ACL Injury and Rehabilitation

Author(s): Ed Laskowski, MD

Originally published:11/10/2011

Last updated:04/14/2016



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 noncontact in nature with activities that involve cutting and pivoting movements or sudden deceleration with change of direction accounting for approximately 70% of acute ACL injuries. 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.

Epidemiology including risk factors and primary prevention

In the United States, there are approximately 100,000 ACL injuries (estimated to be 1 in 3000 individuals) each year. One National College Athletic Association (NCAA) survey reported it to be the second most common injury of the lower extremity. A 3- to 7-times greater injury incidence has been observed in women., with the greatest risk in peri-adolescent athletes.  Both intrinsic and extrinsic risk factors have been identified for ACL injury. Intrinsic factors include sex, hormonal, genetic, neuromuscular, cognitive, anatomic, and history of previous injury.  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, and increased internal rotation of the hip coupled with increased external rotation of the tibia, and increased quadriceps muscle activation.  Neuromuscular control patterns and biomechanical movement patterns have been shown to be amenable to specific prevention programs.


The ACL is a collagenous ligament that originates on the anterior intercondylar region of the tibia and inserts on the posteromedial aspect of the lateral femoral condyle. Although it functions as a single ligament, it contains two main bundles: an anteromedial bundle, which is rigid in flexion, and a posterolateral bundle, which is rigid in extension. Forces on the ACL are highest in the final 30 degrees of knee extension and also with knee hyperextension.

The ACL also provides rotary control and limits internal tibial rotation. It also functions as a secondary restraint to valgus and varus stressors 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 proprioceptive feedback, which is thought to enhance dynamic control. Studies have shown persistent proprioceptive deficits even in a knee that has undergone ACL reconstruction.

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 compared to lateral meniscal tears. An ACL deficient knee is susceptible to instability events (subluxations) if the person returns to aggressive “at risk” activities, including cut/pivot and 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 activity.

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.

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.  In this study, 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.

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 in the future.

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.

Proprioceptive deficits which persist after ACL injury may contribute to increase 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.



An accurate history is one of the essential components of a diagnosis of ACL injury. The patient will usually relate a history of feeling a “pop” or “give-way” of the knee during an aggressive cut, pivot or deceleration movement. Landing awkwardly from a jump or “knee hyperextension” or valgus collapse resulting from a medially directed contact hit to the knee are other mechanisms of injury. The patient usually experiences rapid swelling of the knee, and usually will be unable 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, which challenges the ACL’s ability to control anterior tibial translation in 20-30 degrees of knee flexion with the femur stabilized. 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. Muscle guarding and co-contraction can contribute to make this test less sensitive, especially in the acute to subacute injury period.  The anterior drawer test is less sensitive in detecting ACL injury.

Functional assessment

After acute injury, individuals will likely require assisted ambulation with crutches until weight bearing pain resolves and gait mechanics normalize. A longer period of crutch ambulation also permits bone healing in the case of MRI observed subchondral injury.  A medial/lateral support brace may be used for combined medial collateral or lateral collateral injuries.


Knee radiographs are usually obtained to assess osseous status and to detect evidence of other injuries. Radiographs also can detect associated findings, such as lateral tibial plateau (Segond) fracture or avulsions of the tibial spine. Ultrasound evaluation can be used as a screening tool when physical exam assessment is uncertain. An MRI evaluation is the gold standard imaging study used to confirm ACL injury and 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 (distance hops, vertical jump) can be used to assess dynamic knee stability and is also used to assess return to play readiness after surgical or nonsurgical 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.

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.


ACL injures appear to occur more common when playing American football on artificial turf compared to natural turf. They also 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, since it creates the loss of ability to perform an activity. Significant changes in mood state have been measured after injury, including grief and depression. A support structure entailing family, friends and the medical team is important for successful outcome of both surgical and nonsurgical treatments.  Self-confidence, optimism, and self-motivation have been shown to be predictive of positive outcomes.

Professional Issues

Quick and accurate diagnosis may be clinically suspected but can only be confirmed by MRI. Thus physicians must advocate for their patients to obtain proper diagnostic testing and then decide appropriate surgical vs. non-surgical management. Despite great surgical advances, this injury can still significantly affect an athlete’s career and proper counseling is necessary.


Available or current treatment guidelines

The goal of treatment of ACL injury is to prevent episodes of recurrent knee instability and, theoretically, to protect the cartilage from further trauma. This can be accomplished in two ways: nonsurgical treatment involving activity modification, lower extremity strength and stability training, and appropriate bracing or surgical reconstruction of the ligament. If meniscal repair is performed, ACL reconstruction is the option of choice to protect the meniscal repair. ACL reconstruction is also preferable in cases of functional instability, in most cases of combined and multi-ligament injuries, and in patients with significant meniscal involvement that contributes to knee instability or that results in a mechanical block to knee motion.

At different disease stages

Initial treatment for an ACL injury aims to reduce pain and swelling in the knee, regain normal joint movement (regaining full extension is critical to a successful outcome) and strengthen the musculature around the knee. 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. Athletes who wish to return to activities involving aggressive jumping, cut/pivot or deceleration movements usually pursue surgical reconstruction in order to prevent episodes of instability. Individuals not involved in “at risk” movement patterns can typically maintain knee stability with rehabilitation alone.

Coordination of care

The treatment of ACL injury is inherently multidisciplinary. A coordinated team of physiatrists, orthopedic surgeons, physical therapists, athletic trainers, and, in select cases, sports psychologists, is essential to ensure optimal outcomes.

Patient & family education

Education regarding surgical and nonsurgical treatment options for ACL injury is essential in order for the patient to make a well informed decision. ACL injury can be associated with significant changes in mood state, including depression and grief over loss of sport or functional status. Sports psychology is often useful to provide education regarding coping strategies. The long-time course of rehabilitation after surgical or nonsurgical treatment requires motivation and dedication. Preoperative education of both patient and family can contribute to improved postoperative compliance.

Emerging/unique Interventions

Neuromuscular training is an essential component of a rehabilitation program to optimize performance after ACL injury. Surgical reconstruction provides static stability, but dynamic stability is provided by muscle strength and coordinated muscle firing patterns. 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 sports activities approximately 9 months after ACL reconstruction, and 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 such as single and triple leg hops for distance in addition to psychological readiness. The Lysholm knee score and Tegner activity scales can be used as patient-administered means of functional assessment after ACL injury.


Cutting edge concepts and practice

Surgical reconstruction with “double bundle” technique has been advocated by some as being more “physiologic” and similar to the native ACL. This theoretical rationale has not been proven in clinical studies, and no significant outcome difference has been found when comparing single bundle and double bundle reconstruction techniques. 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.

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,” which is an avulsion fracture off of the lateral tibial condyle 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 design for incorporation into preseason or in-season prevention programs.


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. “Scaffolding” techniques are currently being investigated to assist in the healing of the ACL in the setting of primary repair after an ACL tear.  Further research is needed to better define who will benefit most from 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. Finally, variability exists in the design of ACL prevention programs and the ideal components have yet to be defined.



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Original Version of the Topic:

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

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Ed Laskowski, MD
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