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Biomechanical characterization of three iliac screw fixation techniques: A finite element study
Journal of Clinical Neuroscience xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Journal of Clinical Neuroscience journal homepage: www.elsevier.com/locate/jocn
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Biomechanical characterization of three iliac screw fixation techniques: A finite element study Seil Sohn a,1, Tae Hyun Park f,g,1, Chun Kee Chung b,c,d,e,⇑, Yongjung Jay Kim h, Jong Wuk Jang f, In-bo Han a, Sung Jae Lee g a
Department of Neurosurgery, CHA University, CHA Bundang Medical Center, Republic of Korea Department of Neurosurgery, Seoul National University College of Medicine, Republic of Korea Neuroscience Research Institute, Seoul National University Medical Research Center, Republic of Korea d Clinical Research Institute, Seoul National University Hospital, Republic of Korea e Department of Brain and Cognitive Sciences, Seoul National University College of Natural Sciences, Republic of Korea f R&D Center, Medyssey Co, Ltd, Jechon, Republic of Korea g Department of Biomedical Engineering, College of Biomedical Science & Engineering, Inje University, Gyeongnam, Republic of Korea h Department of Orthopedics, Columbia University, College of Physicians and Surgeons, New York, NY, USA b c
a r t i c l e
i n f o
Article history: Received 12 February 2018 Accepted 11 March 2018 Available online xxxx Keywords: Sacropelvic fixation Spinal deformities Spine S2 alar iliac fixation
a b s t r a c t We aim to characterize the biomechanical properties of a modified iliac screw fixation method compared with the classic iliac screw fixation and the S2 alar iliac screw (S2AI) fixation using a FEM. A three-dimensional, non-linear FEM of lumbosacral spine and pelvis (L1-pelvis) was modified to simulate 3 different iliac screw fixations based on posterior screw fusion. The peak von Mises stress (PVMS) values of the iliac screws in the 3 different iliac screw fixations were recorded in during flexion/extension/axial rotation/lateral bending. The interaction stress which arose between the screw head and the shaft of iliac screws, was also measured for each case. The PVMS values of the 3 different iliac screw fixation techniques were lower than the fatigue strength levels under physiological loadings. PVMS of iliac screws was observed in the screw shaft for S2AI, in the screw neck for the modified iliac screw technique, and in the offset connectors of the classic iliac screw technique. The interaction between the screw head and the neck was compressed in modified iliac screw fixation technique. On the other hand, distraction force was observed in the S2AI technique between the screw head and the screw shaft. This FEM study supports our previous clinical results, which found that the modified iliac screw fixation technique can be an effective alternative sacropelvic fixation technique comparable to the classic iliac screw and the S2AI technique. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction To overcome the complications associated with fusions ending at S1, several methods, such as sacropelvic fixation or the use of a S2 alar screw, have been introduced [1–6]. Modern instrumentation techniques allow for the insertion of screws into the ilium independently of the proximal construct and connection to longitudinal rods by offset connectors [7]. Their
⇑ Corresponding author at: Department of Neurosurgery, Seoul National University College of Medicine, 101 Daehak-no, Jongno-gu, Seoul 110-744, Republic of Korea. E-mail address: [email protected] (C.K. Chung). 1 Seil Sohn and Tae Hyun Park contributed equally to this work.
pullout strength is 3 times than that of Galveston rods. It has also been shown that additional iliac screws may effectively protect S1 screws and enhance the fusion rates at the lumbosacral junction [8–10]. However, this technique is also associated with pain and prominent screw heads, especially in small and thin patients, necessitating implant removal in as many as 22% of cases [11]. Likewise, Tsuchiya et al. [10] reported seven cases of iliac screw breakage and 23 cases necessary removal due to prominence in a 5 year follow-up study of 67 adult patients with spinal deformities. The S2 alar iliac screw (S2AI) method appears to provide a solution to the problem of prominence because the screw head is concealed underneath the posterior superior iliac spine [8]. The
https://doi.org/10.1016/j.jocn.2018.03.002 0967-5868/Ó 2018 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sohn S et al. Biomechanical characterization of three iliac screw fixation techniques: A finite element study. J Clin Neurosci (2018), https://doi.org/10.1016/j.jocn.2018.03.002
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S. Sohn et al. / Journal of Clinical Neuroscience xxx (2018) xxx–xxx
method also allows for the placement of screws with longer and larger diameters through the S2 alar iliac. In addition, as S2AI screws would be in line with S1 screws, the need to use offset connectors is eliminated. However, a recent study revealed that the failure rate of S2AI screws was higher than that of iliac screws with lateral connectors [12]. One of the main causes was the acute angle that develops between the screw head and the shaft of the S2AI screws [12]. We previously reported a modified iliac screw fixation technique which addressed the limitations of these two techniques. That technique showed good clinical results in adult patients [13]. In the present study, we aim to compare our modified iliac screw fixation technique to the classic iliac screw technique and to the S2AI screw fixation technique by using a finite element model (FEM). 2. Methods 2.1. FEM of a normal lumbosacral spine and pelvis To develop a 3-dimensional (3D) FEM of the lumbosacral spine and pelvis, computerized tomography was utilized with 1 mm intervals on the spine (L1-pelvis) of a normal adult person. The FEM consisted of the vertebral body (cancellous and cortical bone), the spinous process, intervertebral discs, and 17 ligaments (the anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, capsular ligament, intertransverse ligament, interspinous ligament, supraspinous ligament, anterior sacroiliac ligament, posterior sacroiliac ligament, interosseous sacroiliac ligament, sacrospinous ligament, sacrospinous ligament, sacrotuberous ligament, superior pubic ligament, arcuate pubic ligament, inguinal ligament, and the iliolumbar ligament). The elastic behavior of the annulus fibers was taken from work by Smit et al. [14] who combined material values from Goel et al. [15] and Shirazi et al. [16]. The nonlinear behavior of the ligaments was incorporated by defining different material properties at different strains. The ligament attachment points were based on anatomical data. Based on the research by Goel et al. [15], the gap between the facet joints was set to 0.5 mm, and the contact direction was set perpendicular to the articular surface. Material properties were selected from various sources in the literature (Tables 1 and 2) [16–23]. The present study used 3-Matics software package (Materialise NV, Leuven, Belgium) and ABAQUS (version 6.5, ABAQUS Inc., Providence, RI, USA). The lumbar FEM for the purposes of the present study was verified in our previous study [24,25]. The lumbosacral spine and pelvis FEM with ligament attachments are illustrated in Fig. 1. This study was approved by the institutional review boards at Seoul National University Hospital (H-1308-124-517).
2.2. Validation of the FEM To validate the FEM, Equivalent loading conditions were applied and the load displacement behavior of the sacroiliac joint (SIJ) was compared to previously published in vitro data by Miller et al. [26]: (1) 42-Nm of flexion, extension, axial rotation, and lateral bending; (2) 294-N of anterior, posterior, superior, and inferior translation (Fig. 2). Good correspondence was observed. 2.3. Lumbosacral spine and pelvis fusion model with three iliac screw fixation techniques The FEM of the intact lumbosacral spine-pelvis was modified to simulate 3 different iliac screw fixation models (L1-pelvis) based on posterior screw fusion. The 3 different iliac screw fixation techniques- the classic iliac screw technique, the S2AI screw fixation technique, and the modified iliac screw fixation techniqueare described in detail in the literature [12,13]. A model of the lumbosacral spine and pelvis fusion with these 3 different iliac screw fixation techniques is illustrated in Fig. 3. 2.4. Loading and boundary conditions A multi-segment spinal fusion model from L1 to the pelvis was used to compare and analyze the peak von Mises stress (PVMS) values of the implants. All nodal points of both acetabula were confined, while the upper endplate of the highest segment was subjected to a pure moment of 10 Nm of flexion/extension/axial rotation/lateral bending. A compressive follower load of 400 N was added to the validated intact lumbar spinal model in the follower load path direction as suggested by Patwardhan et al. [27]. Full fusion between bony structures and instrumentation was assumed. The PVMS of the iliac screw in the 3 different iliac screw fixation techniques were recorded for each loading condition. Table 2 Material properties used in finite element model of pelvic ligaments. Materials
Stiffness coefficient (N/mm)
Reference
Anterior sacroiliac ligament Posterior sacroiliac ligament (long) Posterior sacroiliac ligament (short) Interosseous sacroiliac ligament Sacrospinous ligament Sacrotuberous ligament Superior pubic ligament Accurate pubic ligament Inguinal ligament Iliolumbar ligament
700 1000 400 2800 1400 1500 500 500 250 1000
Zheng et al. (1997)
Phillips et al. (2007)
Table 1 Material properties used in finite element model of lumbosacral spine and pelvis. Part
Materials
Young’s modulus (MPa)
Poisson’s ratio (n)
Reference
Bone
Ilium cortical Ilium cancellous Sacrum cortical Sacrum cancellous Lumbar cortical Lumbar cancellous Sacrum cartilage Ilium cartilage Pubic symphysis Nucleus pulposus Annulus fiber Annulus metrix Endplate
17,000 132 6140 1400 12,000 100 54 54 5 1 450 30 100
0.3 0.2 0.3 0.3 0.3 0.3 0.4 0.4 0.45 0.4999
Dalsta et al. (1995) Dalsta et al. (1995) Hakim et al. (1979) Hakim et al. (1979) Kawahara et al. (2003) Kawahara et al. (2003) Miura (1987) Miura (1987) Shi et al. (2014) Shirazi et al. (1984)
Soft tissue
Please cite this article in press as: Sohn S et al. Biomechanical characterization of three iliac screw fixation techniques: A finite element study. J Clin Neurosci (2018), https://doi.org/10.1016/j.jocn.2018.03.002
S. Sohn et al. / Journal of Clinical Neuroscience xxx (2018) xxx–xxx
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Fig. 1. The finite element lumbosacral spine-pelvis model. a. anterior view, b. posterior view.
Fig. 2. Comparison of sacral displacements under different loads between this study and the experimental results of Millet et al. [26].
Fig. 3. Simulations of 3 different iliac screw fixation techniques. (a) classic iliac screw fixation technique, (b) S2 alar iliac screw fixation technique, (c) modified iliac screw fixation technique.
Please cite this article in press as: Sohn S et al. Biomechanical characterization of three iliac screw fixation techniques: A finite element study. J Clin Neurosci (2018), https://doi.org/10.1016/j.jocn.2018.03.002
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The interaction stress, which developed between the screw head and shaft of the iliac screw during each physiological loading, was calculated at the centroid surface of the iliac screw. 3. Results 3.1. PVMS in the iliac screws of 3 different techniques The PVMS values under the 3 different iliac screw fixation techniques were calculated, as shown in Fig. 4. The PVMS values of these iliac screw fixation techniques were lower than fatigue strength levels under physiological loadings. The PVMS of the modified iliac screw technique was lower than that of classic iliac screw technique and slightly higher than that of S2AI. The locations of PVMS were on the offset connectors, the screw shaft, and the screw neck of the classic iliac screw technique, S2AI,
and modified iliac screw technique, respectively (Fig. 5). The classic iliac screw technique, S2AI, and the modified iliac screw technique had corresponding PVMS values of 126, 63, and 75 MPa in flexion (Fig. 5). Extension stress showed that 96, 43, and 53 MPa in the classic iliac screw, S2AI, and in the modified iliac screw technique, respectively. The axial rotation stress results were 61, 29, and 59 MPa in the classic iliac, S2AI, and modified iliac screw techniques. The lateral bending stress results were 68, 32, and 49 MPa in the classic iliac screw, S2AI, and the modified iliac screw technique correspondingly. 3.2. Interaction stress between the screw head and shaft in S2AI and the modified iliac screw technique The direction of interaction stress between the screw head and the shaft for S2AI and the modified iliac screw technique were
Fig. 4. The peak von Mises stress (PVMS) values among the 3 different iliac screw fixation techniques under physiological loadings (flexion, extension, lateral bending, and axial rotation). Abbreviations: S2AI, S2 alar iliac screw fixation technique; SFx, flexion; Ex, extension; Lb, lateral bending; Ar, axial rotation.
Fig. 5. Contour plots of the PVMS of the iliac screw when tested with flexion load of 10 Nm. (a) 126 MPa was observed at an offset connector in the classic iliac screw fixation technique, (b) 63 MPa was observed at the mid- shaft point in the S2AI screw fixation technique, (c) 75 MPa was observed between the screw head and the shaft in the modified iliac screw fixation technique. The maximum stress sites are indicated by the black arrows.
Please cite this article in press as: Sohn S et al. Biomechanical characterization of three iliac screw fixation techniques: A finite element study. J Clin Neurosci (2018), https://doi.org/10.1016/j.jocn.2018.03.002
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due to screw head deformation in S2AI [12]. On the other hand, the interaction force between the screw head and the shaft was mostly in the same direction in the modified iliac screw fixation technique. The modified iliac screw fixation technique can provide more stability between the screw head and the shaft as compared to S2AI. We hypothesize that the acute angle between the screw head and the shaft in S2AI screws may cause this interaction force direction. 5. Limitations
Fig. 6. The direction of the interaction stress force between the screw head and the shaft was opposite in 2 different techniques. (a) In S2AI, the interaction force between the screw head and the shaft was mostly in the distraction direction. (b) In the modified iliac screw fixation technique, the interaction forces between the screw head and the shaft were in nearly identical directions.
opposite to each other. In S2AI, the interaction force between the screw head and the neck was nearly in the distraction direction (12.14 Nm, Fig. 6). However, the interaction force between the screw head and the neck was mostly compressed in the modified iliac screw technique (15.23 Nm, Fig. 6).
Several limitations of this study should be noted. First, we used a single FE model based on CT scans of a healthy young person without any degenerative changes in the SIJ, and considerations of anatomic variability among individuals were not available. Second, our model only represented the anatomy and biomechanical properties of ligaments and bones. Nevertheless, to the best of our knowledge, this is the first study to verify the effectiveness of the modified iliac screw fixation technique by using a FEM. 6. Conclusions This FEM study verified our previous clinical results which showed that the modified iliac screw fixation technique can be an effective alternative sacropelvic fixation technique compared to the classic iliac screw and the S2AI technique. Further long term follow-up clinical study is warranted. Funding
4. Discussion In the present study, the PVMS of the iliac screw in the modified iliac screw technique was lower than that in the classic iliac screw technique and slightly higher than that in S2AI. However, the interaction stress force in the modified iliac screw fixation technique between the screw head and the neck was mostly compressed, which is better than that in S2AI. This result can verify our previous clinical findings, which demonstrated that the modified iliac screw fixation technique can be an effective alternative sacropelvic fixation [13]. 4.1. PVMS in the iliac screws of the three different techniques In the present study, PVMS was lower than the failure strengths under physiological loading in the 3 different iliac screw fixation techniques. This showed that all 3 different iliac screw fixation techniques can be safe under physiological loading. Under flexion loading, PVMS in the classic iliac screw technique was highest among the three techniques, and it was located at the offset connectors (Fig. 5). This result verified previous clinical studies which found that instrument failures usually occur around the offset connectors [12]. Under flexion loading, PVMS was localized at the screw mid- shaft point in the S2AI technique (Fig. 5). A previous cadaveric study showed that the mean SIJ motion was 1.94° [26]. Therefore, we assumed that micro-motion of the SIJ could affect the PVMS of the screw shaft in the S2AI technique. 4.2. Interaction stress between the screw head and the shaft in S2AI and in the modified iliac screw technique Although the PVMS was lower in the S2AI screw technique than in the modified iliac screw fixation technique, the interaction stress direction was opposite. In S2AI, the interaction force between the screw head and the shaft was mostly in the distraction direction. This indicates that instrument failures usually arise
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B03032980). This work was also supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (HR16C0002). Conflict of interest The authors have no personal financial or institutional interest in any of the drugs, materials, or devices described in this article. Ethics approval This study was approved by the institutional review boards at Seoul National University Hospital (H-1308-124-517). Author’s contributions All authors read and approved the final manuscript. References [1] Moshirfar A, Rand FF, Sponseller PD, Parazin SJ, Khanna AJ, Kebaish KM, et al. Pelvic fixation in spine surgery. Historical overview, indications, biomechanical relevance, and current techniques. J Bone Joint Surg Am 2005;87(Suppl 2):89–106. [2] Shen FH, Mason JR, Shimer AL, Arlet VM. Pelvic fixation for adult scoliosis. Eur Spine J 2013;22(Suppl 2):S265–75. [3] Dubory A, Bachy M, Bouloussa H, Courvoisier A, Morel B, Vialle R. Screw augmentation for spinopelvic fixation in neuromuscular spine deformities: technical note. Eur Spine J 2015;24:2580–7. [4] Grechenig S, Gansslen A, Gueorguiev B, Berner A, Muller M, Nerlich M, et al. PMMA-augmented SI screw: a biomechanical analysis of stiffness and pull-out force in a matched paired human cadaveric model. Injury 2015;46(Suppl 4): S125–8.
Please cite this article in press as: Sohn S et al. Biomechanical characterization of three iliac screw fixation techniques: A finite element study. J Clin Neurosci (2018), https://doi.org/10.1016/j.jocn.2018.03.002
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[5] Koller H, Zenner J, Hempfing A, Ferraris L, Meier O. Reinforcement of lumbosacral instrumentation using S1-pedicle screws combined with S2-alar screws. Oper Orthop Traumatol 2013;25:294–314. [6] Oberkircher L, Masaeli A, Bliemel C, Debus F, Ruchholtz S, Kruger A. Primary stability of three different iliosacral screw fixation techniques in osteoporotic cadaver specimens-a biomechanical investigation. Spine J 2015. [7] Schwend RM, Sluyters R, Najdzionek J. The pylon concept of pelvic anchorage for spinal instrumentation in the human cadaver. Spine (Phila Pa 1976) 2003;28:542–7. [8] Kebaish KM. Sacropelvic fixation: techniques and complications. Spine (Phila Pa 1976) 2010;35:2245–51. [9] Lebwohl NH, Cunningham BW, Dmitriev A, Shimamoto N, Gooch L, Devlin V, et al. Biomechanical comparison of lumbosacral fixation techniques in a calf spine model. Spine (Phila Pa 1976) 2002;27:2312–20. [10] Tsuchiya K, Bridwell KH, Kuklo TR, Lenke LG, Baldus C. Minimum 5-year analysis of L5–S1 fusion using sacropelvic fixation (bilateral S1 and iliac screws) for spinal deformity. Spine (Phila Pa 1976) 2006;31:303–8. [11] Emami A, Deviren V, Berven S, Smith JA, Hu SS, Bradford DS. Outcome and complications of long fusions to the sacrum in adult spine deformity: luquegalveston, combined iliac and sacral screws, and sacral fixation. Spine (Phila Pa 1976) 2002;27:776–86. [12] Guler UO, Cetin E, Yaman O, Pellise F, Casademut AV, Sabat MD, et al. Sacropelvic fixation in adult spinal deformity (ASD); a very high rate of mechanical failure. Eur Spine J 2015;24:1085–91. [13] Sohn S, Chung CK, Kim YJ, Kim CH, Park SB, Kim H. Modified iliac screw fixation: technique and clinical application. Acta Neurochir (Wien) 2016;158:975–80. [14] Smit TH, Odgaard A, Schneider E. Structure and function of vertebral trabecular bone. Spine (Phila Pa 1976) 1997;22:2823–33. [15] Goel VK, Monroe BT, Gilbertson LG, Brinckmann P. Interlaminar shear stresses and laminae separation in a disc. Finite element analysis of the L3–L4 motion segment subjected to axial compressive loads. Spine (Phila Pa 1976) 1995;20:689–98.
[16] Shirazi-Adl SA, Shrivastava SC, Ahmed AM. Stress analysis of the lumbar discbody unit in compression. A three-dimensional nonlinear finite element study. Spine (Phila Pa 1976) 1984;9:120–34. [17] Dalstra M, Huiskes R. Load transfer across the pelvic bone. J Biomech 1995;28:715–24. [18] Hakim NS, King AI. A three dimensional finite element dynamic response analysis of a vertebra with experimental verification. J Biomech. 1979;12: 277–92. [19] Kawahara N, Murakami H, Yoshida A, Sakamoto J, Oda J, Tomita K. Reconstruction after total sacrectomy using a new instrumentation technique: a biomechanical comparison. Spine (Phila Pa 1976) 2003;28:1567–72. [20] Miura H. Biomechanical properties of the sacroiliac joint. Nihon Seikeigeka Gakkai Zasshi 1987;61:1093–105. [21] Zheng N, Watson LG, Yong-Hing K. Biomechanical modelling of the human sacroiliac joint. Med Biol Eng Comput 1997;35:77–82. [22] Phillips AT, Pankaj P, Howie CR, Usmani AS, Simpson AH. Finite element modelling of the pelvis: inclusion of muscular and ligamentous boundary conditions. Med Eng Phys 2007;29:739–48. [23] Shi D, Wang F, Wang D, Li X, Wang Q. 3-D finite element analysis of the influence of synovial condition in sacroiliac joint on the load transmission in human pelvic system. Med Eng Phys 2014;36:745–53. [24] Chen WM, Park C, Lee K, Lee S. In situ contact analysis of the prosthesis components of Prodisc-L in lumbar spine following total disc replacement. Spine (Phila Pa 1976) 2009;34:E716–23. [25] Choi KC, Ryu KS, Lee SH, Kim YH, Lee SJ, Park CK. Biomechanical comparison of anterior lumbar interbody fusion: stand-alone interbody cage versus interbody cage with pedicle screw fixation – a finite element analysis. BMC Musculoskelet Disord 2013;14:220. [26] Miller JA, Schultz AB, Andersson GB. Load-displacement behavior of sacroiliac joints. J Orthop Res 1987;5:92–101. [27] Patwardhan AG, Havey RM, Meade KP, Lee B, Dunlap B. A follower load increases the load-carrying capacity of the lumbar spine in compression. Spine (Phila Pa 1976) 1999;24:1003–9.
Please cite this article in press as: Sohn S et al. Biomechanical characterization of three iliac screw fixation techniques: A finite element study. J Clin Neurosci (2018), https://doi.org/10.1016/j.jocn.2018.03.002
Contents lists available at ScienceDirect
Journal of Clinical Neuroscience journal homepage: www.elsevier.com/locate/jocn
Lab resource
Biomechanical characterization of three iliac screw fixation techniques: A finite element study Seil Sohn a,1, Tae Hyun Park f,g,1, Chun Kee Chung b,c,d,e,⇑, Yongjung Jay Kim h, Jong Wuk Jang f, In-bo Han a, Sung Jae Lee g a
Department of Neurosurgery, CHA University, CHA Bundang Medical Center, Republic of Korea Department of Neurosurgery, Seoul National University College of Medicine, Republic of Korea Neuroscience Research Institute, Seoul National University Medical Research Center, Republic of Korea d Clinical Research Institute, Seoul National University Hospital, Republic of Korea e Department of Brain and Cognitive Sciences, Seoul National University College of Natural Sciences, Republic of Korea f R&D Center, Medyssey Co, Ltd, Jechon, Republic of Korea g Department of Biomedical Engineering, College of Biomedical Science & Engineering, Inje University, Gyeongnam, Republic of Korea h Department of Orthopedics, Columbia University, College of Physicians and Surgeons, New York, NY, USA b c
a r t i c l e
i n f o
Article history: Received 12 February 2018 Accepted 11 March 2018 Available online xxxx Keywords: Sacropelvic fixation Spinal deformities Spine S2 alar iliac fixation
a b s t r a c t We aim to characterize the biomechanical properties of a modified iliac screw fixation method compared with the classic iliac screw fixation and the S2 alar iliac screw (S2AI) fixation using a FEM. A three-dimensional, non-linear FEM of lumbosacral spine and pelvis (L1-pelvis) was modified to simulate 3 different iliac screw fixations based on posterior screw fusion. The peak von Mises stress (PVMS) values of the iliac screws in the 3 different iliac screw fixations were recorded in during flexion/extension/axial rotation/lateral bending. The interaction stress which arose between the screw head and the shaft of iliac screws, was also measured for each case. The PVMS values of the 3 different iliac screw fixation techniques were lower than the fatigue strength levels under physiological loadings. PVMS of iliac screws was observed in the screw shaft for S2AI, in the screw neck for the modified iliac screw technique, and in the offset connectors of the classic iliac screw technique. The interaction between the screw head and the neck was compressed in modified iliac screw fixation technique. On the other hand, distraction force was observed in the S2AI technique between the screw head and the screw shaft. This FEM study supports our previous clinical results, which found that the modified iliac screw fixation technique can be an effective alternative sacropelvic fixation technique comparable to the classic iliac screw and the S2AI technique. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction To overcome the complications associated with fusions ending at S1, several methods, such as sacropelvic fixation or the use of a S2 alar screw, have been introduced [1–6]. Modern instrumentation techniques allow for the insertion of screws into the ilium independently of the proximal construct and connection to longitudinal rods by offset connectors [7]. Their
⇑ Corresponding author at: Department of Neurosurgery, Seoul National University College of Medicine, 101 Daehak-no, Jongno-gu, Seoul 110-744, Republic of Korea. E-mail address: [email protected] (C.K. Chung). 1 Seil Sohn and Tae Hyun Park contributed equally to this work.
pullout strength is 3 times than that of Galveston rods. It has also been shown that additional iliac screws may effectively protect S1 screws and enhance the fusion rates at the lumbosacral junction [8–10]. However, this technique is also associated with pain and prominent screw heads, especially in small and thin patients, necessitating implant removal in as many as 22% of cases [11]. Likewise, Tsuchiya et al. [10] reported seven cases of iliac screw breakage and 23 cases necessary removal due to prominence in a 5 year follow-up study of 67 adult patients with spinal deformities. The S2 alar iliac screw (S2AI) method appears to provide a solution to the problem of prominence because the screw head is concealed underneath the posterior superior iliac spine [8]. The
https://doi.org/10.1016/j.jocn.2018.03.002 0967-5868/Ó 2018 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sohn S et al. Biomechanical characterization of three iliac screw fixation techniques: A finite element study. J Clin Neurosci (2018), https://doi.org/10.1016/j.jocn.2018.03.002
2
S. Sohn et al. / Journal of Clinical Neuroscience xxx (2018) xxx–xxx
method also allows for the placement of screws with longer and larger diameters through the S2 alar iliac. In addition, as S2AI screws would be in line with S1 screws, the need to use offset connectors is eliminated. However, a recent study revealed that the failure rate of S2AI screws was higher than that of iliac screws with lateral connectors [12]. One of the main causes was the acute angle that develops between the screw head and the shaft of the S2AI screws [12]. We previously reported a modified iliac screw fixation technique which addressed the limitations of these two techniques. That technique showed good clinical results in adult patients [13]. In the present study, we aim to compare our modified iliac screw fixation technique to the classic iliac screw technique and to the S2AI screw fixation technique by using a finite element model (FEM). 2. Methods 2.1. FEM of a normal lumbosacral spine and pelvis To develop a 3-dimensional (3D) FEM of the lumbosacral spine and pelvis, computerized tomography was utilized with 1 mm intervals on the spine (L1-pelvis) of a normal adult person. The FEM consisted of the vertebral body (cancellous and cortical bone), the spinous process, intervertebral discs, and 17 ligaments (the anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, capsular ligament, intertransverse ligament, interspinous ligament, supraspinous ligament, anterior sacroiliac ligament, posterior sacroiliac ligament, interosseous sacroiliac ligament, sacrospinous ligament, sacrospinous ligament, sacrotuberous ligament, superior pubic ligament, arcuate pubic ligament, inguinal ligament, and the iliolumbar ligament). The elastic behavior of the annulus fibers was taken from work by Smit et al. [14] who combined material values from Goel et al. [15] and Shirazi et al. [16]. The nonlinear behavior of the ligaments was incorporated by defining different material properties at different strains. The ligament attachment points were based on anatomical data. Based on the research by Goel et al. [15], the gap between the facet joints was set to 0.5 mm, and the contact direction was set perpendicular to the articular surface. Material properties were selected from various sources in the literature (Tables 1 and 2) [16–23]. The present study used 3-Matics software package (Materialise NV, Leuven, Belgium) and ABAQUS (version 6.5, ABAQUS Inc., Providence, RI, USA). The lumbar FEM for the purposes of the present study was verified in our previous study [24,25]. The lumbosacral spine and pelvis FEM with ligament attachments are illustrated in Fig. 1. This study was approved by the institutional review boards at Seoul National University Hospital (H-1308-124-517).
2.2. Validation of the FEM To validate the FEM, Equivalent loading conditions were applied and the load displacement behavior of the sacroiliac joint (SIJ) was compared to previously published in vitro data by Miller et al. [26]: (1) 42-Nm of flexion, extension, axial rotation, and lateral bending; (2) 294-N of anterior, posterior, superior, and inferior translation (Fig. 2). Good correspondence was observed. 2.3. Lumbosacral spine and pelvis fusion model with three iliac screw fixation techniques The FEM of the intact lumbosacral spine-pelvis was modified to simulate 3 different iliac screw fixation models (L1-pelvis) based on posterior screw fusion. The 3 different iliac screw fixation techniques- the classic iliac screw technique, the S2AI screw fixation technique, and the modified iliac screw fixation techniqueare described in detail in the literature [12,13]. A model of the lumbosacral spine and pelvis fusion with these 3 different iliac screw fixation techniques is illustrated in Fig. 3. 2.4. Loading and boundary conditions A multi-segment spinal fusion model from L1 to the pelvis was used to compare and analyze the peak von Mises stress (PVMS) values of the implants. All nodal points of both acetabula were confined, while the upper endplate of the highest segment was subjected to a pure moment of 10 Nm of flexion/extension/axial rotation/lateral bending. A compressive follower load of 400 N was added to the validated intact lumbar spinal model in the follower load path direction as suggested by Patwardhan et al. [27]. Full fusion between bony structures and instrumentation was assumed. The PVMS of the iliac screw in the 3 different iliac screw fixation techniques were recorded for each loading condition. Table 2 Material properties used in finite element model of pelvic ligaments. Materials
Stiffness coefficient (N/mm)
Reference
Anterior sacroiliac ligament Posterior sacroiliac ligament (long) Posterior sacroiliac ligament (short) Interosseous sacroiliac ligament Sacrospinous ligament Sacrotuberous ligament Superior pubic ligament Accurate pubic ligament Inguinal ligament Iliolumbar ligament
700 1000 400 2800 1400 1500 500 500 250 1000
Zheng et al. (1997)
Phillips et al. (2007)
Table 1 Material properties used in finite element model of lumbosacral spine and pelvis. Part
Materials
Young’s modulus (MPa)
Poisson’s ratio (n)
Reference
Bone
Ilium cortical Ilium cancellous Sacrum cortical Sacrum cancellous Lumbar cortical Lumbar cancellous Sacrum cartilage Ilium cartilage Pubic symphysis Nucleus pulposus Annulus fiber Annulus metrix Endplate
17,000 132 6140 1400 12,000 100 54 54 5 1 450 30 100
0.3 0.2 0.3 0.3 0.3 0.3 0.4 0.4 0.45 0.4999
Dalsta et al. (1995) Dalsta et al. (1995) Hakim et al. (1979) Hakim et al. (1979) Kawahara et al. (2003) Kawahara et al. (2003) Miura (1987) Miura (1987) Shi et al. (2014) Shirazi et al. (1984)
Soft tissue
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Fig. 1. The finite element lumbosacral spine-pelvis model. a. anterior view, b. posterior view.
Fig. 2. Comparison of sacral displacements under different loads between this study and the experimental results of Millet et al. [26].
Fig. 3. Simulations of 3 different iliac screw fixation techniques. (a) classic iliac screw fixation technique, (b) S2 alar iliac screw fixation technique, (c) modified iliac screw fixation technique.
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The interaction stress, which developed between the screw head and shaft of the iliac screw during each physiological loading, was calculated at the centroid surface of the iliac screw. 3. Results 3.1. PVMS in the iliac screws of 3 different techniques The PVMS values under the 3 different iliac screw fixation techniques were calculated, as shown in Fig. 4. The PVMS values of these iliac screw fixation techniques were lower than fatigue strength levels under physiological loadings. The PVMS of the modified iliac screw technique was lower than that of classic iliac screw technique and slightly higher than that of S2AI. The locations of PVMS were on the offset connectors, the screw shaft, and the screw neck of the classic iliac screw technique, S2AI,
and modified iliac screw technique, respectively (Fig. 5). The classic iliac screw technique, S2AI, and the modified iliac screw technique had corresponding PVMS values of 126, 63, and 75 MPa in flexion (Fig. 5). Extension stress showed that 96, 43, and 53 MPa in the classic iliac screw, S2AI, and in the modified iliac screw technique, respectively. The axial rotation stress results were 61, 29, and 59 MPa in the classic iliac, S2AI, and modified iliac screw techniques. The lateral bending stress results were 68, 32, and 49 MPa in the classic iliac screw, S2AI, and the modified iliac screw technique correspondingly. 3.2. Interaction stress between the screw head and shaft in S2AI and the modified iliac screw technique The direction of interaction stress between the screw head and the shaft for S2AI and the modified iliac screw technique were
Fig. 4. The peak von Mises stress (PVMS) values among the 3 different iliac screw fixation techniques under physiological loadings (flexion, extension, lateral bending, and axial rotation). Abbreviations: S2AI, S2 alar iliac screw fixation technique; SFx, flexion; Ex, extension; Lb, lateral bending; Ar, axial rotation.
Fig. 5. Contour plots of the PVMS of the iliac screw when tested with flexion load of 10 Nm. (a) 126 MPa was observed at an offset connector in the classic iliac screw fixation technique, (b) 63 MPa was observed at the mid- shaft point in the S2AI screw fixation technique, (c) 75 MPa was observed between the screw head and the shaft in the modified iliac screw fixation technique. The maximum stress sites are indicated by the black arrows.
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due to screw head deformation in S2AI [12]. On the other hand, the interaction force between the screw head and the shaft was mostly in the same direction in the modified iliac screw fixation technique. The modified iliac screw fixation technique can provide more stability between the screw head and the shaft as compared to S2AI. We hypothesize that the acute angle between the screw head and the shaft in S2AI screws may cause this interaction force direction. 5. Limitations
Fig. 6. The direction of the interaction stress force between the screw head and the shaft was opposite in 2 different techniques. (a) In S2AI, the interaction force between the screw head and the shaft was mostly in the distraction direction. (b) In the modified iliac screw fixation technique, the interaction forces between the screw head and the shaft were in nearly identical directions.
opposite to each other. In S2AI, the interaction force between the screw head and the neck was nearly in the distraction direction (12.14 Nm, Fig. 6). However, the interaction force between the screw head and the neck was mostly compressed in the modified iliac screw technique (15.23 Nm, Fig. 6).
Several limitations of this study should be noted. First, we used a single FE model based on CT scans of a healthy young person without any degenerative changes in the SIJ, and considerations of anatomic variability among individuals were not available. Second, our model only represented the anatomy and biomechanical properties of ligaments and bones. Nevertheless, to the best of our knowledge, this is the first study to verify the effectiveness of the modified iliac screw fixation technique by using a FEM. 6. Conclusions This FEM study verified our previous clinical results which showed that the modified iliac screw fixation technique can be an effective alternative sacropelvic fixation technique compared to the classic iliac screw and the S2AI technique. Further long term follow-up clinical study is warranted. Funding
4. Discussion In the present study, the PVMS of the iliac screw in the modified iliac screw technique was lower than that in the classic iliac screw technique and slightly higher than that in S2AI. However, the interaction stress force in the modified iliac screw fixation technique between the screw head and the neck was mostly compressed, which is better than that in S2AI. This result can verify our previous clinical findings, which demonstrated that the modified iliac screw fixation technique can be an effective alternative sacropelvic fixation [13]. 4.1. PVMS in the iliac screws of the three different techniques In the present study, PVMS was lower than the failure strengths under physiological loading in the 3 different iliac screw fixation techniques. This showed that all 3 different iliac screw fixation techniques can be safe under physiological loading. Under flexion loading, PVMS in the classic iliac screw technique was highest among the three techniques, and it was located at the offset connectors (Fig. 5). This result verified previous clinical studies which found that instrument failures usually occur around the offset connectors [12]. Under flexion loading, PVMS was localized at the screw mid- shaft point in the S2AI technique (Fig. 5). A previous cadaveric study showed that the mean SIJ motion was 1.94° [26]. Therefore, we assumed that micro-motion of the SIJ could affect the PVMS of the screw shaft in the S2AI technique. 4.2. Interaction stress between the screw head and the shaft in S2AI and in the modified iliac screw technique Although the PVMS was lower in the S2AI screw technique than in the modified iliac screw fixation technique, the interaction stress direction was opposite. In S2AI, the interaction force between the screw head and the shaft was mostly in the distraction direction. This indicates that instrument failures usually arise
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Please cite this article in press as: Sohn S et al. Biomechanical characterization of three iliac screw fixation techniques: A finite element study. J Clin Neurosci (2018), https://doi.org/10.1016/j.jocn.2018.03.002