Finite element method-based study for effect of adult degenerative scoliosis on the spinal vibration characteristics
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.
Section snippets
Method
Ten 3-D FE spine models were developed to represent five healthy and five pre- surgical scoliotic spines (Fig. 1) based on in vivo computed tomography (CT) scans with a gap of 0.5 mm ([19], [28] submitted). The modeling methods employed to develop these FE spine models were extensively validated in our previous study [28] and have been employed to study the biomechanics of scoliotic spines under static loading conditions ([19] submitted). The FE spine models utilized in this study were described
Results
The vibration-induced resonant translations of different vertebrae of the five healthy and five scoliosis subjects tested in this study were summarized in Fig. 3, where the resonant amplitudes of vertebra L3 in three translational directions (vertical, lateral, and anteroposterior) for healthy and scoliotic subjects were compared with those in the scoliotic subject reported by Li et al. [15]. To illustrate the difference in the resonant amplitudes among different vertebrae, the
Discussions
In this study, five healthy spines and five scoliotic spines were investigated in order to better understand vibrational characteristic difference between the healthy and scoliotic subjects. The effect of inter-subject variation on the simulation results still exist in multiple subject results. The exact values of resonant frequencies and resonant amplitudes obtained in this study were not necessarily able to represent values for all population groups. The scoliotic spines have considerable
Conclusions
In this study, the vibrational characteristics of the healthy and scoliotic spines were compared. Under identical cyclic loads, the scoliotic spines had larger vibration-induced deformations than the healthy spines at the apical scoliotic region. The scoliotic spines are more sensitive than the healthy spines to the cyclic vibrations. More resonant frequencies were predicted in the scoliotic spines than the healthy spines. The scoliotic deformity in the spine was to make the vibrational
Conflict of Interests
None Declared.
Ming Xu is a Ph.D. student at the Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas. He received his B.S. from Wuhan University and MS from Lehigh University.
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Ming Xu is a Ph.D. student at the Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas. He received his B.S. from Wuhan University and MS from Lehigh University.
James Yang is an associate Professor at the Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas.
Isador Lieberman is a spine surgeon at Texas Back Institute.
Ram Haddas is a research director at Texas Back Institute Research Foundation.