the forces being put on it and you could sustain a re-rupture, starting the whole process right from the beginning, only this time with even more scar tissue to begin with. This can become a recurrent problem and may weaken the ligament to such extent that your knee becomes completely unstable.

Generic braces provide a false sense of security and sometimes athletes will return to sport still using a brace. If the muscles around the knee joint are not strong the brace will not prevent re-injury of the knee.

Waiting too long to come for physiotherapy due to the false sense of support the swelling provides. Swelling makes it more difficult to assess and diagnose the knee completely. (See excerpt on swelling in the knee at general knee ligament injuries)

Not attending to your injury apart from physiotherapy: Your physiotherapist will give you a home program you need to follow. We only see you for an hour maybe twice a week. The rest of the time, it is your responsibility to do what is necessary in order for complete healing to occur.

Does an ACL injury need Surgery?

Grade 1 or 2 ACL injuries do not usually need surgery.  Conservative management (bracing and physiotherapy) is generally the best option for these kinds of injuries.  However, surgery may be indicated if the rotational instability (instability with twisting movements) doesn’t improve.  If the medial collateral ligament and medial meniscus were also injured (also known as an “unhappy triad of the knee”) you may also need surgery to repair the other injured structures.

In a grade 3 ligament tear (complete rupture of the ligament) the torn sections cannot re-attach by itself. Surgery is your best option and arthroscopy is the way to go as it is a minimally invasive technique. An ACL can’t be sutured back together, so the surgeon uses a technique called tissue graft.

They may harvest the hamstrings, patella tendon or the adductors to create an artificial tendon attached to the femur, which is like a scaffolding for the ligament to grow on.

This allows the body’s natural form of healing to take place. In the case of a complete ACL rupture, there are usually other structures involved, like the medial meniscus, thus an arthroscopic procedure allows the surgeon to investigate and repair these structures at the same time.

ACL injury

Other medical treatments

  • Knee ranger braces.  Restriction of certain ranges is necessary to protect the ACL to allow it to heal.  This is especially important in the initial phase of healing, but the brace should be removed as the musculature around the knee gets stronger.  The limited ranges should also be adjusted as rehabilitation progresses.
  • Cortisone injections will help decrease swelling and inflammation, which will help with pain relief, but it does not improve the instability caused by the ligament injury.  It is still necessary to see a physiotherapist after cortisone for proper rehabilitation of the knee.
  • Biokineticists can help with rehabilitation in the final stages when there is no pain and you have full range of movement.  It is important not to start rehabilitation with a biokineticist too soon.  If you have pain, stiffness or instability you should see a physiotherapist first.

What else could it be?

  • Knee or patellar dislocations – weakness and laxity of your knee joint.
  • Meniscal injuries – catching during twisting of your knee.
  • Collateral ligaments injury – pain mostly inside or outside of the knee rather than the front (inside or outside ligaments).
  • Medial Collateral Ligament tear (MCL injury) – pain on the inner knee.
  • Knee tendinitis – Patellar tendon pain in the front of your knee.
  • Posterolateral corner of the knee – area of pain more at the back of the knee.
  • Patellar dislocation or fracture – unable to walk and pain with full weight bearing.
  • Femoral, tibial or fibular fractures – deep pain that throbs even when not walking.
  • Bursitis – pain just above or below the kneecap, an irritated and swollen fluid filled sack.
  • Fat pad impingement – pinching of adipose tissue.
  • Other types of knee injuries.

Also known as:

  • Knee sprain.
  • Knee ligament injury.
  • Torn ACL.
  • Anterior knee pain.
  • Twisted knee.
  • ACL sprain.