On the collagen criss-cross angles in the annuli fibrosi of lumbar spine finite element models

Abstract

In the human lumbar spine, annulus fibrosus fibres largely contribute to intervertebral disc stability. Detailed annulus models are therefore necessary to obtain reliable predictions of lumbar spine mechanics by finite element modelling. However, different definitions of collagen orientations coexist in the literature for healthy human lumbar annuli. Therefore, four annulus fibre-induced anisotropy models were built from reported anatomical descriptions, and inserted in a L3-L5 lumbar bi-segment finite element model. Annulus models were, respectively, characterized by radial, tangential, radial and tangential, and no fibre orientation gradients. The effect of rotational and axial compressive loadings was simulated and first, predictions were compared to experimental data. Then, intervertebral disc local biomechanics was studied under axial rotation and axial compression. A new parameter, i.e. the fibre contribution quality parameter, was computed in the anterior, lateral, postero-lateral, and posterior annuli of each model, in function of fibre stresses, radial load distributions, and matrix shear strains. Locally, each annulus model behaved differently, affecting intervertebral disc biomechanics and segmental motions. The fibre contribution quality parameter allowed establishing direct links between local annulus fibre organization and local annulus loadings, while other kinematical and biomechanical data did not. It was concluded that functional relations should exist between local annulus fibre orientations and overall segment morphology. The proposed fibre contribution quality parameter could be used to examine such relations and calibrate lumbar spine finite element models by locally adjusting the annulus bundle criss-cross angles. Conclusions of this study are particularly relevant to patient-specific models or artificial disc designs.

In silico evaluation of a new composite disc substitute with a L3–L5 lumbar spine finite element model

“…Inter-facet volume was filled by the upper and lower cartilage layers modelled as deformable contact bodies. The annulus was modelled as a composite reinforced material with 20 radial mono-oriented fibre layers embedded in a continuum matrix [22].…”

“…For the FJ cartilage a general Mooney-Rivlin model was able to reproduce experimental compressive data [10]. Ligaments and annulus fibres were defined as hypoelastic unidirectional materials with tension-only tangent stiffness that depended on the spine level (ligaments) [28,31] and on collagen Type I and II contents (annulus fibres) [22] (Fig. 1b, c).…”

“…Prosthesis fibre adjustments particularly affected the axial rotation flexibility of the treated segment because facet contact reduced the matrix stiffness influence. Also locally adjusting fibre orientations could help controlling both axial and sagittal motions [22].…”

“…Actually, coupled intersegmental tractions occur [22] and might be strongly affected by the extra stiffening of the device matrix with BW loads. Under sagittal rotations FJ contact forces were affected at both the treated and adjacent levels independently on the amount of ROM restoration probably because of altered intersegmental anteroposterior shear deformations.…”

Analytical evaluation of stresses and displacements of an intervertebral disc

“…The lamellae are anchored into the vertebral endplates by collagen and elastic fibers called Sharpey's fibers (Johnson et al, 1982), which insert into the cartilage endplates. The loads transmitted through the spine cause an increase of the intradiscal pressure, and this pressure transfers load on the AF, the lamellae of which act essentially in tension (Noailly et al, 2011), like in a composite pressure vessel. In typical work activities, important loads may result in compression, flexion, torsion, or a combination of them.…”