Summary

The peak elongation and knee flexion angle at peak elongation are different between the anteromedial and posterolateral bundles of the ACL.

Abstract

Introduction

Although in vivo ACL relative elongation during gait and running has been studied in the past, there is lack of evidence regarding in vivo elongation of the anteromedial (AMB) and posterolateral (PLB) during high-impact activities. 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.

Material And Methods

Written informed consent was obtained from nineteen healthy athletes who performed fast running and single-legged drop landing within a biplane radiography imaging system. Fast running was performed at 5.0 m/s on and instrumented treadmill, and the single-legged drop landing was performed from a 20 cm platform while the participant looked forward during landing. Synchronized biplane radiographs were collected at 150 images per second and 1ms exposure for both activities. ACL elongation was measured as the distance between the femoral and tibial ACL attachment points, identified on MRI and registered to the CT-based subject-specific 3D bone model. CT-based bone models were matched to the biplane radiographs with validated sub-millimeter accuracy. 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. Generalized estimating equations were used to identify differences in the peak relative elongations and differences in tibiofemoral flexion angle at peak elongation between the AMB and PLB. Significance was set at p < 0.05.

Results

Participant’s average age was 20.1 ± 1.3 years and the mean BMI was 24.0 ± 2.8kg/m2. 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 (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).

Conclusion

The 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. Second, the peak elongation is simultaneous during fast running, but 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, and could support an appropriate intraoperative fixation angle for each bundle in anatomical-double bundle ACL reconstructions to better replicate the native AMB and PLB functions.