Finite element method-based study for effect of adult degenerative scoliosis on the spinal vibration characteristics

Abstract

Finite element analysis was used to investigate the responses of five healthy subjects and five adult degenerative scoliosis (ADS) subjects to cyclic vibration. The dynamic responses of the healthy and scoliotic spines to the sinusoidal cyclic vibrations have been investigated in previous studies by simulation or experimental approaches. However, no simulation or experimental results were available for the ADS subjects. The effect of the ADS on the vibrational characteristics of spines remained unknown. The objective of this study was to compare differences of the dynamic responses to the cyclic vibration input between the healthy subjects and subjects with ADS. Based on the simulations results in this study, the scoliotic spines are more sensitive to the cyclic vibrations than the healthy spines. More resonant frequencies were predicted in the scoliotic spines than the healthy spines. The scoliotic deformity in the spine was to make the vibrational response of the spine significantly more complex at the apical scoliotic region. This study suggested that ADS could severely increase spinal response to the cyclic vibrations, which could potentially lead to further scoliotic deformity in the spine.

Introduction

Adult degenerative scoliosis (ADS) is a disorder that results in 3-dimentional abnormal spinal deformity in the adult spines, which is commonly correlated with degenerative intervertebral discs. Since the loads on the scoliotic spines are asymmetric, scoliosis subjects are subjected to higher risk of lower back pain (LBP) than the healthy population [15], [26]. Chronic exposure to whole-body vibration (WBV) on the spine has been identified as one important inducer of LBP [4], [16]. With long-term occupational exposure to WBV, truck drivers and helicopter pilots suffer from spinal degenerative diseases and LBP more commonly than normal populations as results from the degenerative and anatomic changes in the spine especially in the lumbar spine region [4], [14], [18], [27]. Cyclic vibration on the spine could catalyze the spinal tissue fatigue, intervertebral disc degeneration, and eventually the abnormal spinal deformity [25].

To understand the vibration-induced spinal pain and injury mechanism, in the literature, both computational method [2] and experimental approach [12], [13] have been conducted to investigate the spinal vibration characterizations. Compared with experimental studies, computational studies have the advantage of lower cost and higher efficiency. The computational studies of the vibration harmfulness on the spine mainly contained two methods: multi-body dynamic simulations [2], [26] and finite element (FE) analysis [9], [10], [11], [14], [15]. Although with good calculation efficiency, two-dimensional (2-D) and three-dimensional (3-D) multi-body dynamic spine models can’t fully represent the realistic geometries of the vertebrae and other spinal soft tissues [14], which might affect the predicting capability of the biomechanical properties of the models. FE method has been widely utilized to study the spinal biomechanics for the past four decays [22], which is able to accurately represent the complex spinal geometry ([26]; Dreischarf et al [6]). FE analysis can also capture the internal mechanical parameters such as stress and strain, which is difficult to measure in the experimental tests ([6], [22]; Ayturk et al [1]; [26]). It has been shown that dynamic cyclic loads could cause higher responses in stress, intradiscal pressure, disc bulge, and facet joint force compared to the static loads [8]. Li et al. [15] proved that scoliotic spines are more sensitive to the dynamic cyclic load which is used to simulate the WBV environment. There is increasing clinical interest in studying the effect of scoliosis on the vibration characterizations of the human spine [14], [15]. Although FE studies have been employed by researchers to study the vibration-induced responses of healthy subjects [8], [9], [10], [14] and adolescent idiopathic scoliosis subjects [15], no study has been reported to study the vibration characteristics of ADS to the best of our knowledge. Furthermore, all of those previous studies only utilized one deterministic FE spine model [8], [9], [10], [11], [14], [15]. However, great inter-subject variations exist among spines including the spinal geometries and material properties, which raised concerns about the reliability and comparability of the prediction results reported by single deterministic FE spine model [6]. To address this issue, one possible solution is to build a fully probabilistic model (Little and Adam [17]), which has been proved to be difficult and time-consuming [6]. Instead, Dreischarf et al. [6] reported that the combined simulation results obtained from several distinct deterministic FE models could act as one improved and feasible option to overcome the limitation of single deterministic FE model. By investigating multiple healthy and scoliosis subjects, one is able to obtain some insights of vibration characteristics in healthy and scoliotic spines.

The objectives of this study are: (1) to investigate the vibration characterizations of five healthy and five scoliotic spines using FE method; and (2) to reveal the effect of ADS on the spinal vibrational characteristics by comparing the healthy subjects and scoliotic subjects. After the comparison of vibrational characteristics in healthy and scoliotic spines, we should have better understanding of the influence of scoliosis on the spinal response to the WBV and how such influence will affect the spinal health or trigger LBP.