Lower Limb Asymmetry After Anterior Cruciate Ligament Reconstruction in Adolescent Athletes: A Systematic Review and Meta-Analysis

Anterior cruciate ligament (ACL) injury most often occurs during noncontact situations (72% of cases),^1,2 such as landing, side stepping or cutting, and change of direction, in which no direct blow to the knee occurs at the time of injury. After ACL injury, surgery to reconstruct the ruptured ACL is the most frequent option for patients, with 90% of patients electing to proceed.³ Most ACL reconstructions (ACLRs) are performed in the adolescent population.^4,5 After ACLR, adolescent athletes typically aim to return to competitive sport participation. This requires extensive rehabilitation, with the overarching goal of returning the surgical limb to a similar level of function (ie, range of motion, strength, and coordination) as existed before the injury.^6–8 However, preexisting levels of function are usually unknown, so the goal instead becomes returning the surgical limb to symmetric function with the nonsurgical limb,⁶ which is subsequently used by a health care professional as a marker for the athlete's clearance to return to sport.^6,9–11

Despite considerable efforts to improve surgical procedures and rehabilitation practices after ACLR, secondary ACL injury is common in athletes who return to sport.^9,12–15 Between 6% and 29% of patients who undergo ACLR either injure the graft of the surgical limb or sustain an ACL injury to the contralateral limb.^9,12–15 This risk of subsequent ACL injury after reconstruction is highest among adolescent patients.^15–18 Shelbourne et al¹⁶ reported that the prevalence of rupture to either the contralateral or reconstructed ACL within 5 years of ACLR was 9.6% (4.3% for the reconstructed ACL, 5.3% for the contralateral knee) for the general population (N = 1415). This, however, rose to 17.4% (8.7% for both the ACL-reconstructed and contralateral knees) in patients aged <18 years compared with 6.7% in patients aged 18 to 25 years and 3.9% in patients >25 years.¹⁶ Furthermore, Shelbourne et al¹⁶ noted that most secondary ACL injuries occurred during sport participation, with the majority sustained during basketball (52%) or soccer (15%) compared with only 6.6% (9 of 136 secondary ACL injuries) sustained during nonsport-related activities.

The reasons for the high prevalence of secondary injury after ACLR are likely deficiencies in the strength of the muscles controlling knee movement, as well as altered movement and loading patterns when performing athletic activities, such as landing, squatting, and cutting.⁹ Asymmetry in landing mechanics after ACLR has been proposed as a major risk factor for both primary¹⁹ and secondary ACL injury.⁹ Investigators^{8–11,20–24} observed that after ACLR, athletes who exhibited less asymmetry of lower limb biomechanics during sporting activities had a reduced risk of sustaining a second ACL injury. However, one of the main challenges for health care professionals treating patients with ACLR is how to identify asymmetry between limbs and, therefore, properly clear an athlete to return to sport. Some researchers^7,25 have attempted to develop clinical assessment criteria to appropriately define return to sport after ACLR. These criteria typically include consideration of time since surgery; knee-joint stability; symmetry between surgical and nonsurgical limbs in postural control, strength, power, endurance, agility, and hop distance; and qualitative movement analysis during sporting tasks.²⁵ Yet most of these criteria rely on subjective measures and only indirectly assess knee function and loading. For example, a measure of hopping distance provides little information about the lower limb movement and loading patterns that have been identified as risk factors for secondary injury during dynamic movements.^9,22,26 This suggests that the current methods used to clear the athlete to return to play after ACLR may not adequately determine mechanical symmetry between limbs. Consequently, many athletes will likely return to competition prematurely, before their reconstructed knee has regained full function in terms of range of motion, strength, and muscle-activity patterns.

Most researchers who examined asymmetries after ACLR have focused on adult populations.^26,27 In a recent systematic review and meta-analysis, Lepley and Kuenze²⁸ evaluated hip and knee biomechanics after ACLR during double- and single-limb landing tasks. Patients who underwent ACLR demonstrated a reduced knee-extension moment and reduced ground reaction force (GRF) on the surgical side compared with the nonsurgical side, which suggested potential unloading of the surgical limb. In our study, we aimed to build on the work of Lepley and Kuenze²⁸ by focusing on adolescent athletes because of their increased risk of secondary ACL injury and by including more variables in the analysis, specifically ankle biomechanics. During adolescence, the body undergoes considerable change in structure and maturation of motor coordination and control.^29,30 These changes result in altered landing mechanics.^4,5 Given the large number of ACLRs performed during adolescence, asymmetry in lower limb mechanics is likely to have a more profound effect on young athletes wanting to return to sport participation after ACLR. Therefore, the purpose of our systematic review and meta-analysis was to identify reported (1) common biomechanical asymmetries in the literature after ACLR in adolescents during landing movements and (2) timescales for asymmetry to persist postsurgery.

METHODS

Study Selection

We downloaded the article data, including authors, title, abstract, and journal, into RefWorks (ProQuest, Ann Arbor, MI) citation management software; duplicate citations were deleted. This reduced the number of articles to 85. Next, we reviewed article titles and abstracts to apply our inclusion criteria: human adolescents (mean age >11 and <18 years) who had ACLR and ≥1 kinetic or kinematic measure of asymmetry that compared the surgical and nonsurgical limbs. Kinetic variables included GRF, joint moments (turning effects produced by forces acting around joints), and energy absorption (amount of work performed at a joint, calculated from the joint moment multiplied by the change in joint angle). Kinematic variables included joint angles and angular velocities. Lastly, the task performed was required to be either a double-limb (bilateral) or single-limb (unilateral) landing maneuver. Two independent reviewers (G.H., S.C.) assessed all articles, and any disagreements were resolved through oral discussion after all articles had been reviewed.

After the inclusion criteria were applied, 12 papers remained. We inspected the reference lists of each paper and identified 1 more study, bringing the total to 13. The literature search and study-selection procedures are outlined in Figure 1.

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RESULTS

Time After ACLR

Participants were tested at approximately 4,³⁸ 5,^38,40 6,^{38,39,41–43} 7,^23,44 8,^37,40 10,³⁵ and 12^38,39 months after ACLR. Ithurburn et al⁴⁵ reported the time from being cleared to return to sport rather than the time from ACLR (4 weeks after being cleared to return to sport and 2 years later). Schneider et al⁴⁶ simply stated that testing took place within 1 year after ACLR, and Paterno et al⁹ did not clearly specify the time after ACLR (Figure 2).

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Outcome Measures

Most researchers examined asymmetries only in the sagittal plane; only 3 groups^40,41,44 assessed variables in all 3 planes of motion. Kinematic and kinetic joint variables were evaluated 58 times in the sagittal plane (28 kinematic and 30 kinetic analyses) compared with only 14 times in the frontal plane and 6 times in the transverse plane. Differences between surgical and nonsurgical limbs were commonly observed in the sagittal plane: 32% (9/28 included data points) of the time differences for kinematics and 70% (21/30 included data points) for kinetics. Few kinematic variables differed between the surgical and nonsurgical limbs in the frontal and transverse planes (only 1 difference among 13 variables measured), but for kinetic measures in the frontal and transverse planes, 6 differences were demonstrated for only 7 variables measured (5 frontal plane, 2 transverse plane; Tables 1 and 2).

Table 1 Summary of the Kinematic Variables Measured to Assess Biomechanical Asymmetry in Each Study^a

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Table 2 Summary of the Kinetic Variables Measured to Assess Biomechanical Asymmetry in Each Study^a

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With respect to which lower limb joints were assessed for asymmetries, 5 studies^38–41,44 examined the knee and at least 1 other joint (hip or ankle), 5 studies^{9,37,42,43,45} examined only the knee joint, 2 studies^23,35 examined only GRF, and 1 study⁴⁶ examined only leg stiffness.

Most investigators included both kinematic and kinetic measures of asymmetry, with 5 studies^{9,23,35,43,46} examining only kinetic variables. No authors examined purely kinematic measures of asymmetry. Differences between the surgical and nonsurgical limbs were more frequently observed in kinetic variables than in kinematic variables, whereby 76% (44/58 included data points) of the time, a between-limbs difference was present for kinetic variables compared with only 24% (10/41 included data points) for kinematic variables.

The most commonly measured variable was peak vertical GRF, which was assessed in 10^{23,35,37–44} of the 13 studies. This variable was also most often different between limbs, with 9 studies^{23,35,37–43} showing reduced vertical GRF for the surgical limb compared with the nonsurgical limb. The only researchers⁴⁴ who measured peak vertical GRF and did not find a difference between limbs used a single-limb landing task. The meta-analysis indicated asymmetry between the surgical and nonsurgical limbs for peak vertical GRF with large summary effect sizes for double-limb landings (b = −1.67; 95% confidence interval [CI] = −2.53, −0.80; standard error [SE] = 0.44; z = −3.78; P < .001) but a trivial effect size for single-limb landings (b = −0.06; 95% CI = −0.36, 0.23; SE = 0.15; z = −0.42; P = .23; Figure 3A). The second most commonly measured variable to assess asymmetry was peak knee-extension moment, which was measured in 7^{37–40,42,43,45} of the 13 studies and found to be reduced in the surgical limb compared with the nonsurgical limb in 6^{37–40,42,43} of the 7 studies, all of which employed a double-limb landing task (Table 2). The meta-analysis indicated an asymmetry between the surgical and nonsurgical limbs for peak knee-extension moment with a large summary effect size for double-limb landings (b = −2.86; 95% CI = −4.56, −1.17; SE = 0.86; z = −3.31; P < .001; Figure 3B).

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The other kinetic variables in which the meta-analyses identified asymmetry were loading rate, mean knee-extension moment, knee-energy absorption, ankle-energy absorption, mean ankle sagittal-plane moment, and mean knee frontal-plane moment. Large effect sizes were observed for loading rate during double-limb landings (b = −2.08; 95% CI = −3.39, −0.76; SE = 0.49; z = −3.58; P < .001; Figure 3C), mean knee-extension moment during double-limb (b = −1.63; 95% CI = −2.35, −0.92; SE = 0.36; z = −4.48; P < .001) and single-limb (b = −0.90; 95% CI = −1.33, −0.48; SE = 0.22; z = −4.13; P < .001) landings (Figure 3D) and knee-energy absorption during double-limb (b = −1.38; 95% CI = −1.76, −1.01; SE = 0.19; z = −7.23; P < .001) and single-limb (b = −0.99; 95% CI = −1.50, −0.48; SE = 0.26; z = −3.81; P < .001) landings (Figure 3E). Large summary effect sizes were noted during double-limb landings for ankle-energy absorption (b = −1.04; 95% CI = −1.44, −0.64; SE = 0.20; z = −5.08; P < .001 Figure 3F) and mean ankle sagittal-plane moment (b = −1.00; 95% CI = −1.27, −0.74; SE = 0.14; z = −7.42 P < .001; Figure 3G), whereas effect sizes were small or trivial and not different during single-limb landings for ankle-energy absorption (b = 0.17; 95% CI = −0.15, 0.49; SE = 0.16; z = 1.06; P = .29; Figure 3F) and mean ankle sagittal-plane moment (b = 0.43; 95% CI = −0.43, 1.29; SE = 0.44; z = 0.97; P = .33; Figure 3G). Moderate effect sizes were demonstrated for mean knee frontal-plane moment during double-limb (b = −0.68; 95% CI = −0.94, −0.41; SE = 0.14; z = −5.01; P < .001) and single-limb (b = −0.53; 95% CI = −0.82, −0.23; SE = 0.15; z = −3.52; P < .001) landings (Figure 3H).

The only kinematic variables in which the meta-analyses identified differences between the surgical and nonsurgical limbs were peak knee-flexion angle and peak ankle-dorsiflexion angle. A moderate summary effect size was observed for peak knee-flexion angle during double-limb (b = −0.66; 95% CI = −1.02, −0.30; SE = 0.18; z = −3.62; P < .001) and single-limb (b = −0.51; 95% CI = −1.01, −0.01; SE = 0.25; z = −2.02; P = .044) landings (Figure 3I) and for peak ankle-dorsiflexion angle (b = −0.58; 95% CI = −0.89, −0.28; SE = 0.15; z = −3.78; P < .001) during double-limb landings, but only a small summary effect size was detected for this variable during single-limb landings (b = −0.23; 95% CI = −0.65, 0.19; SE = 0.21; z = −1.07; P = .28; Figure 3J).

The meta-analyses indicated no asymmetry between the surgical and nonsurgical limbs for knee flexion at initial ground contact, peak hip-flexion angle, mean hip sagittal-plane moment, and hip-energy absorption during both double- and single-limb landings. Trivial summary effect sizes were present for knee-flexion angle at initial ground contact during double-limb (b = 0.04; 95% CI = −0.34, 0.42; SE = 0.19; z = 0.19; P = .85) and single-limb (b = −0.16; 95% CI = −0.45, 0.14; SE = 0.15; z = −1.05; P = .29) landings, peak hip-flexion angle during double-limb (b = 0.14; 95% CI = −0.10, 0.37; SE = 0.12; z = 1.14; P = .26) and single-limb (b = −0.20; 95% CI = −0.96, 0.57; SE = 0.39; z = −0.50; P = .62) landings, mean hip sagittal-plane moment during double-limb (b = 0.18; 95% CI = −0.08, 0.45; SE = 0.14; z = 1.36; P = .18) and single-limb (b = 0.05; 95% CI = −0.25, 0.34; SE = 0.15; z = 0.32; P = .75) landings, and hip-energy absorption during double-limb (b = −0.05; 95% CI = −0.31, 0.22; SE = 0.14; z = −0.35; P = .73) and single-limb (b = −0.15; 95% CI = −0.99, 0.70; SE = 0.43; z = −0.34; P = .73) landings.

DISCUSSION

In this systematic review and meta-analysis, we aimed to identify the common biomechanical asymmetries during landing movements reported in the literature after ACLR in adolescents. A total of 13 studies were assessed for quality, with 5 studies^{23,37,38,40,41} rated as high quality, 7^{35,39,42–46} rated as moderate quality, and 1⁹ rated as low quality (average score = 10.2/14). Sagittal-plane knee kinetics (8 studies: 4 high,^37,38,40,41 3 moderate,^39,42,43 and 1 low⁹ quality) and vertical GRF (8 studies: 5 high^{23,37,38–41} and 3 moderate^39,42,43 quality) during double-limb landings were the most frequent variables that were both measured and shown to be asymmetric between the surgical and nonsurgical limbs. This result is supported by a recent systematic review and meta-analysis of Lepley and Kuenze,²⁸ who also found asymmetry in these variables during landing in adults with ACLR. The close link between these factors was identified by Dai et al,⁴³ who determined that asymmetries in GRF accurately predicted asymmetries in knee-extension moment. Anterior cruciate ligament reconstruction had a large effect on asymmetry between limbs in peak vertical GRF, peak knee-extension moment, mean knee-extension moment, knee-energy absorption, and loading rate during double-limb landing. In all cases in which asymmetries were identified for these variables, the loading of the surgical limb was less than in the nonsurgical limb. The meta-analyses more often identified asymmetries with larger effect sizes for double-limb landings than for single-limb landings. This may be partly due to the larger number of data points for double-limb landings but also suggests that during double-limb landings, athletes recovering from ACLR typically put less load on their injured limb to protect it from further injury by increasing the load on the uninjured limb. This observation is supported by Lepley and Kuenze,²⁸ who found lower peak knee-extension moments in the surgical limb during double-limb landing but not during single-limb landing. Asymmetry in landing mechanics after ACLR has been proposed as a risk factor for secondary ACL injury⁹ to the surgical or nonsurgical limb, whereby increasing the load on the uninjured limb may increase the risk of injury. The only study⁴⁴ (moderate quality) that measured peak vertical GRF and did not find that it was lower in the surgical than the nonsurgical limb involved a single-legged landing task in which it was not possible to unload the surgical limb by placing additional load on the nonsurgical limb. Anterior cruciate ligament reconstruction had a large effect on asymmetry during single-limb landings only for mean knee-extension moment and knee-energy absorption. This result suggests that, whereas it is not possible to unload the injured limb by increasing loading on the uninjured limb, athletes may still attempt to reduce the peak loading on the knee by spreading the loading over a longer time.

Generally, researchers reported differences far more commonly in kinetic than in kinematic variables. Lepley and Kuenze²⁸ demonstrated no conclusive effect of asymmetry in hip or knee kinematics during landing. The only kinematic variables for which the meta-analyses identified asymmetry were peak knee-flexion angle during both double- and single-limb landings and peak ankle-dorsiflexion angle during double-limb landing, but these variables had only moderate effect sizes. In all studies, comparable 3-dimensional motion-analysis techniques were implemented to measure joint kinematics (between 8 and 10 camera systems sampling at either 120 or 240 Hz). While several different marker sets were used, it is fair to assume that the motion-analysis techniques used by all authors would elicit high-quality kinematic data. Although some investigators reported differences in sagittal-plane kinematics (9 differences at 28 times measured across 8 studies^{37–42,44,45}: 4 high^37,38,40,41 and 4 moderate^39,42,44,45 quality), only once⁴⁴ (moderate-quality study) did a frontal- or transverse-plane kinematic variable differ between limbs in the 13 measured cases. This is, therefore, an area for future work, particularly because frontal- and transverse-plane kinematic variables, such as knee-valgus angle and hip internal rotation, have been strongly implicated as risk factors for ACL injury.^9,47–49 However, these findings relate only to asymmetry between surgical and nonsurgical limbs and not to comparisons between injured and healthy individuals or between pre- and post-ACL injury. Consensus is lacking on the extent of asymmetries in lower limb biomechanics between dominant and nondominant limbs in the general healthy population, with some groups reporting no differences⁵⁰ and others reporting some differences.^47,51 Of the 13 studies included in this systematic review, only 2 (1 high⁴¹ and 1 moderate⁴³ quality) described which limb (dominant or nondominant) was injured, and no apparent control for limb dominance was included in any statistical analyses. The authors who supplied the percentage of injuries to the dominant limb provided values of 48% (11/23)⁴³ and 54% (33/61),⁴¹ supporting previous research that showed no relationship between the side of primary ACL injury and limb dominance.^52–54

Whereas kinetic variables in the frontal and transverse planes were not commonly measured, investigators in the 3 studies (2 high^40,41 and 1 moderate⁴⁴ quality) that did so identified asymmetry. In our review, the only frontal-plane kinetic variable that was different between limbs was mean knee frontal-plane moment (average adduction moment acting about the knee between initial ground contact and maximum knee flexion during the landing maneuver) during double- and single-limb landings, which demonstrated a moderate effect size. This highlights the need for further investigation of frontal- and transverse-plane asymmetries between surgical and nonsurgical limbs.

Although 5 groups (reflecting 3 high-^38,40,41 and 2 moderate-^39,44 quality studies) examined mechanical asymmetries at the hip and ankle joints, most of the asymmetric variables were at the knee joint. The only variables that were asymmetric at the ankle were ankle-energy absorption and mean ankle sagittal-plane moment during double-limb landings, with large effect sizes. No meaningful effect of asymmetry was apparent in peak hip-flexion angle, mean hip sagittal-plane moment, or hip-energy absorption after ACLR. This contrasts with the findings of Lepley and Kuenze,²⁸ who observed large effects for peak hip sagittal-plane moment during double-limb landings in adult participants. Given that the knee was the site of the surgical reconstruction, it is perhaps to be expected that most asymmetry would occur at that joint, which emphasizes the need for rehabilitation practices to focus there, with less concern about compensatory movements in other joints of the kinetic chain.

Among adult participants after ACLR, asymmetries were present in both loading and movement patterns.^{11,28,39,42,55} These asymmetries included reductions in knee flexion, knee sagittal-plane moment, and vertical GRF on the surgical side compared with the nonsurgical side.^{11,28,56–61} We determined that these variables also affected between-limbs symmetry in adolescent participants after ACLR. Therefore, asymmetry in landing biomechanics after ACLR may be similar between adults and adolescents, and differences in secondary ACL injury prevalence between these populations do not necessarily depend on variations in landing asymmetry but may be linked to other factors, such as skeletal maturity and eagerness to return to participation in competitive sports.

For the studies included in the review, participants were tested approximately 4 to 12 months after ACLR. As expected, asymmetries were more commonly noted in participants who were tested closer to the time of surgery. However, authors of the 3 studies (2 high^38,40 and 1 moderate³⁹ quality) that involved >1 test session reported that many asymmetries were still present approximately 8 to 12 months after ACLR. When the same participants were tested on multiple occasions, the asymmetry was reduced as the time after reconstruction increased, showing that any rehabilitation practices the participants were pursuing were effectively reducing the asymmetry in landing mechanics.

CONCLUSIONS

Asymmetries between surgical and nonsurgical limbs were more frequently identified in kinetic than in kinematic variables. The most common asymmetries described were peak knee-extension moment and peak vertical GRF during double-limb landing, both of which were frequently less in the surgical than the nonsurgical limb. In most cases, lower limb kinematics did not display asymmetry. Our meta-analysis indicated that ACLR appeared to have a large effect on asymmetry in peak vertical GRF, peak knee-extension moment, and loading rate during double-limb landings and mean knee-extension moment and knee-energy absorption during both double- and single-limb landings. Large effect sizes were observed for asymmetry in ankle-energy absorption and mean ankle sagittal-plane moment during double-limb landings but not during single-limb landings. Moderate effect sizes were present for asymmetry in mean knee frontal-plane moment and peak knee-flexion angle during both double- and single-limb landings and peak ankle-dorsiflexion angle during double-limb landings. No meaningful asymmetry was demonstrated in peak hip-flexion angle, mean hip sagittal-plane moment, or hip-energy absorption. Our findings suggested that return-to-sport criteria after ACLR should incorporate analysis of the loading experienced by each limb rather than movement patterns alone.