Effect of Graded Facetectomy on Biomechanics of Dynesys Dynamic Stabilization System

ArticleinSpine 37(10):E581-9 · December 2011with27 Reads
DOI: 10.1097/BRS.0b013e3182463775 · Source: PubMed
Finite element (FE) method was used to compare the biomechanics of L3-S1 lumbar spine with graded facetectomy before and after placement of Dynesys. To evaluate the biomechanics of Dynesys as a function of graded bilateral facetectomies. Spinal fusion or posterior dynamic stabilization systems are used to restore stability after facetectomies. The intact FE spine was modified to simulate decompression at L4-L5 with 50% and 75% and total facetectomy with/without dynamic stabilization with Dynesys. Biomechanics of the implanted level was investigated under different physiological loadings. Total facetectomy increased the motion in extension (8.7° vs. 2.7° for intact) and axial rotation (8.4° vs. 2.4° for intact). However the decrease in motion in the Dynesys model ranged from 65% in axial rotation to 80% in flexion for all facetectomies, except in the total facetectomy axial rotation case (motion higher than intact). The center of rotation of dynamic stabilized segment moved inferior/posterior in partial facetectomy and superior/posterior in total facetectomy with respect to the intact and destabilized cases. The Dynesys screws observed peak stresses up to 28% higher than those of a rigid fixation system in certain loadings, such as lateral bending and extension. The critical loosening torque applied to the screws in total facetectomy case was 6 times the partial facetectomy case in axial rotation. Partial facetectomy had a minimal effect on range of motion on the Dynesys-implanted segment. However, in the case of total facetectomy the motion increased by almost 40% in flexion and by 200% in axial rotation. The higher stresses applied to the screws in Dynesys in specific loadings may lead to higher risk of screw failure in Dynesys than in a generic rigid fixation construct.
    • "In our previous study (Haddad et al., 2015; Xu et al., accepted), the IDP predicted in the post-surgical subject was higher than that from the pre-surgical subject for the same person under static loading. This result supports the assertion that RF surgery increases the IDP in the adjacent level (Haddad et al., 2012; Kiapour et al., 2012). Thus, this study showed that the RF surgery could increase the IDP in the adjacent level under dynamic loads similar to static loads. "
    [Show abstract] [Hide abstract] ABSTRACT: 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.
    Full-text · Conference Paper · Aug 2016 · Computer Methods in Biomechanics and Biomedical Engineering
    • "Osseous tissues of the vertebrae and cartilaginous endplates in this study were modeled as isotropic homogeneous linearly-elastic materials (Lu et al. 1996; Kiapour, Ambati, et al. 2012). The intervertebral disc models were based on Schmidt et al. (2012) with the following details: the annulus fibrosus was modeled into ground substance reinforced by collagen fibers. "
    [Show abstract] [Hide abstract] ABSTRACT: Finite element (FE) method is a proven powerful and efficient tool to study the biomechanics of the human lumbar spine. However, due to the large inter-subject variability of geometries and material properties in human lumbar spines, concerns existed on the accuracy and predictive power of one single deterministic FE model with one set of spinal geometry and material properties. It was confirmed that the combined predictions (median or mean value) of several distinct FE models can be used as an improved prediction of behavior of human lumbar spine under identical loading and boundary conditions. In light of this improved prediction, five FE models (L1-L5 spinal levels) of the human lumbar spine were developed based on five healthy living subjects with identical modeling method. The five models were extensively validated through experimental and computational results in the literature. Mesh convergence and material sensitivity analysis were also conducted. We have shown that the results from the five FE models developed in this paper were consistent with the experimental data and simulation results from the existing literature. The validated modeling method introduced in this study can be used in modeling dysfunctional lumber spines such as disc degeneration and scoliosis in future work.
    Full-text · Article · Jun 2016
    • "These treatments are intended to correct sagittal and coronal alignment, restore balance in the spine, prevent further progression of deformity, and protect the spinal cord. However, concerns about the RF remain because of the increased mechanical stress on the adjacent segments and the possible degenerative changes on the fused segments in the long term (Kiapour et al., 2012). Finite element (FE) analysis is a powerful and efficient tool to study the biomechanics of the human lumbar spine. "
    [Show abstract] [Hide abstract] ABSTRACT: Scoliosis is a disorder that results in an abnormal three dimensional curvature of the spine. There are several types of scoliosis based on the etiology, age at onset and location in the spine. The vast majority of patients however are classified as " idiopathic ". Depending on the severity of the curve and the risk of progression treatment recommendations may include any or all of the following; observation, exercise program, bracing, or surgery. Finite element (FE) model studies have provided important insights into the understanding of the functional biomechanics of the lumbar spine. To date there are no substantial FE model studies evaluating the biomechanics of a scoliotic spine based on geometry of real scoliosis patients. The purpose of this study is to assess the difference in range of motion, intradiscal pressure, and facet joint forces through FE model analysis between healthy and scoliosis subjects under the same loading conditions. In addition, for scoliosis subjects pre-and post-surgery the cephalad and caudal levels adjacent to the fusion were evaluated. The lumber spine FE models in this study are based on the modeling methods developed and validated in our previous work. In this study we developed a scoliosis subject-specific pre-and post-surgery FE model, we compared the biomechanical behaviors of scoliosis FE models and validated healthy FE models under identical loading conditions, and we studied the effect of RF on the changes of biomechanical parameters: range of motion (ROM), intradiscal pressure (IDP), and facet joint force (FJF) in adjacent and fused spinal levels. In this study we were able to show the general effect of the RF surgery on scoliotic spines: reducing ROM, IDP, and FJF in the fused level, and increasing IDP and FJF in the adjacent level.
    Full-text · Conference Paper · Jun 2016 · Computer Methods in Biomechanics and Biomedical Engineering
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