Preferential loading of the ACL compared with the MCL during landing: a novel in sim approach yields the multiplanar mechanism of dynamic valgus during ACL injuries

Abstract

Background: Strong biomechanical and epidemiological evidence associates knee valgus collapse with isolated, noncontact anterior cruciate ligament (ACL) injuries. However, a concomitant injury to the medial collateral ligament (MCL) would be expected under valgus collapse, based on the MCL's anatomic orientation and biomechanical role in knee stability. Purpose/

Hypothesis: The purpose of this study was to investigate the relative ACL to MCL strain patterns during physiological simulations of a wide range of high-risk dynamic landing scenarios. We hypothesized that both knee abduction and internal tibial rotation moments would generate a disproportionate increase in the ACL strain relative to the MCL strain. However, the physiological range of knee abduction and internal tibial rotation moments that produce ACL injuries are not of sufficient magnitude to compromise the MCL's integrity consistently.

Study design: Controlled laboratory study.

Methods: A novel in sim approach was used to test our hypothesis. Seventeen cadaveric lower extremities (mean age, 45 ± 7 years; 9 female and 8 male) were tested to simulate a broad range of landings after a jump under anterior tibial shear force, knee abduction, and internal tibial rotation at 25° of knee flexion. The ACL and MCL strains were quantified using differential variable reluctance transducers. An extensively validated, detailed finite element model of the lower extremity was used to help better interpret experimental findings.

Results: Anterior cruciate ligament failure occurred in 15 of 17 specimens (88%). Increased anterior tibial shear force and knee abduction and internal tibial rotation moments resulted in significantly higher ACL:MCL strain ratios (P < .05). Under all modes of single-planar and multiplanar loading, the ACL:MCL strain ratio remained greater than 1.7, while the relative ACL strain was significantly higher than the relative MCL strain (P < .01). Relative change in the ACL strain was demonstrated to be significantly greater under combined multiplanar loading compared with anterior tibial shear force (P = .016), knee abduction (P = .018), and internal tibial rotation (P < .0005) moments alone.

Conclusion: While both the ACL and the MCL resist knee valgus during landing, physiological magnitudes of the applied loads leading to high ACL strain levels and injuries were not sufficient to compromise the MCL's integrity.

Clinical relevance: A better understanding of injury mechanisms may provide insight that improves current risk screening and injury prevention strategies. Current findings support multiplanar knee valgus collapse as a primary factor contributing to a noncontact ACL injury.

Keywords: ACL; MCL; injury mechanism; knee; landing.

Figure 1

Custom-designed drop-stand testing apparatus.

Figure 2

Insertion of the differential variable reluctance transducer (DVRT) on the (A) anterior cruciate ligament’s anteromedial bundle and (B) superficial medial collateral ligament across the joint line.

Figure 3

Increase in the ACL:MCL strain ratio under applied anterior tibial shear forces during simulated neutral landings. *P = .041.

Figure 4

Change in the ACL:MCL strain ratio under applied knee abduction moments with and without additional anterior tibial shear force (134 N for 25 N·m of knee abduction moments and 268 N·m for the rest of the loading conditions) during simulated landings. *P = .011 and ⁺P < .03 for all comparisons.

Figure 5

Change in the ACL:MCL strain ratio under applied internal tibial rotation moments with and without additional anterior tibial shear force (268 N) during simulated landings. *P = .014 and ⁺P < .05 for all comparisons.

Figure 6

Change in the ACL:MCL strain ratio under combined multiplanar loading conditions during simulated landings. Ab, abduction; Ant., anterior; Int. Rot, internal rotation.

Figure 7

Change in the ACL:MCL strain ratio under increased impact severity during simulated landings. Ant., anterior; BW, body weight; Int. Rot, internal rotation.

Figure 8

Relative anterior cruciate ligament and medial collateral ligament strains (compared to baseline) under different modes of loading (⁺P < .003).

Figure 9

The finite element–predicted change in anterior cruciate ligament and medial collateral ligament strains under applied 268 N of anterior tibial shear force combined with (A) continuous ranges of knee abduction moment and (B) internal tibial rotation moment all under simulated muscle loads.