A Simple Loading Model for Stress Analysis of Lumbar Intervertebral Discs in Basic Body Positions

The load acting on human spine is complex and varying in different positions of a body, and direct measurement to obtain the complete load information is not yet possible. The objective of this study was to use a simple model to deliver the realistic load for the stress analysis of the lumbar intervertebral discs in several basic body positions, including standing, sitting, and lying in a supine, prone and lateral position. The validated finite element model of lumbar spine L1-L5 was adapted. And combined with proper constraints, characterized axial force for each body position was applied to the top end of the model according to the normal standing load and the occupied percentage on the experimental data. The stresses and strains in all lumbar intervertebral discs were calculated and their variation tendency was analyzed. The results show that stress distributions in intervertebral discs were similar between standing and sitting, and stress concentration occurred at the anteroinferior and posterior part of annulus fibrosus (AF). Slightly lower stress was found in sitting than in standing. Stress level in each lumbar intervertebral disc was significantly lower in all lying positions than that in standing and sitting. Stress concentration at the AF posterior part partly disappeared for lying. Only a small difference in stress level existed between various lying positions. Moreover, stress level in nucleus pulposus and at the AF anterior part was relatively low, and stress values in intervertebral discs generally increased from L1-2 to L4-5, while stress peak and strain peak were always located at L4-5 intervertebral disc. These features are consistent with those in previous studies mostly. It suggests that our loading model is efficient, the high strain in L4-5 AF may be a reason for leading to high incidence of disk herniation, and more lying supine should be recommended to whom suffering from back problems as a physiotherapy.