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Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: A Finite Element Analysis
Accepted Manuscript Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: a Finite Element Analysis Zhenjun Zhang, Hui Li, Guy R. Fogel, Zhenhua Liao, Yang Li, Weiqiang Liu PII:
S1878-8750(17)32243-X
DOI:
10.1016/j.wneu.2017.12.127
Reference:
WNEU 7140
To appear in:
World Neurosurgery
Received Date: 13 November 2017 Revised Date:
18 December 2017
Accepted Date: 19 December 2017
Please cite this article as: Zhang Z, Li H, Fogel GR, Liao Z, Li Y, Liu W, Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: a Finite Element Analysis, World Neurosurgery (2018), doi: 10.1016/j.wneu.2017.12.127. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Biomechanical analysis of porous additive manufactured cages for lateral lumbar interbody fusion: A finite element analysis
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Zhenjun Zhang1,2, Hui Li3, Guy R. Fogel4, Zhenhua Liao2, Yang Li1,2, Weiqiang Liu1,2 1
Department of Mechanical Engineering, Tsinghua University, Beijing, China
2
Biomechanics and Biotechnology Lab, Research Institute of Tsinghua University in
Naton Science and Technology Group, Beijing, China
4
Spine Pain Begone Clinic, San Antonio, TX, USA
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3
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Shenzhen, Shenzhen, China
Corresponding author:
Weiqiang Liu, Tsinghua University, Haidian District, Beijing, P.R.China; Tel: +86-0755-26551376 Fax: +86-0755-26551380
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E-mail: [email protected]
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This study was supported by the Industry Public Technology Service Platform Project of Shenzhen [Grant Number SMJKPT20140417010001] and the Science and Technology Plan
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Basic Research Project of Shenzhen [Grant Number JCYJ20151030160526024].
No relevant financial activities outside the submitted work.
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ABSTRACT Background
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A porous additive manufactured (AM) cage may provide stability similar to that of traditional solid cages, and may be beneficial to bone ingrowth. The biomechanical influence of various porous cages on stability, subsidence, stresses in cage, and facet contact force has not been
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fully described. The purpose of this study was to verify biomechanical effects of porous AM cages.
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Methods
The surgical FE models with various cages were constructed. The partially porous titanium (PPT) cages and fully porous titanium (FPT) cages were applied. The mechanical parameters
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of porous materials were obtained by mechanical test. Then the porous AM cages were compared to solid titanium (TI) cage and solid PEEK cage. The four motion modes were
were compared.
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Results
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simulated. Range of motion (ROM), cage stress, endplate stress, and facet joint force (FJF)
For all the surgical models, ROM decreased by more than 90%. Compared with TI and PPT cages, PEEK and FPT cages substantially reduced the maximum stresses in cage and endplate in all motion modes. Compared with PEEK cages, the stresses in cage and endplate for FPT cages decreased, whereas the ROM increased. Compared among FPT cages, the stresses in cage and endplate decreased with increasing porosity, whereas ROM increased with increasing porosity. After interbody fusion, FJF was substantially reduced in all motion 2
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modes except for flexion. Conclusions
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Fully porous cages may offer an alternative to solid PEEK cages in lateral lumbar interbody fusion. However, it may be prudent to further increase the porosity of cage.
Key words: biomechanics; facet joint force (FJF); finite element analysis (FEA); lumbar
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interbody fusion; porous cage; range of motion (ROM); subsidence.
Introduction
Lumbar interbody fusion has been used in the treatment of lumbar disease such as spondylolisthesis, trauma, and degenerative disc degeneration. Successful clinical outcomes
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depend on fusion healing. The best opportunity for healing may be the anterior interbody fusion with better loading of the graft and the largest surface area for fusion.1-4
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Laterally-inserted interbody cages may decrease ROM compared to other cages.5-8 The interbody cage fusion improves the loading capacity of the anterior column.2,3 In addition,
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supplemental fixation favorably influences the healing of the lumbar fusion, so bilateral pedicle screw fixation is often used to supplement interbody cage because it can provide multiplanar stability. 4,5,9-11
Traditional solid cage made of titanium or PEEK has been widely used in lumbar fusion.12,13 However, solid cages have high mechanical stiffness, which may affect the loading mechanism of lumbar spine.14,15 A porous AM cage can reduce the mechanical stiffness, may 3
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provide stability similar to that of traditional solid cages, and may be beneficial to bone ingrowth.16 Currently there have been a variety of porous AM cages, such as Posterior Spine
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Truss SystemTM (4WEB Medical, Frisco, TX), CascadiaTM Lateral 3D Interbody System (K2M Group Holdings, Inc., Leesburg, VA), EndoLIF® O-Cage (joimax GmbH, Karlsruhe, Germany), Tritanium® PL Posterior Lumbar Cage (SYK CORP, Kalamazoo, MI), and 3D
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ACT (AK Medical, Beijing, China).
Although some factors such as lumbar instability, bone quality of vertebral trabecular
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and endplate, and previous operation history are known to affect the choice of cage, the biomechanics of cages made of various materials are not fully investigated. Compared with solid cage, partially porous titanium (PPT) cage and fully porous titanium (FPT) cage have a
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porous structure, along with rough surfaces, to allow for bony integration throughout the implant. Some previous studies have investigated the mechanical performance of porous AM cages and evaluated their biomechanics. Lee et al17 developed a porous cage with 50%
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porosity and compared three lumbar fusion techniques by using L3-L4 bone-implant model.
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Kang et al18 investigated the porous biodegradable lumbar interbody fusion cage using a finite element model of mini-pig L2-L5 lumbar spine. Additionally, Tsai et al19 in their recent numerical and experimental study found that porous AM cages with a porosity of between 69% and 80% could provide better biomechanical performances. However, the influence of the various porous cages (solid cage, PPT cage, and FPT cage) and their different porosities on lumbar stability, cage stress, endplate stress, and FJF has not been fully described. Using a FE model to systematically study the biomechanical effects of various cages 4
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may be valuable. Further information to be determined is the influence of various porous structures on the subsidence, which is induced by the changed stiffness in the index disc
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space associated with the interbody cage and supplemented fixation. The aim of this study was to evaluate the lateral lumbar interbody fusion cages by comparing the biomechanical properties of porous cages with that of solid cages, and to verify the biomechanical effects of
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cage porosity.
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Materials and Methods
CT images of intact lumbar spine with interval of 0.7 mm were obtained from a 36 year old woman (weight 52 kg, height 158 cm, excluded from lumbar disease based on visual and radiographic examination). A total of 492 CT images were imported into Mimics (Materialise
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Inc, Leuven, Belgium). In Mimics, the 3D geometry structure was constructed, which consists of vertebrae, intervertebral disc and cartilage endplate. The geometric structure was meshed using Hypermesh (Altair Technologies Inc, Fremont, CA). Lastly, the mesh model
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was imported into Abaqus (Simulia Inc, Providence, RI) to perform FEA. The computer for
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the simulation is ThinkStation (Lenovo, Beijing, China) configured with 24 processors and 64 GB memory.
Figure 1 displayed the FE model of intact lumbar spine L1-L5. The vertebral body was divided into three parts: cortical bone, cancellous bone, and posterior bone. The intervertebral disc was divided into nucleus pulposus and annulus ground. The intact model included 7 kinds of ligaments: anterior longitudinal ligament, posterior longitudinal ligament, ligamenta 5
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flava, interspinal ligament, supraspinal ligament, intertransverse ligament, and capsular ligament. The cortical bone was 1.0 mm thick, and the endplate was 0.5 mm thick. All the
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ligaments were modeled with truss elements (T3D2) which had the tension-only property. The FE model was meshed using the 3D tetrahedral elements except for the ligaments. 195,533 nodes and 841,038 elements were contained in the intact model, which could
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effectively eliminate the influence of meshing on the accuracy of calculation.
The partially porous titanium (PPT) cage was configured as both solid and porous
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structures. The outer part was solid titanium structure, and the inner part is porous titanium structure. The porosity (vol%) and pore size (µm) was chosen according to the previous literatures.19,20 There were three kinds of PPT cages: PPT cage with 65% porosity (PPT65),
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PPT cage with 75% porosity (PPT75), and PPT cage with 80% porosity (PPT80). The fully porous titanium (FPT) cage was configured as porous structure. There were three kinds of FPT cages: FPT cage with 65% porosity (FPT65), FPT cage with 75% porosity (FPT75), and
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FPT cage with 80% porosity (PPT80). The average pore size of all the porous cages was
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350-400 µm. Including solid titanium (TI) cage and solid PEEK (PEEK) cage, there were 8 kinds of cages. All the cage models were 30 mm long, 10 mm wide and 7 mm high. The PEEK cage was made of polyetheretherketone (PEEK), and other cages were made of titanium alloy (Ti6Al4V). The bilateral pedicle screw system was modeled based on EXPEDIUM® 5.5 System (DePuy Synthes Spine, Inc, Raynham, MA). The diameter of pedicle screw was 5.5 mm. The material of pedicle screws was titanium alloy (Ti6Al4V). Figure 2 showed the mechanical test for the porous materials with different porosities. 6
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The static compression testing was carried out according to the ISO13314:2011 standard (Compression test for porous and cellular metals) by using a universal mechanical testing
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machine.21-24 The machine for the compression testing was Instron 8874 (Naton, Beijing, China). The average pore size of all the test samples was 350-400 um. The porosities of the three groups of test samples (5 per group) were 65%, 75%, and 80%. The test results were as
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follows: for the samples with 65% porosity, the average Young's modulus was 2653 MPa and the average plateau stress was 57.2 MPa; for the samples with 75% porosity, the average
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Young's modulus was 1551 MPa and the average plateau stress was 30 MPa; for the samples with 80% porosity, the average Young's modulus was 675MPa and the average plateau stress was 19.1 MPa. The test results were comparable to the previous literature.20 The Young's
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modulus and the plateau stress decreased with increasing porosity. The material properties of components were shown in Table 1.25-34
To validate the intact FE model, the analysis study included two steps of simulation. The
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predicted results were compared with the previous experimental data. The contact surfaces of
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vertebrae and discs were set as tie constraints. The contact between the facet joints was simulated as frictionless surfaces.25-30 Firstly, the range of motion (ROM) of intact lumbar spine L1-L5 under pure moment was predicted. Three different moments (2.5 Nm, 5.0 Nm, and 7.5 Nm) were applied to the upper surface of L1 while the bottom of L5 was fixed. The ROM of the lumbar spine was compared with previous in vitro results.28,29,35 Then the compression displacement and intervetebral disc pressure (IDP) of the motion segment L4-L5 under pure compression were calculated. The upper surface of L4 was loaded with four 7
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preload values (100 N, 200N, 300 N, and 400 N) as described by Berkson et al.36 The compression displacement and IDP of L4-L5 were compared with previous results.28,36,37
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For simulation of the surgical models, the segment L2-L5 was chosen to predict the biomechanics changes of surgical level. The surgical conditions were as follows: the interbody cage was inserted at the L3-L4 disc space laterally, and supplemented with bilateral
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pedicle screw fixation. The FE models of interbody fusion with various cages were shown in Figure 3. All the surgical FE models were constructed based on the validated intact model.
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The surface contact between the vertebrae and discs, and the contact between the facet joints were consistent with that of the validated model. The interfaces of vertebrae and cages were also assigned to tie constraints. The bottom of L5 was fixed in all directions. The
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compressive load of 280 N and the moment of 7.5 Nm were applied to the upper surface of L2 as in previous literature.38,39 The compressive load of 280 N corresponded to the partial weight of a human body, and the moment of 7.5 Nm simulated the motion modes occurred in
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different conditions such as flexion, extension, lateral bending, and axial rotation.
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Considering the symmetry of the sagittal plane, this study simulated the biomechanical properties of surgical FE models in four motion modes: flexion, extension, bending-left, and rotation-left. The ROM, cage stress, endplate stress, and facet joint force (FJF) were analyzed and exported. The predict results of porous cages were compared with that of solid cages. The ROM data were normalized to the intact ROM data. Under the combined loading, the intact L2-L5 model was recalculated. In total, 36 simulation calculations for nine models and four motion modes were performed. Simulation results were in accordance with the 8
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requirements of visualization, and mechanics data was expressed using Von Mises stress contours.
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Results Model validation
Under the pure moment of 7.5 Nm, the L1-L5 ROM was within the range of the
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previous FE and in vitro experimental studies.28,35 The total movement angles were 24.75°in
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flexion-extension, 26.66°in lateral bending, and 18.43°in axial rotation. As was displayed in Figure 4a, all values in different conditions were within the FE and in vitro ranges. The load-deflection curves were shown in Figure 4b, which were comparable to the existing results of previous studies.28,29,35 The rotation angles were 15.04°in flexion, 9.71°in
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extension, 13.45°in bending-left, and 10.33°in rotation-left.
The compression-displacement curves were shown in Figure 4c, which indicated that the axial displacement of L4-L5 segment increased almost linearly with the applied axial
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compressive loading. The results were comparable to the previous in vitro experimental
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study36. The compression-IDP curves were displayed in Figure 4d, which were also compared with the previous FE and in vitro experimental studies.28,37 The predicted results demonstrated that IDP of L4-L5 segment increased almost linearly with the applied axial compressive loading.
Range of motion (ROM)
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Under the combined loading of 280 N and 7.5 Nm, the ROM of surgical models was shown in Figure 5. After inserting the interbody cage, the predicted ROMs for all surgical
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models decreased by more than 90% compared with the intact case. ROMs for three PPT cages were slightly more than that for TI cage, and the porosity of PPT cage did not substantially alter ROM in all motion modes. ROMs for three FPT cages were more than that
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for PEEK cage, and they increased with increasing porosity. Compared among all the surgical models, ROM for TI cage was the minimum and ROM for FPT80 was the maximum
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in all motion modes. Compared with PEEK cage, ROMs for FPT80 increased by 2.55% in flexion, 3.43% in extension, 2.62% in bending-left, and 2.22% in rotation-left, respectively. Cage stress
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The maximum stress in cage (cage stress) was displayed in Figure 6a. Cage stresses for three PPT cages were more than that for TI cage, and the porosity of PPT cage did not substantially alter cage stress in all motion modes. Cage stresses for three FPT cages were
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less than that for PEEK cage, and they decreased with increasing porosity. Compared among
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all the surgical models, cage stress for PPT80 was the maximum and cage stress for FPT80 was the minimum in all motion modes. Compared with PEEK cage, cage stress for FPT80 was reduced by 36.18% in flexion, 49.55% in extension, 33.84% in bending-left, and 28.62% in rotation-left, respectively. In order to compare the stress distribution of the two porous structures, the contour plots of Von Mises stress in PPT75 and FPT75 cages were shown in Figure 6b. The PPT cage may result in a stress shielding effect because of the configuration 10
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of both solid and porous structure. In contrast, the stress for FPT cage was more evenly distributed in the cage in all motion modes because of the fully porous structure.
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Endplate stress Figure 7a depicts the maximum stress in the L3 bottom endplate (endplate stress). After interbody fusion, the predicted endplate stress increased in all motion modes except for
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FPT75 and FPT80 in rotation-left. Endplate stresses for three PPT cages were more than that
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for TI cage, and the porosity of PPT cage did not substantially alter endplate stress in all motion modes. Endplate stresses for three FPT cages were less than that for PEEK cage, and they decreased with increasing porosity. Compared among all the surgical models, endplate stress for PPT80 cage was the maximum and endplate stress for FPT80 was the minimum in
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all motion modes. Compared with PEEK cage, endplate stress for FPT80 was reduced by 37.63% in flexion, 52.40% in extension, 38.51% in bending-left, and 33.12% in rotation-left, respectively. Compared with the intact case, endplate stress for FPT80 remained the same in
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flexion, increased by 15.75% in extension, increased by 54.48% in bending-left, and
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decreased by 26.26% in rotation-left, respectively. The contour plots of Von Mises stress in the L3 bottom endplate for PPT75 and FPT75 cages were shown in Figure 7b. The PPT cage may result in a stress shielding effect, whereas the endplate stress for FPT cage was more evenly distributed in the endplate in all motion modes. Facet joint force (FJF)
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In Figure 8, the FJF of intact and surgical models were displayed. After interbody fusions, FJF at surgical level L3-L4 decreased substantially in all motion modes except for
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flexion. Compared among all the surgical models, FJF for various cages were not substantially changed in all motion modes except for rotation. Compared with TI cage, FJF for PPT cage was not substantially changed in all motion modes. Compared with PEEK cage,
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FJF for FPT80 was reduced by 7.57% in flexion, 11.90% in extension, and 13.13% in
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bending-left, respectively, whereas it increased by 19.24% in rotation-left. Discussion
We employed a FE model to study the biomechanical effect of various cages on ROM, stresses in cage and endplate, and FJF in four loading modes (flexion, extension, bending-left,
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and rotation-left). Such a finite element investigation is not feasible in a cadaver model because of inability to estimate the stresses in various spinal structures.33 The predicted ROMs with various cages were comparable to the previous studies.4,5,10,40 In addition, the
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current study showed the cage stress, endplate stress, and FJF. As was displayed in Figure 5,
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the ROM data were normalized to the intact ROM data. The predicted ROMs for all surgical models were less than 10% of the intact model in all motion modes. That is the maximum stiffness or stability that the cage may provide for the lumbar spine. However, ROM tends to increase further with increasing porosity because of the lower stiffness and greater deformation of cages. As was displayed in Figure 6 and Figure 7, stresses in cage and endplate were sensitive to the various cages. Compared with TI and PPT cages, PEEK and 12
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FPT cages substantially reduced the maximum stresses in cage and endplate in all motion modes. Compared with PEEK cages, the stresses in cage and endplate for FPT cages
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decreased, and the stresses decreased with increasing porosity of cages. The variations of the maximum stresses in the L3 bottom endplate after interbody fusion were listed in Table 2. The endplate stresses were normalized to the intact stress data. Compared with the intact
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model, the endplate stress for PPT80 increased by less than 2-fold in all motion modes. As was displayed in Figure 8, FJF at surgical level decreased substantially compared to the intact
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conditions after inserting the cage at L3-L4 in all motion modes except for flexion. FJF was not substantially changed with various cages except for rotation-left. The ROM, cage stress, and endplate stress at surgical level were directly affected in all
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motion modes, and changed with various cages. By comparing the biomechanics of interbody fusion using different cages, it was shown that FPT cage displayed some advantages, such as the minimum cage stress and the minimum endplate stress in all motion modes. The FPT
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cage reduced the cage stress and endplate stress, which may decrease the risk of subsidence
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of the cages into the endplate and the adjoining vertebral bone over time.33 It was indicated that in performing lateral lumbar interbody fusion, the FPT cage may offer an alternative to PEEK cage.
In the current study, the porous cages (PPT and FPT) were compared with solid cages (TI and PEEK). The Young's modulus of solid titanium was much larger than that of the porous titanium. As was predicted in Figure 6 and Figure 7, PPT cage may result in a stress shielding effect because of the configuration of both solid and porous structure. FPT cage has 13
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advantages in cage stress and endplate stress, which may lead to a smaller risk of subsidence.33 In addition, FPT cage may be lead to better fusion healing because the porous
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structure was beneficial to bone ingrowth.16 According to Figure 5, although ROMs for all the cages decreased by more than 90% compared with the intact case, ROM for FPT cage increased with increasing porosity. Higher porosity may result in greater bone ingrowth but
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diminish mechanical properties.41 Further increased ROM may reduce the stability to affect the healing response of the fusion.4 Clinically, the cages with higher porosity may have better
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bone ingrowth into the cage structure and an enhance bone fusion healing. The improved bone healing could be a reasonable exchange for increased ROM of porous cages. In summary, the porous AM cage may offer an alternative to PEEK cages in lateral lumbar
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interbody fusion, whereas it may be prudent to further increase the porosity of cage. There are some limitations in the present study, such as validating the model by comparing it with other models, using a unique lumbar model, simplifying the material
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properties of some tissues, and ignoring the role of muscles. One limitation is that the
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validation of the model is by comparing it to other models, and not to a biological model. This intact spine model was scanned from a healthy volunteer, and was validated by comparing it to the other FE models and in vitro models. The experimental studies on a biological model are useful to further validate the current FE model. Another limitation is the finite element analysis standard technique of using a unique lumbar model. The geometric model of lumbar spine varies from person to person such as the intervertebral disc space and the gaps between facet joints. But only one model of lumbar spine was chosen in this study. 14
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Furthermore, the material properties of human tissues are poorly understood. In the present model, the material properties were simplified as linear elastic though the components of
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lumbar spine are nonlinear in reality. However, many FEA on lumbar spine have assumed that the components of spine were linear in order to improve the calculation efficiency.26,33,39,42,43 In addition, the muscles were not considered in the present study
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although the muscles play an important role in supporting the stability of lumbar spine. However, the tendency of predicted results with various cages would not be substantially
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changed depending on the individual geometric model, simplified material properties, and model of the muscles.
In conclusion, the predicted results showed that the structure of porous cage can affect
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the biomechanics of lumbar interbody fusion noticeably. Compared among the surgical models with different cages, a porous AM cage showed advantages in cage stress and endplate stress. Compared with PEEK cage, the porous cage has some advantages in
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biomechanics and may lead to better fusion healing in clinical practice. Further clinical
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studies on the effect of porous cages on stability, subsidence, and other clinically relevant parameters are necessary to validate the observations of this finite element study. Acknowledgements
This work was supported by the Industry Public Technology Service Platform Project of Shenzhen [grant number SMJKPT20140417010001] and the Science and Technology Plan Basic Research Project of Shenzhen [grant number JCYJ20151030160526024]. 15
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Conflict of interest
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The authors declare that they have no conflict of interest.
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References 1.
Oxland TR, Lund T. Biomechanics of stand-alone cages and cages in combination with
2.
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posterior fixation: a literature review. Eur Spine J. 2000; 9: S095–S101. Phillips FM, Cunningham B, Carandang G, Ghanayem AJ, Voronov L, Havey RM, Patwardhan AG. Effect of supplemental translaminar facet screw fixation on the stability
preloads. Spine. 2004; 29: 1731–1736.
Tsantrizos A, Andreou A, Aebi M, Steffen T. Biomechanical stability of five stand-alone
M AN U
3.
SC
of stand-alone anterior lumbar interbody fusion cages under physiologic compressive
anterior lumbar interbody fusion constructs. Eur Spine J. 2000; 9: 14–22. 4.
Fogel GR, Parikh RD, Ryu SI, Turner AW. Biomechanics of lateral lumbar interbody
TE D
fusion construts with lateral and posterior plate fixation: Laboratory investigation. J Neurosurg Spine. 2014; 20(3): 1-7. 5.
Cappuccino A, Cornwall GB, Turner AW, Fogel GR, Duong HT, Kim KD, Brodke DS.
6.
AC C
S361–S367.
EP
Biomechanical analysis and review of lateral lumbar fusion constructs. Spine. 2010; 35:
Laws CJ, Coughlin DG, Lotz JC, Laws CJ, Coughlin DG, Lotz JC, Serhan HA, Hu SS. Direct lateral approach to lumbar fusion is a biomechanically equivalent alternative to the anterior approach: an in vitro study. Spine. 2012; 37: 819–825.
7.
Perez-Orribo L, Kalb S, Reyes PM, Chang SW, Crawford NR. Biomechanics of lumbar cortical screw-rod fixation versus pedicle screw-rod fixation with and without interbody support. Spine. 2013; 38: 635–641. 17
ACCEPTED MANUSCRIPT Zhang
8.
Pimenta L, Turner AW, Dooley ZA, Parikh RD, Peterson MD. Biomechanics of lateral interbody spacers: going wider for going stiffer. Scientific World Journal.2012: 381814. Kornblum MB, Turner AW, Cornwall GB, Zatushevsky MA, Phillips FM. Biomechanical
RI PT
9.
evaluation of stand-alone lumbar polyether-ether-ketone interbody cage with integrated screws. Spine J. 2013; 13: 77–84.
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10. Slucky AV, Brodke DS, Bachus KN, Droge JA, Braun JT. Less invasive posterior fixation
Spine J. 2006; 6: 78-85.
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method following transforaminal lumbar interbody fusion: a biomechanical analysis.
11. Ambati DV, Wright EK, Lehman RA, Kang DG, Wagner SC, Dmitriev AE. Bilateral pedicle screw fixation provides superior biomechanical stability in transforaminal lumbar
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interbody fusion: a finite element study. Spine J. 2015; 15: 1812–1822. 12. Cole CD, McCall TD, Schmidt MH, Dailey AT. Comparison of low back fusion techniques: transforaminal lumbar interbody fusion (TLIF) or posterior lumbar interbody
EP
fusion (PLIF) approaches. Curr Rev Musculoskelet Med. 2009; 2: 118–126.
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13. Yang SD, Chen Q, Ding WY, Zhao JQ, Zhang YZ, Shen Y, Yang DL. Unilateral pedicle screw fixation with bone graft vs. bilateral pedicle screw fixation with bone graft or cage: a comparative study. Med Sci Monit. 2016; 22: 890–897. 14. Kim DH, Jeong ST, Lee SS. Posterior lumbar interbody fusion using a unilateral single cage and a local morselized bone graft in the degenerative lumbar spine. Clin Orthop Surg. 2009; 1: 214–221. 15. Melnyk AD, Wen TL, Kingwell SF, Chak JD, Singh V M, Cripton PA, et al. Load 18
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transfer characteristics between posterior spinal implants and the lumbar spine under anterior shear loading: an in vitro investigation. Spine. 2012; 37: E1126–E1133.
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16. Jones AC, Arns CH, Sheppard AP, Hutmacher DW, Milthorpe BK, Knackstedt MA. Assessment of bone ingrowth into porous biomaterials using MICRO-CT. Biomaterials. 2007; 28:2491–2504.
SC
17. Lee YH, Chung CJ, Wang CW, Peng YT, Chang CH, Chen CH, et al. Computational comparison of three posterior lumbar interbody fusion techniques by using porous
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titanium interbody cages with 50% porosity. Comput Biol Med. 2016; 71: 35–45. 18. Kang H, Hollister SJ, Marca FL, Park P, Lin CY. Porous biodegradable lumbar interbody fusion cage design and fabrication using integrated global local topology optimization
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with laser sintering. J Biomech Eng. 2013; 135:101013.
19. Tsai PI, Hsu CC, Chen SY. Biomechanical investigation into the structural design of porous additive manufactured cages using numerical and experimental approaches.
EP
Comput Biol Med. 2016; 76: 14–23.
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20. Lewis G. Properties of open-cell porous metals and alloys for orthopaedic applications. J Mater Sci: Mater Med. 2013; 24: 2293–2325. 21. Wauthle R, Stok JVD, Yavari SA. Additively manufactured porous tantalum implants. Acta Biomaterialia. 2015; 14:217. 22. Campoli G, Borleffs MS, Yavari SA. Mechanical properties of open-cell metallic biomaterials manufactured using additive manufacturing. Mater Design. 2013; 49: 957-965. 19
ACCEPTED MANUSCRIPT Zhang
23. Wang L, Kang JF, Sun CN, Li DC, Cao Y, Jin ZM. Mapping porous microstructures to yield desired mechanical properties for application in 3D printed bone scaffolds and
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orthopaedic implants. Mater Design. 2017; 133: 62-68. 24. ISO 13314:2011. Mechanical testing of metals -- Ductility testing -- Compression test for porous and cellular metals. https://www.iso.org/standard/53669.html.
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25. Shirazi-Adl A, Ahmed AM, Shrivastava SC. Mechanical response of a lumbar motion segment in axial torque alone and combined with compression. Spine. 1986; 11(9):
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914-927.
26. Zhong ZC, Wei SH, Wang JP, Feng CK, Chen CS, Yu CH. Finite element analysis of the lumbar spine with a new cage using a topology optimization method. Med Eng Phys.
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2006; 28: 90-98.
27. Schmidt H, Heuer F, Drumm J, Klezl Z, Claes L, Wilke HJ. Application of a calibration method provides more realistic results for a finite element model of a lumbar spinal
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segment. Clin Biomech. 2007; 22: 377-384.
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28. Dreischarf M, Zander T, Shirazi-Adl A, Puttlitz CM, Adam CJ, Chen CS, et al. Comparison of eight published static finite element models of the intact lumbar spine: Predictive power of models improves when combined together. J Biomech. 2014; 47: 1757-1766.
29. Shirazi-Adl A. biomechanics of the lumbar spine in sagittal/lateral moments. Spine. 1994; 19: 2407-2414. 30. Ayturk UM, Puttlitz CM. Parametric convergence sensitivity and validation of a finite 20
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element model of the human lumbar spine. Comput Method Biomec. 2011; 14: 695-705. 31. Faizan A, Kiapour A, Kiapour AM, Goel VK. Biomechanical analysis of various
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footprints of transforaminal lumbar interbody fusion devices. J Spinal Disord Tech. 2014; 27: E118-E127.
32. Xiao ZT, Wang LY, Gong H , Zhu D. Biomechanical evaluation of three surgical
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scenarios of posterior lumbar interbody fusion by finite element analysis. Biomed Eng Online. 2012; 11: 31.
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33. Vadapalli S, Sairyo K, Goel VK, Robon M, Biyani A, Khandha A, et al. Biomechanical rationale for using polyetheretherketone (PEEK) spacers for lumbar interbody fusion-A finite element study. Spine. 2006; 31: 992-998.
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34. Chosa E, Goto K, Totoribe K, Tajima N. Analysis of the effect of lumbar spine fusion on the superior adjacent intervertebral disk in the presence of disk degeneration, using the three-dimensional finite element method. J Spinal Disord Tech. 2004; 17: 134-139.
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35. Schmidt H, Galbusera F, Rohlmann A, Zander T, Wilke HJ. Effect of multilevel lumbar
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disc arthroplasty on spine kinematics and facet joint loads in flexion and extension: a finite element analysis. Eur Spine J. 2012; 21: S663-S674. 36. Berkson MH, Nachemson A, Schultz AB. Mechanical properties of human lumbar spine motion segments—Part II: responses in compression and shear; influence of gross morphology. J Biomech Eng. 1979; 101: 53-57. 37. Brinckmann P, Grootenboer H. Change of disc height, radial disc bulge, and intradiscal pressure from discectomy. An in vitro investigation on human lumbar discs. Spine. 1991; 21
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16: 641-646. 38. Rohlmann A, Neller S, Claes L, Bergmann G, Wilke HJ. Influence of a follower load on
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intradiscal pressure and intersegmental rotation of the lumbar spine. Spine. 2001; 26: E557-561.
39. Choi J, Shin D, Kim S. Biomechanical effects of the geometry of ball-and-socket
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artificial disc on lumbar spine: A finite element study. Spine. 2017; 42: E332-E339.
40. Beaubien BP, Derincek A, Lew WD, Wood KB. In vitro, biomechanical comparison of an
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anterior lumbar interbody fusion with an anteriorly placed, low-profile lumbar plate and posteriorly placed pedicle screws or translaminar screws. Spine. 2005; 30: 1846-1851. 41. N Yan, TS Hou. Porosity of biomaterials and bone ingrowth. Chinese Journal of
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Biomedical Engineering. 2008; 27: 611-614.
42. Kim KT, Lee SH, Suk KS, Lee JH, Jeong B. Biomechanical changes of the lumbar segment after total disc replacement: charite(r), prodisc(r) and maverick(r) using finite
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element model study. J Korean Neurosurg Soc. 2010; 47: 446-453.
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43. Grauer JN, Biyani A, Faizan A, Kiapour A, Sairyo K, Ivanov A, et al. Biomechanics of two-level Charité artificial disc placement in comparison to fusion plus single-level disc placement combination. Spine J. 2006; 6: 659-666.
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Cross section Poisson ratio
(MPa)
Reference
0.3
—
25,26
Cancellous bone
100
0.2
—
25,26
Posterior bone
3500
0.25
—
25,26
Endplate
4000
0.3
—
27
4.2
0.45
—
25,26
1
0.49
20
0.3
Annulus ground Annulus pulposus
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12,000
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Cortical bone
Area (mm2)
—
28,30
63.7
26,31
0.3
20
26,31
0.3
40
26,31
0.3
40
26,31
15
0.3
30
26,31
58.7
0.3
3.6
26,31
32.9
0.3
60
26,31
3,500
0.3
—
32,33
110,000
0.3
—
32,34
Cage (Porosity 65%)
2,653
0.3
—
Mechanical Test
Cage (Porosity 75%)
1,551
0.3
—
Mechanical Test
Cage (Porosity 80%)
675
0.3
—
Mechanical Test
110,000
0.3
—
32,34
ligament Posterior longitudinal 20 ligament 19.5
Interspinous ligament
11.6
Supraspinous ligament Transverse ligament
Cage (PEEK)
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Cage (TI)
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Capsular ligament
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Ligamentum flavum
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Anterior longitudinal
Pedicle screws (Titanium alloy)
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Table 2. Variation of the maximum stress in the L3 bottom endplate after interbody fusion with respect to intact model.
Intact
1.00
1.00
TI
2.36
4.59
PPT65
2.45
4.75
PPT75
2.46
4.77
PPT80
2.48
4.79
PEEK
1.60 *
FPT65
1.49 *
FPT75
1.30 *
FPT80
1.00 *
Bending-L
Rotation-L
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Extension
1.00
1.00
3.19
1.65 *
3.26
1.79 *
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Flexion
3.26
1.80 *
3.27
1.82 *
2.43
2.51
1.10 *
2.22
2.37
1.02 *
1.79 *
2.06
0.87 *
1.16 *
1.54 *
0.74 *
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Endplate stress
TI: solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous titanium.
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* Endplate stress increased by < 2-fold (The endplate stresses were normalized to the intact stress data).
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Figure 1. Finite element model of the intact lumbar spine.
Figure 2. Mechanical test for the porous materials with different porosities: test
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samples with different porosities (a); mechanical testing machine (b); test results for
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each group of samples (c).
Figure 3. Finite element models of lateral lumbar interbody fusion with various cages. (The dimensions of TI cage, PEEK cage, and FPT cages are the same as PPT cages. TI: solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully
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porous titanium)
Figure 4. Predicted total ROM (a) and load-deflection curves (b) under pure moments,
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and L4-L5 compression-displacement curves (c) and IDP (d) under axial compression.
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Figure 5. ROM at surgical level for various cages in four motion modes. (TI: solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous titanium)
Figure 6. Maximum stress in the cage for various cages in four motion modes (a) and contour plots of Von Mises stress in porous cages (b). (TI: solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous titanium)
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Figure 7. Maximum stress in the L3 bottom endplate for various cages in four motion modes (a) and contour plots of Von Mises stress in endplate for various cages (b). (TI:
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solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous titanium)
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Figure 8. FJF at surgical level for various cages in four motion modes. (TI: solid
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titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous
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titanium)
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with PEEK cage, the porous cages reduced the cage stress and endplate stress in all motion modes. Compared among porous cages, the stresses in cage and endplate decreased with increasing porosity, while ROM increased with increasing porosity.
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Fully porous cages may offer an alternative to PEEK cages in lateral lumbar interbody
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fusion, whereas it is prudent to further increase the porosity of cage.
ACCEPTED MANUSCRIPT Abbreviations and Acronyms AM: Additive manufactured FE: Finite element
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FEA: Finite element analysis FJF: Facet joint force FPT: Fully porous titanium
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IDP: Intervetebral disc pressure
ROM: Range of motion
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TI: Titanium
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PPT: Partially porous titanium
S1878-8750(17)32243-X
DOI:
10.1016/j.wneu.2017.12.127
Reference:
WNEU 7140
To appear in:
World Neurosurgery
Received Date: 13 November 2017 Revised Date:
18 December 2017
Accepted Date: 19 December 2017
Please cite this article as: Zhang Z, Li H, Fogel GR, Liao Z, Li Y, Liu W, Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: a Finite Element Analysis, World Neurosurgery (2018), doi: 10.1016/j.wneu.2017.12.127. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Biomechanical analysis of porous additive manufactured cages for lateral lumbar interbody fusion: A finite element analysis
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Zhenjun Zhang1,2, Hui Li3, Guy R. Fogel4, Zhenhua Liao2, Yang Li1,2, Weiqiang Liu1,2 1
Department of Mechanical Engineering, Tsinghua University, Beijing, China
2
Biomechanics and Biotechnology Lab, Research Institute of Tsinghua University in
Naton Science and Technology Group, Beijing, China
4
Spine Pain Begone Clinic, San Antonio, TX, USA
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3
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Shenzhen, Shenzhen, China
Corresponding author:
Weiqiang Liu, Tsinghua University, Haidian District, Beijing, P.R.China; Tel: +86-0755-26551376 Fax: +86-0755-26551380
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E-mail: [email protected]
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This study was supported by the Industry Public Technology Service Platform Project of Shenzhen [Grant Number SMJKPT20140417010001] and the Science and Technology Plan
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Basic Research Project of Shenzhen [Grant Number JCYJ20151030160526024].
No relevant financial activities outside the submitted work.
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ABSTRACT Background
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A porous additive manufactured (AM) cage may provide stability similar to that of traditional solid cages, and may be beneficial to bone ingrowth. The biomechanical influence of various porous cages on stability, subsidence, stresses in cage, and facet contact force has not been
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fully described. The purpose of this study was to verify biomechanical effects of porous AM cages.
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Methods
The surgical FE models with various cages were constructed. The partially porous titanium (PPT) cages and fully porous titanium (FPT) cages were applied. The mechanical parameters
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of porous materials were obtained by mechanical test. Then the porous AM cages were compared to solid titanium (TI) cage and solid PEEK cage. The four motion modes were
were compared.
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Results
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simulated. Range of motion (ROM), cage stress, endplate stress, and facet joint force (FJF)
For all the surgical models, ROM decreased by more than 90%. Compared with TI and PPT cages, PEEK and FPT cages substantially reduced the maximum stresses in cage and endplate in all motion modes. Compared with PEEK cages, the stresses in cage and endplate for FPT cages decreased, whereas the ROM increased. Compared among FPT cages, the stresses in cage and endplate decreased with increasing porosity, whereas ROM increased with increasing porosity. After interbody fusion, FJF was substantially reduced in all motion 2
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modes except for flexion. Conclusions
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Fully porous cages may offer an alternative to solid PEEK cages in lateral lumbar interbody fusion. However, it may be prudent to further increase the porosity of cage.
Key words: biomechanics; facet joint force (FJF); finite element analysis (FEA); lumbar
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interbody fusion; porous cage; range of motion (ROM); subsidence.
Introduction
Lumbar interbody fusion has been used in the treatment of lumbar disease such as spondylolisthesis, trauma, and degenerative disc degeneration. Successful clinical outcomes
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depend on fusion healing. The best opportunity for healing may be the anterior interbody fusion with better loading of the graft and the largest surface area for fusion.1-4
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Laterally-inserted interbody cages may decrease ROM compared to other cages.5-8 The interbody cage fusion improves the loading capacity of the anterior column.2,3 In addition,
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supplemental fixation favorably influences the healing of the lumbar fusion, so bilateral pedicle screw fixation is often used to supplement interbody cage because it can provide multiplanar stability. 4,5,9-11
Traditional solid cage made of titanium or PEEK has been widely used in lumbar fusion.12,13 However, solid cages have high mechanical stiffness, which may affect the loading mechanism of lumbar spine.14,15 A porous AM cage can reduce the mechanical stiffness, may 3
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provide stability similar to that of traditional solid cages, and may be beneficial to bone ingrowth.16 Currently there have been a variety of porous AM cages, such as Posterior Spine
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Truss SystemTM (4WEB Medical, Frisco, TX), CascadiaTM Lateral 3D Interbody System (K2M Group Holdings, Inc., Leesburg, VA), EndoLIF® O-Cage (joimax GmbH, Karlsruhe, Germany), Tritanium® PL Posterior Lumbar Cage (SYK CORP, Kalamazoo, MI), and 3D
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ACT (AK Medical, Beijing, China).
Although some factors such as lumbar instability, bone quality of vertebral trabecular
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and endplate, and previous operation history are known to affect the choice of cage, the biomechanics of cages made of various materials are not fully investigated. Compared with solid cage, partially porous titanium (PPT) cage and fully porous titanium (FPT) cage have a
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porous structure, along with rough surfaces, to allow for bony integration throughout the implant. Some previous studies have investigated the mechanical performance of porous AM cages and evaluated their biomechanics. Lee et al17 developed a porous cage with 50%
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porosity and compared three lumbar fusion techniques by using L3-L4 bone-implant model.
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Kang et al18 investigated the porous biodegradable lumbar interbody fusion cage using a finite element model of mini-pig L2-L5 lumbar spine. Additionally, Tsai et al19 in their recent numerical and experimental study found that porous AM cages with a porosity of between 69% and 80% could provide better biomechanical performances. However, the influence of the various porous cages (solid cage, PPT cage, and FPT cage) and their different porosities on lumbar stability, cage stress, endplate stress, and FJF has not been fully described. Using a FE model to systematically study the biomechanical effects of various cages 4
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may be valuable. Further information to be determined is the influence of various porous structures on the subsidence, which is induced by the changed stiffness in the index disc
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space associated with the interbody cage and supplemented fixation. The aim of this study was to evaluate the lateral lumbar interbody fusion cages by comparing the biomechanical properties of porous cages with that of solid cages, and to verify the biomechanical effects of
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cage porosity.
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Materials and Methods
CT images of intact lumbar spine with interval of 0.7 mm were obtained from a 36 year old woman (weight 52 kg, height 158 cm, excluded from lumbar disease based on visual and radiographic examination). A total of 492 CT images were imported into Mimics (Materialise
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Inc, Leuven, Belgium). In Mimics, the 3D geometry structure was constructed, which consists of vertebrae, intervertebral disc and cartilage endplate. The geometric structure was meshed using Hypermesh (Altair Technologies Inc, Fremont, CA). Lastly, the mesh model
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was imported into Abaqus (Simulia Inc, Providence, RI) to perform FEA. The computer for
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the simulation is ThinkStation (Lenovo, Beijing, China) configured with 24 processors and 64 GB memory.
Figure 1 displayed the FE model of intact lumbar spine L1-L5. The vertebral body was divided into three parts: cortical bone, cancellous bone, and posterior bone. The intervertebral disc was divided into nucleus pulposus and annulus ground. The intact model included 7 kinds of ligaments: anterior longitudinal ligament, posterior longitudinal ligament, ligamenta 5
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flava, interspinal ligament, supraspinal ligament, intertransverse ligament, and capsular ligament. The cortical bone was 1.0 mm thick, and the endplate was 0.5 mm thick. All the
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ligaments were modeled with truss elements (T3D2) which had the tension-only property. The FE model was meshed using the 3D tetrahedral elements except for the ligaments. 195,533 nodes and 841,038 elements were contained in the intact model, which could
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effectively eliminate the influence of meshing on the accuracy of calculation.
The partially porous titanium (PPT) cage was configured as both solid and porous
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structures. The outer part was solid titanium structure, and the inner part is porous titanium structure. The porosity (vol%) and pore size (µm) was chosen according to the previous literatures.19,20 There were three kinds of PPT cages: PPT cage with 65% porosity (PPT65),
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PPT cage with 75% porosity (PPT75), and PPT cage with 80% porosity (PPT80). The fully porous titanium (FPT) cage was configured as porous structure. There were three kinds of FPT cages: FPT cage with 65% porosity (FPT65), FPT cage with 75% porosity (FPT75), and
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FPT cage with 80% porosity (PPT80). The average pore size of all the porous cages was
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350-400 µm. Including solid titanium (TI) cage and solid PEEK (PEEK) cage, there were 8 kinds of cages. All the cage models were 30 mm long, 10 mm wide and 7 mm high. The PEEK cage was made of polyetheretherketone (PEEK), and other cages were made of titanium alloy (Ti6Al4V). The bilateral pedicle screw system was modeled based on EXPEDIUM® 5.5 System (DePuy Synthes Spine, Inc, Raynham, MA). The diameter of pedicle screw was 5.5 mm. The material of pedicle screws was titanium alloy (Ti6Al4V). Figure 2 showed the mechanical test for the porous materials with different porosities. 6
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The static compression testing was carried out according to the ISO13314:2011 standard (Compression test for porous and cellular metals) by using a universal mechanical testing
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machine.21-24 The machine for the compression testing was Instron 8874 (Naton, Beijing, China). The average pore size of all the test samples was 350-400 um. The porosities of the three groups of test samples (5 per group) were 65%, 75%, and 80%. The test results were as
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follows: for the samples with 65% porosity, the average Young's modulus was 2653 MPa and the average plateau stress was 57.2 MPa; for the samples with 75% porosity, the average
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Young's modulus was 1551 MPa and the average plateau stress was 30 MPa; for the samples with 80% porosity, the average Young's modulus was 675MPa and the average plateau stress was 19.1 MPa. The test results were comparable to the previous literature.20 The Young's
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modulus and the plateau stress decreased with increasing porosity. The material properties of components were shown in Table 1.25-34
To validate the intact FE model, the analysis study included two steps of simulation. The
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predicted results were compared with the previous experimental data. The contact surfaces of
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vertebrae and discs were set as tie constraints. The contact between the facet joints was simulated as frictionless surfaces.25-30 Firstly, the range of motion (ROM) of intact lumbar spine L1-L5 under pure moment was predicted. Three different moments (2.5 Nm, 5.0 Nm, and 7.5 Nm) were applied to the upper surface of L1 while the bottom of L5 was fixed. The ROM of the lumbar spine was compared with previous in vitro results.28,29,35 Then the compression displacement and intervetebral disc pressure (IDP) of the motion segment L4-L5 under pure compression were calculated. The upper surface of L4 was loaded with four 7
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preload values (100 N, 200N, 300 N, and 400 N) as described by Berkson et al.36 The compression displacement and IDP of L4-L5 were compared with previous results.28,36,37
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For simulation of the surgical models, the segment L2-L5 was chosen to predict the biomechanics changes of surgical level. The surgical conditions were as follows: the interbody cage was inserted at the L3-L4 disc space laterally, and supplemented with bilateral
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pedicle screw fixation. The FE models of interbody fusion with various cages were shown in Figure 3. All the surgical FE models were constructed based on the validated intact model.
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The surface contact between the vertebrae and discs, and the contact between the facet joints were consistent with that of the validated model. The interfaces of vertebrae and cages were also assigned to tie constraints. The bottom of L5 was fixed in all directions. The
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compressive load of 280 N and the moment of 7.5 Nm were applied to the upper surface of L2 as in previous literature.38,39 The compressive load of 280 N corresponded to the partial weight of a human body, and the moment of 7.5 Nm simulated the motion modes occurred in
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different conditions such as flexion, extension, lateral bending, and axial rotation.
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Considering the symmetry of the sagittal plane, this study simulated the biomechanical properties of surgical FE models in four motion modes: flexion, extension, bending-left, and rotation-left. The ROM, cage stress, endplate stress, and facet joint force (FJF) were analyzed and exported. The predict results of porous cages were compared with that of solid cages. The ROM data were normalized to the intact ROM data. Under the combined loading, the intact L2-L5 model was recalculated. In total, 36 simulation calculations for nine models and four motion modes were performed. Simulation results were in accordance with the 8
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requirements of visualization, and mechanics data was expressed using Von Mises stress contours.
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Results Model validation
Under the pure moment of 7.5 Nm, the L1-L5 ROM was within the range of the
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previous FE and in vitro experimental studies.28,35 The total movement angles were 24.75°in
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flexion-extension, 26.66°in lateral bending, and 18.43°in axial rotation. As was displayed in Figure 4a, all values in different conditions were within the FE and in vitro ranges. The load-deflection curves were shown in Figure 4b, which were comparable to the existing results of previous studies.28,29,35 The rotation angles were 15.04°in flexion, 9.71°in
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extension, 13.45°in bending-left, and 10.33°in rotation-left.
The compression-displacement curves were shown in Figure 4c, which indicated that the axial displacement of L4-L5 segment increased almost linearly with the applied axial
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compressive loading. The results were comparable to the previous in vitro experimental
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study36. The compression-IDP curves were displayed in Figure 4d, which were also compared with the previous FE and in vitro experimental studies.28,37 The predicted results demonstrated that IDP of L4-L5 segment increased almost linearly with the applied axial compressive loading.
Range of motion (ROM)
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Under the combined loading of 280 N and 7.5 Nm, the ROM of surgical models was shown in Figure 5. After inserting the interbody cage, the predicted ROMs for all surgical
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models decreased by more than 90% compared with the intact case. ROMs for three PPT cages were slightly more than that for TI cage, and the porosity of PPT cage did not substantially alter ROM in all motion modes. ROMs for three FPT cages were more than that
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for PEEK cage, and they increased with increasing porosity. Compared among all the surgical models, ROM for TI cage was the minimum and ROM for FPT80 was the maximum
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in all motion modes. Compared with PEEK cage, ROMs for FPT80 increased by 2.55% in flexion, 3.43% in extension, 2.62% in bending-left, and 2.22% in rotation-left, respectively. Cage stress
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The maximum stress in cage (cage stress) was displayed in Figure 6a. Cage stresses for three PPT cages were more than that for TI cage, and the porosity of PPT cage did not substantially alter cage stress in all motion modes. Cage stresses for three FPT cages were
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less than that for PEEK cage, and they decreased with increasing porosity. Compared among
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all the surgical models, cage stress for PPT80 was the maximum and cage stress for FPT80 was the minimum in all motion modes. Compared with PEEK cage, cage stress for FPT80 was reduced by 36.18% in flexion, 49.55% in extension, 33.84% in bending-left, and 28.62% in rotation-left, respectively. In order to compare the stress distribution of the two porous structures, the contour plots of Von Mises stress in PPT75 and FPT75 cages were shown in Figure 6b. The PPT cage may result in a stress shielding effect because of the configuration 10
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of both solid and porous structure. In contrast, the stress for FPT cage was more evenly distributed in the cage in all motion modes because of the fully porous structure.
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Endplate stress Figure 7a depicts the maximum stress in the L3 bottom endplate (endplate stress). After interbody fusion, the predicted endplate stress increased in all motion modes except for
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FPT75 and FPT80 in rotation-left. Endplate stresses for three PPT cages were more than that
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for TI cage, and the porosity of PPT cage did not substantially alter endplate stress in all motion modes. Endplate stresses for three FPT cages were less than that for PEEK cage, and they decreased with increasing porosity. Compared among all the surgical models, endplate stress for PPT80 cage was the maximum and endplate stress for FPT80 was the minimum in
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all motion modes. Compared with PEEK cage, endplate stress for FPT80 was reduced by 37.63% in flexion, 52.40% in extension, 38.51% in bending-left, and 33.12% in rotation-left, respectively. Compared with the intact case, endplate stress for FPT80 remained the same in
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flexion, increased by 15.75% in extension, increased by 54.48% in bending-left, and
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decreased by 26.26% in rotation-left, respectively. The contour plots of Von Mises stress in the L3 bottom endplate for PPT75 and FPT75 cages were shown in Figure 7b. The PPT cage may result in a stress shielding effect, whereas the endplate stress for FPT cage was more evenly distributed in the endplate in all motion modes. Facet joint force (FJF)
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In Figure 8, the FJF of intact and surgical models were displayed. After interbody fusions, FJF at surgical level L3-L4 decreased substantially in all motion modes except for
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flexion. Compared among all the surgical models, FJF for various cages were not substantially changed in all motion modes except for rotation. Compared with TI cage, FJF for PPT cage was not substantially changed in all motion modes. Compared with PEEK cage,
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FJF for FPT80 was reduced by 7.57% in flexion, 11.90% in extension, and 13.13% in
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bending-left, respectively, whereas it increased by 19.24% in rotation-left. Discussion
We employed a FE model to study the biomechanical effect of various cages on ROM, stresses in cage and endplate, and FJF in four loading modes (flexion, extension, bending-left,
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and rotation-left). Such a finite element investigation is not feasible in a cadaver model because of inability to estimate the stresses in various spinal structures.33 The predicted ROMs with various cages were comparable to the previous studies.4,5,10,40 In addition, the
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current study showed the cage stress, endplate stress, and FJF. As was displayed in Figure 5,
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the ROM data were normalized to the intact ROM data. The predicted ROMs for all surgical models were less than 10% of the intact model in all motion modes. That is the maximum stiffness or stability that the cage may provide for the lumbar spine. However, ROM tends to increase further with increasing porosity because of the lower stiffness and greater deformation of cages. As was displayed in Figure 6 and Figure 7, stresses in cage and endplate were sensitive to the various cages. Compared with TI and PPT cages, PEEK and 12
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FPT cages substantially reduced the maximum stresses in cage and endplate in all motion modes. Compared with PEEK cages, the stresses in cage and endplate for FPT cages
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decreased, and the stresses decreased with increasing porosity of cages. The variations of the maximum stresses in the L3 bottom endplate after interbody fusion were listed in Table 2. The endplate stresses were normalized to the intact stress data. Compared with the intact
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model, the endplate stress for PPT80 increased by less than 2-fold in all motion modes. As was displayed in Figure 8, FJF at surgical level decreased substantially compared to the intact
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conditions after inserting the cage at L3-L4 in all motion modes except for flexion. FJF was not substantially changed with various cages except for rotation-left. The ROM, cage stress, and endplate stress at surgical level were directly affected in all
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motion modes, and changed with various cages. By comparing the biomechanics of interbody fusion using different cages, it was shown that FPT cage displayed some advantages, such as the minimum cage stress and the minimum endplate stress in all motion modes. The FPT
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cage reduced the cage stress and endplate stress, which may decrease the risk of subsidence
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of the cages into the endplate and the adjoining vertebral bone over time.33 It was indicated that in performing lateral lumbar interbody fusion, the FPT cage may offer an alternative to PEEK cage.
In the current study, the porous cages (PPT and FPT) were compared with solid cages (TI and PEEK). The Young's modulus of solid titanium was much larger than that of the porous titanium. As was predicted in Figure 6 and Figure 7, PPT cage may result in a stress shielding effect because of the configuration of both solid and porous structure. FPT cage has 13
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advantages in cage stress and endplate stress, which may lead to a smaller risk of subsidence.33 In addition, FPT cage may be lead to better fusion healing because the porous
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structure was beneficial to bone ingrowth.16 According to Figure 5, although ROMs for all the cages decreased by more than 90% compared with the intact case, ROM for FPT cage increased with increasing porosity. Higher porosity may result in greater bone ingrowth but
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diminish mechanical properties.41 Further increased ROM may reduce the stability to affect the healing response of the fusion.4 Clinically, the cages with higher porosity may have better
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bone ingrowth into the cage structure and an enhance bone fusion healing. The improved bone healing could be a reasonable exchange for increased ROM of porous cages. In summary, the porous AM cage may offer an alternative to PEEK cages in lateral lumbar
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interbody fusion, whereas it may be prudent to further increase the porosity of cage. There are some limitations in the present study, such as validating the model by comparing it with other models, using a unique lumbar model, simplifying the material
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properties of some tissues, and ignoring the role of muscles. One limitation is that the
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validation of the model is by comparing it to other models, and not to a biological model. This intact spine model was scanned from a healthy volunteer, and was validated by comparing it to the other FE models and in vitro models. The experimental studies on a biological model are useful to further validate the current FE model. Another limitation is the finite element analysis standard technique of using a unique lumbar model. The geometric model of lumbar spine varies from person to person such as the intervertebral disc space and the gaps between facet joints. But only one model of lumbar spine was chosen in this study. 14
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Furthermore, the material properties of human tissues are poorly understood. In the present model, the material properties were simplified as linear elastic though the components of
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lumbar spine are nonlinear in reality. However, many FEA on lumbar spine have assumed that the components of spine were linear in order to improve the calculation efficiency.26,33,39,42,43 In addition, the muscles were not considered in the present study
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although the muscles play an important role in supporting the stability of lumbar spine. However, the tendency of predicted results with various cages would not be substantially
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changed depending on the individual geometric model, simplified material properties, and model of the muscles.
In conclusion, the predicted results showed that the structure of porous cage can affect
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the biomechanics of lumbar interbody fusion noticeably. Compared among the surgical models with different cages, a porous AM cage showed advantages in cage stress and endplate stress. Compared with PEEK cage, the porous cage has some advantages in
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biomechanics and may lead to better fusion healing in clinical practice. Further clinical
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studies on the effect of porous cages on stability, subsidence, and other clinically relevant parameters are necessary to validate the observations of this finite element study. Acknowledgements
This work was supported by the Industry Public Technology Service Platform Project of Shenzhen [grant number SMJKPT20140417010001] and the Science and Technology Plan Basic Research Project of Shenzhen [grant number JCYJ20151030160526024]. 15
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Conflict of interest
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The authors declare that they have no conflict of interest.
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References 1.
Oxland TR, Lund T. Biomechanics of stand-alone cages and cages in combination with
2.
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posterior fixation: a literature review. Eur Spine J. 2000; 9: S095–S101. Phillips FM, Cunningham B, Carandang G, Ghanayem AJ, Voronov L, Havey RM, Patwardhan AG. Effect of supplemental translaminar facet screw fixation on the stability
preloads. Spine. 2004; 29: 1731–1736.
Tsantrizos A, Andreou A, Aebi M, Steffen T. Biomechanical stability of five stand-alone
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3.
SC
of stand-alone anterior lumbar interbody fusion cages under physiologic compressive
anterior lumbar interbody fusion constructs. Eur Spine J. 2000; 9: 14–22. 4.
Fogel GR, Parikh RD, Ryu SI, Turner AW. Biomechanics of lateral lumbar interbody
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fusion construts with lateral and posterior plate fixation: Laboratory investigation. J Neurosurg Spine. 2014; 20(3): 1-7. 5.
Cappuccino A, Cornwall GB, Turner AW, Fogel GR, Duong HT, Kim KD, Brodke DS.
6.
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S361–S367.
EP
Biomechanical analysis and review of lateral lumbar fusion constructs. Spine. 2010; 35:
Laws CJ, Coughlin DG, Lotz JC, Laws CJ, Coughlin DG, Lotz JC, Serhan HA, Hu SS. Direct lateral approach to lumbar fusion is a biomechanically equivalent alternative to the anterior approach: an in vitro study. Spine. 2012; 37: 819–825.
7.
Perez-Orribo L, Kalb S, Reyes PM, Chang SW, Crawford NR. Biomechanics of lumbar cortical screw-rod fixation versus pedicle screw-rod fixation with and without interbody support. Spine. 2013; 38: 635–641. 17
ACCEPTED MANUSCRIPT Zhang
8.
Pimenta L, Turner AW, Dooley ZA, Parikh RD, Peterson MD. Biomechanics of lateral interbody spacers: going wider for going stiffer. Scientific World Journal.2012: 381814. Kornblum MB, Turner AW, Cornwall GB, Zatushevsky MA, Phillips FM. Biomechanical
RI PT
9.
evaluation of stand-alone lumbar polyether-ether-ketone interbody cage with integrated screws. Spine J. 2013; 13: 77–84.
SC
10. Slucky AV, Brodke DS, Bachus KN, Droge JA, Braun JT. Less invasive posterior fixation
Spine J. 2006; 6: 78-85.
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method following transforaminal lumbar interbody fusion: a biomechanical analysis.
11. Ambati DV, Wright EK, Lehman RA, Kang DG, Wagner SC, Dmitriev AE. Bilateral pedicle screw fixation provides superior biomechanical stability in transforaminal lumbar
TE D
interbody fusion: a finite element study. Spine J. 2015; 15: 1812–1822. 12. Cole CD, McCall TD, Schmidt MH, Dailey AT. Comparison of low back fusion techniques: transforaminal lumbar interbody fusion (TLIF) or posterior lumbar interbody
EP
fusion (PLIF) approaches. Curr Rev Musculoskelet Med. 2009; 2: 118–126.
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13. Yang SD, Chen Q, Ding WY, Zhao JQ, Zhang YZ, Shen Y, Yang DL. Unilateral pedicle screw fixation with bone graft vs. bilateral pedicle screw fixation with bone graft or cage: a comparative study. Med Sci Monit. 2016; 22: 890–897. 14. Kim DH, Jeong ST, Lee SS. Posterior lumbar interbody fusion using a unilateral single cage and a local morselized bone graft in the degenerative lumbar spine. Clin Orthop Surg. 2009; 1: 214–221. 15. Melnyk AD, Wen TL, Kingwell SF, Chak JD, Singh V M, Cripton PA, et al. Load 18
ACCEPTED MANUSCRIPT Zhang
transfer characteristics between posterior spinal implants and the lumbar spine under anterior shear loading: an in vitro investigation. Spine. 2012; 37: E1126–E1133.
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16. Jones AC, Arns CH, Sheppard AP, Hutmacher DW, Milthorpe BK, Knackstedt MA. Assessment of bone ingrowth into porous biomaterials using MICRO-CT. Biomaterials. 2007; 28:2491–2504.
SC
17. Lee YH, Chung CJ, Wang CW, Peng YT, Chang CH, Chen CH, et al. Computational comparison of three posterior lumbar interbody fusion techniques by using porous
M AN U
titanium interbody cages with 50% porosity. Comput Biol Med. 2016; 71: 35–45. 18. Kang H, Hollister SJ, Marca FL, Park P, Lin CY. Porous biodegradable lumbar interbody fusion cage design and fabrication using integrated global local topology optimization
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with laser sintering. J Biomech Eng. 2013; 135:101013.
19. Tsai PI, Hsu CC, Chen SY. Biomechanical investigation into the structural design of porous additive manufactured cages using numerical and experimental approaches.
EP
Comput Biol Med. 2016; 76: 14–23.
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20. Lewis G. Properties of open-cell porous metals and alloys for orthopaedic applications. J Mater Sci: Mater Med. 2013; 24: 2293–2325. 21. Wauthle R, Stok JVD, Yavari SA. Additively manufactured porous tantalum implants. Acta Biomaterialia. 2015; 14:217. 22. Campoli G, Borleffs MS, Yavari SA. Mechanical properties of open-cell metallic biomaterials manufactured using additive manufacturing. Mater Design. 2013; 49: 957-965. 19
ACCEPTED MANUSCRIPT Zhang
23. Wang L, Kang JF, Sun CN, Li DC, Cao Y, Jin ZM. Mapping porous microstructures to yield desired mechanical properties for application in 3D printed bone scaffolds and
RI PT
orthopaedic implants. Mater Design. 2017; 133: 62-68. 24. ISO 13314:2011. Mechanical testing of metals -- Ductility testing -- Compression test for porous and cellular metals. https://www.iso.org/standard/53669.html.
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25. Shirazi-Adl A, Ahmed AM, Shrivastava SC. Mechanical response of a lumbar motion segment in axial torque alone and combined with compression. Spine. 1986; 11(9):
M AN U
914-927.
26. Zhong ZC, Wei SH, Wang JP, Feng CK, Chen CS, Yu CH. Finite element analysis of the lumbar spine with a new cage using a topology optimization method. Med Eng Phys.
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2006; 28: 90-98.
27. Schmidt H, Heuer F, Drumm J, Klezl Z, Claes L, Wilke HJ. Application of a calibration method provides more realistic results for a finite element model of a lumbar spinal
EP
segment. Clin Biomech. 2007; 22: 377-384.
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28. Dreischarf M, Zander T, Shirazi-Adl A, Puttlitz CM, Adam CJ, Chen CS, et al. Comparison of eight published static finite element models of the intact lumbar spine: Predictive power of models improves when combined together. J Biomech. 2014; 47: 1757-1766.
29. Shirazi-Adl A. biomechanics of the lumbar spine in sagittal/lateral moments. Spine. 1994; 19: 2407-2414. 30. Ayturk UM, Puttlitz CM. Parametric convergence sensitivity and validation of a finite 20
ACCEPTED MANUSCRIPT Zhang
element model of the human lumbar spine. Comput Method Biomec. 2011; 14: 695-705. 31. Faizan A, Kiapour A, Kiapour AM, Goel VK. Biomechanical analysis of various
RI PT
footprints of transforaminal lumbar interbody fusion devices. J Spinal Disord Tech. 2014; 27: E118-E127.
32. Xiao ZT, Wang LY, Gong H , Zhu D. Biomechanical evaluation of three surgical
SC
scenarios of posterior lumbar interbody fusion by finite element analysis. Biomed Eng Online. 2012; 11: 31.
M AN U
33. Vadapalli S, Sairyo K, Goel VK, Robon M, Biyani A, Khandha A, et al. Biomechanical rationale for using polyetheretherketone (PEEK) spacers for lumbar interbody fusion-A finite element study. Spine. 2006; 31: 992-998.
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34. Chosa E, Goto K, Totoribe K, Tajima N. Analysis of the effect of lumbar spine fusion on the superior adjacent intervertebral disk in the presence of disk degeneration, using the three-dimensional finite element method. J Spinal Disord Tech. 2004; 17: 134-139.
EP
35. Schmidt H, Galbusera F, Rohlmann A, Zander T, Wilke HJ. Effect of multilevel lumbar
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disc arthroplasty on spine kinematics and facet joint loads in flexion and extension: a finite element analysis. Eur Spine J. 2012; 21: S663-S674. 36. Berkson MH, Nachemson A, Schultz AB. Mechanical properties of human lumbar spine motion segments—Part II: responses in compression and shear; influence of gross morphology. J Biomech Eng. 1979; 101: 53-57. 37. Brinckmann P, Grootenboer H. Change of disc height, radial disc bulge, and intradiscal pressure from discectomy. An in vitro investigation on human lumbar discs. Spine. 1991; 21
ACCEPTED MANUSCRIPT Zhang
16: 641-646. 38. Rohlmann A, Neller S, Claes L, Bergmann G, Wilke HJ. Influence of a follower load on
RI PT
intradiscal pressure and intersegmental rotation of the lumbar spine. Spine. 2001; 26: E557-561.
39. Choi J, Shin D, Kim S. Biomechanical effects of the geometry of ball-and-socket
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artificial disc on lumbar spine: A finite element study. Spine. 2017; 42: E332-E339.
40. Beaubien BP, Derincek A, Lew WD, Wood KB. In vitro, biomechanical comparison of an
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anterior lumbar interbody fusion with an anteriorly placed, low-profile lumbar plate and posteriorly placed pedicle screws or translaminar screws. Spine. 2005; 30: 1846-1851. 41. N Yan, TS Hou. Porosity of biomaterials and bone ingrowth. Chinese Journal of
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Biomedical Engineering. 2008; 27: 611-614.
42. Kim KT, Lee SH, Suk KS, Lee JH, Jeong B. Biomechanical changes of the lumbar segment after total disc replacement: charite(r), prodisc(r) and maverick(r) using finite
EP
element model study. J Korean Neurosurg Soc. 2010; 47: 446-453.
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43. Grauer JN, Biyani A, Faizan A, Kiapour A, Sairyo K, Ivanov A, et al. Biomechanics of two-level Charité artificial disc placement in comparison to fusion plus single-level disc placement combination. Spine J. 2006; 6: 659-666.
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Cross section Poisson ratio
(MPa)
Reference
0.3
—
25,26
Cancellous bone
100
0.2
—
25,26
Posterior bone
3500
0.25
—
25,26
Endplate
4000
0.3
—
27
4.2
0.45
—
25,26
1
0.49
20
0.3
Annulus ground Annulus pulposus
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12,000
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Cortical bone
Area (mm2)
—
28,30
63.7
26,31
0.3
20
26,31
0.3
40
26,31
0.3
40
26,31
15
0.3
30
26,31
58.7
0.3
3.6
26,31
32.9
0.3
60
26,31
3,500
0.3
—
32,33
110,000
0.3
—
32,34
Cage (Porosity 65%)
2,653
0.3
—
Mechanical Test
Cage (Porosity 75%)
1,551
0.3
—
Mechanical Test
Cage (Porosity 80%)
675
0.3
—
Mechanical Test
110,000
0.3
—
32,34
ligament Posterior longitudinal 20 ligament 19.5
Interspinous ligament
11.6
Supraspinous ligament Transverse ligament
Cage (PEEK)
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Capsular ligament
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Ligamentum flavum
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Anterior longitudinal
Pedicle screws (Titanium alloy)
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Table 2. Variation of the maximum stress in the L3 bottom endplate after interbody fusion with respect to intact model.
Intact
1.00
1.00
TI
2.36
4.59
PPT65
2.45
4.75
PPT75
2.46
4.77
PPT80
2.48
4.79
PEEK
1.60 *
FPT65
1.49 *
FPT75
1.30 *
FPT80
1.00 *
Bending-L
Rotation-L
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Extension
1.00
1.00
3.19
1.65 *
3.26
1.79 *
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Flexion
3.26
1.80 *
3.27
1.82 *
2.43
2.51
1.10 *
2.22
2.37
1.02 *
1.79 *
2.06
0.87 *
1.16 *
1.54 *
0.74 *
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Endplate stress
TI: solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous titanium.
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* Endplate stress increased by < 2-fold (The endplate stresses were normalized to the intact stress data).
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Figure 1. Finite element model of the intact lumbar spine.
Figure 2. Mechanical test for the porous materials with different porosities: test
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samples with different porosities (a); mechanical testing machine (b); test results for
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each group of samples (c).
Figure 3. Finite element models of lateral lumbar interbody fusion with various cages. (The dimensions of TI cage, PEEK cage, and FPT cages are the same as PPT cages. TI: solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully
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porous titanium)
Figure 4. Predicted total ROM (a) and load-deflection curves (b) under pure moments,
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and L4-L5 compression-displacement curves (c) and IDP (d) under axial compression.
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Figure 5. ROM at surgical level for various cages in four motion modes. (TI: solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous titanium)
Figure 6. Maximum stress in the cage for various cages in four motion modes (a) and contour plots of Von Mises stress in porous cages (b). (TI: solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous titanium)
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Figure 7. Maximum stress in the L3 bottom endplate for various cages in four motion modes (a) and contour plots of Von Mises stress in endplate for various cages (b). (TI:
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solid titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous titanium)
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Figure 8. FJF at surgical level for various cages in four motion modes. (TI: solid
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titanium; PEEK: solid PEEK; PPT: partially porous titanium; FPT: fully porous
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titanium)
ACCEPTED MANUSCRIPT Highlights We comprehensively compared the biomechanics of porous cages and solid cages, including ROM, cage stress, endplate stress, and facet joint force. Compared
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with PEEK cage, the porous cages reduced the cage stress and endplate stress in all motion modes. Compared among porous cages, the stresses in cage and endplate decreased with increasing porosity, while ROM increased with increasing porosity.
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Fully porous cages may offer an alternative to PEEK cages in lateral lumbar interbody
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fusion, whereas it is prudent to further increase the porosity of cage.
ACCEPTED MANUSCRIPT Abbreviations and Acronyms AM: Additive manufactured FE: Finite element
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FEA: Finite element analysis FJF: Facet joint force FPT: Fully porous titanium
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IDP: Intervetebral disc pressure
ROM: Range of motion
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TI: Titanium
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PPT: Partially porous titanium