Development and Analysis of a Finite Element Model of the L5/S1 Disk using a Loading Model Simulating Whole-body Vibration

Objectives. This study, being part of a broader program that will cover the entire lumbar region of the human spine, aims at developing a fine partition, three-dimensional, finite element model of the L5/S1 intervertebral disk suitable for step-by-step calculation and video recording of the impacts of vibratory and shock loading that simulate conditions of whole body vibration, time-dependent multidirectional vibrations or continuous (sequential) shocks and permitting a. Identification of the sites of maximum displacement from the corresponding positions of equilibrium in association with the loading size and direction sequence of shocks, b. Graphic and Video Simulation (GVS) of disk motion, and c. Prediction of possible damage due to the afore mentioned conditions. Design. The disk model has 7240 elements and 6900 nodes. Three load cases were studied: a. Constant compressive load of 1600 KPa. b. Linear variation between 200 and 1600 KPa in x-direction c. Linear variation between 800 and 1600 KPa in y-direction. In all three cases the pressure was applied on all the external elements of the model having z>= +0.4 cm. Methods. The model of vibratory and shock loading was selected on the basis of known peak pressure values, and the stress analysis of the disk model was performed for two sequencial peaks, assuming that Vibration Injury Network Appendix W3E to Final Report Biomed 2 project no. BMH4-CT98-3251 2 the disk restitution period (natural period) is larger than the time interval between the two successive loadings. Results. The tentative results show that in both cases the impact of successive loading is additive as far as deformations are concerned and that final damage, by creation of hernia or rapture, occur by lower magnitude but successive loading rather than by higher magnitude isolated shocks without repetition. The second series of experiments tentatively shows that permanent disk damage can occur at even lower magnitude successive loading, particularly at the presence of inhomogeneities of the modulus of elasticity in the disk’s annulus fibrosus.