Anterior Cruciate Ligament (ACL) Injury



Injuries to the ACL are relatively common knee injuries among athletes.[1] They occur most frequently in those who play sports involving pivoting (e.g. football, basketball, netball, soccer, European team handball, gymnastics, downhill skiing). They can range from mild (such as small tears/sprain) to severe (when the ligament is completely torn). Both contact and non-contact injuries can occur, although non-contact tears and ruptures are most common. It appears that females tend to have a higher incidence rate of ACL injury than males, that being between 2.4 and 9.7 times higher in female athletes competing in similar activities.[2][3][4][5][6]

Clinically Relevant Anatomy

Please see these pages for relevant anatomy:

Mechanisms of Injury / Pathological Process

Three major types of ACL injuries are distinguished:[7]

  • Direct Contact
  • Indirect Contact
  • Non-Contact

Most common are the non-contact injuries, caused by forces generated within the athlete’s body while most other sport injuries involve a transfer of energy from an external source.[8] Approximately 75% of ruptures are sustained with minimal or no contact at the time of injury.[9] A cut-and-plant movement is the typical mechanism that causes the ACL to tear, that being a sudden change in direction or speed with the foot firmly planted. Rapid deceleration moments, including those that also involve planting the affected leg to cut and change direction, have also been linked to ACL injuries, as well as landing from a jump, pivoting, twisting, and direct impact to the front of the tibia.[9]

Women are three times more prone to have the ACL injured then men, and is thought to be due to the following reasons:[10]

  • Smaller size and different shape of the intercondylar notch
  • Wider pelvis and greater Q angle
  • Greater ligament laxity
  • Shoe surface interface
  • Neuromuscular factors

A wider pelvis requires the femur to have a greater angle towards the knee, lesser muscle strength provides less knee support, and hormonal variations may alter the laxity of ligaments.[11][12]

Biomechanics of Injury

As 60-80% of ACL injuries occur in non-contact situations, it seems likely that appropriate prevention efforts are warranted. Cutting or sidestep maneuvers are associated with dramatic increases in the varus-valgus and internal rotation moments. The ACL is placed at greater risk with both varus and internal rotation moments. The typical ACL injury occurs with the knee externally rotated and in 10-30° of flexion when the knee is placed in a valgus position as the athlete takes off from the planted foot and internally rotates with the aim of suddenly changing direction (as shown by the figure below).[13][14] The ground reaction force falls medial to the knee joint during a cutting maneuver and this added force may tax an already tensioned ACL and lead to failure. Similarly, in landing injuries, the knee is close to full extension. High-speed activities such as cutting or landing maneuvers require eccentric muscle action of the quadriceps to resist further flexion. It may be hypothesized that vigorous eccentric quadriceps muscle action may play a role in disruption of the ACL. Although this normally would be insufficient to tear the ACL, it may be that the addition of valgus knee position and/or rotation could trigger an ACL rupture.[15]

Non-Contact ACL Mechanism
Non-Contact ACL Mechanism

The athlete could be off balance, held by an opponent, avoiding collision with an opponent, or have adopted an unusually wide foot position. These perturbations contribute to this injury by causing the athlete to plant the foot so as to promote unfavorable lower extremity alignment; this may be compounded by inadequate muscle protection and poor neuromuscular control.[17] Fatigue and loss of concentration may also be a factor. What has become recognized is that unfavourable body movements in landing and pivoting can occur, leading to what has become known as the 'Functional Valgus' or 'dynamic valgus' knee, a pattern of knee collapse where the knee falls medial to the hip and foot. This has been called by Ireland (1996) as the 'Position of No Return', or perhaps it should be termed the 'injury prone position' since there is no proof that one cannot recover from this position.[18] Intervention programs aimed to reduce the risk of ACL injury are based on training safer neuromuscular patterns in simple maneuvers such as cutting and jump landing activities.[19]

Position of No Return
Position of No Return

Grades of Injury

An ACL injury is classified as a grade I, II, or III sprain.[20]

  • Grade I Sprain:
    • The fibres of the ligament are stretched but there is no tear.
    • There is a little tenderness and swelling.
    • The knee does not feel unstable or give out during activity.
    • No increased laxity and there is a firm end feel.
  • Grade II Sprain:
    • The fibres of the ligament are partially torn or incomplete tear with haemorrhage.
    • There is a little tenderness and moderate swelling with some loss of function.
    • The joint may feel unstable or give out during activity.
    • Increased anterior translation yet there is still a firm end point.
    • Painful and pain increase with Lachman's and anterior drawer stress tests.
  • Grade III Sprain:
    • The fibres of the ligament are completely torn (ruptured); the ligament itself has torn completely into two parts.
    • There is tenderness but not a lot of pain, especially when compared to the seriousness of the injury.
    • There may be a little swelling or a lot of swelling.
    • The ligament cannot control knee movements. The knee feels unstable or gives out at certain times.
    • There is also rotational instability as indicated by a positive pivot shift test.
    • No end point is evident.
    • Haemarthrosis occurs within 1-2 hours.

An ACL Avulsion occurs when the ACL is torn away from either the femur or the tibia. This type of injury is more common in children than adults. The term anterior cruciate deficient knee refers to a grade 3 sprain in which there is a complete tear of the ACL. It is generally accepted that a torn ACL will not heal.[21]

Characteristics/Clinical Presentation[1]

  • ­Occurs after either a cutting maneuver or one leg standing, landing or jumping
  • There may be an audible pop or crack at the time of injury
  • ­A feeling of initial instability which may be masked later by extensive swelling
  • Episodes of "giving way" especially on pivoting or twisting motions. Patient has a "trick knee" and a predictable instability
  • ­A torn ACL is extremely painful, particularly immediately after sustaining the injury
  • ­Swelling of the knee, usually immediate and extensive, but can be minimal or delayed
  • ­Restricted movement, especially an inability to fully extend
  • ­Possible widespread mild tenderness
  • ­Tenderness at the medial side of the joint which may indicate cartilage injury

Associated Injuries

Injuries to ACL rarely occur in isolation. The presence and extent of other injuries may affect the way in which the ACL injury is managed.[22]

Meniscal Lesions

Over 50% of all ACL Ruptures have associated Meniscal injuries. If seen in combination with a Medial Meniscus Tear and MCL Injury, it is called O’Donohue’s Triad which has 3 components:[1]

Medial Collateral Ligament Injuries

Associated injury to the MCL (Grade I-III) poses a particular problem due to tendency to develop stiffness after this injury. Most orthopaedic surgeons will first treat MCL injury in a limited-motion knee brace for a period of six weeks, during which time the athlete would undertake a comprehensive rehabilitation program. Only then would ACL reconstruction be performed or be treated.[23]

Bone Contusions and Microfractures

Subcortical trabecular bone injury (bone bruise) may occur due to the pressures exerted on the knee in traumatic injury and are especially associated with ACL rupture. Associated injuries of the menisci and the MCL tend to increase the progression of bone contusion.[24] The focal signal abnormalities in subchondral bone marrow seen on MRI (undetectable on rdiographs) are thought to represent microtrabecular fractures, haemorrhage and edema without disruption of adjacent cortices or articular cartilage.[25] Bone contusions may occur in isolation to ligamentous or meniscal injury.[26]

Occult bony lesions have been reported in 84-98% of the patients with ACL rupture.[24][27][28] The majority of these have lesions of the lateral compartment,[29] involving either the lateral femoral condyle, the lateral tibial plateau, or both. The boney bruising itself is unlikely to cause pain or reduced function.[30] Although the majority of bony lesions resolve, permanent alterations may remain. There is confusion in the literature as to how long these bony lesions remain, but it has been reported that they can persist on MRI for years.[31] Rehabilitation and the long-term prognosis may be affected in those patients with extensive bony and associated articular cartilage injuries. In the case of severe bone bruising it has been recommended to delay return to full weight-bearing status to prevent further collapse of subchondral bone and further aggravation of articular cartilage injury.[31]

Chondral Injury

Hollis et al suggested that all patients following traumatic ACL disruption sustained a chondral injury at the time of initial impact with subsequent longitudinal chondral degradation in compartments unaffected by the initial bone contusion, a process that is accelerated at 5 to 7 years’ follow-up.[32]

Tibial Plateau Fractures

A Tibial Plateau Fracture is a bone fracture or break in the continuity of the bone occurring in the proximal tibia affecting the knee joint, stability, and motion. The tibial plateau is a critical weight-bearing area located on the upper tibia and is composed of two slightly concave condyles (medial and lateral condyles) separated by an intercondylar eminence and the sloping areas in front and behind it. It can be divided into three regions: the medial tibial plateau (the part of the tibial plateau nearest the center of the body and contains the medial condyle), the lateral plateau (the part of the tibial plateau that is farthest away from the center of the body and contains the lateral condyle) and the central tibial plateau (located between the medial and lateral pleateaus and contains intercondylar eminence).

These fractures are also caused by varus or valgus forces combined with axial loading on knee and mostly occur with ACL injuries, rarely alone. The fracture of lateral tibial plateau is also called a Segond fracturewhich most commonly occurs with an ACL injury.

Posterolateral Corner Injury

The stability of the posterolateral corner of the knee is provided by capsular and noncapsular structures that function as static and dynamic stabilizers[33] including the lateral collateral ligament (LCL), the popliteus muscle and tendon including its fibular insertion (popliteofibular ligament), and the lateral and posterolateral capsule. Injuries to this region that result in posterolateral rotatory instability are usually associated with concurrent ligamentous injuries elsewhere in the knee.[34][35][36][37] High-grade posterolateral corner injuries are usually associated with rupture of one or both cruciate ligaments. Importantly, failure to address instability of the posterolateral corner structures increases the forces at the ACL and PCL graft sites, and may ultimately predispose to failure of the cruciate reconstruction.[38][39][40] (See also: Knee Rotary Instability)

Popliteal Cyst

Diagnostic Procedures

An exact diagnosis can be made by the following procedures:

1. PHYSICAL EXAMINATION which includes the following tests:


Radiographs of the knee should be performed when an ACL tear is suspected, including AP (anterior to posterior) view, lateral view, and patellofemoral projection. The standing AP weight-bearing view provides a way of evaluating the joint space between the femur and tibia.[41] It also allows for measurement of the notch width index which provides important predictive values for ACL tears.[42] The patellar tendon and height are measured on lateral radiograph. A tunnel view may also be helpful. The Merchant's radiograph view not only shows the joint space between the femur and patella but also helps to determine whether the patient has patellofemoral malalignment.[43] The presence of the following factors should be noted clearly during review of an x-ray:

  • Notch width index
  • Osteochondral fracture
  • Segond fracture
  • Bone bruise

The Notch width index is the ratio of the width of the intercondylar notch to the width of the distal femur at the level of the popliteal groove measured on a tunnel view roentgenogram of the knee. The normal intercondylar notch ratio is 0.231 ± 0.044. The intercondylar notch width index for men is larger than that for women. It was found that athletes with non-contact ACL injuries had a notch width index that was at least 1 standard deviation below the average, meaning that a person with an ACL injury is more likely to have a small notch width index compared to normal. It is measured with the help of a ruler placed parallel to joint line. The narrowest portion of the notch at the level of ruler is measured.[44]  In more chronic ACL injuries, there may be intercondylar eminence spurring or hypertrophy, or patellar facet osteophyte formation.

Notch Width Index
Notch Width Index
Notch Width Index Measurement
Notch Width Index Measurement

This is also one of the reasons why women are more prone to ACL injuries compared to men. It has also been seen that the value of inner angle of the lateral condyle of femur was significantly higher in women athletes with ACL tear compared to those without. Value of width of intercondylar notch was statistically smaller in athletes with ACL tear, compared to those without. Also it was seen that the inner angle of lateral femoral condyle is a better predictive factor for ACL tears in young female handball players compared to intercondylar notch width.[45]

In more chronic ACL injuries, there may be interchondral eminence spurring or hypertrophy, patellar facet osteophyte formation, or joint space narrowing with marginal osteophytes. It is particularly important in skeletally immature patients to have plain radiographic assessment. This is because there is frequently a ligamentous avulsion in this age group.

ABone bruise is usually present in conjunction with an ACL injury in more than in 80% of cases.[46] The most common site is over the lateral femoral condyle. The bone bruise is most likely caused by impaction between the posterior aspect of the lateral tibial plateau and the lateral femoral condyle during displacement of the joint at the time of the injury. The presence of bone bruise indicates impaction trauma to the articular cartilage.[47] Patients with bone bruises are more prone to develop osteoarthritis later.[48] Bone bruise can be seen most prominently in MRIs.

3. MRI

MRI has the advantage of providing a clearly defined image of all the anatomic structures of the knee. A normal ACL is seen as a well-defined band of low signal intensity on sagittal image through the intercondylar notch. With an acute injury to the ACL, the continuity of the ligament fibers appears disrupted and the ligament substance is ill defined, with a mixed signal intensity representing local edema and haemorrhage.[49]

MRI can diagnose ACL injuries with an accuracy of 95% or better.[50] MRI will also reveal any associated meniscal tears, chondral injuries, or bone bruises.


An adjunct to the clinical special tests in assessing anterior translation is the use of instrumented laxity testing. The most commonly cited arthrometer is the KT1000 (Medmetric, San Diego, California). The arthrometer provides an objective measurement of the anterior translation of the tibia that supplements the Lachman test in ACL injury. It can be particularly useful in the examination of acutely injured patients in whom pain and guarding may preclude evaluation. In such patients the Lachman and other tests can be difficult to perform accurately. The arthrometeric results can be used as a diagnostic tool to assess ACL integrity or as part of the follow up examination after ACL reconstruction.[51] The results of the KT1000 and its sibling. the KT2000 have been noted to be both reliable and accurate.[52]

Differential Diagnosis


The same characteristics for an ACL injury can be found at knee dislocations and meniscal injuries and collateral ligaments injury or posterolateral corner of the knee. Other problems that have to be considered are patellar dislocation or fracture, and a femoral, tibial or fibular fracture.

The differential diagnosis of an acute hemarthrosis of the knee due to ACL in addition to a major ligamentous tear would include meniscal tear or patellar dislocation or osteochondral fracture.

Differentiation can mostly be made based on a thorough examination with particular attention for the mechanism at the time of injury. An additional MRI scan can visualize the injury.

Can you spot the ACL and associated injuries in the MRI below?


The examination of ACL injury can be done in two ways:

  1. Physical/Clinical examination
  2. Examination under anesthesia and arthroscopy

Physical/Clinical Examination:

An organized, systematic physical examination is imperative when examining any joint. Immediately after the acute injury, the physical examination may be very limited due to apprehension and guarding by the patient. While inspecting, the examiner should look for the following:[54]

  • Overall alignment of the knee.
  • Severe distortion of the normal alignment may represent a fracture of the distal femur or proximal tibia or indicate knee dislocation.
  • Any gross effusion, which most commonly be present within a few hours after an ACL injury. Absence of an effusion does not mean that an ACL injury has not occurred; in fact, with more severe injuries that include the surrounding capsule and soft tissues, the hemarthrosis may be able to escape from the knee, and the degree of swelling may paradoxically be diminished. In addition, the presence of swelling and effusion does not guarantee that an ACL injury has occurred. According to Noyes et al, in the absence of bony trauma, an immediate effusion is believed to have a 72% correlation with an ACL injury of some degree.
  • Bony abnormality may suggest an associated fracture of the tibial plateau.
  • Palpation follows inspection and should begin with the uninvolved extremity. Palpation confirms the presence and degree of effusion and bony injury. Subtle effusions missed during inspection should be picked up by the careful manual examination. Palpation of joint lines and collateral ligaments can rule out a possible associated meniscus tear or sprained ligaments.
  • Periarticular tenderness should also be examined.
  • Assessing the patient’s range of motion (ROM) should be carried out to look for lack of complete extension, secondary to a possible bucket-handle meniscus tear or associated loose fragment.
  • Laxity testing should be done either with the special test or with the help of arthrometer.

Examination under anaesthesia and arthroscopy:

Arthroscopy combined with examination under anesthesia is an accurate way to diagnose a torn ACL. It may be indicated in the case whereby the diagnosis is suspected from the patient's history but is not evident on clinical examination. The main value of using arthroscopy on the basis of examination is to diagnose associate joint pathologic conditions such as meniscal tears or chondral fractures.[55][56]

Please see this page for additional information on assessment of the knee: Knee Examination

Medical Management

Please see Anterior Cruciate Ligament (ACL) Reconstruction

Please see Anterior Cruciate Ligament (ACL) Rehabilitation

Physiotherapy Management

Please see Anterior Cruciate Ligament (ACL) Rehabilitation



Given the importance of neuromuscular factors and the etiology of ACL injuries, numerous programs have aimed to improve neuromuscular control during standing, cutting, jumping, and landing.[58] The components of neuromuscular training are:

  • Balance training
  • Landing with increased flexion at the knee and hip
  • Controlling body motions, especially in deceleration and pivoting maneuvers
  • Some form of feedback to the athlete during training of these activities
  • Also following the PEP plan (as per the embedded video)

Clinical Bottom Line

In order to provide the injured athlete with the best care, physiotherapists should have elaborate knowledge of anatomy and functioning of the ACL. The keystone to proper care of an ACL injury is to start from the correct diagnosis within the first hour of injury before the development of significant hemarthrosis. This should also include the detection and diagnosis of associated injuries.[59] Treatment for the injury and the return to play for an athlete depends completely upon the grade and associated injuries.


Anterior Cruciate Ligament Injury

This presentation, created by Terdsak Rojsurakitti, Doctor at Managed Care, discusses anatomy, mechanism of injury, surgical options and rehabilitation of ACL tears. Anterior Cruciate Ligament Injury/ View the presentation

Recent Related Research (from Pubmed)


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