Mechanical Investigation of Damage in Ligaments

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.