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Anterior cruciate ligament injury

147 citationsUpdated Sep 2026

For patients: a plain-language version of this topic is available. See the patient guide.

Overview¶

Anterior cruciate ligament (ACL) injuries present a complex clinical landscape encompassing biomechanical behavior, natural history, and associated pathologies [2]. A classification algorithm serves as an effective tool for prospectively identifying individuals early after injury who desire nonoperative care or must delay surgical intervention, provided they have good potential for successful conservative management [1]. While comprehensive surgical management of concomitant injuries in complex ACL cases yields long-term outcomes equivalent to those with less severe patterns [4], contemporary ACL repair remains an attractive, minimally morbid, biologic alternative for a specific subset of patients [69]. However, well-powered prospective studies are required to directly compare the outcomes of contemporary ACL repair with reconstruction [69], and primary arthroscopic repair using the pulley technique for partial proximal tears should not be widely adopted until such direct comparisons are established [66].

Risk stratification and timing are critical determinants of postoperative success. A history of ACL reconstruction is a risk factor for further injury, with the highest risk occurring in the first year after the procedure [6]. Patients undergoing primary ACL reconstruction who also possess a strong family history of ACL tears are more likely to sustain a postoperative graft rupture or complication requiring surgery [18]. Ideally, particularly in younger patients, ACL reconstruction should not be delayed more than five months from the initial injury [22]. Furthermore, injury chronicity did not adversely affect failure rates or clinical outcomes at minimum 2-year follow-up after ACL primary repair in appropriately selected patients [14], and functional results after ACL repair for acute tears are comparable to those after ACL reconstruction [17].

Assessment and return-to-sport criteria require rigorous standardization. A comprehensive assessment after ACL treatment should provide a complete overview of the treatment result, optimally including early adverse events, patient-reported outcomes, ACL graft failure, and clinical measures of knee function and structure [7]. The minimum follow-up for this assessment is 2 years, with an optimal follow-up rate of 80% [7]. Consensus guidelines provide practical recommendations for reporting outcome entities and suggest preferred tools for reliable and valid assessment [8, 9]. Clinicians must note that limb symmetry indexes can overestimate knee function after ACL injury [32], and current variable return-to-sport criteria utilized in clinical practice may not be stringent enough to achieve safe and successful return-to-sport [32]. Patients with an ACL rupture need to be counselled appropriately, and efforts to prevent these injuries must be redoubled [28].

Anatomy & Pathophysiology¶

Ligament Anatomy¶

The anterior cruciate ligament (ACL) is an intra-articular ligament that is technically extrasynovial because it is surrounded by synovium [100]. It runs from the femur to the tibia in an anterior–medial direction [100]. The ACL has a variable length ranging from 22 to 41 mm [100] and is approximately 32 mm in length [131]. Its width ranges from 7 to 12 mm [100, 131], and it is consistently narrowest in the midsubstance [100]. The ligament is composed of two functional bundles: the anteromedial (AM) and posterolateral (PL) bundles, named according to their tibial insertion [100]. Histologically, the ACL consists of 90% type I collagen and 10% type III collagen [131], with a strength of 2,200 N [131].

The femoral origin of the ACL is on the posteromedial edge of the lateral femoral condyle, posterior to the lateral intercondylar ridge (Resident’s ridge) [100]. The femoral attachment is usually oval in shape [100]. The AM bundle arises from the superior and anterior aspects of the femoral attachment, while the PL bundle arises from the posterior and inferior aspects [100]. These bundles are often separated by the lateral bifurcate ridge, which runs from anterior to posterior on the femur [100]. The lateral intercondylar ridge demarcates the anterior edge of the ACL where the bifurcate ridge separates the two bundles [131]. Using a clock-face description from the posterior outlet of the femoral intercondylar notch, the bulk of the AM bundle is attached between 9:30 and 11:30 o'clock, and the PL bundle is attached between 8:30 and 10:00 o'clock [100]. The distance on the femur between the centers of the AM and PL bundles varies from 8 to 10 mm depending on knee size [100].

The center of the ACL femoral footprint is 43% of the distance from the proximal to distal articular cartilage margin [98]. The center of the anteromedial bundle is 29.5% of the proximal to distal distance of the lateral femoral intercondylar notch, while the center of the posterolateral bundle is 50% of this distance [98]. The posterior edge of the ACL is 2.5 mm from the posterior articular cartilage border [98]. Direct insertion fibers are more critical than indirect insertion fibers in linking ligaments to bone [98]. The ACL inserts more anteriorly on a macroscopic level than on a histologic level [98]. The direct insertion of the ACL is in a narrow area extending from the intercondylar ridge to a second osseous ridge 4 mm posterior [98]. Direct fibers do not continue to the posterior articular cartilage; posterior fibers that extend to the articular cartilage are indirect fibers with a fanlike attachment [98].

The tibial footprint of the ACL is located on the tibial plateau in the anterior intercondylar fossa, between the medial and lateral tibial spines [100]. The tibial insertion is 120% larger than the femoral insertion and is anterolateral to the medial tibial spine [100]. Some ACL fibers pass deep to the transverse meniscal ligament, while others merge with the anterior aspect of the lateral meniscus [100]. The AM bundle attaches to the anteromedial portion of the tibial footprint, and the PL bundle attaches to the posterolateral portion [100]. The center of the PL bundle on the tibia is 4 ± 1 mm from the medial tibial spine, and the center of the AM bundle is 5 ± 1 mm from the medial tibial spine [100].

The ACL receives its blood supply from the middle geniculate artery and its innervation from the posterior articular nerve, a branch of the tibial nerve [131].

Biomechanics & Function¶

The ACL is the main restraint against anterior tibial translation relative to the femur [131]. It stabilizes the knee primarily by resisting anterior translation and secondarily by resisting varus and valgus forces [77]. The ACL also prevents tibial rotation and varus/valgus rotation [131]. The AM bundle is primarily responsible for restraining anterior tibial translation [131]. The AM bundle is tightest in flexion [131, 98] and maintains tension throughout the knee range of motion [98]. The PL bundle is tightest in extension and looser in midflexion [131, 98], allowing for rotation due to its laxity in midflexion [131]. Both bundles are under tension during loading with anterior translation or combined anterior translation and internal rotation [98].

Graft placement anterior to the native ACL footprint can cause impingement in extension, while graft placement posterior to the native ACL footprint can cause impingement on the posterior cruciate ligament [98].

Injury Mechanisms & Risk Factors¶

The most common noncontact ACL injury mechanism involves a deceleration and rotational injury during running, cutting, or jumping activities [33]. The most common contact ACL injury mechanism involves hyperextension and/or valgus forces to the knee by a direct blow [33]. Female athletes have a two-fold to eight-fold higher risk of ACL injury than male athletes when level of competition, age, and time exposed are considered [72]. Differences in neuromuscular firing patterns in the quadriceps and hamstrings between males and females are felt to contribute to the higher injury rate in females [72]. Potential contributing factors to female ACL injury risk include ACL size, notch width anatomy, biomechanics, alignment, muscle strength, hormonal factors, and training [72].

The anatomical parameters of the knee joint (notch width index, ACL width and length) have no role in the cause of an injury [78]. However, combinations of knee joint geometry measurements provide more information about the risk of noncontact ACL injury than individual measures [108]. The aspects of knee geometry that best explain the relationship between knee geometry and ACL injury risk are different between males and females [108]. Altered bone morphological characteristics of the knee are risk factors for ACL tears in both contact and noncontact injuries [157]. Knee torsional alignment is associated with ACL injury, predominantly in the distal femur rather than the proximal tibia [173].

Landing upright appears to be harmful to the ACL by increasing post-impact force and quadriceps muscle activity while decreasing knee flexion angles [174]. Changing toe direction significantly affects knee kinetics and kinematics during landing [95]. Lateral jumps are the most dangerous of stop-jumps regarding knee biomechanics [97]. Increasing the knee flexion angle during jump landing may be an effective intervention to improve knee biomechanical risk factors associated with ACL injury [112]. Frontal plane kinematic temporal relationships at the hip, knee, and ankle differ between genders, with components of dynamic knee valgus peaking during the deceleration phase in women and during the acceleration phase in men [127]. Knee motion and knee loading during a landing task are predictors of anterior cruciate ligament injury risk in female athletes [132]. The early peak knee abduction moment waveform is a novel risk factor predicting anterior cruciate ligament injury in young athletes, and using waveforms instead of discrete peak values of the knee abduction moment may better represent risky movement patterns [120]. Conversely, knee abduction kinematics and kinetics during weight-bearing activities may not be risk factors for future ACL injury, contrary to clinical opinion [130]. Decreasing tibial length relative to femoral length may alter lower extremity biomechanics in a manner that places the ACL at risk for injury [121]. The methods to quantify knee joint laxity may be used in conjunction with measures of neuromuscular control to identify high-risk female athletes [196]. Four typical injury patterns leading to ACL injury in women’s professional football differ significantly in terms of injury mechanisms and match situations, and most common ACL injury patterns identified in women’s professional football have great potential for injury prevention [88].

Pathophysiology & Associated Injuries¶

Most ACL injuries are complete disruptions, although disruption of a single anatomic bundle (anteromedial or posterolateral) may occur [72]. In the skeletally mature patient, the femoral insertion or the midsubstance is the most common site of ACL disruption [72]. The tibial attachment may be avulsed with or without a piece of bone [72]. Approximately 90% of ACL tears can be diagnosed with a pertinent clinical history and a thorough orthopaedic physical examination [77]. ACL tears most commonly involve the mid fibers (approximately 70%) [77]. 7% to 20% of ACL tears occur proximally at the femoral attachment, and 3% to 10% of ACL tears occur distally at the tibial insertion [77].

Pivot-shift bone contusion patterns often appear as bone marrow edema at the lateral condylopatellar sulcus and the posterior aspect of the lateral tibial plateau [77]. The pivot-shift bone contusion occurs when the tibia is displaced anteriorly and impacts onto the lateral femoral condyle [77]. The mechanism of ACL injury can produce an osteochondral impaction fracture known as the deep femoral notch sign [77]. A contrecoup bone contusion in the posterior aspect of the medial tibial plateau is a secondary MRI sign of an ACL tear [77]. A Segond fracture is a small avulsion fracture of the peripheral lateral tibial plateau related to the lateral capsular ligament and/or slips from the iliotibial band or fibular collateral ligament [77]. The Segond sign (avulsion of the anterolateral capsule of the tibia) may be seen on plain radiographs in ACL injury [33]. Bone bruises of the lateral femoral condyle and lateral tibial plateau are noted in up to 80% of ACL injuries [33].

There is a high incidence of associated injuries, including meniscus tears, in ACL injury [33]. Patients with an ACL tear and an osteochondral depression/impaction fracture have an increased incidence of meniscal tears in the same knee and a relatively poor clinical outcome 1 year after ACL reconstruction [77]. An increased volume of bone marrow edema without fracture is not associated with a poor postoperative clinical outcome after ACL reconstruction [77]. Meniscal and anterolateral capsular structures may function as important secondary stabilizers in an ACL-injured knee [91]. The meniscus status has a significant impact on knee kinematics in the ACL-deficient knee [123]. Subtotal medial meniscectomy in knees with ACL deficiency alters knee kinematics, especially at high flexion angles [158].

Osteoarthritis development in injured joints is caused by intra-articular pathogenic processes initiated at the time of injury, combined with long-term changes in dynamic joint loading [5]. Variation in osteoarthritis outcome after ACL injury is reinforced by variables such as age, sex, genetics, obesity, muscle strength, activity, and reinjury [5]. There is a lack of evidence to support a protective role of repair or reconstructive surgery of the ACL against osteoarthritis development [5]. At 10 to 20 years after diagnosis, on average, 50% of those with a diagnosed ACL tear have osteoarthritis with associated pain and functional impairment [5]. Within 10 years after ACL reconstruction, up to 80% show signs of posttraumatic knee osteoarthritis [35]. Patients who had radiographic knee OA 5 years after ACL reconstruction walked with lower knee adduction moments and medial compartment joint contact forces than patients without OA early after injury and reconstruction [181]. Biomechanical factors such as reduced knee adduction moment or reduced knee flexion angle during gait could serve as early indicators for osteoarthritis risk [116]. Clinical measures of knee function were most predictive of subsequent osteoarthritis development following an extended period of rehabilitation early after ACL injury [86]. Synovial fluid biomarkers associated with ACL injury and reconstruction have unknown correlation with subjective and objective measures of knee function and the risk of secondary osteoarthritis [189].

Skeletally Immature Considerations¶

Over a 12-year period (1999 to 2011), the incidence of ACL ruptures increased by more than 400% at Children’s Hospital of Philadelphia compared with tibial spine fractures [21]. The relationship between the femoral origin of the ACL and the distal femoral physeal plate in the skeletally immature knee has been studied anatomically [21]. Three-dimensional intercondylar notch volumes in a skeletally immature pediatric population have been compared between knees with torn and intact ACLs using MRI [21]. Experimental studies in rabbits have examined the effect of trauma to the lower femoral epiphyseal plate and the effect of intra-articular ACL reconstruction on growth plates [21]. All-inside, physeal-sparing ACL reconstruction does not significantly compromise the physis in skeletally immature athletes according to postoperative physeal MRI analysis [21]. Growth disturbances without growth arrest have been reported after ACL reconstruction in children [21]. Substantial overgrowth was seen in two patients and minor limb-length discrepancies were seen in four additional patients following all-epiphyseal reconstructions in prepubescent children [21].

Classification¶

Tear Morphology and Grading¶

Modified Sherman Grading System: This system classifies ACL tears by location into five types based on the percentage of tibial length remaining. Type I is a proximal avulsion with >90% of tibial length remaining, accounting for 8% of all ACL tears [74]. Type II is a proximal tear with 90%–75% length remaining [74]. Type III is a midsubstance tear with 75%–25% length remaining [74]. Type IV is a distal tear with 25%–10% length remaining [74]. Type V is a distal avulsion with <10% length remaining [74]. Medial meniscus tears are most prevalent in type I ACL tears [70]. The plane in which an MRI scan is performed affects the classification of ACL tears [70]. A cadaver study concept allows for a more precise description of ACL rupture patterns [76].

Severity Scales and Associated Injuries¶

Anterior Cruciate Ligament Injury Severity Scale (ACLISS): This tool effectively documents and categorizes the magnitude of associated tissue damage in knees after primary ACL injury and reconstruction [23]. It allows for the easy and rapid identification of different injury severity profiles in patients who underwent primary ACL reconstruction [23]. A classification system for ACL tears can guide a proper surgical plan in the pediatric and adolescent population [96]. Surgeons can counsel patients with complex ACL injuries that comprehensive surgical management of concomitant injuries is associated with good long-term outcomes equivalent to those with less severe injury patterns [4].

Risk Factors and Injury Patterns¶

Risk Factors: An increased lateral femoral condyle ratio is a risk factor for anterior cruciate ligament injury [53]. Lower body mass index, higher Beighton score, complete anterior cruciate ligament tear, and concomitant injuries to the medial collateral ligament, anterolateral complex, and meniscus (especially the posterior horn) are risk factors for preoperative high-grade pivot shift under anaesthesia [94]. Previous injury of any type does not increase the risk of suffering an ACL injury in men’s professional soccer [83].

Injury Patterns: Four distinct patterns of anterior cruciate ligament injury were identified in women’s professional football, which differ significantly in terms of injury mechanisms and match situations [88]. The anatomical parameters of the knee joint, including notch width index, anterior cruciate ligament width, and length, have no role in the cause of an injury [78].

Management Algorithms and Outcomes¶

Osteoarthritis Prevalence: The highest rated studies reported a low prevalence of knee osteoarthritis for individuals with isolated anterior cruciate ligament injury (0%–13%) and a higher prevalence of knee osteoarthritis for subjects with combined injuries (21%–48%) [198]. At 10 to 20 years after diagnosis, on average, 50% of those with a diagnosed anterior cruciate ligament or meniscus tear have osteoarthritis with associated pain and functional impairment [5].

Clinical Presentation¶

History and Mechanism¶

The most common noncontact mechanism for ACL injury involves deceleration and rotational forces during running, cutting, or jumping activities [33]. In contrast, the most common contact mechanism involves hyperextension and/or valgus forces to the knee from a direct blow [33]. Approximately 70% of patients report hearing or feeling a "pop" at the time of injury [72]. Patients often describe a sensation of instability or the knee "giving out" during twisting activities [33]. Substantial knee swelling secondary to hemarthrosis typically occurs within 4 to 12 hours following the injury [33], and almost all patients notice swelling of the knee within 6 to 12 hours of the injury [72]. Patients with chronic ACL injury may report recurrent episodes of knee instability, mechanical symptoms from secondary meniscal tears, or pain and swelling [72]. A history of ACL reconstruction is a risk factor for further injury, with the highest risk occurring in the first year after reconstruction [6].

Physical Examination¶

The Lachman test is the most sensitive and useful test for diagnosing ACL injuries in the acute setting [33, 72]. It is performed with the knee in 20–30 degrees of flexion, applying an anterior force to the tibia while stabilizing the distal femur [33]. Laxity is graded based on increased translation compared to the contralateral knee: * Grade 1: 1–4 mm of increased translation [33]. * Grade 2: 5–9 mm of increased translation [33]. * Grade 3: More than 10 mm of translation [33].

The anterior drawer test is performed with the knee in 90 degrees of flexion and is less sensitive than the Lachman test [33]. The pivot shift test (Losee test) evaluates rotational instability and is considered the most functional test to evaluate knee stability after ACL injury [33]. This test is pathognomonic for ACL injury and is best performed in the chronic setting [72]. A positive pivot shift test occurs when the anterolateral tibial plateau reduces with a visible shift at the lateral joint line at approximately 15 degrees of knee flexion [72]. MCL injury and some meniscal tears may produce a false-negative pivot shift test [33]. Examination under anesthesia is useful for obtaining a more accurate pivot shift test in patients with unclear history or equivocal office examination [33]. Combined medical history and physical examination have strong diagnostic value for ACL rupture when performed by an orthopaedic surgeon [44]. For primary care physicians, only medical history appeared to be of value in diagnosing ACL rupture, whereas physical examination was not [44]. In the acute setting, aspiration of a hemarthrosis can decrease pain and improve the quality of the physical examination [33].

Imaging and Diagnostic Findings¶

MRI can diagnose an ACL tear with 95% or better accuracy [33]. The Segond sign, an avulsion of the anterolateral capsule of the tibia, may be seen on plain radiographs [33]. In more than one-quarter of patients, plain radiographs may help establish the diagnosis of an ACL tear [45]. A positive finding of the tuberculum intercondylare tibiae tertium ought to be included in the group of radiologic signs relevant for the diagnosis of ACL lesion [54]. ALL injuries are not reliably diagnosed on MRI in the setting of an ACL tear, and physicians should not rely on MRI to diagnose an ALL injury in the presence of an ACL injury [46]. MRI evaluation showed ALL injuries are present in 60.2% of acute ACL injuries in adolescent patients [60]. 53% to 56% of patients presenting with initially diagnosed isolated ACL ruptures had concomitant injuries to the Kaplan fiber complex [48]. Isolated ACL tear is less common than combined injuries, which are quite frequent [114]. Instrumented laxity evaluations, such as the KT-1000 arthrometer, can augment the physical examination and provide an objective baseline for future comparison [33].

Risk Factors and Predisposing Conditions¶

An increased lateral femoral condyle ratio is a risk factor for ACL injury [53]. Pre-injury leg length discrepancy is a risk factor for ACL injury in the skeletally immature athlete [52]. A strong family history of ACL tears is associated with a higher likelihood of sustaining a postoperative graft rupture or complication requiring surgery [18]. Marked deviations in posterior tibial slope could be a predisposing risk factor for subsequent contralateral tear [104]. Female sex is associated with greater rotatory knee laxity in collegiate athletes [101].

Investigations¶

Clinical History and Physical Examination¶

Patients frequently report hearing a "pop" at the time of injury, although this history is not specific to ACL tears [33]. Substantial knee swelling secondary to hemarthrosis typically occurs within the first 4–12 hours following an ACL injury [33]. Aspiration of a hemarthrosis can decrease pain and improve the quality of the physical examination in the acute setting [33]. The Lachman test is the most useful test for evaluating anterior laxity of the knee [33]. Lachman test laxity is graded by comparison to the uninjured contralateral knee, with Grade 1 defined as 1–4 mm of increased translation [33]. Grade 2 Lachman laxity is defined as 5–9 mm of increased translation compared to the contralateral knee [33]. Grade 3 Lachman laxity is defined as more than 10 mm of translation compared to the contralateral knee [33].

The pivot shift test (Losee test) evaluates rotational instability associated with an ACL tear [33]. The pivot shift test is considered the most functional test to evaluate knee stability after ACL injury [33]. An examination under anesthesia is useful for obtaining a more accurate pivot shift test in patients with unclear history or equivocal office examination [33]. The ACL Injury Score (ACLIS) was developed to assess the probability of ACL tear and need for specialist referral based on patient-interview information [51]. The ACLIS score includes items such as pivoting/contact activity, perceived cracking sound, sensation of dislocation, joint effusion, suggestive mechanism, inability to resume activity, and immediate sensation of instability [51].

Plain Radiography¶

Plain radiographs of the knee should be obtained to rule out fractures about the knee [33]. In more than one-quarter of patients, plain radiographs may help to establish the diagnosis of an ACL tear [45]. Before skeletal maturity, an avulsion of the tibial insertion of the ACL can be seen radiographically [33]. Multiple radiographic parameters were significantly associated with ACL injuries when compared with age-matched controls [223].

Magnetic Resonance Imaging (MRI)¶

MRI is the imaging study of choice for the diagnosis of ACL pathology [77]. The accuracy of MRI for the diagnosis of ACL tears has been shown to be 95% to 100% [77]. T2-weighted sequences, preferably with fat saturation, are recommended over T1-weighted sequences for ACL evaluation [77]. An intact ACL demonstrates taut fibers paralleling the intercondylar roof on sagittal MRI studies [77]. Confirmation of an intact ACL on both axial and coronal views is critical for accurate diagnosis [77].

The primary MRI feature of a complete ACL tear is the identification of complete disruption of ligament fibers [77]. ACL tears most commonly involve the mid fibers, occurring in approximately 70% of cases [77]. Proximal ACL tears at the femoral attachment occur in 7% to 20% of cases [77]. Distal ACL tears at the tibial insertion occur in 3% to 10% of cases [77]. Increased volume of bone marrow edema without fracture is not associated with a poor postoperative clinical outcome [77].

MRI findings suggesting a partial ACL tear include increased T2 signal/edema along the course of the ligament with predominantly intact fibers [77]. Abrupt angulation of intact fibers or marked attenuation of the fibers are MRI findings suggesting a partial ACL tear [77]. On axial imaging, narrowing of the transverse dimension with a normal anterior-posterior dimension has been described as a stable partial tear [77]. Complete absence of the anteromedial or posterolateral bundle on axial imaging suggests an unstable partial tear [77]. A 2014 study reported that 32% of suspected ACL injuries were single bundle tears [77]. There were three times as many anteromedial bundle tears as posterolateral bundle tears in a 2014 study of suspected ACL injuries [77]. 3-Tesla MRI in the oblique coronal plane yielded significantly greater specificity (92% to 96%) for selective bundle tears than imaging in conventional orthogonal planes (67%) [77].

Pitfalls in ACL diagnosis include increased T2-weighted signal abnormality resulting from chronic mucoid or cystic degeneration of the ACL [77]. An ACL ganglion is best identified on MRI as a globular cystic structure within the ACL with intact fibers [77]. MRI is unreliable for determining the precise location of an ACL tear [145]. MRI accurately predicted the ACL tear type seen arthroscopically in only 55.9% of patients [161]. Poor agreement was found between imaging and arthroscopic assessment of ACL tear location, with MRI predicting intraoperative location in less than half of cases [186]. MRI may not be completely reliable in assessing the degree and location of an ACL tear, and surgeons should not solely rely on MRI for ruling out possible ACL repair [144].

Identification of medial bone bruising on pre-operative MRI following acute ACL rupture should raise suspicion of an associated postero-medial meniscal tear [214]. Spontaneous anterior tibial subluxation on MRI in complete ACL tear was significantly associated with medial meniscal tear and accident-to-MRI time [229]. The diagnostic ability of MRI to predict meniscal injuries present at acute ACL reconstruction was moderate, with performance being poorest at the lateral meniscus [140]. The diagnostic validity of magnetic resonance imaging is similar for meniscal tears in acute knee trauma and in knee symptoms lasting over 6 months in young adults [164]. MRI is a sensitive measure of cruciate and collateral ligament injury in acute knee dislocation [176]. MRI does not reliably diagnose injury to the posterolateral corner or meniscus in acute knee dislocation [176]. MRI findings, including direct signs like fiber discontinuity and indirect signs like bone contusions, correlate well with arthroscopic findings for ACL, PCL, and posterolateral corner injuries [219]. Early recognition of bipolar ACL injury (simultaneous femoral-sided tear and tibial avulsion fracture) via MRI is critical [49]. The presence of high-grade cartilage lesions is significantly increased in an ACL-deficient knee when reconstruction is performed more than 12 months after injury [220]. At the time of acute ACL injuries, posterior root tears of the lateral meniscus do not appear to result in meniscal extrusion on MRI [221]. MRI signal alterations steadily occur within the first 6 months postoperatively after acute meniscus repair, revealing an ongoing healing process [227].

Associated Ligamentous and Soft Tissue Injuries on MRI¶

ALL injuries were not reliably diagnosed on MRI in the setting of an ACL tear [46]. Physicians should not rely on MRI to diagnose an ALL injury in the presence of an ACL injury [46]. 53% to 56% of patients presenting with initially diagnosed isolated ACL ruptures had concomitant injuries to the Kaplan Fiber Complex (KFC) [48]. Standard preoperative MRI scans can reliably be used to visualize Kaplan fiber injury in the majority of pediatric and adolescent patients with ACL tears, especially when performed in the acute setting [159]. The clinical utility of MRI for definitive diagnosis of Kaplan fiber complex injury is questioned, and no correlation has been shown between KFC injury and increased anterolateral rotatory laxity or subjective instability symptoms [226]. Direct and indirect signs of KF injury were established for MRI diagnosis, requiring at least 1 direct sign or 2 indirect signs in any plane [57].

Instrumented Laxity and Other Imaging¶

Instrumented laxity evaluations can augment the physical examination and provide an objective baseline for future comparison [33]. The KT-1000 arthrometer uses a series of standard forces to measure anterior translation of the tibia with the knee in 20–30 degrees of flexion [33]. Dynamic ultrasonography has been reported to have a diagnostic accuracy of 88%, sensitivity of 90%, and specificity of 83% in the evaluation of medial plica syndrome [64]. The clinical utility of imaging-based measurement methods for the determination of ACL injury risk requires more reliable techniques that demonstrate consistency between studies [61]. An increased lateral femoral condyle ratio measured by MRI is associated with higher risk of noncontact ACL injury [228]. Specific osseous morphologic characteristics identified by MRI are associated with ACL injury in male college football players [230]. Osseous morphological risk factors should be considered in individualized anatomic ACL reconstructions to achieve optimal outcomes [231]. AI can extract and synthesize 3D anatomic signatures from routine MRI for ACL injury risk stratification [225].

Treatment¶

Non-Operative¶

Nonsurgical management is reserved for patients with low activity demands or minimal instability complaints [34]. This approach comprises physical therapy to restore range of motion and strength, bracing, and lifestyle modification [34]. Short-term progressive exercise therapy is well tolerated and should be incorporated into early-stage rehabilitation to improve function before reconstruction or as a first step in nonoperative management [139]. Non-surgical treatment may be a strong option when concomitant meniscal or ligamentous injury is absent, function is well conserved on performance tests such as the “hopping test,” and “giving way” is minimal or absent [190]. Prediction models incorporating knee function measures can estimate 2-year prognoses for nonsurgical treatment [26]. However, chronic ACL deficiency increases the risk of meniscal and cartilage pathology [34]. Secondary meniscal tears are most common among patients undergoing delayed surgical or nonoperative treatment [63], and patients treated non-operatively have developed secondary meniscal lesions requiring delayed surgical management [107]. In children and adolescents, initial nonoperative treatment carries a high risk of additional meniscal and chondral injury, potentially resulting in long-term knee impairment [183]. Conversely, conservative management in the paediatric population permits return to normal life and sports activities without a major increase in meniscal tears [149]. Non-ACLR treatment may be viable for older patients or those with lower activity demands to achieve satisfactory return-to-sport outcomes [154]. At 18 months, surgical reconstruction is more effective but more expensive than rehabilitation management for long-standing ACL injury [87]. Patients in the ACLR group reported higher future knee-related self-efficacy and engaged in higher levels of physical activity at 8 and 12 months compared to those treated with rehabilitation alone [117]. Knee laxity increased bilaterally during the first year after non-surgically treated ACL injury [171]. Prolonged nonoperative therapy for complete ACL rupture remains controversial due to the risk of further joint damage and recurrent instability [40]. Patients after nonoperative and delayed treatment experienced more instability, pathological laxity, and inability to return to previous activity levels than those treated with early surgical stabilization [203].

Operative¶

Indications: Surgical treatment is indicated for young patients, those involved in jumping, cutting, and pivoting sports, manual laborers, and patients with concomitant injuries requiring surgery [34]. Early ACL reconstruction is recommended to prevent secondary meniscal tears [162]. In active, skeletally immature patients, surgical management is the preferred strategy to stabilize the knee, protect cartilage and menisci, and allow safe functional participation in cutting and pivoting activities [188]. An ACL injury impairs knee-related quality of life for up to 35 years, with no difference between treatment approaches such as initial repair or later reconstruction compared with nonsurgical treatment [30]. Estimates of patient-reported outcomes have exceeded clinically important thresholds, highlighting the importance of assessing these constructs when managing individuals with ACL injuries [20]. ACLR has a beneficial effect on knee outcomes, with acute-stage ACLR leading to better outcomes in subgroup analysis [122].

Surgical Approach / Technique: The standard of care is anatomic reconstruction using patellar tendon, hamstring, or quadriceps autograft, or allograft tissue [34]. The most common reason for graft failure is tunnel malpositioning; therefore, tunnels should be placed in the center of the anatomic ACL femoral and tibial footprint [34]. Reconstruction can be performed as a single- or double-bundle technique [34]. Double-bundle reconstruction is technically more demanding but may result in improved stability [34]. In a Swedish National Knee Ligament Registry study, single-bundle reconstruction had an increased risk of revision surgery compared with double-bundle (adjusted hazard ratio 1.98, 95% CI 1.12 to 3.51, P = 0.019) [34]. The risk of revision surgery was slightly reduced when an anatomic single-bundle technique was used compared with a nonanatomic single-tunnel technique (adjusted HR 1.87, 95% CI 1.04 to 3.38, P = 0.037) [34]. An osteotomy should be considered if there is evidence of malalignment [34].

Graft Selection: Autograft offers faster healing time, less immune reaction, and no risk of disease transmission, but is associated with increased postoperative pain [34]. Patellar tendon autograft provides bone-to-bone healing and is often considered the benchmark, but has the highest incidence of anterior knee pain [34]. Hamstring autograft uses a smaller incision but can result in permanent decreased hamstring strength, which is a risk factor for graft rupture [34]. Allograft has less donor site morbidity but a higher failure rate in the young athletic population [34]. High-level evidence suggests that allograft has a higher failure rate in young patients than autograft and should be avoided in children undergoing ACL reconstruction [188]. One study of 354 patients showed a 2.6 times risk of graft failure with allograft supplementation compared with no supplementation, when analysis was controlled for age and graft size [188]. Patellar tendon grafts are generally a suboptimal graft choice in young children because of the greater potential of forming a tethering bony bridge across the epiphyseal plate if bone plugs are placed or fixed in the area of the physis [188]. Hamstring, soft-tissue quadriceps tendon, and iliotibial band (ITB) autograft are the most common graft choices in skeletally immature patients [188]. Physeal-sparing techniques have demonstrated successful clinical results, with few reported instances of growth disturbance [188]. Transphyseal ACLR has satisfactory clinical outcomes, with good subjective outcomes, function level, and knee stability [136]. In adolescents approaching skeletal maturity who have minimal growth remaining (skeletal age: older than 13 years for girls, older than 14 years for boys), traditional transphyseal ACL reconstruction can be considered because the potential for clinically significant growth arrest is minimal at this stage of maturity [188]. For younger patients, traditional techniques, particularly those involving the placement of graft bone plugs (eg, patellar tendon) or implants across the physis, can adversely affect the growing athlete’s physis [188].

Repair vs. Reconstruction: Surgical repair of the ACL historically has high failure rates; however, newer methods for structural augmentation of the repair have shown promising results [34]. Proximal ACL tear patterns showed better healing potential with primary repair than distal or midsubstance tears [34]. Some form of internal support of the repair with sutures or scaffolds increased the success rate of ACL repair [34]. Biological characteristics of the repair could be improved by bone marrow access by drilling tunnels or microfracture [34]. Augmentation with platelet-rich plasma was beneficial only in combination with a structural scaffold [34]. Skeletally immature patients had the best outcomes along with patients where the repair was performed in the acute setting [34]. Functional results after ACL repair for acute tears are comparable to those after ACLR [17]. Injury chronicity did not adversely affect failure rates or clinical outcomes at minimum 2-year follow-up after ACLPR [14]. Despite the promising clinical outcomes, the pulley technique for primary arthroscopic repair of partial proximal ACL tears should not be widely adopted unless it has been compared directly with ACL reconstruction [66]. Contemporary ACL repair will apply to a subset of patients and is an attractive, minimally morbid, biologic alternative to ACL reconstruction, though well-powered prospective studies are needed to compare outcomes with reconstruction [69].

Concomitant Injuries: According to current standard indications, 34.9% of all meniscal injuries offer the potential for repair, rising to 55.6% when accompanied by anterior cruciate ligament damage [47]. Clinical outcomes of patients that underwent meniscus repair were better than those that underwent meniscus resection with concurrent ACL reconstruction [134]. Most isolated medial collateral ligament injuries are treated nonsurgically, while concomitant damage to the anterior or posterior cruciate ligaments is a common indication for surgical management of high-grade injuries [166]. Nonoperative and operative treatments of medial collateral ligament injuries lead to equally good results [187]. The treatment strategy of an immediate repair of the MCL and reconstruction of the ACL when conservative treatment has failed seems safe and effective [118]. A LEAP (lateral extra-articular procedure) is not always an innocuous procedure and carries the risk of certain complications, and should not be used ubiquitously in every primary ACLR [41]. The improvement rate was 24.6% among patients who did not receive ACLR in the acute phase and underwent ACLR in the chronic phase regarding anterolateral ligament healing [135].

Rehabilitation and Return to Sport: Postoperative rehabilitation is focused on early range of motion, immediate weight bearing, and regaining quadriceps control [34]. Bracing treatment can be considered postoperatively, but there is no evidence this positively affects outcomes [34]. Functional training after 6 months is thought to aid in injury prevention and return to sports is usually achieved around 9 months [34]. Functional bracing treatment at return to sports has been used but lacks high-level evidence [34]. Only approximately 63% of affected patients actually return to preinjury sports participation after ACL reconstruction [35]. The rate of return to preinjury sports participation rises to 83% in elite athletes following ACL reconstruction [35]. Within 2 years after ACL reconstruction, as many as 20% sustain a second ACL injury to either the surgical or nonsurgical knee, with a slightly higher risk to the nonsurgical knee [35]. The top reasons for reduced sports participation for the patients who do not return to their prior level include fear of reinjury, problems with structure/function of the knee, and family commitments or lifestyle changes [35]. Current literature recommends that full return to levels I or II sport be withheld until at least 9 months [150]. Athletes who have passed all of the criterion-based RTS testing and returned after 9 months are 84% less likely to reinjure their knee within 2 years after ACL reconstruction [150]. An athlete’s reinjury rate is reduced by 51% for each month that RTS is delayed until 9 months, and additional evidence shows similar levels of risk reduction persist until 12 months [150]. Those who do not meet recommended RTS criteria may have up to a fourfold greater risk of rerupture [150]. Not achieving a ≥90% quadriceps index and returning to level I sport before 9 months postoperatively are independent risk factors of rerupture [150]. In assessing at 2-year rates of knee reinjury, patients who returned to level I sport after surgery had 30% rerupture rates, compared with 8% in those who returned to lower level sports [150]. In a sample of more than 100 young athletes who were cleared by their physician or physical therapist for RTS, only 13.9% of the sample passed all combined measures (hop testing and strength), with 27.8% meeting strength cutoffs, and 53% meeting hop testing cutoffs individually [150]. Of a sample of 120 young athletes assessed at the RTS time point after being cleared to return to cutting and pivoting sports, 56% returned to their preinjury level of sport at 1 year, 23% did not resume their preinjury level of sport, and 21% sustained second ACL injuries [150]. Limb symmetry indexes can overestimate knee function after anterior cruciate ligament injury, raising concern whether the variable ACL return-to-sport criteria utilized in current clinical practice are stringent enough to achieve safe and successful return-to-sport [32]. RTS decision making should not be based on time alone [184]. The 2016 consensus group from the First World Congress in Sports Physical Therapy defined an RTS continuum emphasizing a criterion-based progression from return to participation to RTS to return to performance [184]. Evidence strongly supports that patients should pass stringent RTS criteria before beginning on-field rehabilitation and returning to participation [184]. There is emerging evidence that assessment of psychologic readiness may also inform RTS decisions [184]. Altered psychosocial factors and movement patterns have been identified after ACL reconstruction, which can negatively influence rehabilitation outcomes [35]. These impairments are not addressed routinely in most ACL reconstruction rehabilitation protocols [35]. Knee impairments (effusion, pain, loss of ROM, and quadriceps weakness) can negatively affect ACL reconstruction rehabilitation outcomes and are the focus of early rehabilitation [42]. Movement patterns should be assessed for common deviations, particularly in late rehabilitation, when sport-specific tasks imparting high forces to the lower extremity are introduced [42]. Psychosocial factors (such as patient expectations, the fear of reinjury, and self-efficacy) should be monitored throughout ACL reconstruction rehabilitation because they can negatively affect ACL reconstruction rehabilitation outcomes [42]. Clinicians should cautiously progress patients from early to late rehabilitation in the presence of persistent knee impairments because progression too quickly may impede RTS and contribute to the early development of posttraumatic knee osteoarthritis [42]. The AAOS developed Appropriate Use Criteria for ACL injury prevention programs using the RAND/UCLA Appropriateness Method, where 69% of 48 patient scenarios were rated as likely appropriate/helpful for supervised rehabilitation programs [16]. Three ACL intervention programs successfully reduced noncontact ACL injury incidence rates in female adolescent athletes [193].

Complications and Long-Term Outcomes: Complications of ACL reconstruction include infection, loss of motion, arthrofibrosis, infrapatellar contracture syndrome, patellar tendon rupture and patella fracture from graft harvest, complex regional pain syndrome, hardware failure, tunnel osteolysis, local nerve irritation, cyclops lesion, graft failure, and late arthritis [34]. Reconstruction after an anterior cruciate ligament injury does not reduce the osteoarthritis risk but lowers the total knee arthroplasty rate [11]. A comprehensive assessment after ACL treatment should aim to provide a complete overview of the treatment result, optimally including early adverse events, patient-reported outcomes, ACL graft failure, and clinical measures of knee function and structure, with a minimum follow-up of 2 years and an optimal follow-up rate of 80% [7]. The most searched questions on Google regarding ACL tears or surgery relate to indications for surgery, pain, and activities postoperatively [151].

Complications¶

Graft Failure and Revision: Recurrent instability due to graft failure is estimated to occur in 0.7% to 8% of ACL reconstructions [218], though recent reports suggest a range of 10% to 25% failures [218]. Early failure, usually within the first 6 months, most often results from technical errors [218]. Factors potentially involved in failure include surgical technique, graft material selection, graft incorporation problems, secondary restraint integrity, articular and meniscal cartilage condition, postoperative rehabilitation, and patient motivation and expectations [218]. In a series of failed reconstructions, the prevalence of nonanatomical graft placement was identified [10]. Patient predictors of early revision surgery have been identified in a cohort study of 16,930 patients with 2-year follow-up [3]. The likelihood of needing revision ACL reconstruction was 0.82 times lower for every 0.5 mm increase in graft diameter from 7.0 to 9.0 mm [215]. Reports from the Norwegian and Danish ACL registries demonstrated higher risk of revision with hamstring grafts, with hazard ratios of 1.41 to 2.3, respectively [215]. Single-bundle reconstruction had an increased risk of revision surgery compared with double-bundle (adjusted hazard ratio [HR] 1.98, 95% CI 1.12 to 3.51, P = 0.019) in a Swedish National Knee Ligament Registry study [34].

Re-injury Risk: In patients aged 18 years and younger, a further ACL injury occurred in 1 in 3 patients over 15 years after ACL reconstruction [81]. Within 2 years after ACL reconstruction, as many as 20% sustain a second ACL injury to either the surgical or nonsurgical knee [35]. In the 24 months after ACLR and return to sport, patients are at a greater risk to suffer a subsequent ACL injury compared with young athletes without a history of ACL injuries [85]. The frequency of second ACL injuries in the first 12 months after reconstruction and return to sports in young, active patients has been reported to be 15 times greater than the overall frequency of a second ACL injury within 24 months [218]. Risk of ACL graft tear was significantly greater in males as compared to females and in male patients with a chronologic age 16 years as compared to those >16 years [89]. Patients undergoing primary ACLR as well as a strong family history of ACL tears are more likely to sustain a postoperative graft rupture or complication requiring surgery [18].

Infection: In a review of 7096 consecutive arthroscopic ACL reconstructions, 36 cases (0.51%) of postoperative septic arthritis were found [218]. The incidence of postoperative septic arthritis was 0.41% in primary reconstructions and 1.01% in revision reconstructions [218]. Coagulase-negative staphylococci (62.5%) and Staphylococcus aureus (21.9%) were the most common pathogens in postoperative septic arthritis cases [218]. Eradication of postoperative septic arthritis was achieved in all patients after a mean of 2.25 ± 1.22 procedures, with graft retention in all but one patient (97.2%) [218]. Diabetes has been identified to be a significant risk factor for infection following ACL reconstruction [218], with diabetics having an 18.8-times higher odds of postoperative infection than healthy patients [218]. Tobacco use is associated with increased complications after anterior cruciate ligament reconstruction [10]. In a review of 4933 patients after ACL reconstruction, superficial infections occurred in 10 patients (0.20%) and deep infections in 7 patients (0.14%) [218].

Motion Deficits and Stiffness: The most common postoperative complications are motion (primarily extension) deficits and persistent anterior knee pain [218]. Reported frequencies of motion loss after ACL reconstruction range from 1% to 13% [218]. Intraoperative factors associated with motion deficits most often are incorrect tunnel position and inadequate notchplasty [218], which can result in overtightening or impingement of the graft, leading to loss of extension [218]. Collateral ligament surgery and meniscal repair surgery have been reported to contribute to motion loss [218]. Postoperative factors contributing to motion deficits include prolonged immobilization and inadequate or inappropriate rehabilitation [218]. Preoperative effusion, limited range of motion, and concomitant knee ligament injuries make poor postoperative motion more likely [218]. Loss of normal knee motion after anterior cruciate ligament reconstruction is associated with radiographic arthritic changes after surgery [165]. Arthrofibrosis after ACL reconstruction is best treated in a step-wise approach with early recognition and intervention [165].

Pain and Donor Site Complications: Reported frequencies of postoperative pain after ACL reconstruction range from 0% to 34% [218]. The patellar tendon autograft has the highest incidence of anterior knee pain [34]. Hamstring autograft can result in permanent decreased hamstring strength which is a risk factor for graft rupture [34]. Patellar tendon ruptures after anterior cruciate ligament reconstruction have been described with specific tear patterns and surgical repair outcomes [3]. Infections and patellar tendon ruptures after anterior cruciate ligament reconstruction have been compared between ipsilateral and contralateral patellar tendon autografts [3]. Hamstring strength recovery after hamstring tendon harvest for anterior cruciate ligament reconstruction varies between graft types [3].

Thromboembolic Events: In a review of 4933 patients after ACL reconstruction, symptomatic DVT requiring treatment occurred in 27 patients (0.55%) and pulmonary embolism in 6 patients (0.12%) [218]. Risk factors for developing DVT in the 30-day postoperative period included age over 30 years, concomitant high tibial osteotomy, microfracture, hypertension requiring medication, and presence of wound infection [218]. In a study of multiligament knee reconstruction with thromboprophylaxis, the frequency of symptomatic venous thromboembolism was low [3].

Osteoarthritis and Long-Term Joint Degeneration: Current evidence estimates that radiographic knee osteoarthritis appears in over 50% of patients from 10 to 20 years after ACL reconstruction [215]. At 10 to 20 years after the diagnosis, on average, 50% of those with a diagnosed anterior cruciate ligament or meniscus tear have osteoarthritis with associated pain and functional impairment [5]. There is a lack of evidence to support a protective role of repair or reconstructive surgery of the anterior cruciate ligament or meniscus against osteoarthritis development [5]. The current literature does not support the prophylactic benefit of ACL reconstruction in reducing the rate of osteoarthritis after ACL injury [215]. In a cohort of 541 patients, graft choice had minimal effect on the prevalence of osteoarthritis at 10 years after surgery [10]. Athletes with an isolated ACL rupture showed no increased risk of the development of post-traumatic osteoarthritis in the long-term after ACL replacement when compared with the uninjured contralateral knee [156]. Significant factors predictive of osteoarthritis in the long-term were older age at surgery, medial meniscectomy, and knee extension loss [215]. In a 14-year follow-up study of a randomized controlled trial, there was an increased risk of osteoarthritis after anterior cruciate ligament reconstruction [3]. Inflammatory cytokines and biomarkers of cartilage metabolism were altered 8 years after anterior cruciate ligament reconstruction in operated and contralateral knees [3].

Other Considerations: Intraoperative complications include patellar fracture, inadequate graft length, mismatch between the bone plug and tunnel sizes, graft fracture, suture laceration, violation of the posterior femoral cortex, and incorrect femoral or tibial tunnel placement [218]. In a review of 4933 patients after ACL reconstruction, return to the operating room was required in 18 patients (0.36%) [218]. Impingement pressure has been evaluated in anatomical and nonanatomical anterior cruciate ligament reconstruction [10]. Nonanatomic tunnel position in traditional transtibial single-bundle anterior cruciate ligament reconstruction has been evaluated by three-dimensional computed tomography [10]. The effect of tibial drill angles on bone tunnel aperture during anterior cruciate ligament reconstruction has been studied [10]. Adjustable ACL suspension devices demonstrate less reliability in terms of reproducibility and irreversible displacement [3]. The contralateral normal knee anterior cruciate ligament is at a similar risk of anterior cruciate ligament tear (3.0%) as the anterior cruciate ligament graft after primary anterior cruciate ligament reconstruction (3.0%) [179]. In a 5- to 8-year follow-up, one out of every ten patients had a contralateral ACL rupture [75]. Women have a higher incidence of anterior cruciate ligament injury to the contralateral knee than men after reconstruction [202]. In female patients who experienced an ACL injury, a delay in surgery greater than 12 months is associated with a gradual increase in the risk of nonrepairable medial meniscal tear; this risk becomes statistically significant after 24 months [93]. Estimates of patient-reported outcomes at 32 to 37 years after surgical or nonsurgical management of ACL injury exceeded clinically important thresholds [20]. In a study of 3605 military members with knee injuries, 10% had developed posttraumatic arthritis at an average of 4 years after injury; knee dislocation was associated with the highest odds of developing posttraumatic arthritis [92]. Heterotopic ossification has been reported to occur in over 30% of patients with knee dislocations and may result in a stiff knee with loss of motion [92]. In a series of 119 patients with knee dislocations treated at a Level 1 trauma center, 32% had early complications and 9% required amputations [92]. 47 patients (39%) with knee dislocations required at least one unplanned secondary operation to treat instability or other complications [92]. Limitations in knee motion after knee dislocation were associated with high Injury Severity Score, infection, and heterotopic ossification [92].

Recovery¶

Long-term Outcomes and Osteoarthritis Risk: At 10 to 20 years after diagnosis, on average, 50% of individuals with a diagnosed anterior cruciate ligament or meniscus tear develop osteoarthritis with associated pain and functional impairment [5]. Variation in osteoarthritis outcome is reinforced by individual variables including age, sex, genetics, obesity, muscle strength, activity, and reinjury [5]. An ACL injury impairs knee-related quality of life for up to 35 years [30]. There is no difference in knee-related quality of life at 20 to 35 years between treatment approaches involving initial repair or later reconstruction compared with nonsurgical treatment [30]. Patients with ACL injuries treated with ACL reconstruction are less likely to undergo total knee arthroplasty at mid- to long-term follow-up compared with patients treated nonoperatively [79]. Patients with ACL and meniscus injuries treated with ACL reconstruction and meniscectomy are less likely to undergo total knee arthroplasty at mid- to long-term follow-up compared with patients treated nonoperatively [79]. Poor performance on single-legged hop tests is associated with the development of posttraumatic knee osteoarthritis after anterior cruciate ligament injury [86].

Nonoperative Management and Rehabilitation: Good functional performance and knee muscle strength can be achieved and maintained over time in the majority of patients with ACL injury treated with rehabilitation and early activity modification but without reconstructive surgery [147]. A 15-year follow-up study of patients with unilateral nonreconstructed ACL injury initially treated with rehabilitation and activity modification demonstrated good functional performance and knee muscle strength [147]. Surgical reconstruction as a management strategy for patients with long-standing ACL injury is more effective than rehabilitation management at 18 months [87]. Surgical reconstruction as a management strategy for patients with long-standing ACL injury is more expensive than rehabilitation management at 18 months [87]. Prolonged nonoperative therapy for complete ACL rupture remains controversial due to the risk of further joint damage and recurrent instability in skeletally immature patients [40].

Return to Sport and Reinjury Risk: Fifty-five per cent of patients return to competitive sport following anterior cruciate ligament reconstruction surgery [12]. Eighty-three per cent of elite athletes return to preinjury sport after anterior cruciate ligament reconstruction [12]. In the 24 months after ACL reconstruction and return to sport, patients are at a greater risk to suffer a subsequent ACL injury compared with young athletes without a history of ACL injuries [85]. After ACL reconstruction in patients aged 18 years and younger, a further ACL injury occurred in 1 in 3 patients over 15 years [81]. In a 5- to 8-year follow-up, two out of every ten patients had siblings with a history of ACL rupture [75]. Patients undergoing primary ACL reconstruction as well as those with a strong family history of ACL tears are more likely to sustain a postoperative graft rupture or complication requiring surgery [18]. Risk of ACL graft tear was significantly greater in males as compared to females in adolescent athletes [89]. Risk of ACL graft tear was significantly greater in male patients with a chronologic age 16 years as compared to those older than 16 years [89]. Psychological responses matter in returning to preinjury level of sport after anterior cruciate ligament reconstruction surgery [12]. Gait patterns differ between ACL-reconstructed athletes who pass return-to-sport criteria and those who fail [12]. Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport [12].

Timing of Intervention and Secondary Pathology: Patients who waited a long time to have ACL reconstruction had more cartilage damage and loss than patients who did not wait a long time [232]. Increasing time from injury to ACL reconstruction was associated with increased incidence of secondary injury seen in the trochlea, lateral femoral condyle, medial tibial plateau, and medial meniscus [235]. In female patients who experienced an ACL injury, a delay in surgery greater than 12 months is associated with a gradual increase in the risk of nonrepairable medial meniscal tear [93]. The risk of nonrepairable medial meniscal tear in female patients becomes statistically significant after 24 months of delay in surgery [93]. Osteochondral damage begins as early as 7 days after ACL injury [224].

Assessment and Outcome Reporting: The optimal follow-up rate for comprehensive assessment after ACL treatment is 80% [7]. The minimum follow-up duration for comprehensive assessment after ACL treatment is 2 years [7].

Key Evidence¶

[L3] The classification algorithm is an effective tool for prospectively identifying individuals early after anterior cruciate ligament injury who want to pursue nonoperative care or must delay surgical intervention and have good potential to do so. [1] (10.1177/0363546507308190)
[L4] This article provides an overview of the literature on the treatment of anterior cruciate ligament injuries by summarizing the highest level of scientific evidence available, including biomechanical behavior, prevalence, natural history, associated injuries, risk factors, and indications for nonoperative and operative treatments. [2] (10.1177/0363546505279913)
[L3] Surgeons can counsel patients with complex ACL injuries that comprehensive surgical management of concomitant injuries is associated with good long-term outcomes equivalent to those with less severe injury patterns. [4] (10.1177/03635465261443315)
[L4] [5] (10.1177/0363546507307396)
[L2] A history of anterior cruciate ligament reconstruction is a risk factor for further injury, with the highest risk in the first year after reconstruction. [6] (10.1177/03635465010290021301)
[L5] A comprehensive assessment after ACL treatment should aim to provide a complete overview of the treatment result, optimally including early adverse events, patient-reported outcomes, ACL graft failure, and clinical measures of knee function and structure, with a minimum follow-up of 2 years and an optimal follow-up rate of 80%. [7] (10.1177/2325967120934751)
[L5] This consensus paper provides practical guidelines for how the aforementioned entities of outcomes should be reported and suggests the preferred tools for a reliable and valid assessment of outcome after ACL treatment. [8] (10.1136/jisakos-2020-000494)
[L5] This consensus paper provides practical guidelines for how the aforementioned entities of outcomes should be reported and suggests the preferred tools for a reliable and valid assessment of outcome after ACL treatment. [9] (10.1007/s00167-020-06061-x)
[L3] [11] (10.1177/23259671261451241)
[L3] Injury chronicity did not adversely affect failure rates or clinical outcomes at minimum 2-year follow-up after ACLPR. [14] (10.1002/arj.70193)
[L4] ACL repair for proximal ACL tears in the paediatric population demonstrates the potential for excellent outcomes at short-term follow-up. [15] (10.1007/s00167-020-05872-2)
[L5] The AAOS developed Appropriate Use Criteria for ACL injury prevention programs using the RAND/UCLA Appropriateness Method, where 69% of 48 patient scenarios were rated as likely appropriate/helpful for supervised rehabilitation programs. [16] (10.5435/jaaos-d-16-00756)
[L1] Functional results after ACL repair for acute tears are comparable to those after ACLR. [17] (10.1177/2325967120s00326)
[L3] Further, patients undergoing primary ACLR as well as a strong family history of ACL tears are more likely to sustain a postoperative graft rupture or complication requiring surgery. [18] (10.1177/2325967120959665)
[L2] Estimates exceeded clinically important thresholds, highlighting the importance of assessing these constructs when managing individuals with ACL injuries. [20] (10.1177/23259671211021592)
[L3] Ideally, and particularly in younger patients, ACL reconstruction should not be delayed more than five months from injury. [22] (10.1302/0301-620x.95b1.29636)
[L3] The ACLISS allowed to easily and rapidly identify different injury severity profiles in patients who underwent primary ACLR. [23] (10.1007/s00167-023-07311-4)
[L2] Prediction models including knee function measures can estimate 2-year prognoses for nonsurgical treatment. [26] (10.1177/2325967118774255)
[L5] Patients with an ACL rupture need to be counselled appropriately, and efforts to prevent these injuries must be redoubled. [28] (10.1302/0301-620x.104b5.bjj-2022-0239)
[L2] An ACL injury impairs knee-related QoL for up to 35 years, with no difference between treatment approaches (initial repair or later reconstruction compared with nonsurgical treatment). [30] (10.1177/03635465231218237)
[L2] These findings raise concern whether the variable ACL return-to-sport criteria utilized in current clinical practice are stringent enough to achieve safe and successful return-to-sport. [32] (10.2519/jospt.2017.7285)
[L5] Prolonged nonoperative therapy for complete ACL rupture remains controversial due to the risk of further joint damage and recurrent instability. [40] (10.1155/2012/932702)
[L4] However, a LEAP is not always an innocuous procedure and carries the risk of certain complications, and should not be used ubiquitously in every primary ACLR. [41] (10.1016/j.jisako.2022.10.004)
[L3] Combined medical history and physical examination have strong diagnostic value in ACL rupture diagnostics performed by an orthopaedic surgeon, whereas for the primary care physician, only medical history appeared to be of value. [44] (10.1007/s00167-013-2769-4)
[L4] In more than one-quarter of patients, plain radiographs may help to establish the diagnosis of an ACL tear. [45] (10.1007/s00167-014-3022-5)
[L3] However, ALL injuries were not reliably diagnosed on MRI in the setting of an ACL tear, and physicians should not rely on MRI to diagnose an ALL injury in the presence of an ACL injury. [46] (10.1016/j.arthro.2019.09.039)
[L4] According to current standard indications, 34.9% of all meniscal injuries offer the potential for repair, rising to 55.6% when accompanied by anterior cruciate ligament damage. [47] (10.1016/j.arthro.2018.08.051)
[L4] Also, 53% to 56% of the patients presenting with initially diagnosed isolated ACL ruptures had concomitant injuries to the KFC. [48] (10.1177/03635465211015682)
[Case_report] Early recognition via MRI is critical, and primary ACL reconstruction is a reliable treatment when anatomical repair is not feasible. [49] (10.1016/j.jisako.2026.101072)
[L3] [51] (10.1016/j.otsr.2022.103257)
[L3] Recognizing these abnormalities preoperatively is crucial to adequately evaluate for growth disturbance following ACL reconstruction. [52] (10.1177/2325967117s00437)
[L3] The data from this study may help clinicians to identify patients at a greater risk of ACL injury. [53] (10.2106/jbjs.17.01011)
[L3] A positive finding of this structure ought to be included in the group of radiologic signs relevant for the diagnosis of ACL lesion. [54] (10.1177/03635465010290060601)
[L3] [57] (10.1177/0363546520931854)
[L4] MRI evaluation showed ALL injuries are present in 60.2% of acute ACL injuries in adolescent patients. [60] (10.1016/j.arthro.2019.02.034)
[L1] The clinical utility of imaging-based measurement methods for the determination of ACL injury risk requires more reliable techniques that demonstrate consistency between studies. [61] (10.1177/0363546512442307)
[L3] Secondary meniscal tears after ACL injury are most common among patients undergoing delayed surgical or nonoperative treatment of their primary ACL injuries. [63] (10.1177/0363546519844481)
[L4] Despite the promising clinical outcomes, this technique should not be widely adopted unless it has been compared directly with ACL reconstruction. [66] (10.1097/corr.0000000000001118)
[L5] The author believes that contemporary ACL repair will apply to a subset of patients and is an attractive, minimally morbid, biologic alternative to ACL reconstruction, though well-powered prospective studies are needed to compare outcomes with reconstruction. [69] (10.1097/corr.0000000000001244)
[L4] The plane in which an MRI scan is performed affects the classification of ACL tears. [70] (10.1007/s00167-022-07068-2)
[L2] [74] (10.1177/23259671251339491)
[L3] In a 5- to 8-year follow-up, one out of every ten patients had a contralateral ACL rupture, and two out of every ten patients had siblings with a history of ACL rupture. [75] (10.1007/s00167-019-05781-z)
[L5] This concept allows a more precise description of ACL rupture patterns and might lead to a more distinctive approach for reconstructive surgery. [76] (10.1007/s00167-007-0337-5)
[L3] The anatomical parameters of the knee joint (i.e., notch width index, anterior cruciate ligament width and length) have no role in the cause of an injury. [78] (10.1007/s00402-015-2292-9)
[L2] Patients with ACL injuries treated with ACL reconstruction and patients with ACL and meniscus injuries treated with ACL reconstruction and meniscectomy are less likely to undergo TKA at mid- to long-term follow-up compared with patients treated nonoperatively. [79] (10.1016/j.arthro.2025.07.028)
[L4] This document presents survey results from a global group of surgeons regarding their management preferences for partial ACL tears, including treatment choices, rehabilitation protocols, and return-to-sport criteria. [80] (10.1177/23259671241311603)
[L4] After ACL reconstruction in patients aged 18 years and younger, a further ACL injury occurred in 1 in 3 patients over 15 years. [81] (10.1177/0363546515623032)
[L3] This indicates that previous injury of any type does not increase the risk of suffering an ACL injury. [83] (10.1177/2325967116669708)
[L2] In the 24 months after ACLR and return to sport, patients are at a greater risk to suffer a subsequent ACL injury compared with young athletes without a history of ACL injuries. [85] (10.1177/0363546514530088)
[L2] Clinical measures of knee function were most predictive of subsequent OA development following an extended period of rehabilitation early after ACL injury. [86] (10.1177/2325967118810775)
[L1] Surgical reconstruction as a management strategy for patients with long-standing ACL injury is more effective, but more expensive, at 18 months compared to rehabilitation management. [87] (10.1302/0301-620x.106b1.bjj-2023-0175.r1)
[L3] [88] (10.1136/bjsports-2023-107113)
[L3] Risk of ACL graft tear was significantly greater in males as compared to females and in male patients with a chronologic age 16 years as compared to those >16 years. [89] (10.1177/2325967125s00301)
[L3] These structures may function as important secondary stabilizers in an ACL-injured knee. [91] (10.1177/0363546516659649)
[L3] In female patients who experienced an ACL injury, a delay in surgery greater than 12 months is associated with a gradual increase in the risk of nonrepairable medial meniscal tear; this risk becomes statistically significant after 24 months. [93] (10.1016/j.arthro.2022.10.014)
[L1] [94] (10.1002/ksa.12747)
[L4] Changing toe direction significantly affects knee kinetics and kinematics during landing. [95] (10.1007/s00167-013-2815-2)
[L4] A classification system for ACL tears can guide a proper surgical plan. [96] (10.1177/2325967120s00255)
[L3] Jump direction significantly influenced knee biomechanics, suggesting that lateral jumps are the most dangerous of the stop-jumps. [97] (10.1177/0363546505278696)
[L2] These data may be helpful in diagnosing and managing ACL injuries in athletes and could be used in the clinic as a baseline by which to compare and identify patients who might exhibit increased rotatory laxity. [101] (10.1007/s00167-017-4684-6)
[L3] Clinicians should consider routinely assessing PTS in newly presenting patients with an initial ACL tear as marked deviations could be a predisposing risk factor for subsequent contralateral tear. [104] (10.1177/2325967123s00246)
[L3] Patients with ACL tears treated non-operatively developed secondary meniscal lesions requiring delayed surgical management. [107] (10.1007/s00167-018-5201-2)
[L3] Combinations of knee joint geometry measurements provided more information about the risk of noncontact ACL injury than individual measures, and the aspects of geometry that best explained the relationship between knee geometry and the risk of injury were different between males and females. [108] (10.1177/0363546514563277)
[L4] Increasing the knee flexion angle during jump landing may be an effective intervention to improve knee biomechanical risk factors associated with an ACL injury. [112] (10.1177/0363546516634000)
[L4] Isolated ACL tear is less common than combined injuries, which are quite frequent. [114] (10.1177/03635465221092767)
[L3] Biomechanical factors such as reduced KAM or reduced knee flexion angle during gait could serve as early indicators for OA risk and inform targeted rehabilitation interventions. [116] (10.1002/ksa.70183)
[L2] Patients in the ACLR group had higher future knee-related self-efficacy and were active at a higher level of physical activity at 8 and 12 months after treatment. [117] (10.1002/ksa.70042)
[L4] The treatment strategy of an immediate repair of the MCL and reconstruction of the ACL when conservative treatment has failed seems safe and effective. [118] (10.1007/s001670050081)
[L2] Using waveforms, instead of discrete peak values of the knee abduction moment, may better represent risky movement patterns. [120] (10.1002/ksa.12471)
[L3] It is possible that decreasing tibial length relative to femoral length alters lower extremity biomechanics in such a manner that places the ACL at risk for injury. [121] (10.1177/23259671251343811)
[L3] Furthermore, ACLR had a beneficial effect on knee outcomes, with acute-stage ACLR leading to better outcomes in the subgroup analysis. [122] (10.1186/s12891-024-08102-9)
[L5] The meniscus status does have a significant impact on knee kinematics in the ACL-deficient knee. [123] (10.1007/s00264-014-2581-x)
[L5] Frontal plane kinematic temporal relationships at the hip, knee, and ankle differ between genders, with components of dynamic knee valgus peaking during the deceleration phase in women and during the acceleration phase in men. [127] (10.1177/0363546510397175)
[L1] Contrary to clinical opinion, the findings indicate that knee abduction kinematics and kinetics during weight-bearing activities may not be risk factors for future ACL injury. [130] (10.1186/s12891-020-03552-3)
[L2] Knee motion and knee loading during a landing task are predictors of anterior cruciate ligament injury risk in female athletes. [132] (10.1177/0363546504269591)
[L3] Clinical outcomes of patients that underwent meniscus repair were better than those that underwent meniscus resection with concurrent ACL reconstruction. [134] (10.1007/s00167-020-05931-8)
[L4] The improvement rate was 24.6% among patients who did not receive ACLR in the acute phase and underwent ACLR in the chronic phase. [135] (10.1002/ksa.12212)
[L3] Otherwise, transphyseal ACLR has satisfactory clinical outcomes, with good subjective outcomes, function level, and knee stability. [136] (10.1177/2325967116664685)
[L2] Short-term progressive exercise therapy programs are well tolerated and should be incorporated in early-stage ACL rehabilitation, either to improve knee function before ACL reconstruction or as a first step in further nonoperative management. [139] (10.2519/jospt.2010.3345)
[L2] The diagnostic ability of MRI to predict meniscal injuries present at acute ACL reconstruction was moderate, with performance being poorest at the lateral meniscus. [140] (10.1177/23259671221079338)
[L4] MRI may not be completely reliable in assessing the degree and location of an ACL tear, and surgeons should not solely rely on MRI imaging for ruling out possible ACL repair. [144] (10.1177/2325967119s00398)
[L3] MRI is unreliable for determining the precise location of an ACL tear. [145] (10.1177/2325967125s00286)
[L4] Good functional performance and knee muscle strength can be achieved and maintained over time in the majority of patients with ACL injury treated with rehabilitation and early activity modification but without reconstructive surgery. [147] (10.1177/0363546507305018)
[L4] Conservative management of ACL rupture in paediatric population is a valuable treatment which permits return to normal life with sports activities without major increasing of meniscal tears. [149] (10.1016/j.otsr.2018.09.001)
[L4] The most searched questions on Google regarding ACL tears or surgery related to indications for surgery, pain, and activities postoperatively. [151] (10.1016/j.asmr.2023.100805)
[L2] Non-ACLR treatment may therefore be a viable option for selected patients, particularly those who are older or have lower activity demands, to achieve satisfactory RTS outcomes. [154] (10.1177/03635465261451698)
[L4] Athletes with an isolated ACL rupture showed no increased risk of the development of post-traumatic osteoarthritis in the long-term after ACL replacement when compared with the uninjured contralateral knee. [156] (10.1016/j.arthro.2011.09.014)
[L3] Altered bone morphological characteristics of the knee were found to be risk factors for ACL tears in contact injuries as well as noncontact injuries. [157] (10.1177/23259671231179757)
[L5] Subtotal medial meniscectomy in knees with ACL deficiency altered knee kinematics, especially at high flexion angles. [158] (10.1016/j.arthro.2008.09.020)
[L3] Standard preoperative MRI scans can reliably be used to visualize Kaplan fiber injury in the majority of pediatric and adolescent patients with ACL tears, especially when the MRI is performed in the acute setting. [159] (10.1177/23259671221128601)
[L4] MRI accurately predicted the ACL tear type seen arthroscopically in only 55.9% of patients. [161] (10.1177/2325967124s00302)
[L3] Early ACL reconstruction is recommended also for the prevention of secondary meniscal tear. [162] (10.1007/s00402-015-2309-4)
[L2] The diagnostic validity of magnetic resonance imaging is similar for meniscal tears in acute knee trauma and in knee symptoms lasting over 6 months in young adults. [164] (10.1177/0363546508329543)
[L1] [165] (10.1186/s13018-019-1127-8)
[L5] Most isolated medial collateral ligament injuries are treated nonsurgically, while concomitant damage to the anterior or posterior cruciate ligaments is a common indication for surgical management of high-grade injuries. [166] (10.5435/00124635-200903000-00004)
[L4] The most frequently asked questions on Google pertaining to ACL tear and ACL surgery are related to indications/management, technical details, and timeline of recovery. [167] (10.1016/j.asmr.2025.101112)
[L2] Knee laxity increased bilaterally during the first year after non-surgically treated ACL injury. [171] (10.1016/j.jisako.2023.03.097)
[L3] Knee torsional alignment is associated with ACL injury, predominantly in the distal femur rather than the proximal tibia. [173] (10.1186/s13018-024-04609-y)
[L4] Conversely, landing upright appears to be ACL harmful by increasing post-impact force and quadriceps muscle activity while decreasing knee flexion angles. [174] (10.1007/s00167-012-2011-9)
[L2] MRI is a sensitive measure of cruciate and collateral ligament injury in acute knee dislocation; however, it does not reliably diagnose injury to the posterolateral corner or meniscus, and therefore, a higher index of suspicion is required during arthroscopy to prevent misdiagnosis which could affect long-term clinical outcome. [176] (10.1007/s00167-015-3857-4)
[L2] The contralateral normal knee anterior cruciate ligament is at a similar risk of anterior cruciate ligament tear (3.0%) as the anterior cruciate ligament graft after primary anterior cruciate ligament reconstruction (3.0%). [179] (10.1177/0363546507301318)
[L3] Patients who had radiographic knee OA 5 years after ACL reconstruction walked with lower knee adduction moments and medial compartment joint contact forces than did those patients without OA early after injury and reconstruction. [181] (10.1177/0363546515608475)
[L3] Initial nonoperative treatment of ACL tears in children and adolescents carries a high risk of additional meniscal and chondral injury, which may result in long-term knee impairment. [183] (10.1177/2325967114s00037)
[L2] Poor agreement was found between imaging and arthroscopic assessment of ACL tear location, as MRI predicted intraoperative ACL tear location in less than half of cases. [186] (10.1177/23259671251397389)
[L1] Nonoperative and operative treatments of medial collateral ligament injuries lead to equally good results. [187] (10.1177/0363546505284889)
[L4] However, their correlation with subjective and objective measures of knee function, as well as with the risk of secondary osteoarthritis, is unknown. [189] (10.1177/23259671261443910)
[L2] Three ACL intervention programs successfully reduced noncontact ACL injury incidence rates in female adolescent athletes. [193] (10.1016/j.arthro.2013.10.009)
[L3] The methods to quantify knee joint laxity may be used in conjunction with measures of neuromuscular control to identify high-risk female athletes. [196] (10.1177/0363546507313572)
[L1] The highest rated studies reported low prevalence of knee osteoarthritis for individuals with isolated anterior cruciate ligament injury (0%-13%) and a higher prevalence of knee osteoarthritis for subjects with combined injuries (21%-48%). [198] (10.1177/0363546509338827)
[L2] Women have a higher incidence of anterior cruciate ligament injury to the contralateral knee than men after reconstruction. [202] (10.1177/0363546508325665)
[L1] Patients after nonoperative and delayed treatment experienced more instability/pathological laxity and inability to return to previous activity levels than did patients treated with early surgical stabilization. [203] (10.1177/0363546513510889)
[L3] Identification of medial bone bruising on pre-operative MRI following acute ACL rupture should raise suspicion of an associated postero-medial meniscal tear. [214] (10.1007/s00167-019-05490-7)
[L4] MRI findings, including direct signs like fiber discontinuity and indirect signs like bone contusions, correlate well with arthroscopic findings for ACL, PCL, and posterolateral corner injuries, aiding in the diagnosis and surgical planning of knee pathologies. [219] (10.1016/j.csm.2013.03.004)
[L2] The presence of high-grade cartilage lesions is significantly increased in an ACL-deficient knee when reconstruction is performed more than 12 months after injury. [220] (10.1007/s00167-013-2497-9)
[L2] At the time of acute ACL injuries, posterior root tears of the lateral meniscus do not appear to result in meniscal extrusion on MRI. [221] (10.1177/0363546513506551)
[L3] In a large cohort of ACL tears with age-matched controls, the analysis identified multiple radiographic parameters which were significantly associated with ACL injuries when compared with the controls. [223] (10.1016/j.arthro.2020.12.114)
[L5] The data revealed that osteochondral damage begins as early as 7 days after ACL injury, far earlier than the traditionally recognized 4-week timeline. [224] (10.1002/arj.70355)
[L5] Meyer et al provide a strong foundation for the next generation of morphometric ACL injury risk research by showing that AI can extract and synthesize 3D anatomic signatures from routine MRI. [225] (10.1002/arj.70160)
[L5] The authors question the clinical utility of MRI for definitive diagnosis of Kaplan fiber complex injury and caution against using these findings to determine patients for lateral extra-articular augmentation, noting no correlation has been shown between KFC injury and increased anterolateral rotatory laxity or subjective instability symptoms. [226] (10.1177/03635465211049385)
[L4] MRI signal alterations steadily occur within the first 6 months postoperatively, revealing an ongoing healing process that must be considered when discussing return to sport. [227] (10.1007/s00167-018-5335-2)
[L3] This study shows that an increased LFCR is related to an increased risk of noncontact ACL injury as determined by MRI. [228] (10.1186/s12891-022-05134-x)
[Paper] Spontaneous anterior tibial subluxation on MRI in complete ACL tear was significantly associated with medial meniscal tear and accident-to-MRI time. [229] (10.1016/j.otsr.2019.10.025)
[L3] This study identifies specific osseous morphologic characteristics associated with ACL injury in this population. [230] (10.2106/jbjs.15.01163)
[L2] The osseous morphological risk factors should be considered in individualized anatomic ACL reconstructions to achieve optimal outcomes. [231] (10.2106/jbjs.19.00535)
[L4] Patients who waited a long time to have ACL reconstruction had more cartilage damage and loss than patients who did not wait a long time. [232] (10.1177/03635465010290012001)
[L3] Increasing time from injury to ACLR was associated with increased incidence of secondary injury seen in the trochlea, lateral femoral condyle, medial tibial plateau, and medial meniscus. [235] (10.1177/0363546515574061)

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