Planar biaxial extension of the lumbar facet capsular ligament reveals significant in-plane shear forces

Abstract

The lumbar facet capsular ligament (FCL) articulates with six degrees of freedom during spinal motions of flexion/extension, lateral bending, and axial rotation. The lumbar FCL is composed of highly aligned collagen fiber bundles on the posterior surface (oriented primarily laterally between the rigid articular facets) and irregularly oriented elastin on the anterior surface. Because the FCL is a capsule, it has multiple insertion sites across the lumbar facet joint, which, along with its material structure, give rise to complicated deformations in vivo. We performed planar equibiaxial mechanical tests on excised healthy cadaveric lumbar FCLs (n = 6) to extract normal and shear reaction forces, and fit sample-specific two-fiber-family finite element models to the experimental force data. An eight-parameter anisotropic, hyperelastic model was used. Shear forces at maximum extension (mean values of 1.68N and 3.01N in the two directions) were of comparable magnitude to the normal forces perpendicular to the aligned collagen fiber bundles (4.67N) but smaller than normal forces in the fiber direction (16.11N). Inclusion of the experimental shear forces in the model optimization yielded fits with highly aligned fibers oriented at a specific angle across all samples, typically with one fiber population aligned nearly horizontally and the other at an oblique angle. Conversely, models fit to only the normal force data resulted in a broad range of fiber angles with low specificity. We found that shear forces generated through planar equibiaxial extension aided the model fit in describing the anisotropic nature of the FCL surface.

Image-based multiscale mechanical modeling shows the importance of structural heterogeneity in the human lumbar facet capsular ligament

“…Perhaps the most significant limitation is the relatively low depth of imaging. The lumbar FCL is a thick tissue, with the average thickness of 2.5 mm (Claeson and Barocas 2017). Optical penetration depth, however, allowed us to capture its structure for only the first 0.13 mm from the surface.…”

“…Planar biaxial test data from a previous study (Claeson and Barocas 2017) were used; for details of the mechanical tests, the reader is referred to Claeson and Barocas (2017), but the essential elements of that study are summarized briefly here. Six healthy L4–L5 cadaveric lumbar FCLs (right n = 2, left n = 4) from four motion segments (ages 35–65 years) were cleared of the surrounding musculature and tissues, flattened, trimmed to a cruciform shape, and mounted on an Instron-Sacks planar biaxial tester along the bone-ligament-bone axis (lateral–medial, L–M) and the perpendicular axis (superior–inferior, S–I).…”

Strong triaxial coupling and anomalous Poisson effect in collagen networks

“…3D). In the biaxial-shear case, we conducted tests that reproduce the loading conditions similar to those experienced by the collagenrich capsular ligaments of the spine (44,45). We stretched the networks by λ y and λ z in the y and z directions, respectively, and then subjected them to shear loading by moving the top and bottom surfaces of the network in opposite directions along the x axis.…”

Tissue loading and microstructure regulate the deformation of embedded nerve fibres: predictions from single-scale and multiscale simulations

“…The discrete-fibre and multiscale continuum models, each and/or together, can serve as tools to bridge between gross tissue motion and the sensory response. At one end of the bridge, tying this model to local fibre architecture and also local macroscale tissue deformation-obtained experimentally [13,44] or by modelling approaches [10,14]during supraphysiological motions could provide us with a better estimate of the how damaging loads are transmitted to the nerves. At the other end, using the mechanical response of the nerves to predict their electrical response could yield insights on how different body motions could induce injury or pain.…”

“…On the macroscale, spinal motion causes heterogeneous and localized deformation of the FCLs [10][11][12]. Even uniform displacement of the FCL's boundaries during simple ex vivo mechanical tests results in heterogeneous deformation, attributable to the non-uniform fibre structure of their extracellular matrix (ECM) [13][14][15]. On the microscale, the ECM fibres in these ligaments rotate, deform, and transmit local forces and deformations to the embedded sensory components, which can trigger their neural response.…”