Kara Boatwright and Professor Anton Bowden, Mechanical Engineering
Experts have found that 80% of Americans will experience some sort of back pain in their lives. It is a prevalent problem and further information about the motion of the spine is necessary in order to improve techniques of alleviating and someday curing back pain. Ligaments are the connective tissues which connect bones to bones, providing stability and limitations on the movement of bones with respect to each other. The spine, in particular, contains a number of ligaments which largely affect the motion of the spine. Therefore, in order to fully understand the movement of the spine, the mechanical properties of the ligaments must be known.
Ligaments exhibit unique functional properties that serve to stabilize and protect the spine while still allowing tremendous freedom of movement. The functional properties are enabled by the material constitutive properties of ligaments, which are nonlinear, anisotropic, and viscoelastic. Characterizing these material properties is challenging, and is exacerbated by the inadequacies of typical material testing methodologies to grip biological tissues. Because of these difficulties, there are many ligaments which have not yet been adequately characterized. Spinal ligaments have a particularly small amount of information published about them. Our establishment of the material properties of a certain spinal ligament, the anterior longitudinal ligament (ALL), will help further understanding of motion of the spine.
The material properties of the ALL were gathered through the testing of ligament specimens which were individually excised from one cadaveric spinal segment, after which the ligaments were frozen. In preparation for testing, specimens were allowed to reach room temperature and sectioned into multiple samples of uniform thickness using a microtome blade. The specimens were tested using an improved characterization technique called the Anisotropic Quarter Punch Test (AQPT) as shown in Figure 1. Using an electronic level, it was ensured that the stages began level, with zero displacement between them. Samples were loaded onto the test fixture with the fiber direction parallel to the left side of the immovable stage and the matrix direction parallel to the top side of the stage (see Figure 1). This ensured that the experimental model matched the FEA model for accurate data processing. Clamps were then placed over the samples to keep them in place (see Figure 1). Velcro was attached to the bottom of the clamps and the top of the stages in order to avoid slipping of the ligament during testing. The ligament experiences a “quarter punch” during testing as the first stage has a quarter circle shape that clamps a portion of the ligament and as the stage rises, it punches the ligament, stretching it in both the fiber and matrix directions. The stage was elevated until failure of the ligament and both data and images were captured up until failure. Data was collected on the vertical displacement as well as force. The vertical displacement was measured with a LabView interface which recorded the movement of
the motor. Force was measured by a load cell located between the motor and the stage. CCD cameras were mounted to capture picture of the profiles of the ligaments during testing. One camera was mounted perpendicular to the stretch in the fiber direction while another was mounted perpendicular to stretch in the matrix direction. These images allow for comparison and validation of the FEA model, which was created after the experimental testing, where profiles are viewed in the model as well. Once force and displacement data were collected, a FEA model of the testing was created to simulate what happened during testing. The force, displacement, and geometry were inputted and the model was run to find which material parameters would match the testing. The model was created using FEBIO.
At the point where my work on the project halted (due to graduation) we had successfully perfected the testing technique, conducted several successful tests, having collected the data for many specimens. We were also able to run the FEA model in order to obtain material parameters for a few of the specimens. Since then, more ligaments have been tested and the material parameters have been found for all of them. A research paper has been written to report the findings of the project, which will soon be submitted for publication.
The transversely isotropic material characterization of the anterior longitudinal ligament has provided valuable material properties of this important ligament. This information can now be used to continue progress on the study of the movement of the spine. In addition, the AQPT can continue to be utilized. This is highly valuable mainly due to the fact that the AQPT can gather information about all fiber directions of the ligament material, whereas previous testing has mainly been uniaxial, providing information only about one of the fiber directions at a time.