Spinal Ligament Characterization
Spinal ligaments provide passive control of back motion and help protect the spinal cord from injury while allowing for substantial range of motion of the vertebral column. Like all ligaments, spinal ligaments are highly anisotropic due to their fibrous structure. The fiber orientation in each ligament generally represents an adaptation of the ligament to its mechanical environment. In addition, spinal ligament material behavior is nonlinear, viscoelastic, and strain rate dependent. Alterations to ligament kinematics due to age, intervertebral disc degeneration, and the presence of other chronic degenerative and pain-inducing changes are not well understood, even though these processes have a significant effect on ligament behavior, and thus spine kinematics.
Six major ligaments are generally considered to be relevant in the study of lumbar spine biomechanics anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), ligamentum flavum (LF), supraspinous ligament (SSL), intraspinous ligament (ISL), and the facet joint capsule (FJC).
The lumbar spinal ligaments are innervated with mechanoreceptors and mechanonociceptors, which are afferents that respond to their mechanical environment. Evidence exists to support the theory that the lumbar spinal ligaments function proprioceptively and that excitation of ligament mechanoreceptors could contribute to feedback loops initiating paraspinal muscle contraction. Therefore, in addition to the direct contribution of spinal ligaments to passive lumbar spine stability, proper ligament function may play a significant role in contributing to the active support provided by the paraspinal musculature. Elucidating the mechanical behavior of spinal ligaments during different motion types would assist in the study of the normal functioning of the lumbar spine, as well as low back pain mechanisms.Anisotropic Quarter Punch Testing
We are currently in the process of validating a novel technique for testing ligaments known as the anisotropic quarter punch test. This is an extension of the previous work done in anisotropic punch testing. The anisotropic punch method of testing allows us to test the overall response of the ligament. This is in contrast to traditional tensile testing that only tests the material response in one direction and therefore requires multiple tests to gather complete material data for an anisotropic material such as ligaments and tendons. The ability to gather the overall response in one test eliminates the error that can occur when gathering data from different samples. The latest iteration of the anisotropic punch testing is a quarter punch test that uses a smaller sample size than previously required and eliminates some of the challenges with the previous version such as sample placement and alignment. We are currently testing porcine Achilles tendons with this machine and comparing the results with accepted results gathered previously. Once we have verified that this method produces reliable results, we will use it to characterize the anterior longitudinal ligament (ALL) of the spine.
Hyatt, T., Fullwood, D., Bowden, A.E., Bradshaw, R.J., Johnson, O. (2010). Nano-composite sensors for wide range measurement of strain. Proceedings of the SAMPE 2010 Exhibition & Symposium, Seattle, WA, May 17-20, 2010.
Showalter, B.L., M.B. Hunter, and A.E. Bowden (2009). Validation of a modified small punch test for characterizing the material response of spinal ligaments. Proc 55th Annual Meeting of the Orthopaedic Research Society.
This material is based upon work supported by the National Science Foundation under Grant No. 0952758. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).