The Effect of Gender-Related Differences in Knee Morphology on Peak ACL Strain During Repeated Simulated Pivot Landings: An In Vitro Investigation.

Abstract

Anterior cruciate ligament (ACL) injuries are associated with considerable morbidity, including the development of knee osteoarthritis. A smaller ACL cross-sectional volume and a steeper lateral tibial slope have been associated with increased ACL injury risk. A knowledge gap exists as to why adolescent females have a 2- to 8-fold greater ACL injury rate than males, and whether the human ACL is susceptible to a fatigue failure under repeated loading. This dissertation tests the research hypotheses that (1) the female ACL exhibits greater peak strain than the male ACL during a two-times body weight (2*BW) simulated pivot landing because of a smaller ACL cross-sectional area and steeper lateral tibial slope, and (2) the ACL is susceptible to fatigue failure under repeated 3 - 4*BW simulated pivot landings.

Ten male and 10 female knees from age- and size-matched donors were subjected to 3T MR imaging to measure ACL cross-sectional area and lateral tibial slope. Each knee was loaded into a custom testing apparatus which delivered a standardized 2*BW compound impulsive load (compression force, knee flexion moment, internal tibial torque and muscle forces). The quadriceps’ resistance to rapid stretch was modeled using a novel non-linear spring. The 3-D tibiofemoral kinematics and kinetics, muscle forces, and anteromedial ACL relative strain were recorded for 100 ms. Females ACLs exhibited 95% greater peak strain than male ACLs during the pivot landing, and ACL cross-sectional area and lateral tibial slope explained 59% of the variance in peak ACL strain. These results suggest an individual’s knee morphology can predispose them to an ACL injury.

Using similar methods and 10 pairs of knees, the human ACL exhibited fatigue failure under repetitive 3*BW and 4*BW pivot landings. The larger the applied cyclic loading, and the smaller the ACL cross-sectional area, the fewer cycles it took for the ACL to fail. Finally, in 12 female knees increasing quadriceps tensile stiffness by 33% reduced peak ACL strain by 16%, thereby reducing the risk for injury.

Future interventions should target the magnitude and frequency of pivot landing loading cycles and screen for high-risk knee morphologies in order to reduce ACL injury risk.