Effect of Graded Facetectomy on Biomechanics of Dynesys Dynamic Stabilization System
ABSTRACT 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.
- SourceAvailable from: Anand Agarwal
[Show abstract] [Hide abstract]
- "l et al . , 1995 , 2007 ; Dooris et al . , 2001 ) . The model consists of cortical and cancellous vertebral bones , posterior bony elements , annulus , nucleus , facet joints , and seven major ligaments , Figure 1A . This model contains 15 , 136 elements and 19 , 148 nodes and is symmetric with respect to the mid - sagittal plane ( Monore , 1993 ; Kiapour et al . , 2012 ; Dreischarf et al . , 2014 ) . For bony structure and intervertebral disk components , C3D8 brick elements were used . All of the liga - ments were modeled using two nodes tension - only truss elements ( T3D2 ) with non - linear material properties ."
ABSTRACT: Besides the biology, stresses and strains within the tissue greatly influence the location of damage initiation and mode of failure in an intervertebral disk. Finite element models of a functional spinal unit (FSU) that incorporate reasonably accurate geometry and appropriate material properties are suitable to investigate such issues. Different material models and techniques have been used to model the anisotropic annulus fibrosus, but the abilities of these models to predict damage initiation in the annulus and to explain clinically observed phenomena are unclear. In this study, a hyperelastic anisotropic material model for the annulus with two different sets of material constants, experimentally determined using uniaxial and biaxial loading conditions, were incorporated in a 3D finite element model of a ligamentous FSU. The purpose of the study was to highlight the biomechanical differences (e.g., intradiscal pressure, motion, forces, stresses, strains, etc.) due to the dissimilarity between the two sets of material properties (uniaxial and biaxial). Based on the analyses, the biaxial constants simulations resulted in better agreements with the in vitro and in vivo data, and thus are more suitable for future damage analysis and failure prediction of the annulus under complex multiaxial loading conditions.Frontiers in Bioengineering and Biotechnology 06/2015; 3:69. DOI:10.3389/fbioe.2015.00069
[Show abstract] [Hide abstract]
- "Intact L1-S1 Finite Element Model. An experimentally validated nonlinear finite element model of the lumbar spine (L1-S1) was used in this study      . Digitized computerized tomography (transverse sections of 1.5-mm thickness) imaging was used to create the mesh model. "
ABSTRACT: Currently, rigid fixation systems are the gold standard for degenerative disc disease treatment. Dynamic fixation systems have been proposed as alternatives for the treatment of a variety of spinal disorders. These systems address the main drawbacks of traditional rigid fixation systems, such as adjacent segment degeneration and instrumentation failure. Pedicle-screw-based dynamic stabilization is one type of these alternative systems. The aim of this study was to simulate the biomechanical effect of a novel posterior dynamic stabilization system, which is comprised of dynamic (hinged) screws interconnected with a coiled, spring-based dynamic rod (DSDR), and compare it to semi-rigid (DSRR and RSRR) and rigid stabilization (RSRR) systems. A validated finite element (FE) model of L1-S1 was used to quantify the biomechanical parameters of the spine, such as range of motion, intradiscal pressure, stresses and facet loads after single-level instrumentation with different posterior stabilization systems. The results obtained from in vitro experimental intact and instrumented spines were used to validate the FE model, and the validated model was then used to compare the biomechanical effects of different fixation and stabilization constructs with intact under a hybrid loading protocol. The segmental motion at L4-L5 increased by 9.5% and 16.3% in flexion and left rotation, respectively, in DRDS with respect to the intact spine, whereas it was reduced by 6.4% and 10.9% in extension and left-bending loads, respectively. After instrumentation-induced intradiscal pressure at adjacent segments, L3-L4 and L5-S1 became less than the intact in dynamic rod constructs (DSDR and DRRS) except in DRRS model in flexion where the motion was higher than intact by 9.6% at L3-L4 and 11% at L5-S1. The facet loads were insignificant, not exceeding 12 N in any of the instrumented cases in flexion. In extension, the facet load in DSDR case was similar to that in intact spine. The dynamic rod constructions (DSDR and RSDR) led to a lesser peak stress at screws compared with rigid rod constructions (DSRR and RSRR) in all loading cases. A dynamic construct consisting of a dynamic rod and a dynamic screw did protect the adjacent level from excessive motion.Journal of Biomechanical Engineering 03/2014; 136(5). DOI:10.1115/1.4027060 · 1.75 Impact Factor
- Spine 01/2012; 1(120):2. · 2.45 Impact Factor