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      Biomechanical evaluation of three surgical scenarios of posterior lumbar interbody fusion by finite element analysis

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          Abstract

          Background

          For the treatment of low back pain, the following three scenarios of posterior lumbar interbody fusion (PLIF) were usually used, i.e., PLIF procedure with autogenous iliac bone (PAIB model), PLIF with cages made of PEEK (PCP model) or titanium (Ti) (PCT model) materiel. But the benefits or adverse effects among the three surgical scenarios were still not fully understood.

          Method

          Finite element analysis (FEA), as an efficient tool for the analysis of lumbar diseases, was used to establish a three-dimensional nonlinear L1-S1 FE model (intact model) with the ligaments of solid elements. Then it was modified to simulate the three scenarios of PLIF. 10 Nm moments with 400 N preload were applied to the upper L1 vertebral body under the loading conditions of extension, flexion, lateral bending and torsion, respectively.

          Results

          Different mechanical parameters were calculated to evaluate the differences among the three surgical models. The lowest stresses on the bone grafts and the greatest stresses on endplate were found in the PCT model. The PCP model obtained considerable stresses on the bone grafts and less stresses on ligaments. But the changes of stresses on the adjacent discs and endplate were minimal in the PAIB model.

          Conclusions

          The PCT model was inferior to the other two models. Both the PCP and PAIB models had their own relative merits. The findings provide theoretical basis for the choice of a suitable surgical scenario for different patients.

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          Most cited references23

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          Polyetheretherketone as a biomaterial for spinal applications.

          Threaded lumbar interbody spinal fusion devices (TIBFD) made from titanium have been reported to be 90% effective for single-level lumbar interbody fusion, although radiographic determination of fusion has been intensely debated in the literature. Using blinded radiographic, biomechanic, histologic, and statistical measures, we evaluated a radiolucent polyetheretherketone (PEEK)-threaded interbody fusion device packed with autograft or rhBMP-2 on an absorbable collagen sponge in 13 sheep at 6 months. Radiographic fusion, increased spinal level biomechanical stiffness, and histologic fusion were demonstrated for the PEEK cages filled with autograft or rhBMP-2 on a collagen sponge. No device degradation or wear debris was observed. Only mild chronic inflammation consisting of a few macrophages was observed in peri-implant tissues. Based on these results, the polymeric biomaterial PEEK may be a useful biomaterial for interbody fusion cages due to the polymer's increased radiolucency and decreased stiffness.
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            Mechanical behavior of the human lumbar and lumbosacral spine as shown by three-dimensional load-displacement curves.

            The lumbar region is a frequent site of spinal disorders, including low-back pain, and of spinal trauma. Clinical studies have established that abnormal intervertebral motions occur in some patients who have low-back pain. A knowledge of normal spinal movements, with all of the inherent complexities, is needed as a baseline. The present study documents the complete three-dimensional elastic physical properties of each lumbar intervertebral level from the level between the first and second lumbar vertebrae through the level between the fifth lumbar and first sacral vertebrae. Nine whole fresh-frozen human cadaveric lumbar-spine specimens were used. Pure moments of flexion-extension, bilateral axial torque, and bilateral lateral bending were applied, and three-dimensional intervertebral motions were determined with use of stereophotogrammetry. The motions were presented in the form of a set of six load-displacement curves, quantitating intervertebral rotations and translations. The curves were found to be non-linear, and the motions were coupled. The ranges of motion were found to compare favorably with reported values from in vivo studies.
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              Biomechanical rationale for using polyetheretherketone (PEEK) spacers for lumbar interbody fusion-A finite element study.

              To determine the effect of cage/spacer stiffness on the stresses in the bone graft and cage subsidence. To investigate the effect of cage stiffness on the biomechanics of the fused segment in the lumbar region using finite element analysis. There are a wide variety of cage/spacer designs available for lumbar interbody fusion surgery. These range from circular, tapered, rectangular with and without curvature, and were initially manufactured using titanium alloy. Recent advances in the medical implant industry have resulted in using medical grade polyetheretherketone (PEEK). The biomechanical advantages of using different cage material in terms of stability, subsidence, and stresses in bone graft are not fully understood. A previously validated 3-dimensional, nonlinear finite element model of an intact L3-L5 segment was modified to simulate posterior interbody fusion spacers made of PEEK ("E" = 3.6 GPa) and titanium ("E" = 110 GPa) at the L4/5 disc with posterior instrumentation. Bone graft ("E" = 12 GPa) packed between the spacers in the intervertebral space was also simulated. The posterior lumbar interbody fusion spacer with instrumentation and graft represent a simulation of the condition present immediately after surgery. The peak centroidal Von Mises stresses in the graft bone increased by at least 9-fold with PEEK spacers as compared to titanium spacer. The peak centroidal Von Mises stresses in the endplates increased by at least 2.4-fold with titanium spacers over the PEEK spacers. These stresses were concentrated at places where the spacer interfaced with the endplate. The stiffness of the spacer did not affect the relative motion (stability) across the instrumented (L4/5) segment. Spacers less stiff than the graft will: (1) provide stability similar to titanium cages in the presence of posterior instrumentation, (2) reduce the stresses in endplates adjacent to the spacers, and (3) increase the load transfer through the graft, as evident from the increase in stresses in graft.
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                Author and article information

                Journal
                Biomed Eng Online
                Biomed Eng Online
                BioMedical Engineering OnLine
                BioMed Central
                1475-925X
                2012
                18 June 2012
                : 11
                : 31
                Affiliations
                [1 ]State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun, People’s Republic of China
                [2 ]Department of Engineering Mechanics, Nanling Campus, Jilin University, Changchun, 130025, People’s Republic of China
                [3 ]Department of Orthopaedic Trauma, First Hospital of Jilin University, Changchun, People’s Republic of China
                Article
                1475-925X-11-31
                10.1186/1475-925X-11-31
                3430554
                22709659
                93071a26-6f36-45fe-8bfb-9368bf50cbe6
                Copyright ©2012 Xiao et al.; licensee BioMed Central Ltd.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 23 March 2012
                : 18 June 2012
                Categories
                Research

                Biomedical engineering
                autogenous iliac bone,cage,ligaments,spine,peek
                Biomedical engineering
                autogenous iliac bone, cage, ligaments, spine, peek

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