Strain in the transverse plane of the patellar tendon depends on both local composition and structure

Ligaments and tendons are heterogeneous structures composed of two distinct materials at the mm-length scale: fascicles and the interfascicular matrix (IFM). Many studies model the mechanics of ligaments and tendons using a spatially homogeneous, transversely isotropic mechanical model. However, recent studies have demonstrated that these two structures likely need to be modeled separately to explain failure behavior. Additionally, quantification of IFM mechanics would help to better understand the mechanical environment around cells in ligaments and tendons. While much work has been done to quantify fascicle mechanical properties, less has been done to characterize the IFM. This study examines the spatial heterogeneity of strain in transverse slices of sheep patellar tendons loaded in tension using digital image correlation (DIC). Local DIC subset strain was greater for subsets with a greater IFM fraction, which aligns with previous understanding of IFM compared to fascicle stiffness. However, this trend was small, with many fascicle subsets exhibiting large strain, indicating that fascicle and IFM mechanical properties are similar in the transverse plane. In addition to IFM fraction, distance to a fascicle-IFM interface was a significant predictor of local strain. Taken together, these results demonstrate that local strain depends not only on composition but also on structure.