Degeneration of the lumbar spine, and especially how that degeneration may lead to pain, remain poorly understood. In particular, the mechanics of the facet capsular ligament may contribute to low back pain, but the mechanical changes that occur in this ligament with spinal degeneration are unknown. Additionally, the highly nonlinear, heterogeneous, and anisotropic nature of the facet capsular ligament make understanding mechanical changes more difficult. Clinically, MRI-based signs of degeneration in the facet joint and the intervertebral disc (IVD) correlate. Therefore, this study examined how the nonlinear, heterogeneous mechanics of the facet capsular ligament change with degeneration of the lumbar spine as characterized using MRI. Cadaveric human spines were imaged via MRI, and the L2-L5 facet joints and IVD were scored using the Fujiwara and Pfirrmann grading systems. Then, the facet capsular ligament was isolated and biaxially loaded. The nonlinear mechanical properties of the ligament were obtained using a nonlinear generalized anisotropic inverse mechanics analysis (nGAIM). Then a Holzapfel-Gasser-Ogden (HGO) model was fit to the stress-strain data obtained from nGAIM. The facet capsular ligament is stiffer and more anisotropic at larger Pfirrmann grades and higher Fujiwara scores than lower grades and scores. Analysis of ligament heterogeneity showed all tissues are highly heterogeneous, but no distinct spatial patterns of heterogeneity were found. These results show that degeneration of the lumbar spine including the facet capsular ligament appears to be occurring as a whole joint phenomenon and help advance understanding of lumbar spine degeneration.

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Fatigue is the most prevalent failure mode in structural materials, yet remains challenging to study due to the seemingly unpredictable nature of crack initiation. To elucidate the driving forces of crack initiation in ductile polycrystalline metals, we employ a multimodal approach to identify and track grains with a suspected potential to initiate fatigue cracks via a newly founded signature. We discover this crack initiation potential (CIP) signature under the hypothesis that slip localization, a well-known precursor to crack initiation, is linked to intragrain misorientation, which can be quantified through single grain orientation distributions. We verify the CIP signature in an Inconel-718 material via static two-dimensional and three-dimensional electron microscopy and “bring to life” the dynamics of the CIP signature via in-situ synchrotron X-ray diffraction. With this CIP signature, we move to better focus studies of fatigue crack initiation on the individual grains and processes that drive fatigue failure.