Ligament shear strain stiffening arises from unaligned fibers and is amplified by axial strain

The medial collateral ligament is subject to a combination of shear and tensile loading via passive joint rotation or a combination of joint rotation and valgus loading in the knee. However, empirical characterizations of ligament mechanics usually consider only pure tensile or simple shear loading, and such isolated loading conditions may not fully capture the structure-function relationships under physiological conditions. For example, stretch of off-axis ligament fibers may inherently enhance load transfer and shear resistance within the tissue. Our objectives were to characterize the effects of (1) fiber alignment and (2) axial prestrain on the shear behavior of ligaments. We modeled a medial collateral ligament as an incompressible, hyperelastic material with distributed fibers embedded in an isotropic ground matrix. We use high-throughput, probabilistic finite element modeling to determine changes in shear modulus across a broad range of fiber alignments and axial strains observed in musculoskeletal soft tissues. We then used an experimental technique capable of resolving the loaded shear modulus of human medial collateral ligaments. We found that shear modulus increased dramatically (up to 1.5 MPa) in our soft tissue model when fibers were unaligned, and that this effect was amplified when the axial strain in the soft tissue’s shear region was increased. In medial collateral ligaments ex vivo, we found that shear modulus increased by over tenfold at an axial prestrain of 9% relative to the unloaded state. These findings uncover an important structure-function relationship in ligaments that is relevant to the complex loading scenarios these tissues undergo in vivo, and thus should be considered for ongoing analyses of ligament mechanics.