Effects of geometric individualisation of a human spine model on load sharing: neuro-musculoskeletal simulation reveals significant differences in ligament and muscle contribution

Abstract

In spine research, two possibilities to generate models exist: generic (population-based) models representing the average human and subject-specific representations of individuals. Despite the increasing interest in subject specificity, individualisation of spine models remains challenging. Neuro-musculoskeletal (NMS) models enable the analysis and prediction of dynamic motions by incorporating active muscles attaching to bones that are connected using articulating joints under the assumption of rigid body dynamics. In this study, we used forward-dynamic simulations to compare a generic NMS multibody model of the thoracolumbar spine including fully articulated vertebrae, detailed musculature, passive ligaments and linear intervertebral disc (IVD) models with an individualised model to assess the contribution of individual biological structures. Individualisation was achieved by integrating skeletal geometry from computed tomography and custom-selected muscle and ligament paths. Both models underwent a gravitational settling process and a forward flexion-to-extension movement. The model-specific load distribution in an equilibrated upright position and local stiffness in the L4/5 functional spinal unit (FSU) is compared. Load sharing between occurring internal forces generated by individual biological structures and their contribution to the FSU stiffness was computed. The main finding of our simulations is an apparent shift in load sharing with individualisation from an equally distributed element contribution of IVD, ligaments and muscles in the generic spine model to a predominant muscle contribution in the individualised model depending on the analysed spine level.

Designing energy-conserving surrogate models for the coupled,non-linear responses of intervertebral discs

“…We initially intended to also model the L5|S1 IVD but since the FE simulations were restricted to a range below the compression levels reached during gravitational settling process, the MB simulations failed when including a L5|S1 kernel surrogate model into the individualised spine. Instead, we kept the bushing element for L5|S1 from Meszaros-Beller et al (2023).…”

Effect of neglecting passive spinal structures: a quantitative investigation using the forward-dynamics and inverse-dynamics musculoskeletal approach

“…Accurate estimation of joint loading is of high significance to study the biomechanics of the spine. Several musculoskeletal (MSK) spine models have been introduced in literature ( de Zee et al., 2007 ; Christophy et al., 2012 ; Bruno et al., 2015 ; Rupp et al., 2015 ; Cazzola et al., 2017 ; Mörl et al., 2020 ; Guo et al., 2021 ; Silvestros et al., 2022 ; Meszaros-Beller et al., 2023 ) with the goal to predict muscle forces, muscle activation patterns and internal loading conditions during human movement.…”

“…A detailed lumbar spine model with a lumped upper body was introduced by Rupp et al. (2015) , validated with respect to its passive stiffness properties in a lying posture ( Mörl et al., 2020 ) and recently extended to include articulated thoracic vertebrae in order to quantify the load sharing between individual biological structures of the functional spinal unit under gravity ( Meszaros-Beller et al., 2023 ). Another FD lumbar spine model was presented in Damm et al.…”

“…The major aim of this study was to quantify the effect of modelling individual passive structures (i.e., ligaments and IVDs) on the remaining joint forces and torques carried by muscles in the human spine using the ID approach. For this purpose, the recently developed generic spine model ( Hammer et al., 2022 ; Meszaros-Beller et al., 2023 ), implemented in the demoa software environment ( Schmitt, 2022 ), was used. The thoracolumbar spine model including six DOF intervertebral joints, a detailed musculature, intersegmental ligaments and IVDs, previously used in FD simulations of a forward flexion-extension movement ( Meszaros-Beller et al., 2023 ), was transferred into the OpenSim MSK modelling platform.…”

“…For this purpose, the recently developed generic spine model ( Hammer et al., 2022 ; Meszaros-Beller et al., 2023 ), implemented in the demoa software environment ( Schmitt, 2022 ), was used. The thoracolumbar spine model including six DOF intervertebral joints, a detailed musculature, intersegmental ligaments and IVDs, previously used in FD simulations of a forward flexion-extension movement ( Meszaros-Beller et al., 2023 ), was transferred into the OpenSim MSK modelling platform. Using the full kinematic description obtained from the FD simulations, systematic ID analysis was performed in a step-wise approach increasing the model complexity by adding individual elements and compare the ID analysis results to a standard ID “plain” model neglecting the role of spinal ligaments and IVD stiffness.…”