The Spectrum of Anterior Cruciate Ligament Reconstruction Options for the Pediatric and Adolescent Patient: A Narrative Review

As youth sports participation has continued to rise in recent decades, rates of anterior cruciate ligament (ACL) tears in pediatric populations have increased.^1–3 Historically, ACL reconstruction (ACLR) was delayed because of the elevated concern for physeal injury in skeletally immature patients and the risk for leg-length discrepancy (LLD) or angular deformity.^4,5 Although growth disturbances are rare, estimated to occur in up to 2% of patients, they may pose a significant risk of future deformity.⁶ Conversely, nonoperative management has been shown to result in increased rates of meniscal tears or cartilage lesions, high rates of persistent knee instability, and low rates of return to activity.^5,7 Kolin et al⁸ reported that each week surgical intervention was delayed resulted in a 2% higher risk of medial meniscal tear, with each 10-week delay increasing the risk by an estimated 20%.

Nonoperative treatment may be discussed as a possible option in patients who are not returning to sport and are able to avoid cutting and pivoting activities. The work of previous researchers^4,7,9,10 supports nonoperative management in patients with a skeletal age <14 years, partial ACL tears, and a pivot shift of <grade 2 on physical examination. However, these authors did not report on the long-term sequelae of knee instability and the resultant meniscal and chondral injuries. Moreover, for younger, active individuals, especially those wanting to return to higher levels of activity or sport, ACLR is the appropriate option to prevent the poor outcomes of nonoperative management.^5,7 Whereas >90% of adolescent patients returned to a similar sport activity level after ACLR, investigators^11–13 have shown that they are 20% to 30% more likely to incur a reinjury and 7 times more likely to sustain an injury if they return to sport before 9 months.

The purpose of our review was to describe the pediatric ACL in terms of relevant anatomy and biomechanics, physical examination, and diagnostic imaging. In addition, the importance of skeletal age and possible physeal injury is discussed in the context of ACLR options by skeletal age and remaining growth potential.

PEDIATRIC ACL ANATOMY

The ACL is the primary anterior restraint of the knee, partially limiting rotation and translation of the tibia around the femoral axis.¹ It is attached to the lateral femoral condyle posterior to the lateral intercondylar ridge and proximal to the bifurcate ridge.¹⁴ The ACL inserts on the proximal tibia anterior to the retroeminence ridge (Figure 1).^1,7,14 Throughout the range of motion of the knee, the ACL rotates from a more vertical position in extension to a more horizontal position in flexion and is at maximal tension at 30° of flexion.¹ The posterolateral and anteromedial bundles of the ACL are separated by the lateral bifurcate ridge. The 2 bundles are tensioned separately throughout the range of motion of the knee, with the anteromedial bundle achieving the highest strain at high flexion angles and the posterolateral bundle conversely achieving the highest strain at low flexion angles.^1,15

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For pediatric and adolescent patients, the ACL diameter and length grow in proportion with age, and throughout development, the ACL orientation transitions from oblique and anteriorly attached on the tibia to more vertical and posteriorly attached.^16–18 The rates of ACL growth vary during development, with patients aged 1 to 5 years averaging a maximum growth of 2.25 mm per year, which decreases until a plateau at age 12 years.¹⁶ By age 18 years, nearly all patients are expected to have a fully developed ACL.¹⁶ For patients treated with ACLR, the ACL dimensions are important when considering the ACL graft to prevent overstuffing of the intercondylar notch.^16,17,19

In skeletally immature patients, the close proximity of the ACL insertion to the tibial and femoral physes makes surgical planning a challenge (Figure 2).^7,10,20 Drilling holes through the physis for ACL graft placement may cause growth disturbance, and for this reason, surgical techniques have been developed to reduce the risk of clinically meaningful physeal injury. Authors^15,19 of anatomical studies have confirmed that the origin of the ACL insertion on the femoral head is distal to the femoral physis and that the tibial ACL insertion is proximal to the tibial physis. This epiphyseal space is sufficient for placement of graft drill holes, and these distances do not differ across cohorts aged 1 to 15 years.^15,19

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PHYSICAL EXAMINATION

The physical examination for a suspected ACL injury should begin with assessing range of motion, effusion, and laxity using tests including Lachman endpoints, pivot shift, anterior drawer, and muscle strength in the quadriceps and hamstrings.^21–24 The Lachman test is highly effective in identifying ACL injuries, with 87% sensitivity and 93% specificity; however, in follow-up settings, the Lachman test had little relationship with functional testing.^22–24 The pivot shift is an additional examination that is more closely associated with subjective knee function and clinically relevant in follow-up settings.²⁴ False-positive results were associated with isolated posterior cruciate ligament tears, and false-negative results may occur when pain and patient guarding prevent hamstrings relaxation.²² High-grade laxity in all components of the Lachman, pivot shift, and anterior drawer tests are markers of increased odds of an ACL tear.^22,23

IMAGING

Whereas physical examination is usually diagnostic for an ACL tear, imaging for patients with suspected ACL tears is routinely performed for final confirmation of the tear, diagnosis of concomitant injuries, and tunnel positioning during ACLR.^1,4,7,9,10 Radiographs of the knee are used to rule out fracture, but in patients with ACL ruptures, the images are often unremarkable.¹⁰ However, 1 radiographic finding occasionally seen with ACL injury is the Segond fracture, which is an avulsion fracture from the lateral tibial plateau (Figure 3). Magnetic resonance imaging (MRI) is a more useful tool in ACL injury diagnosis, with >90% sensitivity (Figure 1).¹⁰ It can also be used to evaluate patients for common comorbid injuries, such as bone bruises of the lateral femoral condyle and posterolateral tibial plateau (Figure 4), meniscal tears, chondral injuries, and other ligamentous damage. Computed tomography scans are rarely obtained but may be used to identify a possible periarticular fracture.¹

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CONCOMITANT INJURY

Bone bruises in pediatric patients are a common comorbidity with ACL ruptures, resulting from pivot shift and bone-to-bone impact between the tibia and femoral head during injury. Bone bruises of the lateral femoral condyle and lateral tibial plateau are associated with lateral meniscal injuries and high-grade pivot shifts and are pathognomonic for ACL tears (Figure 4).²⁵ Although rarely treated, bone bruises can lengthen the time needed to achieve symmetric range of motion and cause increased pain scores up to 6 months. In skeletally immature populations specifically, the physes provide shock absorption during injury, altering the patterns of bone bruises compared with those in skeletally mature populations and reducing the frequency of bone bruise patterns that cross the epiphysis to the metaphysis.²⁵ The decreased likelihood of effusion crossing the physes could suggest a protective mechanism in skeletally immature patients and reduced comorbidity.²⁵

Diagnosing meniscal tears is important in pediatric populations, as researchers²⁶ have shown the great risk of meniscal injuries accelerating osteoarthritis and increasing rates of ACL reinjury (Figure 5). Magnetic resonance imaging has been an effective tool in diagnosing meniscal tears, but the results vary across anatomic regions.²⁶ Pooled results in adult populations demonstrated a sensitivity and specificity for lateral meniscal root tears of 79.3% and 95.7%, respectively; however, these values were lower in pediatric populations (69% and 88%, respectively).²⁶ These results could be due to more complex anatomy of the posterior lateral meniscal region in pediatric patients, increased vascularization that can produce signal intensity of the meniscus that mimics meniscal injury, or both.^26,27 Thus, the meniscal region should be explored during ACLR to confirm diagnostic imaging findings.

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In a recent study,²⁷ Bernardini et al investigated the prevalence of posterior meniscocapsular tears of the medial meniscus (known as ramp lesions; Figure 6). They found a higher prevalence of ramp lesions in pediatric patients but noted that MRI had a low sensitivity for the diagnosis of these lesions. The authors recommended exploring the posteromedial compartment during ACLR to confirm the diagnosis of ramp lesion.²⁷

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Posterolateral corner (PLC) injuries are uncommon with ACL tears but were observed to occur at higher rates of injury in skeletally immature patients than adult populations.²⁸ Diagnosing PLC injuries using MRI is challenging because of postinjury edema surrounding the anatomically complex region and decreased diagnostic accuracy in skeletally immature populations.²⁸ Posterolateral corner injury is associated with increased rates of ACL graft failure in adult populations, but more research is required to better understand its clinical importance in skeletally immature patients. Given the potential for increased morbidity and likely underreported rates of injury, clinicians should increase their suspicion of PLC injuries in skeletally immature patients via examination and imaging.

PHYSEAL GROWTH AND INJURY

The amount of remaining skeletal growth is particularly important, as physeal injuries of 7% to 9% in animal ACLR models led to growth disturbances.^20,29–32 Skeletal maturity typically occurs around ages 14 to 15 years for girls and ages 16 to 17 years for boys, and the distal femoral and proximal tibial physes grow an average of approximately 10 mm/y and 6 mm/y, respectively.^33–36 In some cases, skeletal and chronologic maturity may not be correlated, and this discrepancy may affect patient outcomes if not accounted for in the choice of ACLR technique. In particular, a patient with an older chronologic than bone age may not be a good candidate for a transphyseal technique.

Many clinical tools are available to assist with determining skeletal age in adolescents, with the tool preferred most commonly by surgeons being Sanders bone age based on wrist and hand radiographs.^10,33,37 Other tools that can be used include the Tanner stages of growth, Risser grade, menarchal history, and growth velocity.^10,33 Also, a new report³⁸ described the measurement of bone age using landmarks on MRI scans of the knee. Given the degree of skeletal growth remaining, the treatment of ACL tears with reconstruction is often classified by age group: prepubescent, adolescent with substantial growth remaining, or adolescent with closing physes. Depending on the growth remaining, ACLR techniques include physeal sparing via iliotibial band (ITB) graft passage in prepubescent patients, epiphyseal or partial transphyseal reconstruction with soft tissue grafts in adolescent patients with growth remaining, and transphyseal adult-like reconstructions in adolescents with closing physes (Table).³⁹

Table Anterior Cruciate Ligament Reconstruction Technique as Recommended by Skeletal Age

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Altered growth after ACLR, physeal arrest, posttraumatic osteoarthritis, knee stiffness, and infection are all concerns with pediatric ACL surgery. In the current literature, the analysis of LLD or angular deformity is limited. Fury et al,⁴⁰ in a systematic review, identified variable reporting and monitoring of growth-related disturbances in the skeletally immature population after ACLR. Given these restrictions, they reported 2% had an LLD >1 cm and 1.3% had a postoperative angular deformity after ACLR.⁴⁰

REHABILITATION AFTER INJURY

Whereas rehabilitation after ACLR does not substantially vary from adult ACLR, surgeons may modify the rehabilitation protocol based on a patient's physical and emotional maturity. In adult populations, early return to weight bearing is emphasized after surgery; however, in pediatric patients, weight bearing may be delayed because of the potential behavioral challenges of maintaining gait mechanics and stressing the graft location through the femoral tunnel.⁴¹ For similar reasons, bracing may be continued throughout rehabilitation in school and other social settings, and delaying the return to sport may reduce the risk of reinjury. As noted, adolescent patients are more likely to return to sport, with 91% of patients returning to similar activity levels,¹³ but patients have a 20% to 30% risk of reinjury^11,13 and are 7 times more likely to sustain ACL tears if they return to sport before 9 months after ACLR.¹² Thus, surgeons have recommended a comprehensive rehabilitation program extending at least 1 year after surgery to address strength, balance, and neuromuscular deficiencies in order to reduce the risk of reinjury.¹¹

SKELETAL AGE AND ACLR

Prepubescent

Prepubescent generally refers to patients in Tanner stage 1 or 2 or to typical skeletal ages of ≤11 years in girls or ≤12 years in boys.^{10,33,39,42–46} The most common techniques in this age group involve ITB reconstructions (Figure 7) and all-epiphyseal reconstructions (Figure 8). Kocher et al⁴⁷ described a nonanatomic combined intra- and extra-articular reconstruction using an autogenous ITB graft. In this procedure, an oblique 6-cm incision is made on the lateral joint line to the superior border of the ITB.⁴⁷ The proximal ITB is then separated from the subcutaneous tissues bluntly, and the anterior and posterior borders of the ITB are incised and carried proximally.⁴⁷ Leaving the distal aspect of the ITB attached to its insertion at the Gerdy tubercle, the surgeon detaches the proximal aspect underneath the skin. After standard arthroscopy, including addressing any concomitant meniscal or cartilage injuries,^{26,27,48,49(p201)} the “over-the-top” position on the femur and “over-the-front” position under the intermeniscal ligament are identified.⁴⁷ The free end of the ITB is brought through the over-the-top position, with a clamp used to loop it posterolaterally over the lateral femoral condyle.^10,47 The ITB is then passed through the intercondylar region, through the joint, and into a trough underneath the intermeniscal ligament.^10,47 This trough allows for a more posterior graft position to more closely approximate anatomic positioning on the tibia.^10,47 The graft is then sutured proximally to the periosteum of the lateral femoral condyle at the insertion of the intermuscular septum.^10,47

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Researchers have begun to investigate outcomes after ITB reconstruction. Kocher et al⁴⁷ described the technique and provided results in a larger cohort of 237 patients over a 23-year period.⁴⁸ No cases of LLD or angular deformity were seen, and in the latter study,⁴⁸ the graft rupture rate was 6.6% at a mean 6-year follow-up. They also reported excellent clinical outcome scores; the main complications were lateral thigh asymmetry or dissatisfaction with the cosmetic appearance of the ITB harvest site in 48% and 17% of patients, respectively.⁴⁸ In a systematic review, Knapik and Voos⁵⁰ found no differences in clinical outcome scores or knee range of motion between all-epiphyseal and ITB reconstructions; however, ITB reconstruction resulted in less LLD and a higher return-to-activity rate.⁵⁰ In a biomechanical study, Suero et al⁵¹ observed that sectioning the ITB in an ACL-deficient knee led to increased anterior tibial translation; yet Kennedy et al⁵² evaluated multiple ACLR techniques and determined that the ITB reconstruction best restored anteroposterior and rotational stability, although it also appeared to overconstrain the knee at some flexion angles.

Another commonly performed ACLR in prepubescent children is the all-epiphyseal ACLR technique (Figure 8), which recreates an anatomic position of the ACL. The technique was first described by Anderson,⁵³ who used radiographic guidance to drill horizontally in the distal femoral epiphysis from lateral to medial, ending in the proximal ACL footprint.^10,53 The tibial tunnel is similarly drilled in the anteromedial tibia under radiographic guidance from anterior to posterior to avoid the physis (Figure 9).^10,53 The hamstrings graft is then secured on the femur using a cortical button and just distal to the tibial physis using a post screw.^10,53 Later authors^10,54,55 modified the technique to create all-epiphyseal tunnels and fixation with cortical buttons or interference screws. Some authors^9,10,56 have raised concerns regarding the tethering force across the physis with the distal post in the Anderson technique,⁵³ leading many surgeons to move toward a completely all-epiphyseal technique with epiphyseal tunnels.

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Interestingly, in an MRI review of 23 patients, all-epiphyseal reconstruction resulted in no femoral physeal injury and in tibial physeal injury of a mean 2.1% by area.⁵⁷ Investigators^{54,55,57–60} of many small case series have reported variable clinical outcomes and rates of LLD after all-epiphyseal reconstruction. Cruz et al used a modified Anderson technique and, in a series of 103 patients with minimum 6-month follow-up, found a 10.7% rerupture rate and <1% rate of LLD.^61,62 Fourman et al,⁶³ in a study with a longer follow-up of a mean 5 years, demonstrated a similar graft rerupture rate (10.5%) but a much higher LLD rate, with 27% having an LLD >1 cm. Patel et al⁴⁹ performed a single-center comparison of 162 patients with all-epiphyseal ACLR and 843 patients with transphyseal ACLR and revealed no difference in rerupture rates (9.8% versus 10.4%, respectively) or in postoperative range of motion or isokinetic strength when controlling for confounding variables. In a recent systematic review, Knapik and Voos⁵⁰ showed that the all-epiphyseal reconstruction group had a lower return-to-activity rate (87% versus 97%) and higher incidence of LLD >1 cm (4% versus 0.8%) compared with the extraphyseal reconstruction group. No differences were present in the rerupture rate (7.9% versus 3.6%) or clinical outcome scores.⁵⁰

Adolescent With Closing Physes

In adolescent populations, most ACL injuries occur in those with <1 year of growth remaining.⁷⁰ Therefore, surgeons can use transphyseal reconstruction methods (Figure 10) without substantial risk for growth disruption in adolescent populations with closing or closed physes (typically girls aged ≥14 years and boys aged ≥16 years with a Tanner stage of 4 or 5).^39,42–46 However, despite a limited amount of growth remaining, the physis must still be respected. Using an ACL graft that is all soft tissue (quadriceps tendon, hamstrings tendon) may carry a lower risk of physeal arrest if this soft tissue graft crosses the physes through the drill hole. In contrast, using an ACL graft that has bone (bone–patellar tendon–bone graft) may have a higher risk of physeal arrest if the bone portion of this graft is placed across the physis in the drill hole. In a survey of high-volume ACLR surgeons, Kocher et al⁹ identified that 11% had seen a growth disturbance related to ACLR in a skeletally immature patient. Because of growth changes after transphyseal reconstruction, patient stratification based on bone age is critical. For patients with 1 to 5 cm of growth remaining (girls aged 11–12 years, boys aged 13–14 years), care must be taken to protect the physes, as clinically important growth disturbance is possible after ACLR. A transphyseal reconstruction may be preferred but should be done in a manner that decreases the physeal injury risk. A quadriceps or hamstrings autograft may be the optimal graft choice in these populations.⁵⁵

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CONCLUSIONS

The unique anatomy of the epiphysis in skeletally immature patients makes ACLR a challenge because of the risk of violating the physes, physeal arrest, and developing LLD or angular deformity. Despite these challenges, early ACLR remains the criterion standard in the literature because of the risks of persistent knee instability and meniscal and chondral injuries associated with nonoperative management. Clinicians should assess both the skeletal age and behavioral maturity of patients when determining the best treatment paths. Depending on these factors, clinicians, parents, and patients should decide on an ACLR approach and treatment plan that accounts for the growth remaining and best protects patients from reinjury after surgery and when returning to sport.