Asymmetric in-plane shear behavior of isolated cadaveric lumbar facet capsular ligaments: Implications for subject specific biomechanical models
Introduction
Low back pain affects up to 80% of adults in the United States in their lifetime (Deyo et al., 2006). The facet (zygapophyseal) joint and its joint capsule, located along the posterior spine, have been the subject of much research as potential sources of spine pain (Dreyer and Dreyfuss, 1996, Jaumard et al., 2011). The joint is formed by the pairs of bony facet surfaces at the interface between adjacent vertebrae, flanking the spinous process. The facet capsular ligament (FCL) is a curved capsular ligament surrounding the facet surfaces and encasing the joint space. The ligament’s major structural components are elastin and collagen fiber bundles, and it is innervated by proprioceptive and nociceptive nerves (Ashton et al., 1992). As a capsular ligament, the FCL encloses the synovial fluid in the facet joint space, whereas its innervation suggests an additional mechanosensory role. Furthermore, the FCL contributes to spinal stability (Sharma et al., 1995). All of these roles – fluid containment, mechanosensation, and stabilization – involve mechanical functions, making FCL mechanics an important area of study.
Previous mechanical studies of the FCL have used uniaxial and biaxial loading (Claeson and Barocas, 2016, Little and Khalsa, 2005). Little and Khalsa showed that the FCL exhibits viscoelastic and anisotropic behavior (Little and Khalsa, 2005), the latter consistent with the ligament's structural anisotropy (Boden et al., 1996). We previously (Claeson and Barocas, 2016) used equibiaxial extension tests in conjunction with a 3D finite element model. To describe the tissue accurately, we needed to include in-plane shear forces in the model parameter optimization (Claeson and Barocas, 2016). This result suggested that the largely unquantified shear properties of the lumbar FCL could be important. The need for detailed, multidirectional studies is further emphasized by the complex geometry of the FCL in conjunction with the complex motions of the spine (Zehr et al., 2019). Therefore, the objective of this study was to characterize the mechanical response of the lumbar FCL during planar shear testing.
Section snippets
Research design
We initially dissected and tested human cadaveric FCL tissue in shear as described below. Upon analyzing the experiments, however, we saw a pronounced asymmetry during positive vs. negative shear, and the asymmetry was not consistent across samples or within donors. Based on the asymmetric response and finite element analysis, we formed a preliminary conclusion that although the primary alignment of collagen fibers is bone-to-bone (roughly medial-lateral), there can be deviations in different
Results
Fig. 3 shows force-displacement results and detailed analysis for a representative shear test. The mesh used for both displacement tracking and finite-element simulation (Fig. 3A) shows the complex sample shape. For this sample, high shear stiffness was observed for positive shear, but almost no shear forces were observed for negative shears up to 20%. Other samples gave qualitatively different results, described in the next paragraph. We further examined behavior at the maximum negative (Fig. 3
Discussion
We used in-plane shear mechanical tests to characterize the lumbar (L4-L5) FCLs. The test setup incorporated both positive shear, similar to that imposed by spinal flexion, and negative shear, similar to spinal extension. Mechanical results showed that not all FCLs had a symmetric response to a symmetric shear test. Samples fell into three categories: high negative shear, high positive shear, and symmetric shear, and that asymmetry remained even after correcting for left vs right side and for
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Funding provided through NIH/NIAMS T32 AR050938 Musculoskeletal Training Grant, NIH/NICHD K12 HD073945 and NIH/NIBIB U01AT010326.
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