Poster Presentation 51st International Society for the Study of the Lumbar Spine Annual Meeting 2025

Introduction

Disc finite element analysis (FEA) is valuable for assessing motion segment mechanics due to disc degeneration or surgical interventions. Several investigators have developed spine FEA, with a range of material models and material properties. However, the model and property choices are not always justified, and it is unknown how changes in the tissue-level material properties impact the overall motion segment’s mechanical behavior. Our objective was to conduct a parametric sensitivity analysis, exploring a range of material property values reported in the literature for annulus fibrosus (AF), to determine which parameters had the most impact on motion segment behaviors. To achieve this, we simulated motion segment loading in torsion and axial creep using FEBio to determine the impact of select parameters on the motion segment response. 

Methods

In this study, we used a non-degenerated intervertebral disc geometry [2], baseline material properties, and AF-NP transition regions based on Newman et al. [1]. Of particular interest was the nonlinear biphasic material model of the inner AF, outer AF, and nucleus pulposus (NP). The material model included: a Holmes-Mow elastic solid with strain-dependent permeability, Donnan equilibrium osmotic swelling, and in the AF only, non-linear fibers [1,2]. Among the many parameters arising from these material models, we performed a one-parameter-at-a-time sensitivity analysis on selected parameters of interest, where each parameter was changed in both the AF and NP (Table 1). Material property ranges were selected from tissue experiments, where available. The mechanical response of the motion segment was analyzed for each parameter variation, with all other parameters held at base value, to assess its impact on the model predictions.

Results

Torsion (Fig 1 (A-E)) and axial creep (Fig 2 (A-E)) responses demonstrate the influence of each of the parameters. In torsion, fixed charge density and fiber modulus had the largest impact on the motion segment response, while the non-fibrillar matrix had minimal effect.  In contrast to torsion, in axial creep, the non-fibrillar matrix parameters, especially permeability, had the largest impact, along with fixed charge density, whereas fiber stiffness had little effect. 

Discussion

This study demonstrated which material parameters have a major influence in modulating the overall motion segment behavior in torsion and axial creep. These results, in combination with future evaluation of the remaining un-studied properties (e.g., matrix modulus, Poisson’s ratio and fiber angle), additional loading conditions (e.g., flexion and lateral bending), and isolating changes in the AF and NP, will be used to determine the most impactful model parameters for describing changing disc mechanical behaviors. These are needed to create models that replicate various stages of degeneration and also, ultimately, to establish patient-specific disc models on a reduced set of material properties that are most influential.

Table 1: List of parameters with their variations used in sensitivity analysis

                                 Figure 1: Torsion response of motion segment with parameter changes

                                 Figure 2: Axial creep motion segment response with parameter changes