Microstructural properties and mechanics vary between bundles of the human anterior cruciate ligament during stress-relaxation

Previous studies have shown different elastic properties between the anteromedial (AM) and posterolateral (PL) bundles of the human anterior cruciate ligament (ACL). However, it is unknown if the two bundles of the ACL exhibit distinct time-dependent properties and microstructural organization, which have important implications for fully understanding the structure-function relationships of this oft-injured ligament. The goal of this study was to quantify the viscoelastic material properties and collagen fiber alignment of the AM and PL bundles in 16 human ACLs during stress-relaxation mechanical tests using the quasi-linear viscoelastic (QLV) model and a quantitative polarization imaging technique. We hypothesized that the AM and PL bundles would exhibit differences in the instantaneous mechanical and organizational properties (i.e., immediately following a step increase in strain), but similar time-dependent changes during stress-relaxation. Results showed that AM samples exhibited larger peak/equilibrium stresses and less stress-relaxation during a 300-hold compared to PL samples. The AM bundle demonstrated stronger and more uniform collagen fiber alignment (i.e., higher degree of linear polarization (DoLP) values and less distributed angle of polarization (AoP) values) compared to the PL bundle, and larger changes in alignment strength during the hold. Results suggest that the AM bundle is the more "dominant" bundle, with significantly different mechanical and material properties in stress-relaxation. While more research is needed to better understand how these findings relate to the pathophysiology of ACL tears and can best guide treatment, the findings provide additional insight into the microstructural properties and biomechanics of the human ACL.