Contralateral Anterior Cruciate Ligament Injuries Following Index Reconstruction in the Pediatric Athlete

Abstract

Purpose of Review

The purpose of this review is to discuss relevant anatomy and pathoanatomy in the knee following anterior cruciate ligament reconstruction, risk factors for contralateral ACL tear, and mechanisms of contralateral injuries in the pediatric population.

Recent Findings

Contralateral ACL rupture rates following ACL reconstruction (ACLR) range from 4% to 42%. Pediatric patients show increased risk compared with adults, likely due to inherent anatomical differences along with biomechanical and neuromuscular changes that occur in both the operative and contralateral knees following index ACLR. Pediatric athletes who return to sport more quickly have been found to be at increased risk for contralateral tears, as have athletes who participate in cutting and pivoting sports. Contralateral tears tend to occur via non-contact mechanisms.

Summary

Pediatric patients are at increased risk of contralateral ACL injury following index ACL reconstruction compared with adults. Further study is warranted to determine appropriate biologic, functional, and rehabilitative parameters gauged toward preventing contralateral ACL tear while minimizing time lost from sport.

Introduction

Anterior cruciate ligament (ACL) tears represent one of the most frequently encountered injuries in athletes, with approximately 200,000 ACL tears diagnosed annually [1]. ACL injury rates remain disproportionately high in the pediatric population (age < 18), occurring in approximately 50:100,000 youth athletes each year [2, 3] (Fig. 1). The diagnosis of ACL injuries and subsequent ACL reconstruction (ACLR) procedures have increased significantly over the last two decades [4]. This increase has been attributed to increasing numbers of pediatric athletes participating in sports, year round sport participation, along with greater emphasis on single-sport specialization, coupled with improvements in advanced imaging and diagnosis of ACL injury [4–7].

Fig. 1.

Fat suppressed, T2-weighted magnetic resonance image of the knee in the sagittal plane of a skeletally immature 12-year-old female (open distal femoral growth plate, white arrow; open proximal tibial growth plate, red arrow) demonstrating tearing of the anterior cruciate ligament (yellow arrow)

Following ACLR, injuries to the contralateral ACL have been reported to occur in 4 to 42% of pediatric patients [8•, 9], accounting for 0.5 to 3% of all ACL injuries in the pediatric population [10]. Contralateral ACL injuries require further surgery, necessitating prolonged rehabilitation and time lost from sport [11, 12, 13••, 14, 15]. To date, few studies have examined contralateral ACL injuries in the pediatric patient following index ACLR. The purpose of this review is to provide a concise overview of the anatomic differences in the pediatric knee, along with the changes in the operative and contralateral knee following ACLR, while evaluating the impact of patient age, time following index ACLR, type of sport and return to sport timing, sex, body mass, and reported mechanisms of injury in the pediatric patient sustaining contralateral ACL injury. By providing clinicians with a better understanding of the risk factors contributing to contralateral ACL injuries following index ACLR, the authors seek to promote further research to optimize the health of youth athletes while minimizing time lost from play.

The Pediatric Anterior Cruciate Ligament

Pediatric Anatomy

Similar to the skeletally mature adult, the primary function of the ACL in the pediatric patient is to coordinate function and provide stability to the knee, preventing anterolateral translation of the tibia relative to the femur. By maintaining both static and dynamic equilibrium of the knee in conjunction with the collateral ligaments and menisci, the ACL allows athletes to meet the demands of sport while minimizing the risk of injury to the articular cartilage [16, 17]. The ACL is composed of an anteromedial and a posterolateral bundle, originating proximally at the posterior-medial aspect of the lateral femoral condyle and inserting distally between the intercondylar eminences on the anterior tibia. The anteromedial band functions to prevent anterior translation of the tibia on the femur, while the posterolateral band stabilizes the knee against rotation [18].

Unlike their adult counterparts, the anatomy of the pediatric knee possesses a number of unique differences contributing to the risk of ACL injuries. During development, the width of the femoral notch has been shown to develop steadily throughout skeletal maturity until approximately age 11 [19]. However, the relationship between the ACL and the femoral notch width index (ratio of the intercondylar width to the bicondylar width) has been shown to increase with increasing age. The presence of proportionally less available space within the notch in younger patients has been hypothesized to account for the higher prevalence of ACLR procedures in pediatric patients [20]. Moreover, the presence of greater posterior tibial slope in pediatric patients has been associated with an increased risk of ACL injury. O’Malley et al. found pediatric patients sustaining ACL injuries possessed significantly higher mean posterior tibial slope (10.0° ± 3°) when compared with a non-injured control group (8.5° ± 3°) (p = 0.128) [21]. Dare et al. similarly demonstrated a significant association between increased lateral tibial slope in pediatric patients sustaining an ACL injury (5.7° ± 2.4°) versus control patients without injury (3.4° ± 1.7°) (p < 0.01) [22]. As such, when compared with adults, important anatomical differences in the pediatric patient must be recognized as potential contributing factors accounting for the higher incidence of ACL injuries, as well as the higher rates of contralateral ACL injury following ACLR.

Management of the ACL-Deficient Pediatric Knee

The current standard of care for pediatric patients following ACL rupture is ACLR using various reconstructive procedures. The proximity of the open, active growth plate at the distal femur and proximal tibia, located in the bone typically reserved for drilling during tunnel preparation and graft fixation, pose a high risk for iatrogenic damage during ACLR [23, 24]. As a result, surgical reconstructive options are largely dictated by patient age and amount of remaining growth at the distal femur and proximal tibia [4, 25], shown to contribute 37% and 28% to the overall growth of the lower extremity, respectively [26]. These techniques include: all-epiphyseal (AE) reconstruction, first described by Anderson et al. [27], consisting of femoral and tibial tunnel drilling isolated to the epiphyses; extra-physeal reconstruction, introduced by Kocher et al. [28], consisting of non-anatomic iliotibial band graft harvest and placement over the top of the femur and under the intermeniscal ligament on the tibia, and ACL repair. While largely abandoned in recent years secondary to high re-rupture and failure rates [29–32], recent advancements in implants [33], techniques [34], and biologic augmentation strategies [35, 36] have renewed interest in ACL repair.

Following injury, surgery is recommended as the pediatric ACL deficient knee is prone to progressive instability with subsequent injury risk to the mensici and chondral surface, potentially resulting in early-onset degenerative changes, inability to return to sport, and poor clinical outcomes with non-surgical management [8•, 13••, 16, 25, 37–39]. The high risk of further intra-articular injury in the pediatric ACL-deficient patient undergoing non-surgical treatment is largely attributed to continued performance of activities secondary to difficulty with pediatric patient compliance, as well as accelerated return to sport prior to adequate biologic recovery following ACLR [4, 13••, 31]. Therefore, operative management is generally recommended in the vast majority of pediatric patients following ACL rupture, yielding excellent outcomes and high return to sport rates following ACLR [40]. However, despite the availability of multiple physeal-sparing techniques, post-operative complications secondary to iatrogenic physeal injury with resultant limb length discrepancy and angular deformities have been reported [8•, 41–43].

Neuromuscular and Biomechanical Changes in the Pediatric Knee Following ACLR

While ACLR effectively restores native knee stability, minimizing the risk of subsequent instability events and intra-articular injury [44], a number of neuromuscular and biomechanical changes have been noted to occur in the both the operative and contralateral knee post-operatively. Alterations in gait kinematics [45, 46], limb symmetry [47••, 48], and quadriceps strength have been reported in the operative knee [49, 50]. These changes increase the forces placed across the ACL graft, increasing the risk for subsequent graft failure [11, 44]. In the contralateral knee, these alterations in strength and neuromuscular control deficiencies in the operative knee have been reported to affect transverse hip moments, frontal plane knee angles, sagittal plane moments, and postural stability in the contralateral knee [8•, 14, 49]. Patients have also been shown to exhibit diminished proprioception in the operative knee for up to 2 years following surgery, increasing the risk for injury to both the operative and contralateral knee during cutting, landing, and pivoting [49]. As such, by compensating movement patterns as a means of offloading the operative limb, the contralateral knee may be at high risk for injury following ACLR, especially in sports-related tasks [12, 47••]. These changes may be further exacerbated by any existing leg-length or angular discrepancies present as a result of ACLR [8•].

Risks Factors for Contralateral ACL Tears in the Pediatric Patient

Patient Age

Despite limited data, pediatric patients have been reported to be at increased risk for contralateral ACL tears following index ACLR when compared with adults. Shelbourne et al. evaluated 1415 patients with minimum 5-year follow-up following ACLR and found a contralateral tear rate of 8.7% (n = 46 of 528) in pediatric patients (age < 18) compared with 4% (n = 14 of 350) of adult patients aged 18 to 25 and 2.8% (n = 15 of 537) in those > 25 years of age [51]. Meanwhile, Webster et al. (n = 517) reported a 300% increased risk of for contralateral tears in patients aged 19 and under (n = 17 of 107; 16%) compared with those aged over 19 (n = 24 of 410; 6%) (p = 0.001) [52]. Despite higher reported rates of contralateral ACL tearing, no study has verified patient age as an independent risk factor for contralateral ACL injury.

Within the pediatric population, no significant differences in contralateral tear rates have been reported based on patient age. A cohort study of 316 pediatric patients organized into 8 separate age groups (11–12, 13, 14, 15, 16, 17, and 18) by Webster et al. found no significant difference in contralateral ACL injury incidence following index ACLR between any age groups [12]. The authors reported that the youngest age group (11–12) possessed the highest contralateral injury rate (29%); however, no potential explanation for this finding was provided [12]. Moreover, Morgan et al. reported no significant difference in contralateral ACL tear rates when comparing patients aged 13–14 (n = 23 patients) with those aged 15–18 (n = 219 patients) (p = 0.83) [25]. Further investigations examining the impact of patient age versus other risk factors unique to the pediatric patient are warranted to better understand the differences between the pediatric and adult patients, as well as pre-adolescent and adolescent patients in regard to contralateral ACL injuries [14, 25, 52, 53•].

Timing Following Index ACLR

Contralateral ACL tears have been shown to occur further out from index ACLR in pediatric patients when compared with ACL graft re-ruptures following ACLR. Morgan et al. reported a contralateral ACL injury incidence of 20% (n = 48 of 242; mean age 16 years, range 13–18 years) occurring at a mean of 71 months following index surgery, compared with a graft rupture rate of 17% (n = 42 of 242) at a mean of 51 months [25]. Furthermore, the authors reported that 70% of graft re-ruptures occurred within 5 years of the index surgery, compared with 50% of contralateral ACL tears. Meanwhile, Placella et al. reported a 42% (n = 10 of 24, mean age 13.2 years) incidence of contralateral ACL ruptures following index ACLR with injury occurring at a mean of 32 months following surgery, while no ACL re-ruptures were noted [9]. Webster et al. also reported a contralateral tear rate of 17.7% (n = 56 of 316) occurring at a mean of 3.7 years after index ACLR, compared with a graft rupture rate of 18% (n = 57 of 316) occurring at a mean of 1.8 years after index ACLR [12].

Sport

When examining injury incidence based on sport, participation in sports requiring cutting and pivoting has been shown to be associated with increased risk for contralateral ACL tearing [25, 47, 51, 52]. Morgan et al. found that athletes returning to International Knee Documentation Committee (IKDC) level 5 sports, such as basketball or soccer, possessed an increased hazard ratio (HR) (HR = 2.3) for contralateral ACL tears (p = 0.05) when compared with pediatric athletes returning to all other sports [25]. Graziano et al. reported in their investigation that the two pediatric patients sustaining contralateral ACL injuries occurred upon returning to lacrosse [47••]. Meanwhile, Shelbourne et al. found that contralateral tears most often occurred in athletes returning to basketball, soccer, and football [51], while Webster et al. reported a significantly greater odds ratio (OR, 4.9; p = 0.001) of sustaining a contralateral ACL injury in athletes of any age returning to a cutting or pivoting sport [52]. Due to the small number of reported contralateral ACL injury cases specific to pediatric patients, additional studies are warranted to further determine which sports pediatric athletes are at greatest risk for sustaining contralateral ACL tears.

Return to Sport Timing

Early return to sport prior to adequate graft incorporation and healing following ACLR has been cited as a substantial risk for athletes sustaining a contralateral ACL injury [13••]. Graft incorporation and healing with religamentization have been shown to require up to 12 months following ACLR, with imaging not mirroring native ACL findings for up to 2 years following surgery [49, 54]. Moreover, strength and functional deficits in the operative knee and their subsequent impact in the contralateral knee following ACLR have been shown to persist for up to 2 years following surgery in pediatric athletes [48, 49]. As such, multiple studies have proposed delaying return to sport for up to 2 years following ACLR to allow for complete biological recovery, minimizing the risk for ACL re-rupture and contralateral ACL injury [13••, 49, 54].

In their study evaluating ACL injuries in 42 adolescent athletes (mean age: 12 years; range, 10–15 years), Graziano et al. reported two instances of contralateral ACL tearing, both occurring in athletes returning to sport less than 12 months of index ACLR [47••]. Meanwhile, Cordasco et al. reported contralateral tearing in a single athlete (4%; n = 1 of 23) playing catch 5 months following revision ACLR before being cleared for return to sport by their surgeon [8•]. Furthermore, the authors stated that successful rehabilitation following ACLR required a minimum of 12 months following surgery prior to return to sport for most patients, with 91% (n = 21 of 23) of athletes successfully returning to sport without further injury when complying with this recommended time-frame (mean rehabilitation time,13.5 months; range) [8•]. Dekker et al. found that of the 13% (n = 11 of 85) of pediatric athletes suffering a contralateral ACL tear following index ACLR, mean time to return of sport (9.6 months) was the only significant predictor for contralateral injury and graft re-rupture [13••]. Meanwhile, athletes returning to sport further out from index ACLR had a significantly decreased incidence of contralateral ACL injuries and graft ruptures (HR = 0.87 per 1-month increase in delay; p = 0.04) [13••]. Capin et al. evaluated 14 female pediatric patients (mean age, 16 years; range, 14–18 years) and found that patients who suffered a contralateral ACL injury or ACL graft re-rupture returned to sport significantly quicker (mean, 6.8 months) following ACLR when compared with patients without additional injury (mean, 9.5 months) (p = 0.024) [55].

Athlete Sex

Despite females possessing higher rates of native ACL injuries as a result of anatomic, biomechanical, neuromuscular, and hormonal differences when compared with males [56–61], scarce data has been published evaluating the influence of patient sex on the incidence of contralateral ACL tears. Shelbourne et al. found that females under age 18 had a significantly higher rate of contralateral ACL injuries following index ACLR when compared with males under age 18 (11.6% versus 4.6%, respectively; p = 0.0099) [51]. In contrast, Morgan et al. reported that pediatric males possessed a higher rate of contralateral ACL injuries (HR = 2.1; p = 0.03) when compared with females [25]. Meanwhile, Webster et al. found no difference in contralateral ACL tear rate when comparing males and females (18.9% versus 18.8%, respectively) [12]. As such, the impact of athlete sex on contralateral ACL risk remains ambiguous and warrants further investigation.

Body Mass

Only a single investigation has examined the impact of body mass index (BMI) on contralateral ACL tear risk in pediatric athletes. Capin et al. reported an association between increased body mass and second ACL injury in 14 pediatric females participating in sports involving jumping, cutting, and pivoting [55]. Patients who suffered either an ACL graft re-rupture or contralateral ACL injury following ACLR possessed a significantly higher BMI (mean, 24.5) when compared with patients without an additional injury (mean, 21.4) (p = 0.039).

Mechanisms of Contralateral ACL Injury

While native ACL ruptures occur primarily as a result of non-contact, twisting injuries to the knee [62, 63], the predominate mechanism of the injury responsible for contralateral ACL injuries following index ACLR is relatively unknown. Graziano et al. reported that in the 2 athletes sustaining contralateral ACL tears following return to sport, both injuries occurred as a result of non-contact mechanisms; however, further injury details were not provided [47••]. Cordasco et al. reported a contralateral ACL injury occurring in a single patient due to a non-contact mechanism while playing catch [8•]. When analyzing athletes of any age, Webster et al. reported no significant differences in contralateral ACL injury incidence following contact versus non-contact mechanisms (9% versus. 8% respectively; p = 0.60) [54]. As such, further reporting of injury mechanism, along with further information regarding the exact mechanisms of non-contact injury responsible for contralateral ACL injuries in pediatric patients, is necessary to help identify and implement injury prevention strategies and methods during athletic participation to minimize injury risks.

Conclusion

The reported rate of contralateral ACL rupture following index ACLR in pediatric patients ranges from 4 to 42%; however, the true injury rate is likely underreported due to the limited number of studies in the literature. Inherent anatomic differences in the pediatric knee, coupled with biomechanical and neuromuscular changes occurring in both the operative and contralateral knee following ACLR, likely contribute to the increased risk for contralateral ACL injury following index ACLR in the pediatric patient when compared with adults. While no significant differences are reported when comparing pre-adolescent with adolescent age groups, contralateral ACL injuries generally occur further out from surgery when compared with ACL re-ruptures. Athletes sustaining contralateral injuries primarily participate in cutting and pivoting sports, with those returning less than 12 months following ACLR at greater risk. While the influence of patient sex on injury risk remains unknown, most reported injuries occur as a result of non-contact mechanisms. Further study is essential to determine the necessary biologic, functional, and rehabilitation parameters necessary to prevent subsequent contralateral ACL injury following index ACLR in pediatric athletes as a means of maintaining athlete health and minimizing time lost from sport due to injury.

Compliance with Ethical Standards

Conflict of Interest

Michael J. Salata, MD, is a paid consultant for Smith and Nephew, Stryker, and Linvatec, outside the current submission. James E. Voos, MD, is a paid consultant for Arthrex, outside the current submission.