The knee is the most complex joint in the human body. It consists of a system of muscle, bone, and ligaments that endures repetitive loading during daily and athletic activities. When this loading is excessive, damage  to the knee occurs leading to a decreased quality of life.The medial collateral ligament (MCL) is one of the 4 major ligaments known to be commonly injured in the knee. The risk of injury to the knee joint increases with the elderly and individuals who experience chronic dehydration. For this reason, the focus of this study is to compare different mechanical quantities that can be used to analyze damage to the MCL. In this study, a novel mechanical testing protocol is used to progressively induce damage in dehydrated rat MCLs by performing tensile tests. This involves stretching the ligaments along their longitudinal axes to consecutive and increasing displacements starting at a 0.4 mm displacement and in increments of 0.2 mm until complete failure occurs. The load and change in length that the ligament experiences are measured at each displacement. Three different methods were evaluated to determine subfailure and damage propagation in rat MCLs: changes in tangent stiffness and chord stiffness, and changes in the load value at the 0.4 mm displacement for each load-displacement curve. The findings of this study indicate that the tangent stiffness and load at the 0.4 mm displacement provide information of the early onset of damage propagation. The decrease in chord stiffness of the ligament does not indicate damage progression in the ligament, but rather is the sign of the imminent failure of the MCL.This study provides insightful data into understanding the subfailure damage in the MCL. / Master of Science

Sprains are the most common injuries to ligamentous tissues. They are classified as first-degree, second-degree, or third-degree sprains depending upon their severity. First-degree sprains are the result of over-stretching of ligaments. Second-degree sprains involve partial tears of the ligaments. In third-degree sprains, the ligaments are completely torn. Although first- and second-degree sprains are not as severe as third-degree sprains, they occur more frequently. The mechanisms leading to sprains are still not well understood. Therefore, histo-mechanical experiments and theoretical studies are needed to advance our current knowledge on the etiology of sprains. In the first part of this study, a structurally-based constitutive equation is proposed to simulate the damage evolution process in ligaments. The ligament is modeled as a bundle of crimped collagen fibers that are assumed to be oriented along one direction, the physiological loading direction. The gradual straightening of collagen fibers determines the nonlinearity in the toe region of the tensile axial stress-strain curve. Straight collagen fibers behave as a linear elastic material. The gradual damage of collagen fibers determines the nonlinearity in the failure region of the tensile axial stress-strain curve. The parameters in the constitutive equation are estimated by curve fitting experimental data on rat medial collateral ligaments (MCLs) published in the biomechanics literature. In the second part of this study, mechanical experiments are performed in order to identify and quantify damage in ligamentous tissues. MCLs, which are harvested from Sprague-Dawley (SD) rats, are subjected to displacement controlled tensile tests. Specifically, the ligaments are stretched to consecutively increasing stretch values until their complete failure occurs. The elongation of the toe region and decrease in tangent modulus of the linear region of the collected stress-strain data are analyzed and two significantly different damage threshold strains are determined. The effect of age and skeletal maturation on the damage evolution process is also investigated by performing mechanical tests on MCLs isolated from two age groups of SD rats. In the third part of this study, scanning electron microscopy (SEM) is used to determine variations in the microstructure of ligaments that are associated with the elongation of the toe region and decrease in tangent modulus of the linear region of the stress-strain curve. MCLs from SD rats are subjected to different threshold strains that produce damage and, subsequently, examined using SEM. By comparing the morphology of collagen fibers and fibrils in undamaged and damaged MCLs, the microscopic variations induced by strain are determined and correlated to the observed macroscopic mechanical damage. / Ph. D.