Anterior Cruciate Ligament (ACL) Injury

Front View of Right KneeLigaments are strong, dense structures made of connective tissue that stabilize a joint. They connect bone to bone across the joint. The knee joint is located where the end of the thigh bone (femur) meets the top of the shin bone (tibia) and four main ligaments connect these two bones:

  • Medial collateral ligament (MCL) runs along the inside of the knee. It prevents the knee from bending in.
  • Lateral collateral ligament (LCL) runs along the outside of the knee. It prevents the knee from bending out.
  • Anterior cruciate ligament (ACL) is in the middle of the knee. It prevents the shin bone from sliding out in front of the thigh bone.
  • Posterior cruciate ligament (PCL) works with the ACL. It prevents the shin bone from sliding backwards under the femur.

The function of the ACL is to provide stability to the knee and minimize stress across the knee joint:

  • It restrains excessive forward movement of the lower leg bone (the tibia) in relation to the thigh bone (the femur).
  • It limits rotational movements of the knee.
  • Hence it holds the knee together during twisting or pivoting movements.

An anterior cruciate ligament injury is the over-stretching or tearing of the anterior cruciate ligament (ACL) in the knee. A tear may be partial or complete. Women are more likely to have an ACL tear than men.

ACL Mechanism of InjuryCause

An ACL injury can occur if a person:

  • Get hit very hard on the side of the knee, such as during a football tackle
  • Overextend the knee joint
  • Quickly stop moving and change direction while running, landing from a jump, or turning

Football, basketball, badminton, netball are common sports linked to ACL tears.

ACL injuries often occur with other injuries. For example, an ACL tear often occurs along with tears to the MCL and the shock-absorbing cartilage in the knee (meniscus).

Most ACL tears occur in the middle of the ligament, or the ligament is pulled off the thigh bone. These injuries form a gap between the torn edges, and do not heal on their own.

Symptoms: Knees become "loose"Signs & Symptoms

Early symptoms:

  • A “popping” sound at the time of injury
  • Knee swelling within 6 hours of injury
  • Pain, especially when trying to put weight on the injured leg
  • Unable to continue with sporting activity

Those who have only a mild injury may notice that the knee feels unstable or seems to “give way” when using it.


An ACL injury is most likely when the patient comes in with the following timeline:

  • Twisting injury to the knee associated with a ‘pop’ or ‘crack’ sound
  • Immediate onset of severe pain and unable to continue with sporting activity (if the person can continue, it is highly unlikely to be an ACL injury)
  • Injured knee becomes swollen within the next 1 to 2 hours
  • Limping over the next 3 to 7 days.
  • Giving way of the knee when walking or jamming/locking of the knee may also be present

An ACL injury can be confirmed by performing a clinical examination of the knee joint. During the physical examination of the affected knee, the doctor is looking for increased laxity in the knee joint. Tests conducted include:

  • Anterior Drawer test – An increased amount of anterior tibial translation compared with the opposite limb or lack of a firm end-point may indicate either a sprain of the anteromedial bundle or complete tear of the ACL. If the tibia pulls forward more than normal, it suggests that the ACL is injured.
  • Lachman’s test – If the ACL is torn, there’ll be an increased forward (upward or anterior) movement of the tibia in relation to the femur (especially when compared to the normal leg) and a soft, mushy endpoint (because the ACL is torn) when this movement ends.
  • Pivot Shift test – the tibia will start forward when the knee is fully straight and then will shift back into the correct position in relation to the femur when the knee is bent past 30 degrees. This is confirmatory of rotatory instability of the injured knee but may be difficult to elicit in a person who is guarding or tensing the knee muscles.

MRI scans of the knee is not usually needed in order to diagnose an ACL tear. It is, however, useful to assess the status of other structures in the knee joint that may be injured as well. These include meniscus tears and tears to other ligaments such as the medial collateral ligament.


The outcome of an ACL injury without surgical intervention varies from patient to patient and depends on the patient’s activity level, degree of injury and instability symptoms.

The prognosis for a partially torn ACL is often favorable, with the recovery and rehabilitation period usually at least three months. However, some patients with partial ACL tears may still have instability symptoms. Close clinical follow-up and a complete course of physical therapy helps identify those patients with unstable knees due to partial ACL tears.

Complete ACL ruptures have a much less favorable outcome. After a complete ACL tear, some patients are unable to participate in cutting or pivoting-type sports, while others have instability during even normal activities, such as walking. There are some rare individuals who can participate in sports without any symptoms of instability (especially those with very strong thigh and hamstring muscles). This depends on the severity of the original knee injury, as well as the physical demands of the patient.

About half of ACL injuries occur in combination with damage to the meniscus, articular cartilage or other ligaments. Secondary damage may occur in patients who have repeated episodes of instability due to ACL injury. With chronic instability, a majority of patients may have meniscus and/or articular cartilage damage down the road after the initial injury.

Non-Surgical Treatment

In non-surgical treatment, progressive physiotherapy and rehabilitation can restore the knee to a condition close to its pre-injury condition. This may be complemented with the use of a hinged knee brace. However, many patients who do not opt to have surgery may suffer from secondary injury to the knee due to repeated instability events.

Surgical treatment is usually recommended in cases of combined injuries (ACL tears accompanied by other injuries in the knee). However, the non-surgery route may be reasonable for certain patients. Non-surgical management of isolated ACL tears may likely be successful or may be the option to go for in patients:

  • With partial tears and no instability symptoms
  • With complete tears and no symptoms of knee instability during low-demand sports, and who are willing to give up high-demand sports
  • Who do light manual labour or live sedentary lifestyles
  • Whose growth plates are still open (eg. children)

Surgical Treatment

ACL tears are not usually repaired using suture to sew it back together, because ACLs repaired in this manner have generally been found to fail over time. Thus, the affected ACL is generally replaced by a substitute graft made of tendon – autografts (other types of tendon taken from the patient himself) or allografts (taken from a cadaver).

Patients treated with surgical reconstruction of the ACL have high long-term success rates. Recurrent instability and graft failure, though do happen, are not common.

The aim of an ACL reconstruction surgery is to prevent instability and restore the function of the torn ligament, thus stabilising the knee. This allows the patient to go back doing sports. There are however certain factors the patient must consider when deciding whether to opt for ACL surgery or not.

Patient Considerations

Adult patients who are active in sports or have jobs that require pivoting, turning or hard-cutting as well as heavy manual labour are encouraged to consider surgical treatment. This includes older patients who were previously excluded from consideration for ACL surgery. It is activity and not age that determines if surgical intervention is considered.

For young children or adolescents with ACL tears, early ACL reconstruction presents a possible risk of growth plate injury, leading to bone growth problems. The surgeon may delay the surgery until the child is closer to skeletal maturity or modify the ACL surgery technique to decrease the risk of growth plate injury.

A patient with an ACL tear and significant functional instability has a high risk of developing secondary knee damage and should therefore consider ACL reconstruction sugery.

It is common to see ACL injuries present together with damage to the menisci, articular cartilage, collateral ligaments, joint capsule, or a combination of the above. For example, often seen in football players and skiers are cases that consists of injuries to the ACL, the MCL, and the medial meniscus.

In cases of combined injuries, surgical treatment may be warranted and generally produces better outcomes. Many cases of meniscus tears may be repaired and heal better if the repair is performed in combination with the ACL reconstruction.

Surgery Types

Patellar Tendon Autograft
The middle third of the patellar tendon of the patient, along with a bone plug from the shin and the kneecap is used in a patellar tendon autograft.

The disadvantages of the patellar tendon autograft may be:

  • Post-surgery pain behind the kneecap
  • Pain when kneeling
  • Slightly increased risk of post-surgery stiffness

Hamstring Tendon Autograft
The semitendinosus hamstring tendon on the inner side of the knee is used in creating the hamstring tendon autograft for ACL reconstruction. Some surgeons use an additional tendon, the gracilis, which is attached below the knee in the same area. This creates a two- or four-strand tendon graft. There can be fewer problems in harvesting of the graft compared to the patellar tendon autograft including:

  • Fewer problems with anterior (front) knee pain or kneecap pain after surgery
  • Less post-surgery stiffness problems
  • Smaller incision
  • Faster recovery

The graft function may be limited by the strength and type of fixation in the bone tunnels, as the graft does not have bone plugs.

Quadriceps Tendon Autograft
The quadriceps tendon autograft is often used for patients who have experienced failed ACL reconstruction before. The middle third of the patient’s quadriceps tendon and a bone plug from the upper end of the knee cap are used. This yields a larger graft for taller and heavier patients.

Potential pitfalls here are:

  • Since there is a bone plug on one side only, the fixation is not as solid as for the patellar tendon graft
  • There is a high possibility of post-surgery anterior (front) knee pain.
  • Patients may find the incision not aesthetically pleasing.

Allograft ACL ReconstructionAllografts
Allografts are grafts taken from cadavers. These grafts are also used for patients who have failed ACL reconstruction before and to repair or reconstruct more than one knee ligament. Advantages of using allograft tissue include elimination of pain caused by obtaining the graft from the patient, decreased surgery time and smaller incisions.

However, allografts are associated with a risk of infection. Also, higher costs involved since donor grafts are imported from AATB accredited tissue banks in USA.

Surgical Technique

An ACL reconstruction surgery is performed via an Arthroscopy (also called arthroscopic surgery) – a minimally-invasive surgical procedure in which an examination and treatment of the damage is performed using an arthroscope, an endoscope that is inserted into the joint through a small incision.

Surgical TechniqueDouble Bundle ACL Recontruction
The natural ACL is made up of 2 anatomically and functionally distinct bundles. They are named the anteromedial bundle (AMB) and the posterolateral bundle (PLB) based on the position of the fibres in the central part of the knee joint.

ACL reconstruction surgeons have traditionally reconstructed the larger and functionally more important bundle and that was the anteromedial bundle (AMB). The understanding of the role of the posterolateral bundle (PLB) was lacking and the ability to reconstruct this bundle was not well accepted until recent years.

Through cadaveric knee anatomical dissection studies as well as biomechanical studies, sports and knee surgeons now have a better understanding of the importance of the once neglected posterolateral bundle (PLB) of the ACL.

The double-bundle technique uses two separate grafts to replicate both the AM and the PL bundles. Because the AM bundle makes a greater contribution to anteroposterior knee stability, while the PL bundle makes a greater contribution to rotational stability, a double-bundle technique may be better able to restore normal knee kinematics.

The reconstructed knee feels more stable when examined and the rotatory control or knee pivoting stability is superior to single bundle reconstructions.

Post-Surgery Recovery

Physical therapy is a crucial part of successful ACL surgery, with exercises beginning immediately after the surgery. Much of the success of ACL reconstructive surgery depends on the patient’s dedication to rigorous physical therapy.

The surgeon may dictate the use of a post-operative brace. Weight-bearing status (use of crutches to keep some or all of the patient’s weight off of the surgical leg) is also determined by physician preference, as well as other injuries addressed at the time of surgery.

The patient may return to sports when there is no longer pain or swelling, when full knee range of motion has been achieved, and when muscle strength, endurance and functional use of the leg have been fully restored.

The patient’s sense of balance and control of the leg must also be restored through exercises designed to improve neuromuscular control. This usually takes four to six months. The use of a functional brace when returning to sports is ideally not needed after a successful ACL reconstruction, but some patients may feel a greater sense of security by wearing one.

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  • 16 years of experience in surgical and non-surgical procedures in sports injuries as a Sports Orthopaedic Specialist in Singapore.
  • Received multiple awards and accolades in the Orthopaedic field.
  • He is a USA Sports Medicine Fellowship trained Orthopaedic Surgeon and is a member of the Arthroscopy Association of North America as well as the International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine.
  • Published many acclaimed articles on Sports Medicine. If you are experiencing one or more of the symptoms mentioned above, it is important to get a prompt and accurate diagnosis to determine the severity of your injury and get proper treatment immediately.


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