A novel finite element model of the ovine lumbar intervertebral disc with anisotropic hyperelastic material properties.

Gloria Casaroli,Hans-Joachim Wilke,Tomaso Villa,Fabio Galbusera,Benedikt Schlager,René Jonas,Lachlan J Smith

doi:10.1371/journal.pone.0177088

Gloria Casaroli, Hans-Joachim Wilke + Show 5 more

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https://doi.org/10.1371/journal.pone.0177088

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Abstract

The Ovine spine is an accepted model to investigate the biomechanical behaviour of the human lumbar one. Indeed, the use of animal models for in vitro studies is necessary to investigate the mechanical behaviour of biological tissue, but needs to be reduced for ethical and social reasons. The aim of this study was to create a finite element model of the lumbar intervertebral disc of the sheep that may help to refine the understanding of parallel in vitro experiments and that can be used to predict when mechanical failure occurs. Anisotropic hyperelastic material properties were assigned to the annulus fibrosus and factorial optimization analyses were performed to find out the optimal parameters of the ground substance and of the collagen fibers. For the ground substance of the annulus fibrosus the investigation was based on experimental data taken from the literature, while for the collagen fibers tensile tests on annulus specimens were conducted. Flexibility analysis in flexion-extension, lateral bending and axial rotation were conducted. Different material properties for the anterior, lateral and posterior regions of the annulus were found. The posterior part resulted the stiffest region in compression whereas the anterior one the stiffest region in tension. Since the flexibility outcomes were in a good agreement with the literature data, we considered this model suitable to be used in conjunction with in vitro and in vivo tests to investigate the mechanical behaviour of the ovine lumbar disc.

Highlights

Human specimens for in vitro experiments on spine biomechanics have been used for decades, despite some intrinsic limitations [1]
Human specimens are characterized by a strong variability in anatomy and physical properties, and since usually harvested from aged subjects they often present degenerated or pathological features
In view of having a numerical model to analyze the stress state generated by the loading conditions applied in a parallel in vitro study [23], the aims of this work are (1) to identify the anisotropic hyperelastic parameters that describe the ovine annulus fibrosus and (2) to obtain a validated biomechanical model of the sheep lumbar intervertebral disc (IVD)

Summary

Introduction

Human specimens for in vitro experiments on spine biomechanics have been used for decades, despite some intrinsic limitations [1]. Human specimens are characterized by a strong variability in anatomy and physical properties, and since usually harvested from aged subjects they often present degenerated or pathological features. The number of specimens usually collected and assessed in the experiments. The use of animal models is characterized by intrinsic disadvantages as genetic, biological, anatomical and postural differences in comparison with humans. Concerning to that, many studies investigated the spinal mobility and failure properties assessing human and animal models, but the use of different species (humans, sheep, calf, pig) [2] caused a lack of comparability between the results [3,4,5,6,7,8]

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