Abstract
Objectives:
It is well known that the anterior cruciate ligament (ACL) comprises two bundles; the anteromedial bundle (AMB) and the posterolateral bundle (PLB). Although in vivo ACL relative elongation during gait and slow jogging has been studied in the past, there is lack of evidence regarding in vivo elongation of the AMB and PLB during high-impact activities. Clinically, it is important for the surgeon to know the knee flexion angle at which each bundle reaches peak elongation so that the grafts can be tensioned accordingly during anatomic double-bundle ACL reconstruction. The purpose of this study was to measure peak relative elongation of the AMB and PLB and identify the knee flexion angle at their peak elongation during fast running and single-legged drop landing. It was hypothesized that there would be no difference between bundles in peak relative elongation and no difference in the knee flexion angle at peak elongation between the AMB and PLB.
Methods:
Written informed consent was obtained from all participants. Nineteen healthy athletes (11 males and 8 females) performed fast running and single-legged drop landing within a biplane radiography imaging system (Figure 1AB). Both knees (total 38 knees) were analyzed for three trials for each activity. The fast running was performed at 5.0m/s on and instrumented treadmill, and single-legged drop landing was performed from a 20 cm platform while looking straight ahead at a target on the wall. Synchronized biplane radiographs were collected at 150 images per second and 1ms exposure for both activities. Tibiofemoral motion was tracked using a previously validated volumetric model-based tracking process that matched CT-based subject-specific 3D bone models to the synchronized biplane radiographs. Knee kinematics were calculated following standard conventions. ACL elongation was measured as the distance between the femoral and tibial ACL attachment points, identified on magnetic resonance imaging (MRI) and registered to the CT based subject-specific 3D bone model. The AMB and PLB centroid-to-centroid distances were calculated from the tracked bone motions, and these bundle lengths were normalized to their respective lengths on MRI to calculate relative elongation. Fast running kinematic data were analyzed over the first 60% of stance (0% = initial contact, 100% = toe-off). Single-legged drop landing kinematics were normalized by setting 0% to initial contact and 100% to maximum knee flexion. Generalized estimating equations were used to account for correlations between knees and bundles within a subject while exploring differences in the peak relative elongations and differences in tibiofemoral flexion angle at peak elongation between the AMB and PLB. Significance level was set as p < 0.05.
Results:
Participant’s average age was 20.1 ± 1.3 years and the mean BMI was 24.0 ± 2.8kg/m2. A total of 57 running and 114 drop landing trials were included in this analysis. The relationship of the relative elongation, timing, and flexion angle of both activities was shown in Figure 2. The peak AMB elongation (2.7% [95% CI: 1.2 - 4.3]) was significantly greater than the peak PLB elongation (0.4 % [95% CI: -1.5 - 2.4]) during fast running (p<0.001). The peak PLB elongation (6.4% [95% CI: 4.8 - 7.9]) was significantly greater than the peak AMB elongation (5.2 % [95% CI: 4.1 - 6.3]) during single-legged drop landing (p<0.01). During fast running, the peak AMB relative elongation occurred at 35° (95%: CI 33 - 37) of knee flexion, and the peak PLB relative elongation occurred at 34° (95%: CI 32 - 36) of knee flexion (flexion angles of peak elongation: p=0.66). During single-legged drop landing, the peak AMB relative elongation occurred at 21° (95%: CI 18 - 24) of knee flexion, and the peak PLB relative elongation occurred at 15° (95%: CI 12 - 18) of knee flexion (p<0.001) (Table 1).
Conclusions:
One important finding of this study was that the relative elongation of the AMB is significantly greater than the PLB in fast running, whereas the relative elongation of the PLB is significantly greater than AMB in single-legged drop landing. A second important finding was that the peak elongation is simultaneous during fast running, but the PLB elongates before the AMB during single-legged drop landing. These results suggest that ACL bundle elongation is influenced by both activity and knee flexion. The fact that the AMB was maximally elongated around 20° and the PLB was maximally elongated at a lower angle during the single-legged drop landing is consistent with previous literature on the kinematics of both bundles, and could support an appropriate intraoperative fixation angle for each bundle. As for clinical relevance, these results provide a better understanding of how much and when the AMB and PLB are elongated during high-impact activities. This information can be useful for further improving graft fixation in anatomical-double bundle ACL reconstructions to better replicate the native AMB and PLB functions.
