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Return to Play Following Anterior Cruciate Ligament Reconstruction

Ellman, Michael B. MD; Sherman, Seth L. MD; Forsythe, Brian MD; LaPrade, Robert F. MD, PhD; Cole, Brian J. MD, MBA; Bach, Bernard R. Jr MD

JAAOS - Journal of the American Academy of Orthopaedic Surgeons: May 2015 - Volume 23 - Issue 5 - p 283–296
doi: 10.5435/JAAOS-D-13-00183
Review Article

In athletes, significant advances in anterior cruciate ligament reconstruction techniques and rehabilitation have led to improved surgical outcomes and increased expectations for return to play. Although an expeditious return to sport has become an achievable and often realistic goal, the factors that most influence safe, timely, and successful return to play remain unknown. The literature offers mainly anecdotal evidence to guide the team physician in the decision-making process, with a paucity of criteria and consensus guidelines available to help determine return to sport. Attempts have been made to introduce criteria-based progression in the rehabilitation process, but validation of subjective and objective criteria has been difficult. Nevertheless, several pertinent factors in the preoperative, intraoperative, and postoperative periods may affect return to play following anterior cruciate ligament reconstruction. Further research is warranted to validate reliable, consensus guidelines with objective criteria to facilitate the return to play process.

From the Steadman Philippon Research Institute, Vail, CO (Dr. Ellman and Dr. LaPrade), University of Missouri, Columbia, MO (Dr. Sherman), and Rush University Medical Center, Chicago, IL (Dr. Forsythe, Dr. Cole, and Dr. Bach).

Dr. Forsythe or an immediate family member serves as a paid consultant to or is an employee of Sonoma, and has stock or stock options held in Jace Medical. Dr. LaPrade or an immediate family member serves as a paid consultant to or is an employee of Arthrex; has received research or institutional support from Arthrex, Smith & Nephew, Ossur, and CONMED Linvatec; and serves as a board member, owner, officer, or committee member of the American Orthopaedic Society for Sports Medicine, the International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine, the Arthroscopy Association of North America, and the European Society for Sports Traumatology, Knee Surgery and Arthroscopy. Dr. Cole or an immediate family member has received royalties from Arthrex and DJ Orthopedics; serves as a paid consultant to or is an employee of Arthrex, DJ Orthopedics, Johnson & Johnson, Regentis Biomaterials, and Zimmer; has stock or stock options held in Carticept Medical and Regentis Biomaterials; received research or institutional support from Johnson & Johnson, Medipost, and Zimmer; and serves as a board member, owner, officer, or committee member of the Arthroscopy Association of North America. Dr. Bach has received research or institutional support from Arthrex, CONMED Linvatec, DJ Orthopedics, Ossur, Smith & Nephew, and Tornier, and serves as a board member, owner, officer, or committee member of the American Orthopaedic Society for Sports Medicine. None of the following authors or any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this article: Dr. Ellman and Dr. Sherman.

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Return to Play−Expectation Compared With Reality

Anterior cruciate ligament (ACL) tears are one of the most common knee ligament injuries in athletes, accounting for up to 64% of all knee injuries in cutting and pivoting sports.1 For athletes who wish to return to play (RTP), the benchmark for treatment of ACL rupture is surgical reconstruction. The purpose of an ACL reconstruction (ACLR) is to eliminate functional instability, thereby reducing the risk of subsequent injury to the menisci and articular cartilage.2 In athletes, advances in ACLR and rehabilitation have led to improved outcomes and heightened expectations for successful and expeditious RTP.

Previous studies, however, suggest that a discrepancy exists between expectations and RTP in athletes, with RTP rates ranging from 60% to 80% in a variety of different sports3-7 (Table 1). Ardern et al4 evaluated 48 studies and 5,770 patients in a systematic review and meta-analysis on RTP following ACLR. Overall, 82% of patients returned to sport, but only 63% were participating in their preinjury sport and 44% had returned to competitive sport. Furthermore, existing high-level literature fails to clearly and consistently define RTP rates.17,18 In a systematic review of 49 level I and II studies of RTP guidelines following ACLR, the description of permission/allowance to return to sport was highly variable and poor; only five studies reported whether patients were able to successfully RTP, and 24% of studies failed to report when patients returned without restrictions.19 Most of these studies also fail to account for variables such as age, gender, timing during the season, the existence of concomitant injuries or persistent knee symptoms, family issues, and contract-specific issues.

Table 1-a

Table 1-a

Table 1-b

Table 1-b

One of the greatest obstacles in establishing consistent RTP rates involves inconsistencies in defining safe RTP. Precise and consistent terminology is essential, yet previous studies differ in their definition of safe and successful RTP.3-7 For example, if the athlete has returned to play, has he or she returned to the same level of competition, and if so, has his or her performance suffered? How does the athlete’s mental state impact RTP? What is the athlete’s risk for reinjury following return? To date, few studies attempt to answer these questions, making the RTP decision-making process challenging. Although attempts have been made to introduce criteria-based progression into the rehabilitation process, validation of subjective and objective criteria for RTP has been difficult.20,21

To answer many of these questions based on the best available evidence in the literature (ie, mostly level IV and level V evidence), the most important preoperative, intraoperative, and postoperative principles that affect RTP after ACLR are presented in Table 2.

Table 2

Table 2

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Preoperative Factors Affecting Return to Play

Following the diagnosis of an ACL tear, preoperative rehabilitation should begin immediately to expedite RTP (Table 3). Preoperative rehabilitation is designed to reduce pain, inflammation, and swelling, restore normal range of motion, improve neuromuscular control, normalize gait, and prevent muscle atrophy.22 Knees reconstructed in the acute setting, before regaining full range of motion (ROM), are at greatest risk for stiffness and arthrofibrosis because the best predictor of postoperative ROM is preoperative ROM.23 Loss of motion can be detrimental to the outcomes of primary ACLR, leading to decreased and delayed athletic functional performance, altered gait and running patterns, and increased patellofemoral contact pressures with subsequent joint degeneration.24 The optimal timing for surgery should be after the athlete has regained full ROM, typically between 1 and 4 weeks postinjury, or as early as 2 to 3 days postinjury with an aggressive preoperative rehabilitation program and/or early knee aspiration of hemarthrosis to improve ROM, stimulate early quadriceps function, and improve pain control.23

Table 3-a

Table 3-a

Table 3-b

Table 3-b

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Intraoperative Factors Affecting Return to Play

Anatomic Graft Position and Graft Tensioning

The anatomic position of the femoral and tibial tunnels is perhaps the most critical factor that leads to improved patient outcomes following ACLR.25 The surgeon must place a strong emphasis on anatomic tunnel placement and appropriate graft tensioning to ensure optimal graft isometry and function.26

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Graft Choice and Return to Play

The ideal graft is one that allows for secure fixation, has minimal morbidity, and allows for early, safe, postoperative rehabilitation and timely RTP.27 The options for available grafts fall into two general categories: autograft and allograft. For autograft reconstruction, there are multiple options for use, but the most common are bone-patellar tendon-bone (BTB), hamstring, and quadriceps tendon.

Many studies demonstrate predictable and safe outcomes and timely RTP using BTB autograft in elite athletes.28-31 The advantages of BTB autograft include excellent graft strength/stiffness and stable interference screw fixation, allowing for bone-to-bone healing within the ACL tunnels.29,31 In animal studies, bone graft incorporation has been reported to occur as early as 6 weeks postoperatively, in comparison with 8 to 12 weeks with hamstring (ie, soft tissue) autograft.32 Whereas incorporation of the graft does not equate to maturation, earlier graft incorporation may allow for more aggressive rehabilitation protocols and a more rapid RTP.

In a prospective, randomized study by Wipfler et al,33 the authors compared hamstring autograft with BTB autograft and reported no significant objective differences between the two groups at long-term follow-up. With isokinetic testing, quadriceps strength was close to normal (96%) in both groups, but hamstring strength was predictably lower in the hamstring tendon group (100.3% versus 95.1%). Kneeling, knee walking, and single-leg hop tests showed better results in the hamstring group,33 revealing the potential morbidity of the BTB group in athletes who require increased patellofemoral contact forces, such as wrestlers. Other studies, however, have shown that peak flexion torque at high angles is reduced after hamstring autograft harvest compared with BTB harvest,34,35 bringing into question the use of hamstring autograft in athletes who participate in sports that require cutting or jumping activities. Leys et al36 demonstrated a higher retear rate with hamstring autograft (17%) compared with BTB autograft (8% [P < 0.07]); these findings have been corroborated by others.37

In patients with genu recurvatum (ie, hyperlax female gymnasts), Goldblatt et al38 and Ghodadra et al39 have shown that hamstring autografts tend to stretch out over time, whereas BTB autografts stretch less. Studies have also demonstrated that smaller hamstring autografts (<8 mm in diameter)40,41 in younger patients (<20 years)41,42 are associated with worse clinical outcomes and with a greater chance for failure and revision; the use of larger hamstring autografts (>8 mm) is recommended to optimize outcomes. Finally, athletes, such as skiers and soccer players, require the medial knee stabilizers for sport-specific tasks, potentially making hamstring autografts less of an ideal graft choice in this population. Therefore, given that decreased hamstring strength (relative to quadriceps strength) has been reported as a risk factor for ACL injury,43 and that a greater percentage of patients who underwent reconstruction with BTB autografts returned to sport compared with patients with hamstring autografts,44 the authors prefer to use BTB autograft in this population.

The use of allograft for ACLR is controversial in the young athlete. Although decreased donor site morbidity and earlier return of dynamic muscle strength may facilitate functional return, slower graft ligamentization requires a prolonged period of protection to prevent catastrophic graft failure.27,45,46 Clinical studies report increased rates of allograft failure compared with autograft in the young, active population;28-31 in one study, allograft failure rates exceeded 40%.28 In a study of ACLR in military cadets, 33% of athletes with allograft ACLR experienced graft failure at 1-year follow-up (compared with 2% with autograft), while more than half of the allograft patients experienced graft failure at 2-year follow-up (compared with only 6% in the autograft group).28 However, these studies did not standardize for graft processing (ie, irradiation), surgical technique, and rehabilitation protocols, and a specific patient age at which allograft failure rates are equivalent to autograft failure rates is currently unknown.28-31 Higher-quality trials are necessary to determine the safety and efficacy of allograft ACLR in the young, active population. At present, the authors do not recommend routine use of allograft ACLR in this population based on concerns for higher graft failure and the requirement for delayed RTP.

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Postoperative Factors Affecting Return to Play

The importance of a strict and reproducible rehabilitation program is paramount to the RTP process; several authors have reported that RTP is more dependent on the rehabilitation program than on the technique or the graft choice used intraoperatively.47,48 Shelbourne and Nitz49 were the first clinicians to describe an accelerated rehabilitation program for athletes following ACLR. The main differences between traditional and accelerated programs are the rate of progression through the various phases of rehabilitation and the period of time recommended before return to sports.22,45 Beynnon et al50 reported that rehabilitation with an accelerated protocol (ie, 19 weeks) compared with a nonaccelerated protocol (ie, 32 weeks) resulted in no differences in subjective and objective outcomes following ACLR with patellar tendon autograft, thus spurring the movement toward accelerated rehabilitation programs for athletes. A modified version of the Shelbourne and Nitz49 accelerated protocol, as described by Wilk et al,45 has been adopted at the senior authors’ institution and is presented in Table 3. Prior to advancement to the next phase, specific criteria must be met with regard to ROM, strength, neuromuscular control, proprioception, functional tests, clinical examination, endurance, and subjective knee scores. Table 4 summarizes criteria to permit RTP based on the authors’ experience with a large number of reconstructions in high-level athletes. Unfortunately, few of these criteria have been validated in the literature.

Table 4

Table 4

To date, the effect of an accelerated rehabilitation protocol in patients undergoing hamstring ACLR has not been demonstrated in a controlled study, yet Fujimoto et al51 reported this protocol may lead to significant graft laxity over time. Further studies are warranted to elucidate the effects of an accelerated program following ACLR using hamstring grafts.

It is important to remember that although the protocol provided in Table 3 addresses a timeline for progression, each patient differs in terms of functional status, and little evidence supports the use of time as a basis for progression after ACLR.52 Furthermore, this protocol is based largely on the authors’ experiences and lower level evidence rather than high-level studies, given the lack of level I and level II evidence in the literature on RTP following ACLR. Therefore, these time periods are simply guidelines, and progression through each stage should proceed via both patient-specific functional advancement and the time necessary for biologic healing of the graft.

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Sport-specific Training and Return to Play

Assuming the criteria listed in Table 4 are met, the athlete may return to limited practice, and if no setbacks are encountered, the athlete may return to full activity without limitations. Importantly, athletes are likely more susceptible to reinjury as they fatigue, elucidating the importance of incorporating a program of endurance exercises before full RTP.53 In a systematic review by Harris et al,19 the authors reported that 51% of studies allowed RTP without restrictions at 6 months postoperatively, while 86% of studies permitted RTP at 9 months. The authors prefer to wait at least 6 months before allowing RTP without restrictions to allow for graft healing and to decrease the risk for early graft failure, despite limited evidence for a strict timeline for RTP.

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Objective Criteria as Guidelines for Return to Play

Few objective validated testing measures are available to guide the physician for the RTP decision-making process. For example, Harris et al19 reported on 49 level I and II studies; 90% and 65% of the studies failed to use objective criteria or any criteria, respectively, to permit return to sport. Barber-Westin and Noyes,52 in a systematic review of 264 studies, reported that 105 studies (40%) failed to provide any measures for RTP after surgery, and only 35 studies (13%) included objective criteria that consisted of the categories of muscle strength or thigh circumference, general knee examination, single-leg hop tests, Lachman rating, or validated questionnaires. These findings demonstrate a lack of objective assessment before release to athletics.

One of the major obstacles in determining objective RTP criteria is identifying the criteria purpose. For example, should criteria be used to determine the athlete’s physical ability to RTP (often the athlete’s preference), or should criteria be used to determine the athlete’s safety following RTP (often the surgeon’s preference)? These criteria remain undefined in the literature. In addition, an athlete may not be able to RTP, despite passing all objective criteria, because of his or her mental state and/or expectations, further challenging the use of objective criteria to help determine RTP. In a review of nearly 6,000 patients after ACLR, only 44% of patients were able to return to competitive sport, despite the fact that 90% of patients had normal or nearly normal function using objective outcome scores and that 85% of patients had normal or nearly normal function on the basis of activity measures, such as the International Knee Documentation Committee subjective knee evaluation form.4

Nevertheless, several studies have attempted to define objective criteria to guide RTP. One measure is hop testing, a functional rehabilitation test that may signal the capacity for successful RTP.54 The most common hop tests used in clinical practice are the single-leg hop for distance, the single-leg triple hop for distance, the single-leg timed hop, the single-leg crossover hop for distance, and the vertical jump test.

In a cohort study of patients after ACLR using BTB autograft, Hopper et al55 suggested using a specific series of hop tests to assess functional recovery to resume play. Using a score of ≥85% as a criterion for normative limb symmetry, patients achieved passing scores in a 6-minute timed hop at week 18, in the stair hop and the vertical hop at week 26, and in the crossover hop at week 39, suggesting that these criteria may be used to assess RTP in athletes.55 As noted, the crossover hop was not achieved until week 39 (almost 10 months postoperatively), but several athletes have successfully returned to sport earlier than this timeline suggests.

In a study by Yosmaoglu et al,56 the authors theorized about a series of hop tests that would provide a reliable and valid performance-based outcome measure following ACLR. Brosky et al57 reported a high intra-rater reliability using functional hop tests, the Biodex isokinetic dynamometer, and the KT-1000, suggesting these tests may be used in combination to evaluate progress. Wilk et al,58 in combining three different measures, reported a positive correlation between isokinetic knee extension peak torques, subjective knee scores, and three different single-leg hop tests (ie, hop for distance, timed hop, and crossover triple hop), suggesting that these tests may be used in conjunction to predict progression and RTP.

Paterno et al59 and Hewett et al60 first reported that dynamic valgus observed on drop vertical tests is a significant risk factor for ACL injury, reinjury, and contralateral ACL injury, and McLean et al61 described a method of measuring dynamic valgus in basketball players using two-dimensional video analysis. The authors suggested that optimizing neuromuscular control of the hip and knee following ACLR, via a decrease in dynamic valgus, may decrease the risk of knee injury following RTP.59

During rehabilitation, hop testing provides a reliable and valid outcome measure that replicates the demands of high-level activities; however, it may not be sensitive enough to identify some functional limitations associated with untested multiplanar movements.62,63 Currently, the use of hop tests or dynamic valgus measures is institution-specific and has yet to be adopted as consensus guidelines. No one single outcome criterion has been correlated with successful RTP. Most clinicians prefer to use a combination of functional, clinical, and subjective testing to determine readiness to RTP. At the senior authors’ institution, a functional sports assessment has been created based on the best available literature, yet it is largely based on level IV and V evidence (Table 5). Prior to RTP, an athlete must complete the functional sports assessment; the team physician then determines readiness for RTP. Unfortunately, given the lack of a specific biologic time point for safe RTP, the head team physician often has difficulty making this determination. At the time of clearance, a conversation between the team physician and the athlete, family, and trainer should address several of these unknown variables, including the risk of reinjury despite being cleared to play, the biologic healing that will continue to progress after RTP, and the importance of persistent rehabilitation to enhance neuromuscular control beyond the first year after surgery.

Table 5

Table 5

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Variations in Rehabilitation

Variations to the provided standard rehabilitation protocol exist because of a variety of factors. Patient-specific factors play a role in the progression of rehabilitation, such as motivation or desired activity level, timing of injury, contract or family issues, and concomitant injuries/surgeries. The age of the athlete, the stage of an athlete’s career, the time of the season, and the level of athletics (eg, recreational, professional) all play a significant role in the rehabilitation and decision-making process. More specifically, concomitant injuries are common in athletes with ACL tears and have the potential to profoundly delay the postoperative course. Brophy et al16 reported that a history of meniscectomy, but not ACLR, shortens the expected career of a professional football player, and a combination of ACLR with meniscectomy may be more detrimental to an athlete’s durability than ACLR alone.

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Functional Bracing After Return to Sport

The role for functional ACL braces during and after return to sport remains controversial. The general trend is to use braces during sports for at least the first year after surgery; however, the literature behind this is lacking.48 Studies show that the use of a brace can improve early coordination and jumping mechanics, while providing a positive psychological effect.64 Furthermore, braces have been shown to be effective in preventing recurrent ACL injury in skiers.65 In contrast, other studies have shown no increase in stability or in speed to RTP, with some studies even suggesting that bracing may potentially decrease speed and turning quickness.66,67 Despite these controversies, the authors recommend use of a functional brace during the acute transition back to sports and permanent use in skiers during ski-related activities.

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Psychological Factors Affecting Return to Play

Perhaps the most important factor in determining an athlete’s RTP is his or her psychological state. Each athlete varies in his or her subjective sense of stability, confidence, and comfort with RTP. Gobbi and Francisco68 prospectively analyzed the effects of various subjective scoring systems on RTP after ACLR using either patellar tendon or hamstring grafts. Using the International Knee Documentation Committee subjective knee form and the Lysholm, Noyes, and Tegner subjective knee evaluation scales, the authors found no significant differences between athletes who returned to sports and athletes who did not return to sports. However, in a psychiatric questionnaire, the authors found significant differences between the two groups, and along with other clinicians,4,15,69,70 have suggested that psychological factors significantly affect RTP in athletes. Two other validated methods to identify each patient’s psychological status postoperatively include the Tampa Scale of Kinesiophobia71 and the ACL-Return to Sport after Injury scale;72 however, these methods are not routinely used.

Therefore, in an athlete with a healed graft and a fully rehabilitated knee, psychological factors, such as fear of reinjury or poor performance, should not be overlooked because they may prevent a return to the playing field. Multiple studies have reported that fear of reinjury, rather than clinical findings of instability or pain, is the single greatest reason for failure to RTP.15,73 To date, however, a single, validated psychological tool to assess a patient’s psychological state is not used clinically.

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Sport-specific Outcomes Following ACLR

The Multicenter Orthopaedic Outcomes Network database allows for the analysis of sport-specific outcomes and factors influencing RTP after ACLR. Brophy et al8 analyzed factors influencing RTP in soccer and reported that 72 of 100 soccer athletes (72%), with a mean age of 24.2 years, returned to soccer; however, only 36% of the returnees were still playing at 7-year follow-up, suggesting that continued participation declines over time. Based on multivariate analyses, RTP was less likely in older athletes and females, and graft choice had no effect on RTP.8 In another study, McCullough et al10 retrospectively analyzed the percentage of high school and collegiate American football players for RTP at their previous level of competition. Of 147 athletes, RTP rates were similar (63% and 69% in high school and collegiate athletes, respectively), and both rates were lower than estimated. Based on player perception, only 43% of the players felt they were able to return at the same performance level, 27% felt they did not perform at a level attained before their ACL tear, and 30% were unable to RTP. At both levels of competition, fear of reinjury or further damage was cited by approximately 50% of the players who did not RTP.10

Existing studies vary with regard to the definition of RTP, the type and level of athlete, outcome measurements, and the length of follow-up (Table 1). In their systematic review, Ardern et al4 found a significantly higher rate of reported RTP in studies with a mean follow-up of <24 months compared with studies with a mean follow-up of ≥24 months, implying that there may be a rapid decline in sports participation after 2 years. Patients who have successfully returned to sports may subsequently give up that sport or reduce their participation to a lower level over time.74 Further research is warranted to identify pertinent factors and sport-specific outcomes for RTP following ACLR.

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Risk of Reinjury Following ACLR

One of the greatest concerns with RTP is the risk of reinjury to the ipsilateral reconstructed graft or to the contralateral native ACL. Graft re-rupture rates after RTP range from 5% to 25%; higher rates are found in younger athletes involved in cutting or pivoting sports compared with athletes who participate in straight-line activities or who are jumpers.24,30 In a systematic review, Wright et al75 reported an overall risk of hamstring or BTB graft rupture of 5.8% (range, 1.8% to 10.4%) at a minimum 5-year follow-up and an overall rate of contralateral ACL rupture of 11.8% (range, 8.2% to 16%). The exact time at which the risk of reinjury to the ipsilateral knee (ie, ACL graft) is equal to injury to the contralateral ACL is unknown. As stated, perhaps the greatest risk of graft failure is the use of allograft in young patients,76 particularly within the first year of reconstruction,28 and there is a trend toward a higher failure rate for hamstring (secondary to stretching or retear) compared with BTB autograft.36 Therefore, many surgeons prefer to use BTB autograft to minimize the risk of reinjury in young athletes.

Although older athletes have a lower rate of return to their preinjury sport,3 younger athletes (<25 years) are at an increased risk for reinjury and revision surgery.29,77 Bourke et al78 reported that male gender and a positive family history of ACL injury were associated with an increased risk of ACL graft rupture. Paterno et al59 identified specific biomechanical predictive factors for reinjury, including dynamic valgus with the vertical drop test, valgus malalignment, greater asymmetry in internal knee extensor moment at initial contact, increased hip rotation moment, and a deficit in single-leg postural stability of the involved limb (measured by the Biodex stability system). With regard to the timing of RTP, Shelbourne et al79 found no difference in graft rupture rates in patients who returned to sport before and after 6 months following surgery; their findings were corroborated by Glasgow et al.80

With regard to ACL injury to the contralateral knee, recent data suggest that if athletes fail to achieve full strength and neuromuscular control before RTP from their index surgery, the contralateral knee is at increased risk of an ACL rupture.81 In addition, female gender, a positive family history, a return to preinjury sport, and an ACLR with a patellar graft have been identified as risk factors for injury to the contralateral knee,80,81 although there are also studies that do not identify gender as a risk factor.82

A significant risk of injury to the menisci and cartilage is possible with continued sports participation following ACLR,18 although this topic has been less well-defined. Overall, the variability between studies suggests that further investigation into the risk of reinjury to the ipsilateral graft, the contralateral ACL, and other structures in the knee is still required.

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Summary

Return-to-play decision making is a challenging and often stressful process for the team physician. Unfortunately, there remains a paucity of objective criteria and consensus guidelines to facilitate the decision-making process, and the time required to return an athlete to play with an equal or lesser chance of reinjury to the reconstructed knee compared with the contralateral knee remains unknown. Perhaps the most important, yet overlooked factor in determining an athlete’s RTP is his or her psychological state; the fear of reinjury has been found to play a major role in preventing return to sport. Further research is warranted to validate reliable, consensus guidelines with both subjective and objective criteria before allowing an athlete to RTP.

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References

Evidence-based Medicine: Levels of evidence are described in the table of contents. In this article, references 35, 38, and 50 are level I studies. References 6, 19, 23, 25, 28, 29, 32, 33, 36, 37, 44, 46, 55-57, 59, 60, 62, 63, 68, 69, 75, 79, and 82 are level II studies. References 3, 4, 8-10, 16, 21, 30, 31, 34, 39-42, 51, 53, 54, 58, 64, 65, 67, 72, 77, and 80 are level III studies. References 2, 5, 7, 11-15, 52, 61, 70, 71, 73, 76, and 78 are level IV studies. References 1, 17, 18, 20, 22, 24, 26, 27, 43, 45, 47-49, 66, 74, and 81 are level V expert opinion.

References printed in bold type are those published within the past 5 years.

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