Development of Methods for Characterizing Ankle Ligament Viscoelastic Properties

Abstract

The goal of this work is to develop methods for determining the viscoelastic properties of ankle ligaments. The tibiocalcaneal ligament was used for the method development and for the preliminary mechanical testing presented in this work. This document focuses primarily on the development of several novel approaches to: morphological length and cross-sectional area measurements, in situ pose maintenance during ligament extraction and testing, and automated mechanical testing. Additionally, preliminary ligaments are mechanically tested using an exhaustive battery to determine their strain rate-dependent properties. Traditional caliper length (CL) measures are augmented by novel ligament linear length (LL) measures determined by a straight line between the centroids of insertion surfaces, and iterative length (IL) measures which refine the LL to define a length that wraps about the center of the ligament body along its longitudinal axis. Cross-sectional area was estimated from an elliptical and rectangular fit to caliper measurements and from computed tomography (CT) scans. The mechanical properties from these length estimates are compared to caliper length measurements. Caliper length estimates were ~ 25% > LL and IL which were similar in length (≤ ~10%). There was ~80% variation in cross-sectional area measurements. This resulted in ~8-15% average change in modulus of elasticity. Peak force and hysteresis increased by 80% and 250% respectfully between length estimate strain levels. Ligament differed from in situ pose by about 23o (± 6.6o) and 1.4 mm (± 4.13 mm). Minimal loading rate-dependent differences in stiffness, modulus, and peak force were measured. However, a general rate dependent modulus change was observed from the plotted relationship. Temperature was maintained within ± 1.3 oC of the target temperature (37 oC) and specimens remained moist for the duration of mechanical tests. A quasi-linear viscoelastic (QLV) model was successfully fit to the data with minimal error (RMSE < 0.1) in most tests. There was a notable challenge relating to the mechanical testing system that led to the actuator not reaching the intended strain level at higher strain rates. This is a common challenge with mechanical testing, and additional steps are proposed to address this limitation in ongoing, further testing is required to determine the mechanical characteristics of these ligaments.