Anterior cruciate ligament injury

INTRODUCTION — The anterior cruciate ligament (ACL) is an important stabilizing ligament of the knee that is frequently injured by athletes and trauma victims. There are between 100,000 and 200,000 ACL ruptures per year in the United States alone [1-3].

This topic review will discuss the presentation, evaluation, and management of ACL injuries. A discussion of the general approach to the patient with knee pain, including descriptions of examination techniques, and discussions of other specific knee injuries are found separately:

●Knee pain and examination in adults: (see "Physical examination of the knee" and "Approach to the adult with knee pain likely of musculoskeletal origin" and "Approach to the adult with unspecified knee pain" and "Musculoskeletal ultrasound of the knee")

●Knee pain in children: (see "Approach to acute knee pain and injury in children and skeletally immature adolescents" and "Approach to chronic knee pain or injury in children or skeletally immature adolescents")

●Soft tissue knee injuries: (see "Meniscus injury of the knee" and "Medial (tibial) collateral ligament injury of the knee" and "Posterior cruciate ligament injury" and "Lateral collateral ligament injury and related posterolateral corner injuries of the knee" and "Patellofemoral pain")

ANATOMY AND FUNCTION — The anatomy and biomechanics of the knee joint are discussed in detail separately; the anatomy and function of the ACL are reviewed below. (See "Physical examination of the knee", section on 'Anatomy' and "Physical examination of the knee", section on 'Biomechanics'.)

The primary function of the ACL is to control anterior translation of the tibia. The ACL also is a secondary restraint to tibial rotation as well as varus or valgus stress [4]. The ACL originates at the posteromedial aspect of the lateral femoral condyle. It courses distally in an anterior and medial fashion to the anteromedial aspect of the tibia between the condyles. The position on the tibia is approximately 15 mm behind the anterior border of the tibial articular surface, and medial to the attachment of the anterior horn of the lateral meniscus (figure 1 and picture 1 and figure 2) [5]. The ACL is often said to be comprised of two bundles: an anteromedial bundle that is tight in flexion and a posterolateral bundle that is tight in extension. The blood supply to the ACL is from branches of the middle geniculate artery and its innervation comes from the posterior articular nerve, a branch of the tibial nerve [6].

EPIDEMIOLOGY — The ACL is the most injured knee ligament. In the United States alone there are between 100,000 and 200,000 ACL ruptures per year, with an annual incidence in the general population of approximately 1 in 3500, although the actual incidence may be higher [1,2,7-9]. Data are limited by the absence of any standard surveillance mechanism for the general population. Registries exist for several national health services and for injuries sustained by United States college and high school athletes, but the latter accounts for a small percentage of the total number of injuries [10-12]. Overall, available data suggest the incidence of ACL tears has increased in patients of all ages over the past decades [12-19].

The majority of ACL tears occur from noncontact athletic injuries. According to the National Collegiate Athletic Association (NCAA) injury surveillance system, which has tracked all injuries associated with United States college athletics since 1988, American football players sustain the greatest number of ACL tears, but these are predominately contact injuries. Female athletes sustain higher rates of ACL injury per athletic exposure across sports and countries [10-12,20-25]. One athlete participating in a single game or practice equals one exposure. Among skiers, recreational alpine skiers have the highest incidence of ACL rupture, while expert recreational skiers the lowest [26]. Competitive alpine skiers sustain ACL injuries at a high rate [27]. Participants in women's ice hockey and men's baseball have a low incidence [28].

With certain sports, female gender confers significantly greater risk of ACL rupture regardless of age [12,21,29-34]. In addition to gymnasts, female soccer and basketball players sustain significantly more ACL injuries than their male counterparts (incidence ratios of approximately 3.5 and 2.7 for each sport, respectively) [10,11,26,35]. Although the overall incidence of ACL injuries is roughly equal for female and male United States college athletes, this stems from the disproportionate number of contact injuries among male American football players. (See 'Risk factors' below.)

RISK FACTORS — A number of potential risk factors for noncontact ACL injuries have been identified. These include anatomic, neuromuscular, biomechanical, and external factors. Risk factors are discussed in detail separately. (See "Anterior cruciate ligament (ACL) injury prevention", section on 'Risk factors for noncontact ACL injury'.)

MECHANISM AND PRESENTATION — ACL injuries can occur by a variety of mechanisms, including both high-energy (eg, motor vehicle collision, downhill skiing) and low-energy (ie, noncontact field sports) [3]. Low-energy injuries may involve contact (eg, blow to the lateral knee), but noncontact injuries are more common, accounting for approximately 70 percent of ACL tears [33,36]. The most common mechanism involves a low-energy, noncontact injury sustained during an athletic activity.

Noncontact mechanism — The typical mechanism for a noncontact ACL injury involves a running or jumping athlete who suddenly decelerates and changes direction (eg, cutting) or pivots or lands in a way that involves rotation and lateral bending (ie, valgus stress) of the knee and anterior translation of the tibia on the femur. According to several studies using video to assess the biomechanics of ACL tears, the majority of injuries are associated with a valgus position with the knee, minimal knee flexion, and internal rotation of the tibia [37-40]. Dynamic valgus collapse of the knee appears to be more common in female athletes and may contribute to their higher injury rates. Sports associated with ACL injuries often involve pivoting and sudden changes in direction, and include alpine skiing, soccer (football), basketball, and tennis (table 1). (See 'Epidemiology' above and 'Risk factors' above.)

Contact mechanism — Contact-related ACL injuries usually occur from a direct blow causing hyperextension or valgus deformation of the knee. This is often seen in American football when a player's foot is planted and an opponent strikes him on the lateral aspect of the planted leg [41].

ACL injuries also occur during high speed motor vehicle collisions. Such injuries are often missed in the multiple trauma patient because clinicians concentrate appropriately on managing life-threatening injuries, and the tertiary trauma examination may be delayed.

Signs and symptoms — Patients who sustain a noncontact ACL injury often complain of feeling a "pop" in their knee at the time of injury, acute swelling thereafter, and a feeling that the knee is unstable or "giving out." Nearly all patients with an acute ACL injury manifest a knee effusion from hemarthrosis. Conversely, approximately 67 to 77 percent of patients presenting with acute traumatic knee hemarthrosis have an ACL injury [42,43].

Often after the initial swelling has improved, patients are able to bear weight but complain of instability. Movements such as squatting, pivoting, and stepping laterally, and activities such as walking down stairs, in which the entire body weight is placed on the affected leg, most often elicit such instability.

Associated injuries — Other structures are often damaged during an acute ACL injury [44-46]. Associated structures that are commonly injured include the meniscus, joint capsule, articular cartilage, subchondral bone (bone bruise or fracture), and other ligaments [47-50]. Such injuries may be more frequent if the mechanism involves significant force (eg, contact injury). One small study suggests that weightbearing motion in the uninjured knee does not appear to be adversely affected, and thus it can be used for comparison during the physical examination (assuming it has not been injured previously and functions normally) [51].

PHYSICAL EXAMINATION — Evaluation of the knee includes an appropriate history and physical examination. In patients with a possible ACL injury, the clinician should inquire about the timing of the injury, the mechanism, joint swelling, functional ability (eg, can the patient walk, climb stairs), joint instability (eg, is the knee giving out), and associated injuries. (See 'Mechanism and presentation' above.)

An appropriate examination includes inspection, palpation, testing of mobility, strength, and stability, and performance of special tests of ACL integrity. Depending upon the patient and the time elapsed since the acute injury, the knee examination may be limited by pain or hemarthrosis. Although an ACL tear can generally be diagnosed clinically, magnetic resonance injury (MRI) is often used to assist diagnosis. Performance of the knee examination is discussed in detail separately. (See "Physical examination of the knee".)

One key to an accurate knee examination is to evaluate the unaffected knee for comparison. Many patients have increased laxity that is not pathologic. When evaluating for an ACL injury, it is often best to examine the patient immediately after the injury is sustained. This avoids the difficulty of trying to evaluate a knee with a significant hemarthrosis, which can develop within a few hours.

Many tests for detecting ACL injury are described. Three such tests, the Lachman, anterior drawer, and pivot shift, are the most sensitive and specific [52-54]. Of these tests, the pivot shift is the least sensitive and the most difficult to perform. We suggest that clinicians perform the Lachman and anterior drawer maneuvers; sports medicine specialists may wish to perform the pivot shift.

The Lachman test is performed by placing the knee in 30 degrees of flexion and then stabilizing the distal femur with one hand while pulling the proximal tibia anteriorly with the other hand, thereby attempting to produce anterior translation of the tibia (picture 2). An intact ACL limits anterior translation and provides a distinct endpoint. Increased translation compared with the uninjured knee and a vague endpoint suggests ACL injury.

The anterior drawer test is performed with the patient lying supine and the knee flexed at 90 degrees. The proximal tibia is gripped with both hands and pulled anteriorly, checking for anterior translation. Often, the clinician sits on the foot while performing the test to provide stability (picture 3). The test is positive if there is anterior translation. Comparing the degree of translation with the uninjured knee is helpful.

The pivot shift test can be difficult to perform in the awake patient due to guarding, and is sensitive only in a fully relaxed and cooperative patient. A positive test is highly specific, albeit insensitive, for ACL rupture [52,55]. The test is performed with the knee starting in extension. The clinician holds the lower leg with one hand and internally rotates the tibia, while placing a valgus stress on the knee using the other hand (figure 3). This causes subluxation in the ACL-deficient knee. While maintaining the forces described, the clinician flexes the knee. In the ACL-deficient knee this causes a reduction of the subluxed tibia, which the clinician senses as a "clunk," and which constitutes a positive test.

It is important to evaluate for posterior translation of the tibia prior to performing the drawer test. A false positive anterior drawer test can occur if a posterior cruciate ligament (PCL) injury exists. Posterior sag from the PCL injury will give the clinician the sensation of anterior tibial translation, when in fact the knee is returning to a neutral position. Sag exists if one tibia lies below the other when observing the legs from the side with the knees flexed to 90 degrees. In addition, examination of the ACL-deficient knee may also reveal lateral instability [56].

The Lever test has demonstrated sensitivity and specificity comparable with the Lachman, pivot shift, and anterior drawer tests [57,58]. This test may be particularly helpful when examining larger athletes. The Lever test is performed with the patient supine and knee extended (figure 4). The examiner places a fist below the proximal third of the patient's calf, elevating the knee off the examining table, and then presses down on the distal third of the patient's quadriceps. If the ACL is intact, downward pressure on the quadriceps causes the ipsilateral heel to rise, as the lever arm is intact. If the ACL is torn, disrupting the lever arm, downward pressure causes posterior displacement of the thigh only, and the heel will not rise.

Preliminary evidence suggests the Forced Active Buckling Sign may be useful but further study is needed [59].

A meta-analysis of the efficacy of the tests used to diagnose ACL injury found the Lachman to be the most useful, with a sensitivity of 85 percent and a specificity of 94 percent for ACL rupture [52]. The pivot shift had a sensitivity of 24 percent and specificity of 98 percent. The anterior drawer had a sensitivity of 92 percent and specificity of 91 percent in chronic conditions but was not accurate in acute injury [41,52]. Other reviews have reported similar results [54,60].

The KT-1000 knee ligament arthrometer (picture 4) is a device that provides an objective measurement of anterior-posterior translation and is often used in studies evaluating ACL tears. This machine is seldom used in clinical practice because physical examination is generally reliable. Due to the high sensitivity of the Lachman and the high specificity of the pivot shift, we suggest performing both tests to confirm an ACL rupture. The combination of a positive Lachman and a negative pivot shift can mean the ACL is partially torn [41].

It is important to evaluate the other knee structures that can sustain injury in conjunction with the ACL. Test the stability of the medial and lateral collateral ligaments by applying gradual varus and valgus stress. Test the posterior collateral ligament by performing a posterior drawer test. Assess for meniscal injury by palpating the medial and lateral joint lines, and performing the appropriate examination maneuvers. Examination techniques for meniscal injury are described separately. (See "Meniscus injury of the knee", section on 'Physical examination'.)

DIAGNOSTIC IMAGING — Plain radiographs are often performed following traumatic knee injuries to rule out fractures but cannot be used to diagnose ACL tears. In some cases, an avulsion fracture of the anterolateral tibial plateau at the site of attachment of the lateral capsule (the so-called Segond fracture) is identified on plain film (image 1). Such an injury suggests the presence of an associated ACL rupture [61-64].

In the United States, magnetic resonance imaging (MRI) is the primary modality used to diagnose ACL rupture (algorithm 1). In parts of Europe, ultrasound is often used to assist in the diagnosis. Knee arthrograms are only performed in patients in whom MRI is contraindicated and physical examination is inconclusive.

MRI is both highly sensitive and specific in the diagnosis of complete ACL rupture (image 2). A systematic review using arthroscopy as a gold standard found MRI to have a sensitivity of 86 percent and a specificity of 95 percent for ACL tear [44]. Diagnostic studies, again using arthroscopy as the gold standard, describe sensitivities as high as 92 to 100 percent and specificities as high as 95 to 100 percent [65-67]. MRI is less accurate in differentiating complete tears from partial tears, and in detecting chronic tears. In addition, the capacity of MRI to reveal associated meniscal injury is limited [68]. (See "Meniscus injury of the knee", section on 'Imaging'.)

In some parts of Europe and elsewhere, ultrasound is widely used to aid in the diagnosis of ACL tear. Like MRI, ultrasound is best at detecting complete ACL rupture (image 3). Ultrasound is inexpensive, rapid, and painless, and multiple studies report high sensitivity, specificity, and positive predictive value for complete ACL tears [69,70]. Sensitivity is likely more limited than MRI. The accuracy of ultrasound is highly user dependent.

Multidetector computed tomography (MDCT) is not used to evaluate ACL injury. Data suggest MDCT is accurate at detecting an intact ACL but is unreliable for determining whether an ACL tear is present [71].

DIAGNOSIS — A definitive diagnosis of ACL tear is made by diagnostic imaging (magnetic resonance imaging [MRI] is most accurate (algorithm 1)) or knee arthroscopy. However, in many instances, the clinical presentation can establish the diagnosis without the need for imaging. ACL tears sustained through a noncontact injury are most common and are suspected on the basis of a suggestive history (sudden change of direction while moving rapidly or landing from a jump during sport causing the knee to "pop" or give out) and clinical findings (acute knee effusion; positive Lachman, pivot shift, and anterior drawer tests). Contact injuries typically stem from a direct blow causing hyperextension or valgus deformation of the knee, and are often associated with injuries to other structures.

TREATMENT

Acute management — Acute management consists of rest, ice, compression of the injured knee, and elevation of the affected lower extremity. Crutches may be needed acutely to avoid weight-bearing, particularly if the knee is unstable. Over-the-counter analgesics are generally sufficient to control pain. While nonsteroidal antiinflammatory drugs (NSAIDs) provide effective short-term pain relief, their effect on ligament and bone healing remains unclear. This issue is discussed separately. (See "Nonselective NSAIDs: Overview of adverse effects", section on 'Possible effect on tendon injury'.)

Operative or nonoperative treatment? — Appropriate treatment for an ACL injury depends upon the extent of injury, patient characteristics and activities, and available resources [3,72]. It is important that the patient feels comfortable discussing the available treatment options with their surgeon and that issues such as patient expectations, rehabilitation, and potential complications are addressed in such discussions. The risks associated with nonoperative management and the potential complications of surgery are reviewed below. Should nonoperative management prove unsuccessful, delayed surgical repair is a viable option [73-75]. (See 'Injury risk associated with nonoperative management' below and 'Complications of surgery' below.)

Determining the need for surgery — ACL injuries can be managed operatively or nonoperatively. Most active, younger patients and high-level athletes opt for surgical reconstruction. In general, patients with an ACL injury should be referred to an orthopedist to discuss treatment options. Patients who decide not to pursue surgical management should be referred to a knowledgeable physical therapist or athletic trainer for rehabilitation. (See 'Rehabilitation' below.)

The decision to have surgery is based upon multiple factors, including the patient's level of activity, functional demands placed on the knee, and the presence of associated injuries to the meniscus or other knee ligaments. Other factors, such as age and occupation, also play a role. Patients with injuries to multiple knee structures (eg, ACL plus meniscus or medial collateral ligament) generally need surgical reconstruction due to the increased instability of the knee, which typically causes substantial activity limitations and mechanical symptoms (eg, locking, giving out), and because such injuries probably increase the risk for developing osteoarthritis (OA). (See 'Risk of osteoarthritis' below.)

In addition, surgical reconstruction of the ACL is appropriate for patients who [72]:

●Participate in high-demand sports or occupations (ie, those involving cutting, jumping, pivoting, and quick deceleration)

or

●Experience significant knee instability (eg, knee gives out while climbing stairs) [73]

Traditionally, anterior translation of more than 5 mm with testing on a KT1000 or comparable device has been used as a criterion for surgery. However, some studies question the use of static translation as an accurate predictive tool for knee function and the need for surgical reconstruction [76]. Some experts believe a positive pivot shift test three months following injury best predicts the future need for surgical repair [77].

There are no long-term studies that directly compare the rates of return to sport between athletes treated operatively and nonoperatively. Nevertheless, in our experience, athletes who participate in sports involving rapid deceleration, pivoting, and change in direction have a better chance of returning to play if they undergo ACL reconstruction [78]. Overall, patients who fare worst with nonoperative treatment are high-level athletes and young athletes [47,79].

According to a systematic review of 69 studies involving 7556 participants, 81 percent of patients treated with ACL reconstruction returned to some type of athletic activity, 65 percent attained their preinjury level of competition, and 55 percent of high-level athletes successfully returned to competition [80]. These rates are relatively low given that approximately 90 percent of patients achieve normal or near normal knee function following surgery, suggesting that other factors, such as fear of reinjury, play an important role in athletes' decision-making about return to play. According to the review, factors associated with successful return to preinjury levels of activity include symmetric performance of unilateral hopping exercises, younger age, male gender, playing sports at an elite level, and a positive psychological outlook.

Elite athletes return to sport at substantially higher levels than recreational athletes. In another systematic review limited to elite athletes, 83 percent of patients returned to sports, and in most studies they performed comparably with matched, uninjured controls [81]. Financial remuneration may be one important incentive. Motivation is another important factor that likely influences whether an athlete returns to high-level sport. While elite athletes may have some advantages, such as access to high-quality medical and rehabilitation services, their psychological investment in sport probably explains much of this difference.

Less active patients who do not participate in sports that involve squatting, pivoting, and lateral movement have less risk of developing further knee injury. Such patients may be candidates for nonoperative management. The patients best suited for nonoperative management are described below. (See 'Patients amenable to nonoperative treatment' below.)

Theoretically there is no age cut-off for surgery. Although patients older than 55 years rarely undergo ACL reconstruction, the decision whether to perform surgery depends upon the patient's condition including symptomatic knee instability, activity level, and the surgeon's judgment. Observational studies suggest that ACL reconstruction is generally successful in patients older than 40 years [82-85].

Injury risk associated with nonoperative management — When deciding to treat a complete ACL rupture nonoperatively, it is important to understand the possible sequelae. Although rigorous prospective studies are scant limiting our ability to draw firm conclusions, the ACL-deficient knee is associated with an increased risk for meniscal tear, articular cartilage injury, chronic knee pain, and decreased activity [86-92].

Risk of osteoarthritis — Whether the absence of the ACL itself increases the long-term risk for OA is a subject of debate. Some observational studies suggest that a major factor in determining the risk for OA is the degree of joint trauma sustained during the initial injury that caused the ACL to rupture. We believe the risk for OA is likely multifactorial and that the severity of the initial trauma, extent of meniscal injury, quality of the surgical repair, knee biomechanics, and subsequent patient activity all play a role [87,93-98].

Multiple studies and systematic reviews have been performed to try to determine the risk of developing OA following ACL injury. Notable findings include the following:

●Higher-quality studies with a minimum radiographic follow-up of 10 years report the prevalence of knee OA in patients with isolated ACL injury to range from 0 to 13 percent, lower than previously thought [86].

●The prevalence of knee OA is higher (rates range between approximately 20 and 50 percent) in patients with associated injuries, particularly meniscal tear [88,91,95,97,99,100].

●Surgical repair is not associated with a reduction in the long-term prevalence of knee OA, supporting the theory that the risk for OA is multifactorial [101]. Selection bias may play a role, as patients who choose surgical repair tend to be more active and place greater demands on the repaired knee.

●Many studies of OA risk are retrospective and of limited quality. Moreover, multiple radiologic classification schemes have been used to determine the presence of OA, making comparisons among studies difficult.

Degenerative OA may occur regardless of the treatment approach [87,90,101]. According to multiple studies, the risk of OA in a knee with a surgically repaired ACL is several times that of the uninjured knee in the same patient [90,100]. A review of 53 studies, including over 185,000 patients with an isolated ACL injury and over 725,000 patients with a combined ACL and meniscal injury, reported comparable risks for developing knee OA (odds ratio [OR] for isolated ACL injury 4.2, 95% CI 2.2-8.0; OR for combined injury 6.4, 95% CI 4.9-8.3) [102]. Given the limitations of the included studies (described further below), we believe these numbers provide only a rough approximation of risk and that individual patient and clinical circumstances must be considered.

An important limitation of nearly all studies included in these systematic reviews is the inability to account for patients' activity levels following injury. Activity is typically higher in patients who undergo surgical repair, which increases the risk for developing OA. In addition, surgical repair may be more common in patients whose initial knee injury was more extensive, another likely risk factor for OA. It seems unlikely that surgical repair itself increases the risk for OA, but it may be a marker for these other factors.

Another limitation of many surveillance studies is their limited timeframe; a longer period (eg, over 20 years from the time of injury) may be needed to reveal signs of OA in patients managed conservatively. To address concerns about the limited timeframe of many surveillance studies, researchers performed a systematic review of 29 studies with a minimum of 10 years of follow-up that included 1585 patients treated with surgical reconstruction and 685 patients treated nonoperatively [103]. Notable findings included the following:

●Patients treated with primary surgery (ie, surgery was performed soon after injury) had less need for subsequent knee surgery, including meniscal surgery.

●Patients managed nonoperatively had a greater decline in their level of activity (as determined by the Tegner score), although the absolute level of activity at final follow-up did not differ significantly between the two cohorts.

●The rate of radiographically evident OA did not differ between the operative and nonoperative cohorts (35.3 and 32.8 percent, respectively).

A cohort study involving the radiographic assessment of 423 knees a minimum of 20 years following surgical reconstruction of a torn ACL reported that 28.6 percent had developed moderate to severe OA [104]. Statistically significant factors associated with OA included older age at surgery, medial meniscectomy, and limited knee extension when discharged following surgery.

Risk of subsequent knee injury — Secondary meniscal injury is a relatively common sequela following an ACL tear and an important risk factor for knee OA [97,105]. Although the quality of available evidence is limited, the risk of secondary meniscal injury appears to be greater in patients whose surgery is delayed for several months or longer and in those managed nonoperatively [105-109]. These findings are supported by several observational studies, including a database study of 1398 patients with ACL injuries in which the rates of subsequent meniscal tear in the injured knee were 7 percent in patients whose ACL was repaired within six months of injury but 33 percent among those whose surgery was delayed and 19 percent among those managed nonoperatively [106]. A large majority of secondary meniscal injuries require surgical treatment. In addition to the increased risk of secondary meniscal injury, delayed surgery may increase the proportion of meniscal injuries that are not amenable to repair and must be resected [110].

Patients amenable to nonoperative treatment — A minority of patients with ACL injury are capable of returning to sustained, high-level athletic activity without surgical repair [111]. Assessment to identify these patients soon after their injury is likely to be more accurate when several tests of dynamic neuromuscular function are used [76,112,113]. While a significant number of these athletes may later choose to undergo surgical repair, identification of those capable of performing without surgery gives them the option of continuing to compete, once symptoms have subsided, while surgery would preclude early participation in competitive sports.

This approach is supported by a prospective observational study of 345 consecutive patients, all active in sports that place significant demands on the knee, who sustained an isolated ACL rupture and were tested within seven months of injury [76]. Dynamic functional testing (a series of specific hopping tests) better predicted those patients capable of returning to preinjury levels of athletic performance without ACL repair than did traditional isolated testing of joint laxity or strength.

In this study, 88 of 146 athletes who attained a minimum level of strength and knee mobility with preliminary rehabilitation and passed dynamic functional testing chose rehabilitation as the primary treatment for their ACL injury. Ten-year follow-up data were available in 61 of 63 athletes who returned to full sporting activity: 25 continued without surgical repair, while 36 ultimately underwent ACL reconstruction. Long-term follow-up studies are needed to confirm these results. The results of this study and a randomized trial described elsewhere in this review suggest that there is a subset of active patients, albeit not yet clearly defined, for whom nonoperative treatment is a viable approach [74,76]. Further research is needed to delineate this group of patients.

Patients with low functional demands and athletes who participate in sports that do not place high demands on the knee, such as those involving linear, non-deceleration activities, may be treated nonoperatively [6,72]. With some activity modification and proper rehabilitation, such patients can achieve good results [114,115]. We believe these patients should work with a qualified physical therapist following their injury to improve the strength and proprioception needed to support the injured knee, and thereby reduce the risk of degenerative disease and further injury. (See 'Rehabilitation' below.)

Surgical management

Graft selection — ACL reconstruction is generally performed with arthroscopy using a tendon graft to replace the ruptured ACL. Graft selection remains a source of debate among orthopedic surgeons and details are beyond the scope of this discussion. What follows is a brief description for the nonsurgeon of the types of grafts available. Of note, patient factors (eg, prior knee injury, comorbidity), resources, and surgeon training and preference all factor into graft selection. In addition, surgical technique, especially proper graft positioning, plays a significant role in surgical success or failure regardless of graft type.

Both native (autograft) and cadaver (allograft) tendons can be used for ACL reconstruction. Autografts may be taken from the patient's patellar, hamstring (semitendinosus and gracilis), or quadriceps tendons. Most young athletes who participate in high-risk sports receive autografts. The advantages of autografts include faster healing, lower risk of reinjury, and no risk of infection from graft tissue. Disadvantages include harvest site morbidity, longer time in surgery, and constraints around tissue choice (eg, size, harvest site).

Allografts are usually taken from an Achilles or patellar tendon, but the quadriceps, hamstring, and tibialis tendons may be used. Allografts are typically reserved for middle-aged athletes who engage in low-impact sports. In most ways, allografts perform comparably to autografts, but rates of reinjury of the reconstructed ACL are higher in younger patients [116-118]. The advantages of allograft include reduced surgical time, reduced harvest site morbidity, and access to a range of sizes. Other disadvantages include potential disease transmission, immunologic reactions, slower remodeling and integration, and cost [119]. The risk of infection from an allograft is extremely low. Although reports exist of HIV and hepatitis transmission, no transmissions have been reported since 2002 [120,121]. Clinically significant bacterial infections occur in less than 1 percent of cases [122,123].

The most common autografts are the patellar tendon and the hamstring tendon. Neither graft has clearly demonstrated superior functional outcomes in controlled trials [116,124-128]. Potential advantages of the patellar graft include increased initial strength and stiffness compared with an uninjured ACL. In addition, patellar tendon grafts include a portion of bone at either end, which allows for bone-to-bone healing in the femoral and tibial tunnels made during surgery and earlier graft fixation [120]. The main disadvantage is pain at the harvest site. Systematic reviews confirm that reconstruction using the patellar tendon graft results in greater anterior knee pain compared with other grafts [129-131]. Such pain usually resolves after the first year, but not in all cases. While patellar tendon grafts provide greater stability than traditional hamstring grafts, this does not appear to be the case when multiple-strand hamstring grafts are used [125,131,132]. Patellar tendon grafts are associated in some studies with increased rates of OA of the knee, but a myriad of other factors are involved in the development of OA and it remains unclear if patellar tendon graft surgery contributes directly [133-135].

The hamstring graft has several advantages. Use of the hamstring tendon eliminates patellar tendon morbidity, primarily anterior knee pain. Hamstring donor site pain usually resolves by three months, and hamstring strength returns to normal by approximately 12 months [130,136]. The hamstring graft can be folded and the resulting multiple-strand grafts are stronger and stiffer, providing greater knee stability [137]. As hamstring grafts are comprised entirely of tendon, one potential disadvantage is the need for healing between a tendon and an osseous tunnel. As a result, initial fixation may be slower and ultimately weaker than the bone-to-bone healing of a patellar tendon graft [120,138].

The quadriceps tendon graft has been used less often for ACL reconstruction, but the procedure is becoming more common as more trials supporting this approach are published [139-141]. Advantages of the quadriceps tendon graft include avoiding injury to the infrapatellar branch of the saphenous nerve, which may occur with hamstring tendon grafts, and sparing the area around the tibial tubercle during surgery [142]. Repair with the quadriceps tendon causes less harvest site pain compared with patellar tendon grafts [143]. Like hamstring tendons, the quadriceps tendon can be folded to form a multiple-strand graft, thereby improving graft strength, and it includes a bone fragment at one end allowing for bone-to-bone healing at that end of the graft. In a systematic review of twenty-seven clinical studies involving 2856 ACL reconstructions, no significant differences in functional outcomes, including graft failure, were identified in patients treated with a quadriceps tendon graft compared with either patellar or hamstring tendon grafts [139].

Evidence is growing that supplementing ACL reconstruction with an anterolateral stabilizing procedure reduces reinjury rates, but prospective controlled studies are lacking [144,145].

Timing and preparation for surgery — The best time to undergo ACL reconstruction remains unclear [146]. We believe the condition of the injured knee is the most important factor when determining the timing of surgery. The knee should exhibit full range of motion with no significant effusion and adequate strength at the time of reconstruction. Observational studies suggest that surgery performed prematurely increases the risk of arthrofibrosis (scarring, abnormal tissue, and adhesions that develop within the knee joint) [147,148]. One such study found that 70 percent of patients with signs of knee swelling and inflammation at the time of ACL reconstruction went on to develop arthrofibrosis. Once the knee effusion has resolved and mobility restored, early repair may result in better long-term knee function [75,149,150]. According to a systematic review of primarily retrospective data, meniscal tears were more likely to occur if surgery was delayed longer than three months [151]. Often, our patients undergo two to four weeks of "prehabilitation" to maximize strength and motion prior to surgery [152]. Such brief delays do not appear to be detrimental [153].

In one randomized trial involving young healthy adults with acute uncomplicated ACL injuries, no difference in symptoms or patient perceptions of knee function were noted at 2-, 5-, and 10-year follow-up between patients treated with structured rehabilitation and early reconstruction and those treated with structured rehabilitation and optional delayed reconstruction [74,154]. At 10 years, over one-half of those assigned to optional reconstruction had undergone surgical repair. The authors claim that the latter approach could substantially reduce the number of ACL surgeries without adversely affecting outcomes. Delayed reconstruction may increase the risk of further knee injury (eg, medial meniscal tear) and prolongs the time before an athlete can return to full activity [151,155-158]. A similar trial published subsequently reported no clinically significant differences in outcome between patients treated with early surgery and those treated with rehabilitation and elective repair [75]. (See 'Risk of osteoarthritis' above.)

Complications of surgery — Potential short-term complications following ACL reconstruction include infection and deep vein thrombosis. Data from surgical registries and case series suggest that the rate for each is below one percent [159,160]. Loss of joint mobility, primarily loss of full extension, occurs in approximately 8 percent of patients. The risk of motion loss is reduced by appropriate pre- and postoperative physical therapy to maximize knee mobility. Anterior knee pain is common, especially among those receiving a patellar graft, but pain generally subsides over one to two years.

Important longer-term complications of ACL reconstruction include knee OA (which is discussed above), graft failure, and arthrofibrosis. ACL reconstruction may not reduce the long-term risk of developing OA. Graft failure rates are reported to be approximately 6 percent and may be due to poor operative technique, unrecognized concomitant injury, and reinjury, possibly related to premature return to a high-risk sport [160-162]. (See 'Risk of osteoarthritis' above and 'Return to sport and other activity' below.)

Arthrofibrosis is not well understood but consists of scarring, abnormal tissue growth, and adhesions that develop within a joint following trauma or surgery and that restrict motion and cause pain [163]. Following ACL reconstruction, arthrofibrosis develops in approximately 2 to 10 percent of patients [164-168]. Risk factors are not well studied, but limited knee mobility prior to surgery and poor compliance with physical therapy following surgery may increase risk. Treatment usually entails arthroscopic surgery to debride the joint followed by physical therapy and results in improved knee motion for most patients, but high-quality evidence is lacking [164].

Partial tear — In most cases, incomplete tears of the ACL can be managed nonoperatively with an emphasis upon physical therapy and proper sport-specific biomechanics [169]. Clinical findings suggestive of a partial ACL tear include an asymmetric Lachman test, a negative pivot shift test, and KT-1000 arthrometer testing that demonstrates no more than 3 mm of anterior-posterior translation.

A hinged knee brace may be worn during the early stages of rehabilitation. There is no evidence that wearing a brace upon returning to full activity reduces the risk of progression to a complete tear, but some clinicians suggest bracing. Once the strength and motion of the injured leg equals that of the opposite leg, the patient may return to sport. Symptom progression depends upon the extent of the tear and the patient's activities. Patients should be referred to an orthopedic surgeon if symptomatic instability develops. Studies of primary repair of partial ACL tears are ongoing. (See 'Rehabilitation' below.)

PEDIATRIC CONSIDERATIONS — The overriding clinical question with children and adolescents who have sustained a complete tear of the ACL is whether to perform surgical repair. We recommend surgical management for the large majority of these patients. The risk of growth disturbance or other complications from surgery is low [170,171]. One notable exception is the adolescent whose growth plates are expected to close within six to nine months. In such cases, we prefer to delay surgery until the growth plates close, and to restrict the patient's activity in the interim.

Our preference for surgical repair is supported by multiple studies reporting better functional outcomes and lower complication rates among patients managed surgically [172-174]. One meta-analysis with results from six studies involving 217 children and adolescents comparing operative and nonoperative treatment, and five studies involving 353 children and adolescents comparing early to delayed reconstruction, reported that multiple, clinically important complications occurred significantly more frequently among patients treated nonoperatively [172]. The following findings were emphasized:

●According to three studies, clinically significant knee instability developed in 13.6 percent of patients managed surgically compared with 75 percent of those managed nonoperatively.

●According to two studies, the incidence of meniscal tear was substantially greater among patients treated nonoperatively (35.4 percent versus 3.9 percent among patients treated surgically). Subsequent meta-analyses have confirmed that delayed surgery substantially increases the risk for meniscal injury [173,174].

●According to two studies, no patient treated nonoperatively was able to return to their previous level of activity compared with 85.7 percent of those treated surgically.

Following surgery, the importance of proper rehabilitation and allowing adequate time for healing before resuming sport is no different for children and adolescents. If anything, clinicians should err on the side of delaying return to play given that reinjury rates are higher among younger athletes. In a systematic review of studies of pediatric ACL repair, the reinjury rate was 8.7 percent [171]. A very small percentage of patients whose ACL is repaired while their growth plates are open will develop a leg length discrepancy [175]. (See 'Rehabilitation' below.)

REHABILITATION

Principles — Novel approaches to ACL rehabilitation develop continually. Nevertheless, several principles of rehabilitation have been shown consistently to be important for complete recovery [176]. As an example, full range of motion, especially in knee extension, should be promoted immediately following ACL reconstruction. The inability to regain normal knee motion is associated with an increased risk of osteoarthritis (OA) [177].

Closed kinetic chain exercises to strengthen the hamstring and quadriceps muscles are effective for initial rehabilitation [1,178]. Closed kinetic chain exercises require that both feet be planted and remain in a fixed position throughout the exercise (eg, squat). During open kinetic chain exercises the feet are not planted and change position.

Controversy continues about the role of open kinetic chain (ie, open chain) exercises in ACL rehabilitation. Based upon limited evidence, we believe that strenuous open chain exercises may be added to the rehabilitation program no sooner than six weeks following surgery [178-180]. However, specific open chain exercises that do not stress the knee or surgical graft may be used immediately following surgery. These exercises include straight leg raises (picture 5), quad sets (picture 6), and calf pumps (simple dorsiflexion and plantarflexion of the ankle to work the calf muscles).

Exercises to enhance balance, proprioception, and core strength should be incorporated into postoperative rehabilitation, as should training to improve sport-specific biomechanics [181,182]. The hamstrings are the primary muscle group that supports the ACL and thus hamstring strength is a critical aspect of rehabilitation. Patients who opt for nonoperative management also benefit from all the exercises described and should participate in a comprehensive rehabilitation program following injury.

Motivated patients can perform postoperative rehabilitation effectively on their own with no difference in long-term outcomes [183]. Patients wishing to perform rehabilitation independently must be given clear instructions explaining how to perform the exercises correctly and should demonstrate proper technique to a knowledgeable clinician before beginning. Different muscle groups manifest relatively greater weakness postoperatively depending upon the site of the autograft. Specific rehabilitation protocols based on the autograft site have been developed [184].

A number of devices have been used as part of rehabilitation, but often there is little evidence of effectiveness. A systematic review found no benefit from the use of passive-motion machines following surgery [185]. Use of a brace after surgery is based upon surgeon and patient preference. Systematic reviews of bracing following ACL reconstruction report no clear evidence of improved outcome or reduced risk of subsequent injury among patients using a brace [186-188].

Return to sport and other activity — Little high-quality research is available to help determine when patients can safely return to full activity and sport [189-192]. However, reinjury rates following surgery are significant (approximately 20 percent, but higher in younger athletes) and premature return undoubtedly increases the risk for reinjury and graft failure [175,193-195]. Rates of return to sport factor into the decision to undergo surgical repair and are discussed above. (See 'Determining the need for surgery' above.)

We believe that athletes may safely return to sport once their repaired knee demonstrates strength, proprioception, and function roughly equal to the unaffected knee. This determination assumes that the uninured knee and lower extremity possess adequate strength and function. Assessment of the unaffected extremity should include strength of the hamstrings, quadriceps, and hip musculature, and the patient should demonstrate excellent single-leg stability. If such is the case, it is reasonable for an athlete to return to play if they meet the following criteria:

●Lower extremities demonstrate approximately equal strength in all major muscle groups and movements

●Balance on one leg is roughly equivalent with eyes open and closed

●Ability to perform dynamic movements (eg, hopping, rapid change of direction) in all directions is approximately equal for each lower extremity

●Sport-specific movements can be performed at full speed (this must be achieved gradually) without producing pain, instability, or limping

No time limit should be placed on achieving these goals, and no player should be permitted to return to play without achieving them.

Given the location and space constraints of some medical offices, it can be difficult for some clinicians to perform an adequate physical assessment and determine whether a patient is fully recovered and ready to return to full activity or full sport. Particularly in such circumstances, it is important to maintain good, regular communication with the physical therapist or athletic trainer supervising the patient’s rehabilitation. They are in a better position to assess such things as the patient's ability to perform the dynamic movements required of their sport.

We tell our patients to expect a return to full activity and sports between 8 and 12 months following surgery, depending upon their baseline function, sport, and compliance with a sound rehabilitation program. However, a minimum of 10 months before return to play may reduce the risk of reinjury, particularly for high-risk sports, by ensuring that rehabilitation goals are achieved and the graft is fully incorporated. In some cases, 18 months or longer may be required for a graft to be fully incorporated, and rehabilitation of the affected extremity to be completed.

Despite the complex demands placed on the knee during participation in high-risk sports and the substantial risk of reinjury following ACL repair, the published criteria used to determine when an athlete is ready to return to play consist largely of time and the absence of significant discrepancies in strength [191]. A systematic review of over 264 studies addressing return to play after ACL reconstruction identified only 35 studies with objective criteria for return [196]. In many studies, time from surgery was the sole factor. A systematic review of studies of return-to-sport test batteries found that meeting these criteria was not associated with a significant reduction in subsequent knee injury but was associated with a lower risk of ACL graft rupture [190]. However, the authors' meta-analysis found that, even in studies reporting a reduction in the rate of graft rupture, there was a statistically significant increased risk of contralateral ACL tear among patients who met the test battery criteria. (See 'Risk of reinjury' below.)

Additional research is needed to identify the most useful criteria for determining when an athlete is ready to return to sport with minimal risk of reinjury, graft failure, and contralateral ACL tear. Such criteria are likely to involve a combination of factors involving knee motion, strength of supporting muscles, and neuromuscular function. Of note, an athlete's subjective sense that they are ready to return to play does not correlate with objective assessments of strength, power, and agility and should not be relied upon to determine fitness for sport [197].

Some patients are now returning to full activity at six months (and some high-level athletes sooner) following reconstructive surgery. For selected athletes eager to return to competition, early participation may not be disadvantageous, provided an appropriate and rigorous rehabilitation program is completed and appropriate functional milestones are achieved [178]. However, studies supporting early participation involve small numbers of patients, and athletes should be aware that this approach entails a risk of reinjury [161,195]. Expedited returns occur before reconstructed ACL grafts are completely incorporated into the knee. Athletes who participate in accelerated rehabilitation programs may continue to demonstrate some abnormal joint motion and relative weakness for up to 22 months following surgery. Although studies are limited, early return to full sport following ACL reconstruction may increase the risk for knee OA [198].

RISK OF REINJURY — Following surgical repair, the risk of rupturing the ACL graft and for tearing the uninjured ACL are both substantial [175,193,194]. This fact highlights the importance of completing a robust rehabilitation program before returning to sport. (See 'Return to sport and other activity' above.)

In a systematic review and meta-analysis of 24 studies involving elite athletes, the rate of graft rupture was 5.2 percent (95% CI 2.8-8.3) [81]. In another systematic review and meta-analysis of 14 studies, the overall rate of graft rupture was 7 percent (95% CI 5-8), but among patients under 25 years climbed to 10 percent (95% CI 7-14) [194]. In addition, tear rates for the uninjured ACL were 11 percent (95% CI 7-16) in this younger age cohort.

According to a systematic review of 71 studies involving over 600,000 athletes, factors associated with an increased risk for re-tear include male sex (OR 1.27, 95% CI 1.14-1.41), younger age (OR 1.07, 95% CI 1.05-1.08), preoperative high-grade knee laxity (OR 2.30, 95% CI 1.46-3.64), return to a high activity level or sport (OR 2.03, 95% CI 1.15-3.57), and concomitant medial collateral ligament injury (OR 1.62, 95% CI 1.31-2.00) [199].

PREVENTION — The overall toll of ACL reconstruction is high and this has stimulated research into the prevention of noncontact ACL injuries. Studies have focused on various aspects of physical training, particularly neuromuscular training, and on extrinsic supports. The prevention of noncontact ACL injuries is discussed in detail separately. (See "Anterior cruciate ligament (ACL) injury prevention".)

EXPERIMENTAL TREATMENTS — Numerous possible treatments are being studied. Future approaches to ACL reconstruction may include repair of the injured ACL, synthetic replacements, and bioengineered ACL reconstruction [200,201]. Management with a systematic bracing protocol may provide an effective, non-surgical approach to treatment [202].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Anterior cruciate ligament injury" and "Society guideline links: Knee pain" and "Society guideline links: Meniscal injury".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topics (see "Patient education: Anterior cruciate ligament tear (The Basics)" and "Patient education: Knee pain (The Basics)")

●Beyond the Basics topics (see "Patient education: Anterior cruciate ligament injury (Beyond the Basics)" and "Patient education: Knee pain (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

●Epidemiology – The anterior cruciate ligament (ACL) is the most commonly injured knee ligament. Noncontact, low-energy injuries incurred during athletic activity account for the majority of ACL tears. Female athletes are at increased risk. (See 'Epidemiology' above and 'Risk factors' above.)

●Mechanism of injury – The typical mechanism for a noncontact ACL injury involves a running or jumping athlete who suddenly decelerates and changes direction (eg, cutting) or pivots or lands from a jump in a way that involves rotation or lateral bending (ie, valgus stress) of the knee. (See 'Mechanism and presentation' above.)

●Presentation – Patients who sustain an ACL injury often complain of feeling a "pop" in their knee at the time of injury, acute swelling thereafter, and a feeling that the knee is unstable or "giving out." Nearly all patients with an acute ACL injury manifest a knee effusion from hemarthrosis. (See 'Mechanism and presentation' above.)

●Physical examination – The Lachman (picture 2), pivot shift (figure 3), anterior drawer (picture 3), and lever (figure 4) tests are the most useful examination techniques for detecting ACL injury. The Lever test may be useful, particularly for larger athletes. When evaluating a patient for ACL injury, it is important to look for associated injuries (eg, meniscal tear) and to examine the unaffected knee for comparison. Many patients have increased laxity that is not pathologic. (See 'Physical examination' above.)

●Diagnostic imaging – Plain radiographs cannot be used to diagnose ACL rupture. Magnetic resonance imaging (MRI) is both highly sensitive and specific. Ultrasound is accurate for diagnosing complete tears. (See 'Diagnostic imaging' above.)

●Management – ACL injuries can be managed operatively or nonoperatively. Although rigorous studies are few, the ACL-deficient knee is associated with an increased risk for further injury (eg, meniscal tear), chronic pain, and decreased level of activity. Premature, degenerative osteoarthritis (OA) may occur regardless of the treatment approach depending on the extent of injury. (See 'Operative or nonoperative treatment?' above.)

Patients with injuries to multiple knee structures (eg, ACL plus meniscus or medial collateral ligament) or who experience significant knee instability (eg, knee gives out while climbing stairs) generally need surgical reconstruction. Young athletes and athletes who participate and wish to continue in high-demand sports (ie, those involving cutting, jumping, pivoting, and quick deceleration) generally need surgical reconstruction.

Different tissue grafts can be used for ACL reconstruction. Graft selection and the timing of surgery are discussed in the text. (See 'Graft selection' above and 'Timing and preparation for surgery' above.)

●Prevention – Focused neuromuscular training designed to prevent noncontact ACL tears reduce injury risk, particularly among women participating in high-risk sports. We strongly encourage athletes who participate in sports that place them at high risk for ACL injury to participate in a well-designed, neuromuscular, injury-prevention program. (See "Anterior cruciate ligament (ACL) injury prevention".)