Sensitivity of Intervertebral Disc Finite Element Models to Internal Geometric and Non-geometric Parameters

Abstract

Finite element models are useful for investigating internal intervertebral disc (IVD) behaviours without using disruptive experimental techniques. Simplified geometries are commonly used to reduce computational time or because internal geometries cannot be acquired from CT scans. This study aimed to (1) investigate the effect of altered geometries both at endplates and the nucleus-anulus boundary on model response, and (2) to investigate model sensitivity to material and geometric inputs, and different modelling approaches (graduated or consistent fibre bundle angles and glued or cohesive inter-lamellar contact). Six models were developed from 9.4 T MRIs of bovine IVDs. Models had two variations of endplate geometry (a simple curved profile from the centre of the disc to the periphery, and precise geometry segmented from MRIs), and three variations of NP-AF boundary (linear, curved, and segmented). Models were subjected to axial compressive loading (to 0.86 mm at a strain rate of 0.1/s) and the effect on stiffness and strain distributions, and the sensitivity to modelling approaches was investigated. The model with the most complex geometry (segmented endplates, curved NP-AF boundary) was 3.1 times stiffer than the model with the simplest geometry (curved endplates, linear NP-AF boundary), although this difference may be exaggerated since segmenting the endplates in the complex geometry models resulted in a shorter average disc height. Peak strains were close to the endplates at locations of high curvature in the segmented endplate models which were not captured in the curved endplate models. Differences were also seen in sensitivity to material properties, graduated fibre angles, cohesive rather than glued inter-lamellar contact, and NP:AF ratios. These results show that FE modellers must take care to ensure geometries are realistic so that load is distributed and passes through IVDs accurately.

Automatic generation of subject-specific finite element models of the spine from magnetic resonance images

“…A limitation of this study is that not all biological variation that is relevant for the biomechanics of the spine are included in the presented segmentation and morphing strategy. The annulus fibrosus and the nucleus pulposus are morphed as one, even though changes in size and location of the nucleus can lead to significant changes in, e.g., predicted peak force under compression ( Du et al, 2021 ; Liu and El-Rich, 2020 ). A T2-weighted MR sequence could be added for the separate segmentation of the nucleus pulposus ( Castro et al, 2014 ) so that it can also be included in the morphing.…”

The Radial Bulging and Axial Strains of Intervertebral Discs during Creep Obtained with the 3D-DIC System

“…Based on this, a novel structural model for the elastic fibers in the NP were proposed. With the rapid development of computational simulations, finite element modeling (FEM) with rational simplifications [ 59 ] can provide information on the underlying tissue that cannot be assessed from in vitro experiments [ 15 , 60 ] or original images [ 61 ]. Komeili et al [ 62 ] and Castro et al [ 63 ] systemically developed spinal models with linear elastic, hyper-elastic, and biphasic material constitutive models and applied various physiological conditions.…”

Development of a Computational Model of the Mechanical Behavior of the L4–L5 Lumbar Spine: Application to Disc Degeneration

“…In addition, a significant simplification of the geometry of model samples in finite element models leads to non-physiological stress distribution. Therefore, defining a more realistic internal and external geometry of the IVD can significantly improve the quality of the simulation results of the corresponding FE model, and it is likely that a more realistic geometry leads to more accurate stress distribution in the IVD [ 73 ].…”