Finite Element Method-Based Analysis for Effect of Vibration on Healthy and Scoliotic Spines

Whole body vibration (WBV) could increase the risk of spine disorders in human spine. Scoliosis is a disorder that results in abnormal three dimensional deformity of the human spine. Rigid fusion (RF) surgery is a common treatment for scoliosis subjects. Subjects with scoliosis have been proven to be more sensitive to WBV than healthy subjects. To date nobody has investigated the effect of WBV on post-surgical of scoliosis subjects although pre-surgical scoliosis subjects have been investigated. Finite element (FE) studies have provided important insights into the understanding of the functional biomechanics of the lumbar spine. The purpose of this study is to analyze the mechanical responses of the healthy, pre- and post-surgical scoliotic spines to vibration using FE methods. The FE models employed in this study were developed using extensively validated modeling methods. Vibration modal analysis was performed to obtain the first-order resonant frequencies in vertical direction in this study: 14.3 Hz in healthy subject, 3.9 Hz in pre-surgical scoliosis subject and 28 Hz in post-surgical scoliosis subject. A cyclic axial load of 40 N at 5 Hz was applied to all three FE models to represent the WBV in vehicle seats under normal driving condition. Intradiscal pressure (IDP) and disc bulge in the adjacent level (L₁-L₂) for the post-surgical scoliosis subject and healthy and pre-surgical scoliosis subjects were recorded. The IDP and disc bulge in post-surgical scoliosis model were larger than those in healthy and pre-surgical scoliosis subjects. To compare the effect of dynamic and static loads on the spine, static compressions of 360 N and 440 N were applied to the three models corresponding to the minimum and maximum magnitudes of dynamic loads. The differences in IDP and disc bulge under dynamic loads and corresponding static loads in the pre-surgical scoliosis subject was larger than those in both healthy and post-surgical scoliosis subjects. The pre-surgery scoliosis subject has the lowest resonant frequency which is lower than the resonant frequency of the healthy subject. RF surgery considerably increased the resonant frequency of scoliotic spine, which makes the resonance frequency higher in the post-surgery scoliosis subject than that for healthy subjects.