Finite element investigation of the effect of nucleus removal on vibration characteristics of the lumbar spine under a compressive follower preload

Previous studies have reported the effect of removing the nucleus on biomechanical responses of the human spine to static loadings. However, few studies have dealt with the whole-body vibration condition. The purpose of this study was to investigate the effect of a single-level (L4-L5) nucleus removal on vibration characteristics of the whole lumbar spine in the presence of a physiologic compressive preload, and also to evaluate the preload effect on the vibration characteristics. A 3-D non-linear finite element model of the lumbar spine (L1 to sacrum) subjected to the physiologic conditions of a compressive follower preload was developed and validated. Comparative studies on forced vibration responses between the intact and denucleated models were conducted. The results from the forced-vibration (transient dynamic) analyses considering axial cyclic loading indicated that the nucleus removal increased the dynamic responses at all disc levels. For example, at the denucleated L4-L5 level, after nucleus removal the maximum response values of disc bulge and von-Mises stress in annulus increased by 63.9% and 110.5% respectively, and their vibration amplitudes increased by 97.9% and 139.7% respectively. At other levels, the predicted maximum response values and vibration amplitudes of the stresses and strains also produced 3.1-7.5% and 10.8-30.6% increases respectively due to the nucleus removal, and a relatively larger increase was observed at level L5-S1. It was also found that increasing the preload increased the stresses and strains at all levels but decreased their vibration amplitudes. Nucleus removal at a single level deteriorates the effects of vibration on whole lumbar spine. Also, increasing the preload alters vibration characteristics of the spine. These findings may be useful to provide a guideline for the patients suffering from lumbar disc degeneration to minimize the risk of further injury and discomfort.