Material properties of bovine intervertebral discs across strain rates

Abstract

The intervertebral disc (IVD) is a complex structure responsible for distributing compressive loading to adjacent vertebrae and allowing the vertebral column to bend and twist. To study the mechanical behaviour of individual components of the IVD, it is common for specimens to be dissected away from their surrounding tissues for mechanical testing. However, disrupting the continuity of the IVD to obtain material properties of each component separately may result in erroneous values. In this study, an inverse finite element (FE) modelling optimisation algorithm has been used to obtain material properties of the IVD across strain rates, therefore bypassing the need to harvest individual samples of each component. Uniaxial compression was applied to ten fresh-frozen bovine intervertebral discs at strain rates of 10-1/s. The experimental data were fed into the inverse FE optimisation algorithm and each experiment was simulated using the subject specific FE model of the respective specimen. A sensitivity analysis revealed that the IVD's response was most dependent upon the Young's modulus (YM) of the fibre bundles and therefore this was chosen to be the parameter to optimise. Based on the obtained YM values for each test corresponding to a different strain rate (ε̇), the following relationship was derived:YM=35.5lnε̇+527.5. These properties can be used in finite element models of the IVD that aim to simulate spinal biomechanics across loading rates.

Annulus fibrosus functional extrafibrillar and fibrous mechanical behaviour: experimental and computational characterisation

“…Given the calibration of the annulus in tension, the nonlinear behaviour of the fibres is more prominent to the specimen behaviour, resulting in a ratio k 1 / k 2 lower than those reported here. Other studies that performed direct-controlled calibration of intervertebral disc behaviour for more than one specimen either assumed axial symmetry of bovine discs and relatively low axial compressive strains [ 8 ] or used a relatively low numbers of human discs (three) for a large number of unknowns (six parameters, varying with the degree of degeneration of each disc) [ 4 ]. For bovine discs, the axial symmetry is a relatively weak assumption, with the endplates showing different concavity in the proximal and distal sides.…”

“…Moreover, at larger strain levels, several of the assumptions in Newell et al . [ 8 ], such as linearity of the fibres or rigidity of the bone, are no longer valid. Previous work validating finite-element models of functional spinal units had shown that nonlinear material models were required at axial compression higher than 15% apparent strains [ 5 ].…”

“…The micro-CT images were scaled to 0–255 greyscale and specimen-specific geometries were derived using image processing tools in Simpleware ScanIP 7.0 (Synopsys, Mountain View, USA) with images down-sampled to an isotropic 0.5 mm resolution. As soft tissues cannot be distinguished in the micro-CT images, a systematic protocol was developed to create the annulus and nucleus regions of the disc based on the diameter ratio measured in the first part of this study and assuming relatively straight sides from one endplate to the other, which is a good approximation for bovine caudal discs [ 8 ]. Finite-element models were produced using the segmented geometry and underlying greyscale, with a homogeneous linear tetrahedral mesh with a size based on previous studies [ 5 ].…”

A Novel Modelling Methodology Which Predicts the Structural Behaviour of Vertebral Bodies under Axial Impact Loading: A Finite Element and DIC Study

“…A further point to highlight with respect to the dynamic simulations is that bone cement and polyoxymethylene specimen holders were defined as rigid bodies [ 60 ]. As the rigid body constraints were set at the centres of mass of the objects, the reaction forces from the caudal constraints were not available for these simulations; therefore, experimental load data from the bottom load cell were used to produce load-displacement curves.…”

The two Poisson’s ratios in annulus fibrosus: relation with the osmo-inelastic features

“…Indeed, under mechanical loading, the fluid, flowed out, leads to chemical imbalances that generate fluid inflow [1,2]. The time-dependent response of the disc tissues is largely governed by the interactions between the inelastic features of the tangled ECM (representing all non-fibrillar "solid" components) and the surrounding physiological fluid by osmotic effect [3][4][5][6][7][8][9][10][11]. The resulting chemo-mechanical couplings in the disc tissues are now well accepted but not fully understood [12,13] and corresponding data are rare in the literature.…”