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Abstract
Musculoskeletal soft tissue injury, dysfunction, and pathology represent a large portion of clinical visits and are an increasingly costly societal burden. Tendons and ligaments are soft tissues that connect muscle to bone and bone to bone, respectively, and are both composed of a dense hierarchical arrangement of fascicles, fibers, and fibrils. Tendon and ligament health is critical to maintaining their physiological function, however there are currently a lack of objective metrics for gauging the mechanics of these tissues in vivo. Shear wave tensiometry is an emerging method for gauging the mechanics of tendons based upon the speed of a propagating shear wave measured using a wearable sensor. More specifically, shear wave speed is proportional to increases in both the axial stress and shear modulus of the tissue, meaning that both loading and mechanical properties can be inferred from a wearable measurement. However, it is unknown whether shear wave tensiometry can be applied to tissues other than tendon, and very little is known about the capacity of shear wave speeds to actually infer mechanical properties that correspond to useful clinical metrics. Accordingly, the aims of this thesis are to (1) investigate macroscopic sources of variability in shear wave propagation in tendons and ligaments, (2) investigate microscopic sources of variability in shear wave propagation in tendons and ligaments, and (3) apply the newfound knowledge from Aims 1 and 2 to an important clinical problem related to musculoskeletal soft tissue health. To accomplish these aims, we first perform ex vivo experiments in porcine collateral ligaments to verify that tensiometry is useful as a measurement technique in ligament-type tissues, in addition to tendon. Following this study, we implemented a novel finite element modeling technique to systematically study the sensitivity of shear wave speeds to tissue factors using high-throughput computing. To this end, we studied the effect of surrounding subcutaneous tissues and large sub-tissue constituents on shear wave propagation, which is commonly observed in superficial soft tissues such as the Achilles tendon. To accomplish Aim 2, we use this modeling technique with a microstructure-informed constitutive model to study changes in fiber alignment and their effect on regional tissue mechanics and the resulting shear wave propagation, and demonstrate that shear wave speeds track regional stress and increases in shear modulus that occur with unaligned fibers. We then perform extensive ex vivo experiments to demonstrate that the tangential shear modulus in ligament is indeed axial load-dependent, and should be considered when making tensiometry measurements. Finally, we measured shear wave speeds in tendon fascicles during fatigue loading, which is a known cause of tendinopathy in the young, old, and active population. We show that shear wave speeds are useful as a surrogate measure of tendon microdamage and failure. Altogether, these studies demonstrate the potential of using shear wave speed to track subtle changes to the tendon microenvironment, which regulates the health and function of the entire tissue. Future studies should leverage the techniques developed in this thesis to better understand tendon mechanics ex vivo, but also use this information as a guideline with which to implement tensiometry in a research or clinical environment in vivo.
