ACL and MCL Strains During High Knee Abduction and Anterior Tibial Shear Loads: Implications for ACL Injury Mechanisms

Abstract

PURPOSE: Although the ACL is vulnerable to anterior tibial (AT) loads, abduction (AB) collapse may be a more common ACL injury mechanism. Intuitively, if an injury occurs by an AB mechanism, concomitant injury to the MCL would be expected due to its anatomic alignment. However, combined ACL/MCL injuries occur in less than 30% of all ACL injuries. The purpose of this study was to analyze strains in the ACL and MCL during AB to determine how an ACL injury could occur without concomitant MCL injury. METHODS: A finite element knee model created from CT and MRI images was validated with cadaveric studies available in the literature1 and used for further analyses. The ACL and MCL strains were analyzed during flexion angles (0-40 degrees) for AB (10 Nm), AT (100 N) and combined AB/AT loads. RESULTS: The model results were within one standard deviation of the cadaveric results with correlation coefficients of 0.925 (ACL) and 0.999 (MCL). Although ACL strains increased during AB loads, the MCL strains were almost 2-fold higher than ACL strains. In contrast, ACL strains during AB/AT were 1.5-fold higher than during AT alone, with ACL strains on average 1.4-fold higher than the MCL strains for AB/AT conditions. CONCLUSIONS: These findings indicate that the ACL was disproportionally loaded relative to the MCL during combined abduction and anterior tibial loads at low knee flexion angles. These disproportionate ACL loads relative to MCL may help explain the high prevalence of reported isolated ACL tears in absence of concomitant MCL injury. Reference: 1. Markolf KL et al. Journal of Orthopaedic Research, 1995, Supported by NIH, R01-AR049735, RO1- AR05563, and R01-AR056259, and the American College of Sports Medicine Foundation Plus One Active Research Grant on Wellness Using Internet Technology.