Introduction

Management of anterior cruciate ligament (ACL) injury has evolved significantly in the past several decades. Prior fundamental questions, such as anatomic versus nonanatomic reconstruction, anteromedial versus transtibial drilling, and use of allografts in young patients, have largely been answered. Nevertheless, countless other questions have been investigated over the past several years. This study seeks to summarize the recent literature on the clinical care of ACL injury and highlight topics of particular interest.

Evaluation

Physical examination

Physical examination is an essential element in the setting of suspected ACL injury and involves the anterior drawer, Lachman, and pivot-shift tests (Fig. 1). The anterior drawer test has been shown to have poor sensitivity (49%) and specificity (58%) in the acute setting [16]. The anterior drawer test performs substantially better in the chronic setting and when secondary restraints are concomitantly injured [16, 145]. The Lachman test has much higher sensitivity (86%) and specificity (91%) [145]. The Lachman test only assesses the anteromedial bundle of the ACL and does not evaluate rotatory knee laxity [48]. The pivot-shift test is useful to assess rotatory knee laxity by applying a valgus stress and axial load while internally rotating the tibia. Because it tests lateral compartment rotatory laxity related to ACL insufficiency, the pivot shift is able to assess the posterolateral bundle of the ACL more directly than both the anterior drawer and Lachman tests [62]. The pivot shift demonstrates very high specificity (98%), but low sensitivity (32%) [16]. Because of the relative complexity of the pivot-shift test, examiner experience and technique play a major role in testing accuracy [121, 156]. Computer tablets recording the pivot-shift test have been shown to be reproducible and valid in quantifying tibial translation and thus the degree of rotatory knee laxity [82, 83].

Fig. 1
figure 1

Assessment of ACL injury with the Lachman and pivot-shift tests. A The Lachman test evaluates the amount of anterior tibial translation and firmness of the endpoint relative to the contralateral side, with increased translation and a soft endpoint in a positive test. B The pivot-shift test involves the examiner providing a valgus (1) and internal rotation (2) force with the proximal hand while axially loading the ankle with the distal hand (3) to flex the knee. C In a positive test, as the knee flexes to roughly 30 degrees, the anteriorly subluxated tibia reduces with a visible glide or clunk posteriorly (4)

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Imaging

Magnetic Resonance Imaging (MRI) can confirm equivocal clinical diagnoses, identify associated injuries, provide information about prognosis via detection of intra-articular damage, and assist during preoperative planning. The presence and location of a bone bruise can provide insight into the injury mechanism and associated injuries [39]. For example, a lateral bone bruise has been associated with a higher prevalence of lateral meniscal tears [39, 176, 190]. A medial bone bruise is associated with medial meniscus tears, with medial meniscus injury present in 66% of a series of ACL injuries with both lateral and medial bone contusions on MRI [18, 190]. Bone bruises have been shown to be associated with increased risk of chondral damage and subsequent development of osteoarthritis (OA) [59, 166]. At 12-month follow-up after ACL reconstruction (ACL-R), MRI has been shown to detect cartilage alterations including fibrillation, thinning, and areas of cartilage loss overlying areas of bone bruising seen on preoperative MRI [166]. Imaging findings of cartilage destruction have been supported by histological data and joint fluid analysis that suggest chondral damage [170].

Beyond identification of intra-articular pathology, MRI allows the surgeon to estimate autograft size, ACL insertion site size, and notch width [8]. MRI evaluation of the size of the patellar tendon, quadriceps tendon, and hamstring tendon is reliable with moderate-to-good accuracy when compared with intraoperative graft measurement [162, 192]. The size of the ACL insertion site on the tibia, as well as the femur, can be useful if the surgeon considers double bundle ACL-R in the setting of a large (> 18 mm) insertion site to better replicate the insertion [67, 146]. Notch width can also factor into graft size, with narrower notches better accommodating a smaller graft [174].

Table 1 summarizes the key facts on evaluation of ACL injury.

Table 1 ACL injury evaluation fact box
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Consideration of operative or nonoperative management

ACL-R is generally the preferred treatment for restoration of knee stability in young and active patients, particularly those hoping to return to sports that require pivoting or change of direction [122]. In older and less active patients, and/or those participating in linear activities alone, nonoperative management may be more strongly considered [48]. Evaluating the data on the merits of operative or nonoperative management requires evaluation of objective knee laxity, subsequent injuries, and information about patient-reported outcomes (PROs).

The most consistent data on the topic are related to the evaluation of objective knee laxity. There is substantial evidence that ACL-R significantly decreases anterior tibial translation and pivot shift [60, 114, 153, 172]. These findings are the basis for concerns about possible subsequent injuries and worse outcomes related to nonoperative management.

Without the primary restraint of the ACL to anteroposterior translation and rotation, increased load is placed on the secondary stabilizers to these motions, often leading to injury. This is demonstrated in a recent systematic review of long-term outcomes of nonoperative and operative management of over 2200 ACL injuries, showing significantly higher rates of subsequent meniscus surgery in the nonoperative group (29.4 vs 13.9%) [29]. This in part contributed to significantly higher rates of repeat knee surgery in the nonoperative group (24.9 vs 12.4%). Similarly, a database study of nearly 1400 ACL injuries found that long-term (mean 8–14 year follow-up) rates of secondary meniscal tears were 7% in the acute ACL-R group, 19% in the nonoperative group, and 33% in the delayed ACL-R group, with significant differences between the groups [69]. On the other hand, a Level 1 study of 121 patients at 5-year follow-up showed no difference in subsequent meniscus surgery between operative and nonoperative groups, though there was significant crossover from the nonoperative to operative group [60].

Progression of OA is an important indicator of the success of operative and nonoperative management. Many studies have found no differences in terms of short-to-long-term rates of OA [29, 69, 114, 172, 181]. Increased rates of OA were found in a systematic review of 1,397 patients in studies with greater than 10-year follow-up (OR 1.56), which was not seen in studies with shorter follow-up, i.e., 5–10 years [153]. A study of nonoperatively-managed ACL-injured patients at 20-year follow-up showed arthritic changes in 82.4% [64]. Comparative high-level long-term studies are needed to evaluate the risk of OA with nonoperative versus operative management of ACL injuries.

In terms of patient outcomes, the literature is mixed. The first randomized controlled trial on nonoperative versus operative management of ACL tears showed no difference in KOOS, SF-36, or Tegner scores among 121 ACL-injured patients at 5 years [60]. The study was limited by 51% of the nonoperative cohort crossing over into the operative cohort, but their “as-treated analysis” showed the same findings. A smaller randomized study with 10-year follow-up found significantly higher IKDC Subjective Knee Form (SKF) Scores in the operative group (86.6 vs 77.5) [172]. A Level 2 study of 105 combined operative and nonoperative ACL-injured patients showed that at 5 years, there were no differences in performance in single-leg hop tests, return to preinjury activity level, IKDC SKF, and most KOOS subscales [180].

Screening tests to identify potential copers (able to return to sport without ACL-R) incorporate objective knee function testing, PROs, and episodes of functional instability [86]. Outcomes of these tests are mixed and suggest that a minority of ACL-injured patients are able to return to cutting and pivoting sports without ACL-R [131]. With nonoperative treatment of the torn ACL, most patients will become ‘adapters’ (willing to reduce activity levels to prevent instability) and ‘non-copers’ (unable to avoid instability or return to prior level without ACL-R) [119].

Amidst at times conflicting data, an international group of experts on ACL management evaluated the literature and published consensus statements on the management of ACL injuries in 2019 (Fig. 2) [48]. Notably, there was unanimous agreement in the following: operative and nonoperative treatments are both acceptable treatment options; in patients wishing to return to jumping/cutting/pivoting sports, operative treatment is preferred; and in patients wishing to return to straight plane activities, nonoperative treatment is an option, however it is less appropriate if the patient experiences recurrent functional instability in daily life.

Fig. 2
figure 2

Reproduced from: Treatment after anterior cruciate ligament injury: Panther Symposium ACL Treatment Consensus Group. Diermeier T, et al. Knee Surgery, Sports Traumatology, Arthroscopy 28(8):2390–2402©2020 with permission from BMJ Publishing Group Ltd

Consensus statements of the Panther Symposium on nonoperative and operative management of ACL injury.

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In the pediatric population, nonoperative management shows consistently inferior outcomes with significantly higher recurrent functional instability (75 vs 14%), higher subsequent medial meniscus tears (35 vs 4%), and lower rates of return to previous levels of sport (0 vs 86%) [140]. A more recent systematic review confirmed widely disparate return to play rates [22].

Table 2 summarizes the key facts on considerations of operative versus nonoperative management of ACL injury.

Table 2 Operative versus nonoperative management of ACL injury fact box
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Considerations in unique populations

Literature on ACL-R outcomes largely focuses on patients from skeletal maturity up to middle age. However, increased appreciation of the importance of the ACL has led to more research on ACL-R in younger and older populations that may also benefit from the procedure. Additionally, as female participation in sports continues to increase, there has been a strong push to evaluate the role of sex on outcomes after ACL-R. These three subsets of ACL-R patient populations warrant focused discussion.

The prevailing dogma of the twentieth century was that pediatric ACL rupture was an uncommon injury, owing mainly to pediatric skeletal immaturity and generalized joint laxity [42]. However, pediatric competitive athletics are increasing, and in turn, the incidence of ACL-Rs in patients aged 3–20 years increased from 17.6 to 50.9 per 100,000 over the past 20 years [50]. This paradigm shift in the management of pediatric ACL injuries has been substantiated by studies demonstrating that opting for nonoperative or delayed management of pediatric ACL injury is associated with subsequent meniscal and chondral injuries [7, 52, 103], worse PROs, increased rates of knee laxity and concomitant ligamentous injuries, and lower rates of return to preinjury activity level [53].

It is now widely accepted that early autograft reconstruction is the standard of care in pediatric and adolescent ACL injuries, with allografts having up to three times the failure rate as autografts [55, 140]. The techniques for creating the tunnels for pediatric ACL-R are more widely debated [135]. There are three commonly accepted techniques: physeal-sparing reconstruction with the tunnel(s) avoiding the physis of both the tibia and femur, partial transphyseal tunnels crossing the tibial physis but not the femoral physis, and transphyseal reconstruction with tunnels through both the tibial and femoral physes [135]. To date, clinical and biomechanical studies comparing these techniques are scarce, equivocal, and lack the power to guide clinical practice [135]. Additionally, there are no known randomized clinical trials comparing these techniques, likely due to the ethical dilemmas surrounding randomization of the pediatric population to surgical techniques. However, given the increased incidence of these injuries in the setting of increased pediatric participation in sports, it is expected that this topic will be at the forefront of pediatric ACL-R research in the coming years. Collaboration via multi-center studies and registries is of great benefit to pediatric ACL-R research by allowing for increased sample sizes and thus power.

The middle-aged population is another unique demographic group for ACL-R. Prior literature set a threshold of 40 years of age for an “older” population [14, 20, 34, 100]. With physical activity becoming more important to the middle-aged population, recent studies have pushed the ‘older’ threshold to 50 years of age and found that patients in this age group benefit biomechanically, clinically, and functionally after ACL-R [19, 34, 38, 175]. A recent systematic review evaluating 16 studies of 470 ACL-Rs in patients aged 50–75 found that surgical intervention significantly improved clinical and functional outcomes in all of the reviewed studies, concluding that increased age is not a contraindication to ACL-R [36]. These findings have expanded the indications for ACL-R and have opened the door for larger scale clinical studies that will continue to shape indications and techniques to optimize ACL-R in a population that, two decades ago, was unlikely to have been offered surgery.

Sex has also become an increasingly studied factor in ACL injury management over the past two decades, particularly in terms of rehabilitation and prognosis. It is widely accepted that females are at an increased risk for ACL injury [3, 77, 138]. Recent studies have demonstrated that females have inferior outcomes after ACL-R, with a recent systematic review and meta-analysis evaluating 135 publications and over 120,000 patients showing inferior PROs in females [163]. Males have greater strength recovery of the hamstrings and quadriceps after ACL-R [65, 95, 195]. One study evaluating 320 patients who underwent primary ACL-R with soft tissue quadriceps tendon found that female patients had greater deficits in quadriceps strength and extension range of motion up to six months after surgery compared with males [85], while another found significantly greater hamstring strength deficits in females after ACL-R with a hamstring tendon graft up to one year postoperatively [40]. Lastly, females have consistently been shown to have lower rates of return to sport relative to their male counterparts after ACL-R [11, 163]. There may be a higher rate of “psychological readiness” that allows males to return to their preinjury level of play earlier than females after ACL-R [179].

Table 3 summarizes the key facts on ACL injury management considerations in unique populations.

Table 3 ACL injury management considerations in unique populations fact box
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Graft selection

ACL graft selection must consider not only the biological and biomechanical properties of the graft itself, but also the clinical demands, characteristics, and expectations of the patient.

Bone-patellar tendon-bone (BTB) autograft

The most commonly utilized ACL-R graft historically was the BTB autograft [88]. A key advantage is more robust and rapid initial fixation compared with soft tissue grafts due to the retention of a native tendon-bone interface [129]. Patellar tendon grafts create a pure bony interface that has been demonstrated to be stronger than the fibrovascular scarring after soft tissue-to-bone healing [182]. Static laxity testing (instrumented laxity testing, Lachman, pivot shift) has consistently been superior in BTB compared with hamstring grafts [115, 116]. In a recent large registry study, risk of revision for graft rupture was twice as high among patients treated with hamstring compared with BTB [134]. PROs, patient satisfaction, and time to return to sport are similar between BTB and hamstring [81, 107, 110, 115, 116, 168]. Disadvantages with BTB autograft include graft-tunnel mismatch, donor-site morbidity with anterior knee pain (up to 32%) [188], and patella fracture (rare) [127, 161].

Quadriceps autograft

Quadriceps tendon autograft with or without bone block has recently gained popularity [152]. Comparing all soft tissue versus bone block, there is no difference in graft rupture, similar PROs, and less rotatory laxity with all soft tissue quadriceps autograft [37]. Quadriceps autograft has performed well compared with other graft options in terms of laxity (instrumented laxity testing, Lachman, pivot shift), range of motion, PROs, and overall patient satisfaction [152]. There is less donor-site morbidity, decreased rate of anterior knee pain, and consistently larger graft cross-sectional area compared with BTB [70, 96]. Comparing quadriceps autograft versus hamstring shows superior performance of quadriceps tendon on PROs, restoration of laxity as measured with the KT-1000 knee arthrometer, and higher likelihood of a negative Lachman test [27].

Hamstring autograft

Hamstring autograft is the most frequently used graft choice, though there are some signs of decline in its use [12]. Hamstring autograft has greater ultimate tensile load, stiffness, and cross-sectional area compared with both BTB and the native ACL [113, 182]. Hamstring harvest is faster than BTB and does not disrupt bone but requires tendon-to-bone healing after implantation. Tendon-to-bone healing has demonstrated slower in-growth and lower initial pullout strength compared with BTB [171]. Slower graft healing may be responsible for increased early re-rupture rate after hamstring autograft ACL-R compared with BTB [111, 134, 139], though there is high-level data that they have similar failure rates overall [33]. An increased early re-rupture rate with hamstring autograft compared with BTB is especially concerning for higher risk patients, particularly young women and those that play sports requiring cutting and pivoting [144].

A large study from the Swedish National Knee Ligament Registry demonstrated that for every 0.5 mm increase in hamstring graft diameter, the risk of revision surgery decreased by 0.86 times [155]. Grafts 8.5 mm or greater in diameter have significantly lower risk of revision than grafts less than 8 mm [154]. The risk associated with the use of smaller diameter hamstring grafts is of particular concern in patients younger than 20 years old [35]. Hamstring weakness after graft harvest can be present even at 5 year follow-up after ACL-R [102]. For this reason, some researchers recommend avoiding hamstring autograft in high-level athletes [2]. Tunnel widening is seen with use of hamstring autografts at a higher rate than after reconstruction with BTB autograft [58, 113], however, no long-term differences are found in terms of PROs or risk of OA [107, 168].

Allograft

Allograft is a relatively common graft choice, with use in 30% of primary ACL-Rs in the Multicenter Orthopaedic Outcomes Network (MOON) from 2002 to 2008 [84]. Allograft options for ACL-R include hamstring, quadriceps, BTB, tibialis anterior, tibialis posterior, and Achilles tendons. Among these choices, looped tibialis anterior tendon has shown the highest load to failure, while quadriceps tendon has shown the highest stiffness [5, 101]. There is very limited data on the clinical difference between allografts, with one large registry study showing BTB allograft had a higher rate of revision surgery (HR 1.79) compared with soft tissue allografts [164]. The primary concern with allograft is an increased re-rupture rate in young patients (up to 30%) [55, 89, 125]. This discrepancy narrows with increased patient age until around age 40, when re-rupture rates are similar for autografts and allografts (Fig. 3) [89]. Graft rupture risk is up to 3–5 times higher for BTB allograft compared with BTB autograft [97, 99] and roughly twice as high with allografts overall compared with autografts [91, 194].

Fig. 3
figure 3

Reproduced from: Kaeding CC, et al. Sports Health 3(1):73–81. Allograft versus autograft anterior cruciate ligament reconstruction: predictors of failure from a MOON prospective longitudinal cohort. ©2011 by SAGE Publications

Risk of graft failure in the Multicenter Orthopaedic Outcomes Network (MOON) cohort by age and autograft versus allograft.

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A limiting factor of allograft ACL-R is graft processing. There is a dose-dependent relationship between gamma irradiation processing and increased risk of graft failure [47, 148]. Non-irradiating chemical processing techniques have been shown to similarly negatively affect both allograft load to failure and stiffness [101]. There is general consensus that sterilization techniques, particularly irradiation, negatively impact biomechanical properties of allograft and have the potential to lead to lower PROs, failure to restore normal stability, and increased revision rate [130, 169]. The variability in irradiation complicates the literature on autograft versus allograft. A large meta-analysis showed significant differences between autografts and irradiated allografts, but no such differences between autografts and non-irradiated allografts [194]. There is a need for larger scale studies on non-irradiated allografts.

Table 4 summarizes the key facts on graft selection in ACL-R.

Table 4 Graft selection in ACL reconstruction fact box
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Associated procedures

Whether in isolated ACL injuries or ACL tears with concurrent pathology, some surgeons consider associated procedures alongside ACL-R to increase the likelihood of successful ACL-R. The anterolateral complex and reconstruction of its components are the most notable in recent literature. The anterolateral ligament (ALL) has gained extensive traction in the orthopedic sports surgery community in the past several years, though there is evidence questioning if it is a true anatomic structure (Fig. 4) [68, 73, 120]. In recent years, several biomechanical studies investigated the biomechanical properties of the anterolateral complex and its kinematic function in association with ACL injuries [72, 94]. Biomechanical studies have shown that the ALL acts merely as a secondary stabilizer to anterior tibial translation with the knee in flexion and to pivot shift in the ACL-injured knee [160, 165].

Fig. 4
figure 4

Anatomic dissection of the anterolateral complex of the knee. A The superficial iliotibial band (sITB) is reflected, exposing the lateral joint capsule and the ITB insertion on the distal femur (Kaplan fibers, KF). The deep ITB (black arrow demonstrates its course) merges with the sITB. The capsulo-osseous layer of the ITB is also seen (black triangle). B Separation of the deep ITB (black arrow) and capsulo-osseous layer (black triangle) show the convergence of multiple layers of the ITB distally before inserting upon Gerdy’s tubercle (GT). Reproduced by permission from: Springer Nature. Knee Surgery, Sports Traumatology, Arthroscopy. The anterolateral complex of the knee: a pictorial essay. Herbst E, et al. ©2017

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ACL-R with combined ALL reconstruction may decrease ACL-R failure rate. Minimum 2-year follow-up of 92 patients who underwent associated ACL-R with hamstring autograft and ALL reconstruction reported good subjective and objective outcomes, 1.1% failure rate, and 8.4% grade 1 pivot shift [159]. In a retrospective cohort study at a mean 9-year follow-up with a propensity matched isolated ACL-R group, a similar patient cohort reported decreased rates of revision ACL-R (3.5% with ALL reconstruction vs 17.4% without) and no differences in clinical outcomes between ACL-R with and without ALL reconstruction [158].

Lateral extra-articular tenodesis (LET) has been used for decades and received increased attention recently [6, 104]. Despite encouraging early postoperative results of isolated LET for ACL injury historically, some authors reported progressive knee laxity, clinical failures, and evidence of OA at longer-term follow-up [6, 141]. Following those reports and the progression of arthroscopic ACL-R, surgeons shifted towards isolated intra-articular ACL-R, and therefore most LET procedures were abandoned for years.

The re-emergence of LET has led to recent data on biomechanical and clinical outcomes of combined ACL-R and LET, with mixed results. LET leads to no difference in rotatory knee laxity or lateral compartment translation when added to ACL-R in vivo at time zero [149], and LET has also been shown to stretch out by one year postoperatively [30]. In a randomized study of BTB autograft ACL-R with and without additional LET, there were no differences in terms of clinical outcomes or failure rate at an average follow-up of 20 years [26]. However, LET had an increased risk of lateral compartment OA (59 vs 22%). In a prospective study of hamstring autograft over-the-top ACL-R with LET with minimum 20 years of follow-up, 86% of patients reported good or excellent clinical outcomes [191]. There was a positive pivot shift in 12% of patients, graft re-rupture in 2%, and increased medial OA compared with the contralateral knee only in patients who underwent concurrent medial meniscectomy.

The multi-center randomized controlled STABILITY trial investigated the effect of the LET (modified Lemaire technique) on a cohort of 618 high risk patients undergoing hamstring autograft ACL-R [63]. At 2-year follow-up, the LET group showed a relative risk reduction of 0.38 in clinical failure and 0.67 in graft rupture when compared with isolated ACL-R. A follow-up multi-center study, the STABILITY 2 Trial, is underway comparing clinical outcomes and ACL-R failure rates in 1,200 patients in four groups—ACL-R with BTB versus quadriceps tendon, with or without a LET (ClinicalTrials.gov Identifier: NCT03935750).

Beyond soft tissue procedures, osteotomies play a role in ACL-R in the setting of bony morphology that predisposes patients to graft failure or osteoarthritis. Increased posterior tibial slope is a risk factor for failure of ACL-R [31, 66]. Slope-reducing tibial osteotomy, or deflexion osteotomy, biomechanically decreases graft forces [87, 189] and has been shown to clinically improve knee stability and PROs in patients with increased posterior tibial slope following revision [4, 41] and more recently even primary ACL-R [157].

Table 5 summarizes the key facts on associated procedures with ACL-R.

Table 5 Associated procedures with ACL reconstruction fact box
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Areas of uncertainty

As knee surgeons and researchers continue to learn more about the ACL and outcomes after reconstruction, there has been increased interest in developing new surgical techniques to further restore native function, return patients to preinjury activity, and reduce the incidence of knee OA. Of the wide variety of new techniques, ACL repair and ACL augmentation have received the most interest and scrutiny from the orthopedic community.

ACL repair, initially described in 1903 [143], gained popularity in the 1970’s. However, poor long-term outcomes were reported in the 1980’s and early 1990’s that stifled its use, in part due to simple operative techniques, wide array of clinical indications, and a precursory understanding of ACL healing [57, 92]. Amidst renewed interest in repair, various new ACL repair techniques have demonstrated promising short-term outcomes in tightly selected groups of patients. ACL repair techniques include suture anchor fixation into the femoral footprint [49], independent suture reinforcement [74, 185], dynamic intraligamentary stabilization [54], and bridge-enhanced ACL repair (BEAR) [117]. Each ACL repair technique has been reported in small case series or short-term outcome studies, and the literature has demonstrated mixed results, with reported failure rates from 0 to 60% [1, 80, 118]. A study of suture ligament augmentation, involving repair of femoral-sided ACL avulsions with sufficient length and quality of the remnant ACL with bridging suture tape, found graft failure for ACL repair to be 11 times greater than that of ACL-R (49 vs 5%) in adolescent patients [61]. A recent systematic review demonstrated ACL repair survivorship to be as low as 60%, with a reoperation rate over 50% [123]. Further large-scale long-term outcomes studies are warranted before ACL repair can be advocated in the general population.

Table 6 summarizes the key facts on areas of uncertainty in management of ACL injury.

Table 6 Areas of uncertainty in management of ACL injury fact box
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Rehabilitation after ACL reconstruction

Rigid rehabilitation protocols that centered around time intervals following ACL-R have gradually been replaced by criteria-based guidelines [28]. One of the first key criteria after ACL-R is achieving full passive and active knee extension. Loss of knee extension leads to abnormal joint biomechanics and subsequently abnormal articular cartilage contact pressures and inhibition of quadriceps activation [71, 150]. Loss of 3–5 degrees of knee extension, including loss of hyperextension, has an adverse effect on PROs, leads to increased risk of osteoarthritic changes [151], and is a common cause of repeat surgery [187]. Full knee extension should be achieved preoperatively, given that preoperative extension loss is associated with postoperative extension loss [142]. Postoperatively, restoration of full passive and active knee extension symmetrical to the contralateral normal knee should be restored within the first several weeks after ACL-R. Strategies to achieve full passive knee extension include prolonged stretching under low load and sleeping in a postoperative brace locked in full extension [28]. Use of patellofemoral joint mobilization to restore normal superior translation of the patella is critical to achieving full active knee extension without a quadriceps lag. Exercises to regain flexion, including wall slides and stationary bicycle, should begin shortly after ACL-R.

Although controversy remains in terms of the precise timing of re-initiation of weight-bearing following isolated ACL-R, clinical practice guidelines recommend either early full weight-bearing exercises or immediate postoperative weight-bearing as tolerated [108, 173, 186]. Many studies have failed to demonstrate any advantage to the use of postoperative knee braces [17, 79, 112].

Quadriceps activation and strengthening are a major focus of ACL-R rehabilitation. Managing an effusion promotes quadriceps activation and function since joint effusion is sensed by capsule mechanoreceptors with subsequent inhibitory signals to the quadriceps muscle [126]. Early quadriceps strengthening is key because quadriceps atrophy is associated with deficits in performance-based functional tests and PROs [105] and contributes to long-term deficits in extension strength [167, 184]. Deficits in quadriceps strength, defined as a quadriceps limb symmetry index < 85% at the time of return to sports, has been associated with elevated MRI chondral T2 relaxation times five years after return to sports, suggesting that quadriceps strength deficits may contribute to the future development of knee cartilage degeneration and OA [21].

Rehabilitation and psychological/neurological factors

Beyond physical factors, psychological factors such as motivation and self-efficacy have been shown to play an important role in outcomes following ACL-R [32]. Psychological readiness to return to sport has shown the strongest association with return to preinjury activity level [9], suggesting a benefit to focusing on the patient’s psychological state during rehabilitation [10].

The systemic neurologic response to injury, and the relative failure of traditional neuromuscular rehabilitation to address changes after injury, may help explain part of the greater relative risk of ipsilateral ACL re-injury of patients after ACL-R [132, 133, 183]. Deficient ligament mechanoreceptors and downstream effects of inflammation and joint effusion may impact the central nervous system [93, 128, 147]. In addition to afferent disruptions, patient experience-driven factors such as pain, compensatory mechanical patterns, and postoperative rehabilitation can lead to neuroplastic alterations, including decreased neural excitability [105, 136, 137]. Decreased excitability of the motor cortex after ACL-R increases the required stimulus in the motor cortex needed to activate the quadriceps and to control the knee in space [15, 105].

Table 7 summarizes the key facts on rehabilitation after ACL reconstruction.

Table 7 Rehabilitation after ACL reconstruction fact box
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ACL injury prevention

There has been a progressive increase in interest in primary ACL injury prevention. As complete prevention is not possible, a more accurate and updated term is ACL injury reduction strategies. Given that most ACL injuries are noncontact or indirect contact, including 88% of football/soccer ACL injuries, some, or even many, of these injuries may be prevented [43, 109]. Well-known neuromuscular and biomechanical factors associated with ACL injuries, such as dynamic knee valgus loading, shallow knee flexion, and homolateral/ipsilateral trunk tilt [43, 109], are the target of neuromuscular interventions in ACL injury reduction programs [75]. Neuromuscular training (NMT) programs, such as the FIFA 11 + , have been shown to be effective in reducing primary ACL injuries, with a 50% reduction of all ACL injuries and a 67% reduction of noncontact ACL injuries in female athletes [178]. The effectiveness of the programs is correlated with athlete compliance and frequency.

Given the demonstrated effectiveness of injury reduction programs, there is increased focus on removing barriers to implementation. A common approach is to implement NMT in early- or preadolescent athletes to engrain it in their athletic routine. Preventative training may be targeted based on risk profile [76]. As specific biomechanical variables [78, 106, 193] are correlated with increased risk of ACL injury, the adoption of movement analysis of jumping [78, 124] and cutting tasks [44, 51] has been suggested to target NMT on the athlete’s movement profile. Low external knee abduction moment (KAM) tasks are preferred to limit ACL loading. Technique training aims to reduce high KAM (Fig. 5A) to low KAM (Fig. 5B) at 90° change of direction. While literature theoretically supports this approach, its effectiveness has yet to be proven [46]. There is no consensus on the utility of screening, with particularly low utility seen when limiting testing to jumping tasks [98].

Fig. 5
figure 5

Improper and proper biomechanics during a change of direction task. A High knee abduction moment (KAM) change of direction task pre-training. B Low KAM change of direction after eight weeks of targeted NMT based on proper technique

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Secondary ACL injury reduction strategies are important because subsequent ACL injury risk is high, ranging from 7.9% in the MOON cohort [90] to up to 42% in the young female population that returns to play football/soccer [56, 177]. Of particular focus are young active patients [177] and those who have sustained noncontact injuries [45]. Optimization of mid- and late-stage rehabilitation after ACL-R targets neuromuscular function, starting from the recovery of isolated muscle strength and finishing with complete sport specific reconditioning [23]. A targeted NMT program should start as soon as the patient achieves adequate strength (isokinetic deficit < 20% for knee extensors and flexors) [24, 25], which can lead to low re-injury rate [13].

Table 8 summarizes the key facts on prevention of ACL injury.

Table 8 ACL injury prevention fact box
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Conclusion

The ACL is perhaps the most studied musculoskeletal structure in the human body. The explosion of literature on the ACL over the past two decades has shed light on how important a biomechanical role it serves both in isolation as well as in concert with other bony and soft tissue structures of the knee. Since the concept of individualized ACL-R in the early 2000’s, investigations have informed orthopedic surgeons worldwide about the importance of understanding the bony and ligamentous anatomy of the knee, proper graft selection for ACL-R, and postoperative rehabilitation strategies to optimize clinical outcomes and patients’ return to their previous activities.

Amidst this influx of knowledge, this is an exciting time for ACL research. The years to come will continue to explore more established ACL-R techniques but also produce valuable data on outcomes of newer techniques. To truly understand the merits of new technology and techniques in ACL reconstruction, researchers must strive for high quality clinical studies with long-term clinical follow-up.