Returning Athletes to Sports Following Anterior Cruciate Ligament Tears

Abstract

Purpose of Review

The purpose of this review is to discuss treatment options, rehabilitation protocols, return-to-play criteria, and expected outcomes after non-operative and operative treatment of anterior cruciate ligament (ACL) tears among an athletic population.

Recent Findings

Non-operative treatment may be a viable option for some athletes with an ACL tears but can be difficult to predict “copers,” and those that resume to sports return at lower performance level and/or less intense activities. Most studies assessing function after ACL reconstruction demonstrate favorable outcomes using patient-reported outcome studies. However, return-to-play and graft re-rupture rates vary substantially based on patient characteristics and level and type of athletic activity. Grafts used to reconstruct ACL produce similar objective outcomes and favorable patient-reported outcomes but have variable re-rupture rates depending on study and differ largely on morbidity associated with graft harvest.

Summary

Various treatment methods including non-operative and operative techniques have been demonstrated to be efficacious in returning athletes to athletic activity depending on patient age and level of activity. Adherence to fundamental rehabilitation principles and accepted return-to-play guidelines can optimize outcomes and limit re-injury to the injured or contralateral limb.

Background

Anterior cruciate ligament (ACL) injuries are a common injury and are estimated to occur in over 120,000 Americans per year [1]. A large percentage of ACL tears occur in adolescent and collegiate athletes [2], many of whom wish to return to athletic competition. The risk of ACL injury among adolescent athletes is 0.69 per 10,000 athletic exposures [3]. Adolescent females, especially those participating in soccer, basketball, and lacrosse, are particularly susceptible to ACL injury and have 1.5 higher risk of injury based on gender alone [3]. Among National Collegiate Athletic Association (NCAA) athletes, ACL tears are the most commonly observed injury requiring surgery and are estimated to occur in 8.0 per 10,000 athletic exposures; however, this injury had the lowest rates of return to sports in this cohort [4]. Given the high incidence of this injury among athletes and the plethora of available information regarding ACL tears, our purposes are to review rehabilitation protocols and expected return-to-play outcomes and functional performance after non-operative and operative treatment of ACL tears among an athletic population.

Non-operative Management

People who suffer isolated ACL injuries, have limited functional activities, and/or are middle-aged or older can be candidates for non-operative management of ACL tears [5–9]. “Copers” may adequately compensate for their injury and often have positive outcomes [10, 11•], while “non-copers” are those who require surgical stabilization for optimal outcomes [11•, 12, 13]. “Copers” and “non-copers” should be identified early after injury with multiple functional and neuromuscular tests [11•, 12–16] to determine those who are candidates for conservative management [12, 14]. Identifying “copers” and “non-copers” can be challenging: studies have demonstrated that about half of those initially identified as “non-copers” responded well to rehabilitation alone and became “copers” [11•, 17].

Athletes competing in high-level athletics or pivoting sports, such as soccer, football, and basketball, have historically been considered poor candidates for non-operative management [6, 7, 9, 18, 19] likely due to finding that higher intensity activities increase the risk of further joint damage [19]. However, for some athletes, rehabilitation alone can provide functionality for both short- and long-term competitions [7, 19, 20], especially given that many of those with ACL reconstruction return with lower performance than pre-injury levels of play [21••, 22•, 23]. Many patients require activity modification with less intense exercise, sports that do not require pivoting or cutting, and/or avoidance of the activity that caused the injury [8, 24]. Rehabilitation with optional delayed ACL reconstruction can produce similar outcomes as early surgical reconstruction, and, in one study of young, active adults, only 39% proceeded with ACL reconstruction after undergoing a course of rehabilitation [17]. Non-operative management with rehabilitation may provide sufficient function [8, 20, 25, 26] and similar outcomes and quality of life as surgical intervention in a select cohort of patients [27–30]. However, the non-operative approach to ACL tears may not be as favorable among children and adolescent athletes: a recent meta-analysis comparing operative versus non-operative treatment following ACL tears suggested that early surgical stabilization is preferred to non-operative or delayed treatment because of increased risk of residual instability/pathologic laxity and meniscus tears in those initially treated non-operatively [31]

Rehabilitation focuses on restoring muscle performance, cardiovascular endurance, agility and coordination, and sports-specific skills [15]. Acutely, cryotherapy and compression [12] are used to reduce effusion, and quadriceps strengthening is initiated with limited motion between 30 and 100° of flexion [32]. Next, neuromuscular training focuses on increasing range of motion, strength, and dynamic weight-bearing while resolving effusion [7, 33]. Lastly, the final phase of rehabilitation is return-to-sports readiness, which re-integrates functional, sports-specific exercises to improve cardiovascular conditioning and performance until symmetry on functional testing at least 90% compared to the contralateral limb is achieved [7]. During rehabilitation, it is essential that patients do not have any episodes of the knee giving away, which suggests significant joint laxity [7]. Return to work or less intense sports can be initiated as early as 4 weeks post-injury and after ten physical therapy session but often requires more rehabilitation with more physically intense occupations [15, 34]. No rigorous studies have discussed timing of return to sport in non-operatively managed athletes, but a case report suggests return-to-sport is possible in less than 8 weeks with rehabilitation [35]. In general, the timing of rehabilitation and return to activities following non-operative management of an ACL rupture should be individualized. In most instances, return to pivoting sports occurs within 1 year [36].

Operative Management

Generally, younger patients who participate in high-demand sports or occupations or have feelings of knee instability and/or those who have failed non-operative treatment are candidates for surgical treatment [23]. Individuals who participate in high-level athletics or that require pivoting and cutting, including football, soccer, and basketball are thought to derive benefit from early ACL reconstruction [23]. Despite rehabilitation, roughly 18% of patients fail non-operative treatment [37] and require delayed ACL reconstruction, which has been demonstrated to improve function and stability [17, 25, 26].

Early ACL reconstruction has not clearly demonstrated better patient-reported outcomes compared to delayed reconstruction [17], and timing of ACL reconstruction has not been proved to impact objective or subjective patient outcomes [38]. Both pre- and post-operative rehabilitations with resistive and proprioception training have been shown to have improved outcomes following ACL reconstruction [11•, 39, 40]. About 80% of athletes who undergo ACL reconstruction return to sport [41, 42••, 43, 44••] and feel more confident to compete [45••] but without statistically improved outcomes than those treated non-operatively [17]. Injury to reconstructed ACL and/or contralateral ACL occur in variable frequency depending on graft choice and patient characteristics, such as age, gender, and sports activities [23, 46, 47•, 48]. It has been speculated that the majority of ACL graft failures are a result of technical errors and patient-specific anatomic factors [49•].

In addition to high-demand athletes, ACL reconstruction has proven effective in most populations, including pediatric patients and middle-aged adults. Pediatric patients who had delayed surgery or were treated conservatively were found to be 33 times more likely to have persistent instability or laxity and 12 times more likely to develop a medial meniscus tear [31] Caution should be taken to avoid physeal injury when treating a patient with open physes, but overall, early surgical intervention provides superior outcomes [31, 50]. ACL reconstruction with allograft [51] or autograft [52] is safe for people over 40 years of age, and surgery has demonstrated favorable outcomes in patient satisfaction and stability without increased risk of complication [5, 51–54].

Graft options

Multiple graft options, including patellar tendon autograft, hamstring tendon autograft, quadriceps tendon autograft, allografts, and synthetic devices, exist to reconstruct the ACL. Biomechanically, each graft ideally has an ultimate tensile load that surpassed the native ACL (ultimate load = 2160 N) [55]. At the time of implantation, quadrupled hamstring tendon autograft has the highest tensile load (4090 N) [56], followed by patellar tendon autograft (2977 N) [57] and quadriceps tendon autograft (2352 N) [58]. Though properties change during remodeling, patellar tendon autografts demonstrate almost identical ultimate stress, ultimate strain, and Young modulus compared to the native ACL [59]. Allografts have varying mechanical properties depending upon tissue used but these properties can be significantly altered by sterilization and storage processes [60].

Patellar tendon autograft has been used as a graft choice for almost a century [61] and is the commonly used graft [62]. While the graft allows for bone-to-bone healing and integration [63], it is associated with the morbidities such increased anterior knee pain, pain with kneeling, patellar fracture, and a large anterior incision [58, 63]. Those undergoing ACL reconstruction using BPTB grafts are more prone to develop post-operative patellar hypomobility; therefore, rehabilitation should focus on maintaining patellar mobility [64]. Average return to sport following ACL reconstruction using BPTB grafts typically occurs between 6 and 12 months after surgery [21••] but should be individualized with considerations for physical and psychological factor [64]. Many have advocated for delaying return to sports for at least 9 months post-operatively to reduce the risk of re-injury and allow for increased strength symmetry [65–67, 68••].

Hamstring autografts, including semitendinosus and gracilis tendons, have also been used for reconstructions since the late 1920s [61]. Hamstring autograft popularity has increased in the past decade due to improved stiffness and fixation techniques [58] and studies demonstrating similar clinical outcome [69, 70] but less harvest site morbidity than BPTB graft reconstructions [58, 62, 63]. Hamstring autografts are often used in pediatric patients with open physeal plates [58], older patients, and those with less demanding activities [62]. Drawbacks of hamstring autografts include slower initial healing, increased postoperative laxity, and decreased knee flexion strength [63]. To limit donor-site morbidity, hamstring-strengthening exercise should be avoided for at least four weeks post-operatively [64]. Similar to BPTB, return to sports has averaged 6 and 12 months but many suggest return should be delayed until at least 9 months post-operatively [21••, 67, 68••].

Quadriceps tendon autografts have recently become more popular as a graft option for ACL reconstruction; as such, limited long-term follow-up data is available [62]. Quadriceps tendon grafts have been reported to cause less anterior knee and kneeling pain without decreasing stability when compared to BPTB grafts [71, 72]. During rehabilitation, protocols may have to be modified due to delayed return to quadriceps musculature strength after using quadriceps tendon autograft for ACL reconstruction [64].

Allografts, including Achilles, patellar, hamstring, quadriceps, tibialis anterior and posterior, and peroneus longus tendons, are a common graft choice for ACL reconstruction in select populations [63]. Benefits of allografts include reduced harvest site morbidity, decreased surgical time, and ease of rehabilitation [62, 73]. However, allografts have been associated with higher graft re-tear rates, delayed remodeling, potential disease transmission and immune reactions, and higher cost [58, 63]. Allografts have been demonstrated to result in unacceptably high failure rates in pediatric patients and therefore should be avoided in these patients [50]. In a study of 2683 patients with a minimum of 2 years follow-up, ACL reconstructions performed using allograft were 5.2 times more likely to re-tear compared to patellar tendon and hamstring autograft tendon reconstructions, which did not differ significantly between each other [74].

Repair techniques [75••] and alternative graft options, such as collagen and synthetic grafts [58, 76, 77•, 78], have been preliminarily studied as options to treat ACL tears. To date, these repair techniques and grafts have not produced predictably successful long-term results and their role in treatment of ACL tears is continuing to be defined [79]. The role of osteotomy and anterolateral ligament (ALL) reconstruction to augment primary ACL reconstruction has not been completely elucidated [80–82]. A recent prospective, randomized study of 107 male athletes demonstrated decreased laxity, decreased re-rupture rates, and improved functional outcomes scores at a mean of 5 years among those who underwent ACL reconstruction combined with ALL reconstruction [81].

Rehabilitation Protocol

The goal of rehabilitation after ACL reconstruction is to restore the knee range of motion and lower extremity strength and function to pre-injury levels while minimizing excessive strains on the graft during the healing process and reducing risks of re rupture [73].

Pre-operative

The goal of pre-surgery rehabilitation is to decrease pain and swelling and improve range of motion and quadriceps strength prior to proceeding with surgery. Of these, restoration of full extension is particularly important: pre-operative extension deficit is associated with increased risk of extension deficits after ACL reconstruction [83, 84]. When combined with post-operative rehabilitation protocols, prehabilitation has been shown to improve patient-reported knee function at 12 weeks post-operatively [85] and up to 2 years after ACL reconstruction [86]. Prehabilitation also provides the practical advantage of providing clear instructions about post-operative exercises and expectations and crutch training prior to surgery. These have been shown to be effective at decreasing post-operative perceptions of pain, increased early post-operative rehabilitation, less time using crutches, and improved subjective knee function scores [87].

Post-operative

Most post-operative ACL reconstruction rehabilitation protocols are divided into phases with specific goals that must be accomplished prior to proceeding to the next.

Phase I

Phase I is typically geared towards decreasing post-operative pain and swelling and maintaining full knee extension. Compressive wraps, elevation, and cryotherapy are combined with medications to help improve post-operative pain [88]. Once pain is adequately controlled, range of motion exercises including patellar mobilization can be initiated. Range of motion exercises should be performed actively and passively and focus on maintaining full extension with target range of motion from 0 to 90° by the end of the first post-operative week [89]. Isometric quadriceps and hamstring strengthening including straight leg raises, closed-chain kinetic exercises (e.g., mini-squats from 0 to 30° of knee flexion), ankle pumps, and hamstring curls are carefully integrated to minimize strain on the graft [78, 90]. Phase 1 typically spans 7 to 10 days and the goal is for patients to be weight-bearing as tolerated with emphasis on achieving normal gait patterns [89].

Phase II

In phase II, adequate pain control and swelling minimalization with cryotherapy and compression are important to avoid range of motion loss, further quadriceps dysfunction, and altered gait mechanics. During this phase, the graft is remodeling and most vulnerable to injury; therefore, attention should be directed at limiting exercises that could compromise the graft [91]. Knee flexion is carefully increased beyond 90° while maintaining full knee extension. Gait training without crutches on flat surfaces or on a treadmill is continued. Quadriceps and hamstring strengthening are carefully increased via isometric isotonic and isokinetic exercises to minimize graft strain. Mini-squats starting from 0 to 60° and advancing up to 90° of flexion, pool walking, stationary bikes, and limited-arc leg presses from 0 to 60° of flexion are introduced. Specific exercises for phase II also include stair-stepping machines beginning at 4 weeks post-operatively and straight-line jogging and stationary biking starting at 8 weeks post-operatively [89, 90, 92, 93].

Phase III

During phase III, which typically occurs from post-operative weeks 9 to 16, the tensile strength of the graft is increasing and allows for increased strengthening, neuromuscular, and balance training [94]. Lateral lunges and single-leg balance exercises help improve knee proprioception and co-activation of the quadriceps and hamstring muscles. Plyometric drills allow for dynamic stabilization and neuromuscular control of the knee joint and are used to train the lower extremity to produce and dissipate forces to avoid injury [89, 90]. Endurance training, including jogging on even surfaces, swimming, and elliptical trainers, is introduced when adequate neuromuscular control is demonstrated.

Phase IV

Phase IV, usually beginning at month four post-operatively, is aimed at maximizing endurance and strength in anticipation of return to play. In this phase, plyometric and other lower extremity strengthening exercises are advanced. Sports-specific agility training, including running, jumping, and cutting, are tailored to the athlete and are slowly integrated into workouts. The intensity of these activities is slowly increased in a controlled setting.

Perturbation training and blood-flow restrictive (BFR) therapy have emerged as adjuncts in ACL rehabilitation [95, 96]. Perturbation training programs aim to improve neuromuscular control and proprioception by exposing athletes to directional forces while standing on uneven surfaces [95]. In a randomized trial of 74 patients undergoing ACL reconstruction, those who underwent post-operative perturbation training as a component of their rehabilitation had improved functional outcome scores compared to the group that had standard protocol [97]. Blood flow restriction therapy involves selectively applying an external device (e.g., cuff) to restrict afferent and efferent blood flow to a muscle group. Precise role for BFR following ACL reconstruction is being defined but some have advocated for its use in early stages of rehabilitation to limit pain during resistance training [98].

Return-to-Play Criteria

Multiple metrics are used to gauge readiness for return to play. Prior to any consideration of returning an athlete to sports activity, the knee should be pain-free and devoid of swelling and demonstrate full range of motion. Decisions to allow return to play should be based on graft maturity, objective parameters, and psychological readiness [99].

Graft Maturity

Time from surgery is often used as a surrogate for graft maturation. Based on imaging studies, grafts have similar intra-articular appearance to native ACL between 6 and 12 months post-operatively [94]. Previously, 6 months had been the minimal period of post-operative recovery at which surgeons would consider allowing return to play [21••]. More recently, many have advocated delaying return to sports activity for a minimum of 9 months post-operatively following ACL reconstruction [67, 68••].

Objective Parameters

Multiple objective parameters, including strength and functional test measurements, are used to gauge physical readiness to return to athletic activity. Quadriceps and hamstring muscles are important dynamic stabilizers and should be not only have strength but balance comparable to the contralateral limb [83]. Multiple objective functional tests, such as single-leg hop, have been suggested to assess readiness to return to play [100–103]. Additionally, single-leg squat test is frequently used to assess postural control and dynamic hip function [104, 105]. Commonly used objective tests and suggested threshold for return to play are outlined in Table 1.

Table 1.

Common objective parameters for allow to return to play after ACL reconstruction

Test	Threshold for return to play
Isokinetic quadriceps strength	> 80% contralateral limb at 180° per second [103]
Isokinetic hamstring strength	> 110% contralateral limb at 180° per second [103]
Quadriceps peak torque-to-body weight ratio	> 55% at 180° per second [103]
Hamstring-to-quadriceps ratio	> 70% at 180° per second [103]
Single-leg hop	> 85–90% contralateral limb [100,102.]
Triple-leg hop	> 85–90% contralateral limb [100, 102]

Psychological Readiness

Several studies have shown that fear of re-injury and reduced confidence are associated with decreased likelihood to return to sports and pre-injury activity levels even in the absence of any physical or functional impairments [106–111]. Although the athlete might be physically ready and meeting all rehabilitation milestones and objective testing, psychological readiness may lag and prevent athletes from returning to sports by 12 months [112]. It is therefore important to address the athlete’s psychological readiness to return to sport throughout the rehabilitation process. Pre-surgical education involving videos of patients who have completed post ACL reconstruction rehabilitation can help reduce perceptions of pain and earlier functional achievement up to 6 weeks post ACL reconstruction and is recommended by some throughout the post-operative rehabilitation process [87]. Health coaching is another technique that has been shown to help increase physical activity in patients with chronic diseases and patients with musculoskeletal injuries [113–115]. This technique can be employed during the rehabilitation of a patient and can help in addressing the psychological aspects related to the patient's injury and recovery [113].

Outcomes

The three main domains for assessing the success of treatment following ACL tear include return-to-play rates, incidence of secondary ACL injuries (re-tear), and patient-reported or functional outcome measures such as the International Knee Documentation Committee (IKDC) questionnaire or Lysholm score [116, 117••]. Without critically evaluating each of these domains, the success of ACL treatment may be misrepresented. For example, in a recent meta-analysis of 5770 patients who had undergone ACL reconstruction, 90% had normal or nearly normal function using objective outcome scores, but that is contradicted by the fact that only 44% of patients were able to return to competitive sport [118]. These limitations evaluations often skew comparisons between operative and non-operative treatment strategies. In a comparison study of 138 matched patients who sustained ACL injuries, those treated operatively and non-operatively did not differ significantly pre-injury or 12 months after injury in types or frequency of sports played and functional testing. However, the study did not objectify the sport activity intensity level which may differ significantly between groups [36] Evaluating all domains together provides a more comprehensive assessment of the overall outcome following treatment of an ACL injury.

Return-to-Play Rates

Counseling patients regarding the likelihood of returning to play following an ACL injury is challenging: reported rates are highly variable and often lower than expected, with between 44 and 81% of competitive athletes returning to their pre-operative level of play following ACL reconstruction [117••, 119••, 120]. Unfortunately, this outcome measure is inconsistently reported in the existing literature, with only 10% of level I ACL reconstruction studies including return-to-play rates [121]. Nevertheless, the likelihood of returning to play should be considered and can be analyzed based on a multitude of factors, including patient demographics, concomitant injuries, specific sport played, graft selection, and psychological factors [66]. The effect each of these individual factors has on return-to-play rates remains unclear. For example, in their meta-analysis of sixty-nine articles, Ardern et al. found that younger patients were more likely to return to sport following ACL reconstruction [120]. In contrast, King et al. report only a weak correlation between younger age and return to sport [117••], and Nwachukwu et. al. found no correlation [122]. The effect of gender on return to play after ACL reconstruction is similarly unclear: some studies suggest females have an inferior ability to return to sports while others report no difference [117••, 123, 124]. Patients with medial or lateral meniscal injuries or grade 3–4 medial femoral chondral injuries at the time of surgery may have lower return-to-play rates [117••]. Studies have also suggested that the use of BPTB autograft may increase return-to-play rates compared to other graft options [119••, 122]. In a recent meta-analysis comparing BTB to hamstring tendon autograft, the authors found an 81% return-to-play rate for BTB and only a 70.6% return-to-play rate for the hamstring option, although this difference may be related to specific sports and individual patient characteristics [119••].

One major factor that has consistently been shown to affect return to play rates is the psychological aspect to recovery. There have been multiple studies demonstrating that fear of re-injury, rather than persistent pain or instability, is the primary factor preventing athletes from returning to sport [125••, 126]. In a retrospective analysis of high school and collegiate football players who underwent ACL reconstruction, fear of further knee damage was cited by 50% of those that did not return to play [127].

Re-injury Rates

Risk of graft failure ranges from 5 to 28% but is variable based upon graft selection and sports participation: higher rates seen in younger patients treated with allograft and in those participating in cutting and pivoting sports [66, 128, 129, 130••]. Biomechanical risk factors play a role in re-injury following ACL reconstruction. Deficits in single-leg postural stability, increased hip rotation moment, valgus malalignment, and dynamic valgus with the vertical drop test have been associated with increased risk of a second ACL tear [131].

There have been a plethora of studies examining re-tear rates based on graft selection. A recent meta-analysis involving over 47,000 patients demonstrated a slightly lower incidence of graft re-tear when BPTB autograft is used versus hamstring autograft to reconstruct ACL (2.80% versus 2.84%, respectively) [132]. In addition, a recent cohort study assessing a total of 770 high school and college aged athletes demonstrated a 2.1 times lower revision rate with BPTB compared to hamstring tendon autograft [133••]. Conversely, a meta-analysis comparing quadriceps tendon (QT) to BPTB and hamstring autografts showed no difference in graft survival rates based on donor site [134••].

Although many athletes focus on preventing re-injury of their reconstructed knee, the risk of contralateral ACL tear is double the risk of ACL graft rupture in the ipsilateral knee and should be taken into consideration during post-operative rehabilitation [135]. The use of post-operative functional bracing to protect the reconstructed knee has not been proven to decrease re-tear rates [136], except in skiers who were 2.7 times less likely to suffer an ACL graft re-tear if functional bracing was used [137].

Patient-Reported and Functional Outcome Measures

Objective functional outcome measures, such as knee laxity (determined by KT-1000, pivot shift, or Lachman), range of motion, and donor-site morbidity, are commonly used to assess the success of treatment following ACL injury. Interestingly, objective functional outcome measures have not been shown to correlate with post-operative patient satisfaction [138]. Although these objective functional outcome measures are often used in graft selection comparison studies, no consistent differences in knee laxity or post-operative range of motion have been identified between reconstructions performed with QT, BPTB, or hamstring autografts [139, 140, 141••, 142]. In a comparison study between those treated operatively and non-operatively following ACL rupture, functional outcomes were not significantly different between groups despite those treated non-operatively having significantly more anterior tibial translation measured by KT1000 [36].

Donor-site morbidity does vary between graft choices. The use of BPTB can result in anterior knee pain, patella fracture, and extension lag. Although patella fracture is rare (0.4–13%) and extension lag often resolves by long-term follow-up, the anterior knee pain can be quite common and debilitating and occurs in up to 52% of patients who have undergone BPTB autograft [143]. Use of the hamstring autograft can result in sensory disturbances from injury to the saphenous nerve and weakness in knee flexion [144–146]. QT harvest can result in hematoma formation, donor-site cosmetic defects, and patella fractures, although these may be observed less frequently than with other grafts [139, 147•].

Patient-reported outcomes (PROs) frequently used to assess outcomes following treatment of ACL injuries include Lysholm score, the Knee Injury and Osteoarthritis Outcome Score (KOOS), and the IKDC questionnaire, while Tegner and Marx Activity Rating scores quantify activity level. Reporting both PRO and activity rating scores are important to assess outcomes following ACL injury treatment, because PROs may overestimate function if the patient has not returned to more strenuous activities. When comparing PROs between available graft options, there have been no consistent differences demonstrated between BTB, QT, and hamstring tendon [139, 148, 149••]. Early accelerated rehabilitation following ACL reconstruction produced improved IKDC subjective scores compared to non-accelerated rehabilitation protocols [150].

Conclusions

Anterior cruciate ligament injuries are a common injury and a major cause of missed games and disability among athletes. Various treatment methods including non-operative and operative techniques have been demonstrated to be efficacious in returning athletes to athletic activity depending on patient age and level of activity [7, 19, 20, 21••, 22•]. Adherence to rehabilitation guidelines and return-to-play parameters can optimize outcomes and limit re-injury [86, 99]. While most studies following ACL reconstruction demonstrate favorable PRO regardless of graft type [139, 148, 149••], return-to-play [117••, 119••, 120] and graft re-rupture rates [66, 128, 129, 130••] vary substantially and largely depend on study population and level and type of athletic activity.

Declarations

Conflict of Interest

Anna M. Ptasinski, Mark Dunleavy, and Temitope Adebayo declare that they have no conflict of interest.

Robert A. Gallo has stock options from Kalibur Labs, editorial board member of Current Reviews in Musculoskeletal Medicine and Sports Medicine and Arthroscopy Reviews, and is committee member of American Society of Orthopaedic Sports Medicine and American Board of Orthopaedic Surgery.

Human and Animal Rights and Informed Consent

This article does not contain any studies with human or animal subjects performed by any of the author.