Intrasubstance Anterior Cruciate Ligament Injuries in the Pediatric Population.

Pediatric intrasubstance anterior cruciate ligament (ACL) tears have a significant epidemiologic impact as their numbers continue to grow globally. This review focuses on true pediatric intrasubstance ACL tears, which occur >400,000 times annually. Modifiable and non-modifiable risk factors include intercondylar notch width, ACL size, gender, landing mechanisms, and hormonal variations. The proposed mechanisms of injury include anterior tibial shear and dynamic valgus collapse. ACL tears can be associated with soft tissue and chondral defects. History and physical examination are the most important parts of evaluation, including the Lachman test, which is considered the most accurate physical examination maneuver. Imaging studies should begin with AP and lateral radiographs, but magnetic resonance imaging is very useful in confirming the diagnosis and preoperative planning. ACL injury prevention programs targeting high risk populations have been proven to reduce the risk of injury, but lack uniformity across programs. Pediatric ACL injuries were conventionally treated nonoperatively, but recent data suggest that early operative intervention produces best long term outcomes pertaining to knee stability, meniscal tear risk, and return to previous level of play. Current techniques in ACL reconstruction, including more vertically oriented tunnels and physeal sparing techniques, have been described to reduce the risk of physeal arrest and limb angulation or deformity. Data consistently show that autograft is superior to allograft regarding failure rate. Mean durations of postoperative therapy and return to sport were 7 ± 3 and 10 ± 3 months, respectively. These patients have good functional outcomes compared to the general population yet are at increased risk of additional ACL injury. Attempts at primary ACL repair using biological scaffolds are under investigation.

Introduction

Pediatric intrasubstance anterior cruciate ligament (ACL) tears are of growing interest in the orthopedic and sports communities, with particular emphasis on prevention and surgical management of these injuries. Classically, the tibial eminence avulsion fracture has been considered the pediatric “equivalent” of the adult intrasubstance ACL tear. Recent literature, however, is pointing toward an increase incidence of a pediatric intrasubstance tear as well.12 This review will focus on the “true” intrasubstance ACL tears in the pediatric age group.

Demographics

Incidence

The incidence of intrasubstance ACL tears in all age groups has been estimated at >400,000/year in the western literature.12 In the pediatric population, two nationwide registries from Sweden and New Zealand found the annual incidence to be 65–144/100,000 for patients over the age of 10. Exceedingly, few tears were found in patients below the age of 10 in both studies.34 A recent retrospective review of 20-year insurance claim data from a high income nation found a similar incidence of an average of 121/100,00 person-years, with an annual increase of 2.3%/year.2 The incidence of intrasubstance ACL tears in Asian regions including India remains unknown.

Risk factors

Risk factors for pediatric intrasubstance ACL include non-modifiable and modifiable patient characteristics. Non-modifiable risk factors include anatomic considerations such as intercondylar notch width, ACL size, female gender.5 Modifiable risk factors include “cutting” sports, landing mechanics, and hormonal variations during the menstrual cycle.6

Gender bias

Females were found to have a significantly increased ACL tear rate relative to males, especially when they were younger than 17 years.2 Overall female incidence was 129 tears per 100,000 patient years, peaking at age 16 with 392/100,000. Male incidence was 114/100,000 overall, peaking at age 17 with 492/100,000.2 Multiple mechanisms have been proposed to explain this [Table 1].5678

Table 1

Gender difference in risk factors for anterior cruciate ligament tear (adapted with permission from Orthobullets.com)

Genetics

Both protective and harmful genetic factors in ACL ruptures have been scrutinized, particularly variants of the collagen-encoding genes COL1A1, COL3A1, and COL5A1. While some articles have mentioned an association between ACL rupture in adults and presence of polymorphisms, two recent review articles were unable to conclusively link any specific gene to an increased risk of ACL rupture.5678

Financial impacts

Pediatric ACL reconstructions in New York State increased from 18/100,000 population in 1990, to 51/100,000 population in 2009.9 The long term impact of pediatric ACL tears is incompletely understood. Within 14 years of sustaining an ACL tear, about 78% of adults had radiographic evidence of arthritis in the affected knee.1011 The link between meniscal damage and development of arthritis is although well documented, however long term followup of adults with ACL injury has not addressed the association between ACL tears and the development of symptomatic arthritis.12 In addition, long term costs associated with ACL reconstruction in adults may be as high as $38,000.13 Pediatric-specific data on costs and long term morbidity from intrasubstance ACL tears, especially symptomatic arthritis requiring treatment, still remains incomplete. However, an association has been described between delayed operative management and meniscal tears.14

Pathomechanics

Anatomy

The ACL is a strong band of connective tissue traveling from the posteromedial aspect of the lateral femoral condyle to the anterior tibial plateau. It is comprised two functional bundles, the anteromedial bundle (AM) and the posterolateral (PL) bundle, named for their tibial insertions. Historically, the AM bundle was considered the main functional portion, but recently, more focus is being placed on the PL bundle and its effect on rotational stability. The AM bundle is tight in flexion and contributes primarily as a restraint to anterior translation of the tibia, while the PL bundle is tightest in extension.15

The ACL receives its blood supply from the middle geniculate artery, a branch of the popliteal artery.16 Tibial nerve branches are responsible for innervation of the ligament, mainly serving proprioceptive and vasomotor sensory functions. There are essentially no pain fibers within the ACL, thus pain after an ACL tear develops only after hemarthrosis development within the knee.17

Collectively, the physes surrounding the knee joint are the most active of any joint in the body. The distal femoral physis contributes to over 37% of total limb length, with the proximal tibial physis contributing to an additional 25%.1 Thus, pediatric ACL tears present a challenge, as iatrogenic damage to an open physis can lead to severe sequelae.

Mechanism of injury

Acute ACL injuries are most often noncontact injuries, during lateral pivoting, landing, or deceleration maneuvers, with the knee in shallow flexion and the foot planted.18 These mechanisms lead to two predominant theories of ACL loading-anterior tibial shear and dynamic valgus collapse. Dynamic valgus collapse is theorized to be a greater issue in female.19

Associated injuries

Both soft tissue and osteochondral injuries have been described to have an association with pediatric ACL tears.202122 Impaction forces at the time of injury lead to bony edema secondary to trabecular microfracture. This is classically seen in the middle third of the lateral femoral condyle and the posterior third of the lateral tibial plateau.2023 While it is unclear whether these findings independently lead to long term sequelae, subchondral changes can persist on magnetic resonance imaging (MRI) for several years after injury.2324

Soft tissue injuries, including medial meniscus, lateral meniscus, and medial collateral ligaments (MCL) injuries can all have significant long term effects if they are unrecognized.25 The percentage of associated meniscal pathology varies by study, with acute ACL tears having a higher associated lateral meniscal injury and medial meniscal pathology having a higher association with chronic tears.26 Furthermore, successful repair of ACL-related meniscal injury is less successful with an increased delay in surgery.27 Combined pediatric ACL/MCL tears have few reports in the literature, but valgus instability may result if MCL injury is missed.2829

Diagnosis

Diagnosis of ACL tears begins with a thorough history, including possible risk factors, mechanism, and duration of symptoms. Acute ACL injuries are often described as having a distinct “pop,” with a subsequent feeling of knee instability. There is a successive pain after an effusion develops. Between 47% and 65% of pediatric patients will develop a posttraumatic hemarthrosis.29

A subsequent physical examination should begin with an examination of the uninjured side. This may calm anxious pediatric patients through demonstration of maneuvers, while also establishing patient-specific “normal.” Inspection and palpation of the joint should be performed to assess for effusion as well as specific areas of tenderness. The range of motion may be limited by anxiety and pain, but gentle active and passive evaluation is necessary to assess for accompanying meniscal pathology; the patient should also be evaluated for hypermobility (via Beighton score) and associated injuries. The Lachman test, anterior drawer test, and pivot shift test are all specific tests utilized to diagnose ACL tears. With a sensitivity of 85% and specificity of 94%, the Lachman is considered as the most accurate test. While the pivot shift test has a very high specificity (98%), the poor sensitivity (24%) as well as the reproduction of knee instability make it a more limited test in the pediatric population.30313233 The knee arthrometer (e.g., KT-1000) is an objective tool that can be used to assess the amount of tibial translation compared to the healthy joint.34

Imaging studies should begin with standard anteroposterior and lateral radiographs. This can allow for the evaluation of tibial eminence fractures, status of the physis, and any anatomic variations. Often, a history and physical examination is all that is necessary to diagnose an ACL injury. However, in a pediatric patient where an accurate examination is difficult, and in a patient with an equivocal examination, MRI can be utilized to aid in diagnosis [Figure 1]. It is also helpful in preoperative planning, as it can also show other associated soft tissue injuries.35

Figure 1

Proton-density magnetic resonance imaging (a-f) with fat saturation (except c and e) showing the spectrum of anterior cruciate ligament injuries in the sagittal plane in pediatric patients aged 13-16 years (adapted from Jaremko et al.) (a) intact anterior cruciate ligament; (b) thin, but intact anterior cruciate ligament; (c) surgically confirmed high grade partial anterior cruciate ligament tear with lax fibers; (d) full-thickness midsubstance anterior cruciate ligament tear with some intact fibers near the tibial attachment (arrow); (e) full-thickness tear; (f) full-thickness tear with anteriorly flipped distal ligament fibers (arrow) and anterior tibial translation

Management

Prevention

In response to an increase in pediatric ACL injuries and recognition of their importance, particularly in female athletes, there has been an increase in neuromuscular ACL injury prevention programs. These programs target modifiable risk factors, including biomechanical patterns and neuromuscular functional changes.36 While ACL prevention programs have been accepted as effective in reducing primary injury, there are limited data on the uniformity of these programs. Components of these prevention programs include: balance, plyometrics (jump training), strength, agility, and stretching.37 In addition, there is a discrepancy between the frequency and the duration that athletes perform these exercises. A metaanalysis performed by Taylor et al.36 included 13 studies on ACL injury prevention programs, demonstrating that there was a statistically significant decrease in all ACL injuries (odds ratio [OR]: 0.61, 95% confidence interval [CI]: 0.44–0.85) and noncontact ACL injuries (OR: 0.35, 95% CI: 0.23–0.54) post training. Currently, there is no defined program composition for ACL injury prevention; however, it was noted that balance training– unless performed to correct other lower extremity biomechanics38 did not demonstrate a decrease in OR of injury, whereas a combination of agility and strength training demonstrated beneficial results.37

Nonoperative management

Until recently, pediatric ACL tears were treated by limiting activity and sports play, bracing, and extensive rehabilitation due to the concerns of physeal arrest, limb length discrepancy or angular deformity after reconstruction. However, data specific to this population with nonoperative treatment demonstrates an increased risk in symptomatic medial meniscal tears, cartilaginous injury, continued laxity/instability, and an inability to return to prior level of play.39 The preferred intervention has shifted recently toward early operative reconstruction due to these reasons.

Risk factors associated with the pediatric population in regards to meniscal and cartilaginous injuries in ACL deficient knees are increased age, male gender, one or more episode of instability, and an increased delay for surgery.22384041 Anderson and Anderson reported that delayed ACL reconstruction increased the risks of secondary meniscal and chondral injuries in children.14 Of particular concern is an increase in bucket handle meniscus tears which have a poor prognosis after surgical repair, therefore predisposing these patients to future arthritic changes of the knee.1421 In addition, patients treated nonoperatively or with delayed treatment typically had more instability and laxity with an inability to return to prior level of activity or sports than patients treated with early surgical intervention <12 weeks.39 These outcomes have led to the belief that early recognition and intervention of a pediatric ACL injury is necessary.

Operative management

Overview and technical considerations

Figure 2 demonstrates the currently preferred treatment algorithm for ACL reconstructions.1 A variety of techniques are available depending on the patient's skeletal maturity, including physeal sparing, partial physeal, and transphyseal [Table 2].1

Figure 2

Algorithm for treatment of skeletally immature patients with either partial or complete anterior cruciate ligament tear (Adapted with permission from Fabricant et al.)

Table 2

Outcomes of anterior cruciate ligament reconstruction in pediatric patients using all-epiphyseal, extraphyseal, and transphyseal techniques (adapted with permission from Fabricant et al.)

The transphyseal technique involves traversing both the tibial and femoral physis with a graft. Growth disturbance or angular deformity were only demonstrated when a method of fixation was passed across the physis, i.e., bone plug, staple, and screw. Considerations regarding the radius of the graft tunnel and drill angle are made when sizing and positioning the graft to decrease the possible risk of physeal damage. An increase in deformity is noted when >7% of the total physeal volume is involved while preparing the tunnel. A decrease from an 11 mm to 6 mm diameter graft decreases physeal percentage from 7.8% to 2.3%, and a more vertically placed tunnel also decreases physeal damage by approximately 0.2% for every 5° angular increase. Thus, recommendations are more central and vertically placed, and a smaller diameter tunnel specific to pediatric patients.1

Partial physeal techniques involve sparing of either the tibial or femoral physis, recommended for use on Tanner stages I-V.1 The graft choice is primarily by surgeon preference; however, the autologous hamstring is most often used. It has been shown that allograft use is associated with a higher rate of failure.42

The physeal sparing technique is suggested for use in Tanner stage I or II patient.1 It utilizes an all epiphyseal trajectory of drilling or an extraarticular over the top method of fixation. The over the top method is performed by harvesting the middle one-third of the iliotibial band proximally, leaving an attachment to the Gerdy tubercle distally. This graft is then brought through the knee posteriorly and under the intrameniscal ligament anteriorly on the tibia. Fixation of the graft is performed by suturing it to the intermuscular septum and periosteum on the femoral side and the periosteum on the tibial side. Different physeal sparing techniques are demonstrated in Figure 3.

Figure 3

Diagrammatic representation of physeal sparing techniques for anterior cruciate ligament reconstruction in skeletally immature patients (Reproduced with permission from Fabricant et al.). (a) Kocher technique combines intraarticular with extraphyseal fixation. (b) Anderson technique is intraarticular both for femur and tibia, extraphyseal fixation on the tibia. (c) Ganley technique involves intraarticular all epiphyseal fixation utilizing interference screws. (d) Cordasco-Green utilizes intraarticular all epiphyseal with suspensory fixation

Graft selection

As described above under technical considerations, various surgical techniques favor specific graft choices; however, others leave room for surgeon preference. Table 3 demonstrates the graft choices in relation to the surgical techniques as well as the pros and cons for specific graft choices. Current literature demonstrates favorable outcomes for autograft selection of iliotibial band, hamstrings, or bone-patella tendon- bone. Allograft hamstring selection demonstrated an unacceptable risk for rupture, four times the rate of hamstring autograft.42 Occasionally, autograft hamstring tendons may be very small in this population and may need to be augmented with allograft.43

Table 3

Graft selection in reference to surgical technique, pros and cons

Postoperative rehabilitation and return to sports

Postoperative rehabilitation

Postoperative ACL rehabilitation is focused on closed-chain, progressive exercise. Mean duration of physical therapy was 7 ± 3 months with a mean time to return to sport of 10 ± 3 months.44 Accelerated rehabilitation programs (19 weeks) were found to have similar results in knee laxity, patient satisfaction, clinical assessment, and function as compared to nonaccelerated (32 weeks) programs.45

Return to sports

Return to sport generally is recommended after completing a postoperative rehab course of approximately 6 months. While the majority of patients returned to sports, on an average, they participated in fewer sports postoperatively. In the study by Dekker et al.44 there was 32% prevalence of a second ACL injury either ipsilateral, contralateral or both. The only factor noted to place the patient at an increased risk for repeat injury was early return to sport <6 months. No clinical significance was noted with graft type.

Outcomes

ACL reconstruction with the use of autograft has become the gold standard for treating ACL tears in the young and active population.464748 Return to sport rates after ACL reconstruction are noted to be high, ranging from 80% to 100% as noted in Table 2, without the continued instability noted in patients treated nonoperatively. Cassard et al.49 noted that Lysholm Knee scores in patients who have undergone an ACL reconstruction ranged from 85 to 100 which is similar to that of a population of healthy knees. A recent metaanalysis of 53 studies of ACL reconstruction in skeletally immature individuals found an overall rate of growth disturbance of approximately 2.6%.50 The authors did not find a statistically significant difference between physeal-sparing and physeal-crossing techniques for growth disturbance, which they defined as a leg length difference of >1 cm or an axis deviation (varus, valgus or recurvatum) of >3°. Physeal-sparing techniques were associated with decreased postoperative complications; however, the authors acknowledged that the metaanalysis was limited by a wide variability of outcome reporting among the individual studies. However, patients who undergo ACL reconstruction still have a relatively high rate of premature OA when compared to the general population.51 This is hypothesized to be associated with the loss of the native biomechanics and proprioception of the ACL after reconstruction. In addition, there is a higher risk of rupture in the ipsilateral, contralateral or bilateral knee in comparison to an adult.44 In summary, due to minimal findings of limb length discrepancy, angular deformity, continued instability, or inability to return to play, reconstruction of ACL injuries in the skeletally immature patient is currently the treatment option of choice.

Anterior cruciate ligament repair

Recently, there has been increased interest in ACL repair as well.5253 ACL repair, as compared to reconstruction, is believed to allow for some conservation of the native biomechanics and proprioception of the ligament. However, ACL repair is far from a new concept. The first described ACL repair in print was described by Robson in 1895. Robson described a suture repair of the ACL and PCL in a minor who was noted to walk without a limp and continue in his arduous occupation after 6 years followup.54 In the 1930s to 50s pioneers such as Palmer and O’Donoghue described early ACL suture repair through bony tunnels.55 All of these techniques stressed the importance of immobilization, usually for 4–6 weeks with the knee in 30° of flexion. In the 1980s two randomized clinical trials compared ACL suture repair against nonoperative management.4756 Although there were some noted differences, such as the repair group showing overall greater stability and preservation of menisci, both studies reported no significant difference between repair and nonoperative management in patient function and satisfaction. As a result, greater emphasis was placed on augmenting repair with graft, such as iliotibial band and BPTB. Overtime, these augmentation procedures transformed into reconstructions without the need for ACL repair.

One of the probable reasons that ACL repair failed in the past is due to repair site gapping. Even with meticulous suture repair, midsubstance tears are bound to have micro gapping.52 Research over the last decade has been fierce in developing a collagen scaffold, impregnated with cells and growth factors to allow for ligamentous healing within the joint.

Currently, the only forms of ACL tears amenable to repairs are those that have avulsed off either at the proximal or distal ends of its origin. It is estimated that roughly 10% of ACL tears are avulsed from the femoral origin. In these cases, if the majority of the ACL remnant is found to be of viable tissue intraoperatively, an ACL repair through femoral and tibial drill holes can be attempted. We believe that in a specific population of pediatric patients with femoral avulsions of the ACL, an arthroscopic ACL repair with Fibertape augmentation can be an effective means of treatment with good functionality and the possibility of a decreased incidence of premature OA.5758

Future Directions

Analyzing long term sequelae of ACL tears and the consequences of different repair and reconstruction techniques would greatly improve strategies for management. Animal models exploring the use of mesenchymal stem cells or mesenchymal progenitor cells demonstrated accelerated healing.59 Gene therapy and tissue engineering are some key areas of research in ACL management. A collagen-silk composite scaffold was found to have sufficient mechanical support similar to the properties of the native ACL. Longer term animal studies, as well as human trials, are necessary.58 Standardization of exercise regime in the prevention aspect is also important.

Conclusions

Pediatric intrasubstance ACL tears are increasing in incidence, particularly among female athletes. Numerous programs promote ACL tear prevention such as proprioceptive training and hamstring activation exercises in combination with strength training. Nonoperative management has been largely supplanted by operative management, with a variety of techniques including physeal-sparing and transphyseal reconstruction.73

Financial support and sponsorship

No financial support or sponsorship was utilized for any aspect of this study.

Conflicts of interest

There are no conflicts of interest.