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The anterior cruciate ligament (ACL) is the primary restraint to anterior tibial translation. It is a collagenous structure and has two major bundles that function as a unit to provide isometric knee stabilization.


The ACL contributes to knee stability, especially with respect to dynamic cut/pivot and deceleration movements. Most ACL injuries are traumatic and noncontact in nature, with activities that involve cutting and pivoting movements, sudden deceleration with change of direction or landing from a jump with the knee close to full extension and valgus, accounting for approximately 70% of acute ACL injuries.

Epidemiology including risk factors and primary prevention

In the United States, there are approximately 250,000 ACL injuries each year. It has been reported that high school female athletes have an overall higher rate of injury per hours of exposure when compared to male counterparts in sports such as soccer and basketball.22 For men, football and lacrosse have been described as high risk.13 Both intrinsic and extrinsic risk factors have been identified for ACL injury. Intrinsic factors include sex, hormonal status, genetics, neuromuscular deficits, cognitive, anatomic, and history of previous injury.1,22 Extrinsic factors include level and type of activity, type of playing surface, environmental conditions, and equipment used. Neuromuscular control patterns that have been identified as contributing to ACL injury include increased dynamic knee valgus, decreased hip and knee flexion, increased internal rotation of the hip coupled with increased external rotation of the tibia, and increased quadriceps muscle activation. Neuromuscular control and biomechanical movement patterns have been shown to be modifiable risk factors amenable to specific prevention programs.1,22


The ACL is a collagenous structure that originates at the posteromedial aspect of the lateral femoral condyle and inserts anteriorly to the intercondylar eminence of the tibial articular surface. Although it functions as a single ligament, it contains two main bundles: an anteromedial bundle, which is taut in flexion, and a posterolateral bundle, which is taut in extension. Forces on the ACL are highest in the final 30 degrees of knee extension and with knee hyperextension.

The ACL also provides rotary control and limits internal tibial rotation. It functions as a secondary restraint to valgus and varus stress throughout the range of motion.

The main blood supply of the ACL arises from the middle geniculate artery, and innervation is via a branch of the tibial nerve, the posterior articular nerve. In addition to static stability, the ACL contributes to proprioceptive feedback, which is thought to enhance dynamic control. Studies have shown persistent proprioceptive deficits even in a knee that has undergone ACL reconstruction.2

Disease progression including natural history, disease phases or stages, disease trajectory (clinical features and presentation over time)

Acute ACL injury is associated with a high incidence of meniscal injuries with higher rates of lateral compared to medial tears, whereas chronic ACL injury is associated with articular cartilage injury and higher rates of medial meniscal tears.1 An ACL deficient knee is susceptible to instability events (subluxations) if the person returns to aggressive high-risk activities, including cut/pivot, rapid deceleration, and change of direction activities. Repeated episodes of instability are thought to lead to meniscal and articular cartilage injury and to compromise in ability to return to high demand athletic activities.

A smaller percentage of ACL-injured individuals may experience “functional” instability, with give-way episodes occurring during activities of daily living. A large segment of persons with ACL injuries can function well and limit or prevent subluxation events by avoiding high risk activities.1

The development of osteoarthritis in isolated ACL injuries is reported to be lower than in combined ACL and meniscal injuries (0%-13% vs 21%-48%). ACL reconstruction is not associated with a significant reduction in the prevalence of knee osteoarthritis, and a recent study found that a younger age and a return to a high level of activity are factors associated with secondary ACL injury.3 In recent studies, nearly 25% of young athletic patients who sustain an ACL injury and return to high-risk sport will sustain another ACL injury at a later time.18,19

Specific secondary or associated conditions and complications

Meniscal injury is commonly associated with ACL injury and is a risk factor for the development of osteoarthritis.3

Quadriceps weakness, which may be due to muscle inhibition or impaired neural activation, has been documented after ACL injury in both the injured and uninjured legs and is also thought to be a contributing factor to the development of osteoarthritis.12,17 Loss of motion, in particular knee extension, has also been associated with osteoarthritis. Proprioceptive deficits which persist after ACL injury may contribute to increased risk of knee instability and subsequent chondral injury, though there is limited evidence that proprioceptive deficits as measured by commonly used tests adversely affect function in ACL deficient and ACL reconstructed individuals.8

Essentials of Assessment


An accurate history is an essential component of the diagnosis of ACL injury. The patient will usually relate a history of feeling a “pop” or “give-way” of the knee. Noncontact mechanisms include an aggressive cut, pivot, deceleration movement, landing awkwardly from a jump, or “knee hyperextension”. Valgus collapse resulting from a medially directed contact hit to the knee is another mechanism of injury. The majority of ACL injuries during recreational skiing result from a mechanism of internal rotation of the tibia with the knee flexed beyond 90 degrees, which has been termed the “phantom foot” mechanism. The rigid ski boot serves to amplify anterior directed force in falls that involve sitting back or attempted recovery from a fall with aggressive quadriceps contraction. The patient usually experiences rapid swelling of the knee, and inability to return to sport or activity due to pain, swelling, and a feeling of instability.

Physical examination

The diagnosis of ACL injury can be assessed by performing the Lachman test and the Pivot shift test. The Lachman’s test challenges the ACL’s ability to control anterior tibial translation in 20-30 degrees of knee flexion with the femur stabilized.16 This test has high sensitivity (87%) and specificity (97%) for ACL tear.17 Increased tibial excursion without an end point indicates ACL disruption. Hamstring relaxation is key to accurately performing the Lachman test, as the hamstrings are antagonists of anterior tibial translation and muscle guarding with co-contraction can contribute to making this test less sensitive. The Pivot shift test is a dynamic test that produces subluxation and reduction of the tibial plateau when applying rotation and valgus pressure to the knee. It has been reported to have a sensitivity of 49% and specificity of 98%.17 The anterior drawer test is less sensitive in detecting acute ACL injuries. To rule out combined injuries, patellar apprehension test (for patellar instability), McMurray’s test (for meniscal injury), and valgus and varus testing (for collateral ligament instability) should be included in the examination.

Functional assessment

After acute injury, individuals will likely require assisted ambulation with crutches until pain with weight bearing resolves and gait mechanics normalize. Abnormal gait patterns after crutches are discontinued include hip retraction and reduced hip and knee flexion of the affected leg which affect limb advancement.

Following injury or surgical reconstruction, a functional evaluation of the patient may reveal difficulty with single leg control, valgus collapse when attempting a single leg squat or stepping down from a bench, and knee extension and valgus collapse upon landing from a jump.16,24 These biomechanical abnormalities may be found both in the injured and non-injured limbs.


Knee radiographs are initially obtained to assess osseous status and to detect evidence of other injuries. Radiographs can also detect associated findings, such as lateral tibial condyle avulsion (Segond) fracture or avulsions of the tibial spine. Point-of-care ultrasound evaluation can be used as a screening tool when physical exam assessment is uncertain and to evaluate soft tissue injuries. MRI is the gold standard imaging study used to confirm ACL injury. It has been reported to have a very high sensitivity (97%) and specificity (100%) for ACL injury.17 MRI can also be used to evaluate for associated meniscal, ligament, and articular cartilage injury.

Supplemental assessment tools

Ligament arthrometry (KT-1000, KT-2000) can be used to objectively measure and compare anterior tibial translation. Functional testing (vertical jump, triple hop, triple crossover hop, side hop, running T test and isokinetic dynamometer)24,20 can be used to assess dynamic knee stability and is also used to assess return to play readiness after surgical or non-surgical treatment. However, despite years of research, there is no true single functional test that will objectively determine when an athlete is ready to return to play. For this reason, it is recommended to perform at least 4-5 functional testing maneuvers to challenge knee function when assessing return to sports.20

Early predictions of outcomes

ACL injuries associated with “functional” episodes of instability during activities of daily living and those associated with meniscal tears, especially those that are repairable, may benefit from consideration of earlier surgical intervention. Achievement of full knee extension and symmetric quadriceps strength prior to surgical intervention have been shown to correlate with good clinical results two years following ACL reconstruction.16

A positive pivot-shift test is closely related to patient reported instability during dynamic cutting, pivoting, and deceleration movements, and associated with failure to return to pre-injury sport level as well as higher incidence of osteoarthritis. Kinetic analysis using a dynamic platform can be an objective and useful tool to measure the rotational stability of the ACL by studying the lack of stability in vivo in ACL-deficient knees.21


ACL injuries appear to occur more commonly when playing American football on artificial turf compared to natural turf and when playing basketball or handball on synthetic rubber surfaces when compared to wooden floors.1 Also, footwear with longer and higher number of cleats provide higher torsional resistance with the ground and is associated with injury. In addition, they are common in skiing due to the long lever arm of the ski and the rigid ski boot which transmit significant rotational and translational forces to the knee.

Social role and social support system

ACL injury can be devastating to the athlete as it may result in the loss of ability to return to sports or perform at the same level when compared to pre- injury. Significant changes in emotional state have been measured after injury, including grief and depression.4 A support structure that consists of family, friends, and the medical team is important for the successful outcome of both surgical and non-surgical treatments. Psychologic factors such as self-confidence, optimism, and self-motivation have been shown to be predictive of positive outcomes.4

Professional issues

Quick and accurate diagnosis of ACL and associated secondary injuries may be clinically suspected but can be confirmed by MRI. Physicians must advocate for their patients to obtain proper diagnostic testing and recommend appropriate surgical vs. non-surgical management. Despite great surgical advances, this injury can still significantly affect an athlete’s career and result in long term consequences such as osteoarthritis, thus proper counseling is necessary.

Rehabilitation Management and Treatments

Available or current treatment guidelines

The goal of treatment of an ACL injury is to prevent episodes of recurrent knee instability and to protect the joint from further trauma. This can be accomplished in two ways: non-surgical treatment which includes activity modification, lower extremity strength and stability training, and appropriate bracing,7 or surgical treatment which includes reconstruction or repair of the ligament.7,11 In the early stages of management, avoidance of high-risk activities is important to prevent recurrent episodes of instability and further damage to other knee structures such as the meniscus and articular cartilage.23 There is scientific evidence of ACL healing, documented using MRI, with rehabilitation alone in one third of ACL tears with good functional outcomes at 2 years and no meaningful difference at 5 years when compared to ACL reconstruction.6

At different disease stages

Initial treatment for an ACL injury aims to reduce inflammation, pain, and swelling as well as to improve strength, neuromuscular control, and gait. Regaining normal joint movement (especially full extension) is critical to a successful outcome.16,22,23

The decision of whether to pursue surgery plus rehabilitation or rehabilitation and bracing is based upon several factors, including the extent of associated damage to the knee and the patient’s willingness to modify his or her activities. Young and competitive athletes who wish to return to activities involving aggressive jumping, cut/pivot, or deceleration movements and patients with meniscal tears amenable to repair usually pursue surgical reconstruction. Individuals not involved in “at risk” movement patterns can typically maintain knee stability with rehabilitation alone.

ACL rehabilitation protocols consist of 3 phases (acute, recovery, functional) followed by a structured return to play (RTP) phase.2,16,22,23,24 In patients who require ACL reconstruction, a pre-surgical phase is added. Progression of these programs should be based on achievement of specific goals and not on time elapsed from injury or surgery.

Rehabilitation of individuals who are treated without surgery is started with modalities to reduce pain and swelling and integrate exercises to achieve full motion and normal strength, followed by progression to functional exercises that include dynamic strengthening in activity-specific ranges of motion as well as neuromuscular training programs. Return to activity should be based on specific criteria including no symptoms at rest or with activity, normal strength and range of motion, and performance on functional tests.22,23,24 In specific populations such as active adults, delaying ligament reconstruction for 6 months in those patients that develop symptoms with activity or re-injure their knee has been found to have similar results at 5 years following surgery when compared to early rehabilitation plus surgical reconstruction.7

Pre-operative rehabilitation is recommended prior to surgical intervention with goals of reducing knee swelling and achieving full motion, in particular knee extension and normal strength.

Post operative rehabilitation programs should focus on early weight bearing, reducing pain and swelling, and improving range of motion and quadriceps activation in the acute phase. Modalities recommended in this phase include cryotherapy (to reduce pain and swelling) and neuromuscular electrical stimulation (to improve quadricep muscle activation)14.The recovery phase should emphasize strengthening programs with closed and open kinetic chain exercises, neuromuscular control, and balance/proprioceptive exercises. The functional phase integrates advanced strengthening exercises and sports specific training.23 Return-to-play decisions (RTP) are based on physical findings on examination, physical readiness (assessed by isokinetic testing and hop tests), as well as psychological readiness.5

Coordination of care

The treatment of ACL injury is multidisciplinary. A coordinated team of physiatrists, sports medicine specialists, orthopedic surgeons, physical therapists, athletic trainers, sports psychologists, and exercise physiologists is essential to ensure optimal outcomes.

Patient & family education

Education regarding surgical and non-surgical treatment options for ACL injury is essential for the patient to make a well-informed decision. The patient should be aware that RTP to pivoting sports after an ACL reconstruction requires at least 9 months of rehabilitation, passing specific criteria, and continuation of neuromuscular training after return to sport.9 Education about modifiable factors, such as neuromuscular control and landing biomechanics, are critical to decrease the risk of recurrence.5 Sports psychology is often useful to provide education regarding fear of activity or re-injury and address appropriate coping strategies. The prolonged      course of rehabilitation after surgical or non-surgical treatment requires motivation and dedication. Pre-operative education of both patient and family can contribute to improved post-operative compliance.

Emerging/unique interventions

Neuromuscular training is an essential component of the ACL rehabilitation programs to correct biomechanical deficits, optimize performance, and prevent recurrence of injury. Suboptimal neuromuscular patterns contribute to movement flaws which can increase the risk of ACL injury, especially in female athletes, and correction of these flaws is associated with a reduced incidence of ACL injury. Most individuals return to sport activities 9-12 months after ACL reconstruction. In recent years there has been a shift towards a slower return-to-play to ensure graft maturity, neuromuscular control, appropriate strength and stability, and psychologic readiness. The criteria for return to sports after ACL injury involves many different parameters, including both objective and functional performance measures in addition to psychological readiness.2,5,16,22,23,24 Psychological readiness, as measured by the ACL return to sports after injury scale (ACL-RSI), has been found to be a positive predictive factor in returning to sport competition.

Cutting Edge/Emerging and Unique Concepts and Practice

Two and three -dimensional movement analysis has been shown to be important in identification of movement flaws which can predispose to initial ACL injury or graft re-tear, and these movement analyses have become essential components of ACL rehabilitation and prevention programs.23 The Anterolateral Ligament (ALL) has recently been the topic of renewed interest and study as a stabilizer of medial rotation of the knee and a contributor to a positive pivot shift test if torn. Avulsion of the ALL is likely responsible for the Segond fracture seen in many ACL injuries. Further research is ongoing with respect to the role and importance of ALL reconstruction after ACL injury. A significant amount of research continues to focus on the factors involved in primary and secondary prevention of ACL injury, and on optimal neuromuscular training program designs for incorporation into preseason or in-season prevention programs.

Platelet rich plasma (PRP) is a technique that has been gaining popularity to aid in the healing process of cartilage, tendon, and ligament pathologies. The use of PRP has been investigated as a stimulator of the healing process in ACL reconstructions. It has been shown to have a beneficial influence in vascularization, inflammation, and ligamentization of the tendon graft in the early stages of healing.15 Future research in this area should focus on optimization of PRP preparations (platelet concentrations, leukocyte rich or poor concentrations), timing and frequency of procedures.

A traditional rehabilitation program following ACL-Reconstruction (ACLR) is directed towards tissue healing, decreasing muscle atrophy, and regaining optimal ROM in the early phases and emphasis on muscle re-education in later phases. The use of low load blood flow restriction is a modality that has been gaining attention as it seems to improve quadricep and hamstring strength, particularly when the patient has knee pain with higher exercise loads. Additionally, augmentative rehabilitation interventions using motor learning, sensory motor disturbance, virtual reality, and neurocognition are being integrated more commonly to rehabilitation programs to improve the central nervous system’s neuroplastic response.10

Gaps in the Evidence-Based Knowledge

Research is underway to determine whether primary repair of the ligament can be accomplished in a way that would provide improved outcomes over current reconstruction techniques, the latter of which carry significant rates of long-term degenerative change. BEAR (bridge-enhanced ACL repair) “scaffolding” techniques are currently being investigated in humans to assist in the healing of the ACL in the setting of primary repair after an ACL tear and initial reports show similar two-year results in clinical, functional, and patient-reported outcomes compared with ACLR.18 Further research is needed to better define who will benefit most from primary repair or ACL reconstruction, which patients will have a greater risk of degenerative disease, who will best be able to “cope” with the injury and be successfully managed in non-operative fashion, and which functional tests are the best predictors of good outcome. ACL rehabilitation protocols continue to undergo refinement, and the optimal duration of such programs is undergoing clarification. In addition, variability exists in the design of ACL prevention programs with the ideal components and timing of the interventions yet to be defined. Finally, the role of regenerative techniques in patients who choose non-surgical treatment or surgical reconstruction is yet to be determined.


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Original Version of the Topic

Ed Laskowski, MD. ACL Injury and Rehabilitation. 11/10/2011.

Previous Revision(s) of the Topic

William F. Micheo, MD, Odrick R Rosas, MD, PhD, Alexandra Rivera, MD, Anthony Lombardi, MD. ACL Injury and Rehabilitation. 7/23/2020

Author Disclosure

William F. Micheo, MD
Nothing to Disclose

Anthony L. Lombardi, MD
Nothing to Disclose

Carlota Martín, MD
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Gerardo Miranda, MD
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