Influence of passive elements on prediction of intradiscal pressure and muscle activation in lumbar musculoskeletal models

Background and objective: The objective of this study was to investigate the effect of incorporating various passive elements, which could represent combined or individual effects of intervertebral disc, facet articulation and ligaments, on the prediction of lumbar muscle activation and L4-L5 intradiscal pressure. Methods: The passive elements representing the intervertebral disc, facet articulations, and ligaments were added to the existed lumbar musculoskeletal model with nonlinear rotational stiffness or force-strain relationships. The model was fed with kinematics of trunk flexion, extension, axial rotation and lateral bending to calculate muscle activation and L4-L5 intradiscal pressure. Results: In the trunk axial rotation, the intradiscal pressure values predicted by the models with elements representing facet articulation were much higher than that predicated by models removing these elements. In the trunk flexion, the models with passive elements showed lower muscle activation of extensors than model with no passive elements. At the end of trunk flexion, extension, axial rotation and lateral bending, the intradiscal pressure values predicted by model with intact passive elements were 120.6%, 92.5%, 334.8% and 74.9% of the values predicted by model with no passive elements, respectively. Conclusions: Caution must be taken while modeling facet articulation as elements with rotational stiffness, as they may lead to overestimation of intradiscal pressure in trunk axial rotation. The inclusion of ligaments as spring-like elements may improve the simulation of flexion-relaxation phenomenon in trunk flexion. Future models considering detailed properties of passive elements are needed to allow more access to understanding the mechanics of the lumbar spine.