- Email: [email protected]
Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study
Journal of Clinical Neuroscience xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Clinical Neuroscience journal homepage: www.elsevier.com/locate/jocn
Experimental study
Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study Je Beom Hong a,1, Dong Min Son b,1, Tae Hyun Park b,c, Su Heon Woo c, Sung Jae Lee b, Un Yong Choi d, In-bo Han d, Chun Kee Chung e,f,g,h, Yongjung Jay Kim i, Seil Sohn d,2 a
Department of Neurosurgery, Kangbuk Samsung Hospital, Sungkyunkwan University School of Medicine, Seoul, Republic of Korea Department of Biomedical Engineering, College of Biomedical Science& Engineering, Inje University, Gyeongnam, Republic of Korea R&D Center, Medyssey Co, Ltd, Jechon, Republic of Korea d Department of Neurosurgery, CHA University, CHA Bundang Medical Center, Republic of Korea e Department of Neurosurgery, Seoul National University College of Medicine, Republic of Korea f Neuroscience Research Institute, Seoul National University Medical Research Center, Republic of Korea g Clinical Research Institute, Seoul National University Hospital, Republic of Korea h Department of Brain and Cognitive Sciences, Seoul National University College of Natural Sciences, Republic of Korea i 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 29 October 2019 Accepted 11 January 2020 Available online xxxx Keywords: Cross-link Rod fracture Pedicle subtraction osteotomy Spine Finite element analysis
a b s t r a c t Purpose: The purpose of this study is to investigate the effect of the cross-link position on the rod fracture phenomenon during pedicle subtraction osteotomy (PSO) surgery using finite element model (FEM). Methods: A three-dimensional finite element model of a lumbar spine with sagittal imbalance was constructed using computed tomography data of a 65-year-old female patient. After simulating the standard PSO at the L4 level, we constructed four models, specifically a model without a cross-link and three models with a cross-link at three different sites. The peak von Mises stress (PVMS) of the rod around the PSO site was measured after applying physiological loads (flexion, extension, axial rotation, and lateral bending) in each model. Results: The measured PVMS outcomes at the PSO site were 135.8, 135.9, 208.9, and 384.7 MPa for model 1, 2, 3, and 4 during flexion, and 180.0, 180.1, 210.1, and 445.7 MPa during extension. These results show that when the cross-link is located at the PSO site, the rod stress at the PSO site increases significantly during flexion and extension. As the cross-link moved away from the PSO site, the effect on the rod stress decreased. When the cross-link was placed two levels away from the PSO site, the rod stress was scarcely affected. Conclusion: When the cross-link during PSO surgery was positioned two levels away from the PSO site, the risk of rod fracture did not increase. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Pedicle subtraction osteotomy (PSO), originally introduced by Thomasen [1], can increase lordosis by approximately 30 to 40 degrees and is known to be an effective treatment for fixed sagittal imbalance [2–5]. However, multiple complications can occur [6,7]. The risk of both rod fracture and late complications such as pseudoarthrosis due to rod stress reportedly ranges from 15.8% to 22% [3,4,8–10].
1
Je Beom Hong and Dong Min Sohn contributed equally to this work. Department of Neurosurgery, CHA University College of Medicine, 59, Yatap-ro, Bundang-gu, Seongnam-si, Gyeonggi-do, 13496, Korea. E-mail address: [email protected] (S. Sohn)
Various attempts and numerous studies have been conducted to prevent these complications. Studies of different rod materials, contours and diameters, and trial with a wide variety of rod models have been done [9,11–14]. However, there remains no established surgical technique to prevent rod failure. Cross-links are also used by many surgeons, but little research has been done on the proper location of a cross-link during PSO. In this study, we used finite element model (FEM) to investigate the effects of the cross-link position on the rod stress during lumbar PSO. 2. Methods
2
A three-dimensional finite element lumbar spine model with sagittal imbalance was constructed using computed tomography
https://doi.org/10.1016/j.jocn.2020.01.053 0967-5868/Ó 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: J. B. Hong, D. M. Son, T. H. Park et al., Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2020.01.053
2
J.B. Hong et al. / Journal of Clinical Neuroscience xxx (xxxx) xxx
Fig. 1. Finite element model (FEM): a) distribution of the material properties based on Hounsfield units, and b) preoperative three-dimensional computed tomography.
Fig. 2. Four different finite element lumbar spine models according to the cross-link position for each model.
Fig. 3. Stress distribution of the FEM under physiological loadings (flexion, extension).
data of a 65-year-old female and Mimics (Materialise US, Plymouth, MI, USA) software. The mesh was generated using 3-matics (Materialise, Ann Arbor, MI, USA) software. The material properties (density, stiffness, poisson’s ratio) were applied to each element of the lumbar spine. Each value was obtained from a study by Rho et al. [15]. Osteotomy was performed with a 30° wedge on L4 of the intact lumbar model (Fig. 1). A pedicle screw system (IliadÒ,
Medyssey, Korea) was applied to L1-L5 according to standard surgical techniques (Without a cross-link, Model 1). To compare the effects of different cross-link positions, four models were created by applying a cross-link to three different sites. In the model 1, we did not position a cross-link. We positioned a cross-link at the L1/2 level in the model 2, at the L2/3 level in the model 3, and at the L3/5 level (PSO level) in the model 4 (Fig. 2).
Please cite this article as: J. B. Hong, D. M. Son, T. H. Park et al., Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2020.01.053
3
J.B. Hong et al. / Journal of Clinical Neuroscience xxx (xxxx) xxx
Fig. 4. Comparison of the peak von Mises stress (PVMS) values for each model at the pedicle subtraction osteotomy (PSO) site. Table 1 The peak von Mises stress (PVMS) values at Pedicle Subtraction Osteotomy site in each type model.
Type Type Type Type
1 2 3 4
Flexion
Extension
Axial rotation
Lateral bending
135.8 135.9 208.9 384.7
180.0 180.1 210.1 445.7
29.0 29.0 29.1 29.1
67.8 67.8 72.6 68.1
Surface-to-surface contact was defined along the PSO site after osteotomy with a friction coefficient of 0.46 [11]. The lower endplate of L5 was fixed and a compressive follower load of 400 N was applied (Fig. 3.) [16]. Pure moment of 10Nm was applied to the upper endplate of L1 in the direction of flexion, extension, axial rotation, and lateral bending for a physiological load [11]. A tie contact was applied assuming that the bony structure and the parts of the implant were in a fully fused state. For each loading condition, we measured the peak von Mises stress (PVMS) applied to the rods around the PSO site using Abaqus (version 6.14, Dassault Systèmes, France) software. 3. Results The PVMS outcomes at the L3/5 (PSO site) are shown in Fig. 4 and Table 1. During flexion, the PVMS values were 135.8, 135.9, 208.9, and 384.7 MPa for the model 1, 2, 3, and 4 models, respectively. During extension, the PVMS values were 180.0, 180.1, 210.1, and 445.7 MPa for the model 1, 2, 3, and 4, respectively. During axial rotation, the PVMS values were 29.0, 29.0, 29.1, and 29.1 for the model 1, 2, 3, and 4, respectively. During lateral bending, the PVMS values were 67.8, 67.8, 72.6, and 68.1 MPa for the corresponding models. With regard to model 4, where cross-links were located at the PSO site, the largest PVMS increase of the PSO site rod was found during flexion and extension loading. In model 2 and model 3, during flexion and extension, the PVMS values of the rod at the PSO site decreased when the cross-link was located away from the PSO site. In particular, we found that the rod stress in the model 2 was nearly identical to that in the model 1. Besides, during axial rotation and lateral bending, the position of the cross-link did not affect the PVMS of the rod stress significantly. 4. Discussion Surgeons often use a cross-link as a supplement to increase the stability of the rods during PSO. A cross-link is known to increase the stiffness of the pedicle screw system during axial rotation and to provide stability to surgical constructs [17–19]. However,
other studies have reported that cross-links do not make a difference in clinical outcomes [20,21]. In our previous study, we showed that the risk of a rod fracture is increased when a cross-link is applied to the PSO site [22]. Similarly, in the current study, the PVMS values increased by 283.3% (from 135.8 to 384.7 MPa) in flexion and 247.6% (from 180.0 to 445.7 MPa) in extension when cross-links were placed at the PSO site. Furthermore, here we used three different cross-link positions. As the position of the cross-link moved away from the PSO site, the increment in the PVMS of the rod at the PSO site decreased during flexion and extension. In addition, the cross-link when located two levels away from the PSO site had little effect on the rod stress. Through this study, we can suggest that the location of the crosslink is safe when it is two segments away from the PSO site. We also considered how meaningful the range of stress values in this study would be in actual clinical situations. When the material used in rods is titanium alloy, the yield stress is known to be 795 MPa [23]. This value is larger than the range of PVMS values for each model in this study. However, because stress can be repeatedly exerted for extended amounts of time in clinical situations, the results of this study are meaningful. Several limitations of this study should be noted. First, we used a single FEM based on the computed tomography of one patient. Therefore, our model may not be applicable to every sagittal imbalance situation. Second, our model only represented the anatomy and biomechanical properties of bones. Nonetheless, to the best of our knowledge, this is the first biomechanical study to elucidate the effect of the cross-link position during lumbar PSO surgery. 5. Conclusions A cross-link two levels away from the PSO site does not increase the risk of rod fracture. Funding This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded
Please cite this article as: J. B. Hong, D. M. Son, T. H. Park et al., Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2020.01.053
4
J.B. Hong et al. / Journal of Clinical Neuroscience xxx (xxxx) xxx
by the Ministry of Education (NRF-2018R1D1A1B07048252) and 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 (grant number: HI16C1559). 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. Declaration of Competing Interest The authors have no personal financial or institutional interest in any of the drugs, materials, or devices described in this article. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jocn.2020.01.053. References [1] Thomasen E. Vertebral osteotomy for correction of kyphosis in ankylosing spondylitis. Clin Orthop Relat Res 1985;142–52. [2] Kim YJ, Bridwell KH, Lenke LG, Cheh G, Baldus C. Results of lumbar pedicle subtraction osteotomies for fixed sagittal imbalance: a minimum 5-year follow-up study. Spine (Phila Pa 1976) 2007(32):2189–97. [3] Berjano P, Bassani R, Casero G, Sinigaglia A, Cecchinato R, Lamartina C. Failures and revisions in surgery for sagittal imbalance: analysis of factors influencing failure. Eur Spine J 2013;22(Suppl 6):S853–8. [4] Bridwell KH, Lewis SJ, Lenke LG, Baldus C, Blanke K. Pedicle subtraction osteotomy for the treatment of fixed sagittal imbalance. J Bone Joint Surg Am 2003:454–63. [5] Gupta S, Gupta MC. The nuances of pedicle subtraction osteotomies. Neurosurg Clin N Am 2018;29:355–63. [6] Charles YP, Yu B, Steib JP. Sacroiliac joint luxation after pedicle subtraction osteotomy: report of two cases and analysis of failure mechanism. Eur Spine J 2016;25(Suppl 1):63–74. [7] Smith JS, Sansur CA, Donaldson 3rd WF, Perra JH, Mudiyam R, Choma TJ, et al. Short-term morbidity and mortality associated with correction of thoracolumbar fixed sagittal plane deformity: a report from the Scoliosis Research Society Morbidity and Mortality Committee. Spine (Phila Pa 1976) 2011(36):958–64.
[8] Smith JS, Shaffrey CI, Ames CP, Demakakos J, Fu KM, Keshavarzi S, et al. Assessment of symptomatic rod fracture after posterior instrumented fusion for adult spinal deformity. Neurosurgery 2012;71:862–7. [9] Smith JS, Shaffrey E, Klineberg E, Shaffrey CI, Lafage V, Schwab FJ, et al. Prospective multicenter assessment of risk factors for rod fracture following surgery for adult spinal deformity. J Neurosurg Spine 2014;21:994–1003. [10] Barton C, Noshchenko A, Patel V, Cain C, Kleck C, Burger E. Risk factors for rod fracture after posterior correction of adult spinal deformity with osteotomy: a retrospective case-series. Scoliosis 2015;10:30. [11] Luca A, Ottardi C, Sasso M, Prosdocimo L, La Barbera L, Brayda-Bruno M, et al. Instrumentation failure following pedicle subtraction osteotomy: the role of rod material, diameter, and multi-rod constructs. Eur Spine J 2017;26:764–70. [12] Hallager DW, Gehrchen M, Dahl B, Harris JA, Gudipally M, Jenkins S, et al. Use of supplemental short pre-contoured accessory rods and cobalt chrome alloy posterior rods reduces primary rod strain and range of motion across the pedicle subtraction osteotomy level: an in vitro biomechanical study. Spine (Phila Pa 1976) 2016(41):E388–95. [13] Gupta S, Eksi MS, Ames CP, Deviren V, Durbin-Johnson B, Smith JS, et al. A novel 4-rod technique offers potential to reduce rod breakage and pseudarthrosis in pedicle subtraction osteotomies for adult spinal deformity correction. Oper Neurosurg (Hagerstown) 2018;14:449–56. [14] Tang JA, Leasure JM, Smith JS, Buckley JM, Kondrashov D, Ames CP. Effect of severity of rod contour on posterior rod failure in the setting of lumbar pedicle subtraction osteotomy (PSO): a biomechanical study. Neurosurgery 2013;72:276–82. discussion 83. [15] Rho JY, Hobatho MC, Ashman RB. Relations of mechanical properties to density and CT numbers in human bone. Med Eng Phys 1995;17:347–55. [16] 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. [17] Brodke DS, Bachus KN, Mohr RA, Nguyen BK. Segmental pedicle screw fixation or cross-links in multilevel lumbar constructs. A biomechanical analysis. Spine J 2001;1:373–9. [18] Scheer JK, Tang JA, Deviren V, Buckley JM, Pekmezci M, McClellan RT, et al. Biomechanical analysis of revision strategies for rod fracture in pedicle subtraction osteotomy. Neurosurgery 2011;69:164–72. discussion 72. [19] Kuklo TR, Dmitriev AE, Cardoso MJ, Lehman Jr RA, Erickson M, Gill NW. Biomechanical contribution of transverse connectors to segmental stability following long segment instrumentation with thoracic pedicle screws. Spine (Phila Pa 1976) 2008(33):E482–7. [20] Dhawale AA, Shah SA, Yorgova P, Neiss G, Layer Jr DJ, Rogers KJ, et al. Effectiveness of cross-linking posterior segmental instrumentation in adolescent idiopathic scoliosis: a 2-year follow-up comparative study. Spine J 2013;13:1485–92. [21] Garg S, Niswander C, Pan Z, Erickson M. Cross-links do not improve clinical or radiographic outcomes of posterior spinal fusion with pedicle screws in adolescent idiopathic scoliosis: a multicenter cohort study. Spine Deform 2015;3:338–44. [22] Park TH, Woo SH, Lee SJ, Sohn DM, Chung CK, Kim YJ, et al. Cross-link is a risk factor for rod fracture at pedicle subtraction osteotomy site: a finite element study. J Clin Neurosci 2019;66:246–50. [23] Wedemeyer M, Parent S, Mahar A, Odell T, Swimmer T, Newton P. Titanium versus stainless steel for anterior spinal fusions: an analysis of rod stress as a predictor of rod breakage during physiologic loading in a bovine model. Spine (Phila Pa 1976) 2007(32):42–8.
Please cite this article as: J. B. Hong, D. M. Son, T. H. Park et al., Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2020.01.053
Contents lists available at ScienceDirect
Journal of Clinical Neuroscience journal homepage: www.elsevier.com/locate/jocn
Experimental study
Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study Je Beom Hong a,1, Dong Min Son b,1, Tae Hyun Park b,c, Su Heon Woo c, Sung Jae Lee b, Un Yong Choi d, In-bo Han d, Chun Kee Chung e,f,g,h, Yongjung Jay Kim i, Seil Sohn d,2 a
Department of Neurosurgery, Kangbuk Samsung Hospital, Sungkyunkwan University School of Medicine, Seoul, Republic of Korea Department of Biomedical Engineering, College of Biomedical Science& Engineering, Inje University, Gyeongnam, Republic of Korea R&D Center, Medyssey Co, Ltd, Jechon, Republic of Korea d Department of Neurosurgery, CHA University, CHA Bundang Medical Center, Republic of Korea e Department of Neurosurgery, Seoul National University College of Medicine, Republic of Korea f Neuroscience Research Institute, Seoul National University Medical Research Center, Republic of Korea g Clinical Research Institute, Seoul National University Hospital, Republic of Korea h Department of Brain and Cognitive Sciences, Seoul National University College of Natural Sciences, Republic of Korea i 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 29 October 2019 Accepted 11 January 2020 Available online xxxx Keywords: Cross-link Rod fracture Pedicle subtraction osteotomy Spine Finite element analysis
a b s t r a c t Purpose: The purpose of this study is to investigate the effect of the cross-link position on the rod fracture phenomenon during pedicle subtraction osteotomy (PSO) surgery using finite element model (FEM). Methods: A three-dimensional finite element model of a lumbar spine with sagittal imbalance was constructed using computed tomography data of a 65-year-old female patient. After simulating the standard PSO at the L4 level, we constructed four models, specifically a model without a cross-link and three models with a cross-link at three different sites. The peak von Mises stress (PVMS) of the rod around the PSO site was measured after applying physiological loads (flexion, extension, axial rotation, and lateral bending) in each model. Results: The measured PVMS outcomes at the PSO site were 135.8, 135.9, 208.9, and 384.7 MPa for model 1, 2, 3, and 4 during flexion, and 180.0, 180.1, 210.1, and 445.7 MPa during extension. These results show that when the cross-link is located at the PSO site, the rod stress at the PSO site increases significantly during flexion and extension. As the cross-link moved away from the PSO site, the effect on the rod stress decreased. When the cross-link was placed two levels away from the PSO site, the rod stress was scarcely affected. Conclusion: When the cross-link during PSO surgery was positioned two levels away from the PSO site, the risk of rod fracture did not increase. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Pedicle subtraction osteotomy (PSO), originally introduced by Thomasen [1], can increase lordosis by approximately 30 to 40 degrees and is known to be an effective treatment for fixed sagittal imbalance [2–5]. However, multiple complications can occur [6,7]. The risk of both rod fracture and late complications such as pseudoarthrosis due to rod stress reportedly ranges from 15.8% to 22% [3,4,8–10].
1
Je Beom Hong and Dong Min Sohn contributed equally to this work. Department of Neurosurgery, CHA University College of Medicine, 59, Yatap-ro, Bundang-gu, Seongnam-si, Gyeonggi-do, 13496, Korea. E-mail address: [email protected] (S. Sohn)
Various attempts and numerous studies have been conducted to prevent these complications. Studies of different rod materials, contours and diameters, and trial with a wide variety of rod models have been done [9,11–14]. However, there remains no established surgical technique to prevent rod failure. Cross-links are also used by many surgeons, but little research has been done on the proper location of a cross-link during PSO. In this study, we used finite element model (FEM) to investigate the effects of the cross-link position on the rod stress during lumbar PSO. 2. Methods
2
A three-dimensional finite element lumbar spine model with sagittal imbalance was constructed using computed tomography
https://doi.org/10.1016/j.jocn.2020.01.053 0967-5868/Ó 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: J. B. Hong, D. M. Son, T. H. Park et al., Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2020.01.053
2
J.B. Hong et al. / Journal of Clinical Neuroscience xxx (xxxx) xxx
Fig. 1. Finite element model (FEM): a) distribution of the material properties based on Hounsfield units, and b) preoperative three-dimensional computed tomography.
Fig. 2. Four different finite element lumbar spine models according to the cross-link position for each model.
Fig. 3. Stress distribution of the FEM under physiological loadings (flexion, extension).
data of a 65-year-old female and Mimics (Materialise US, Plymouth, MI, USA) software. The mesh was generated using 3-matics (Materialise, Ann Arbor, MI, USA) software. The material properties (density, stiffness, poisson’s ratio) were applied to each element of the lumbar spine. Each value was obtained from a study by Rho et al. [15]. Osteotomy was performed with a 30° wedge on L4 of the intact lumbar model (Fig. 1). A pedicle screw system (IliadÒ,
Medyssey, Korea) was applied to L1-L5 according to standard surgical techniques (Without a cross-link, Model 1). To compare the effects of different cross-link positions, four models were created by applying a cross-link to three different sites. In the model 1, we did not position a cross-link. We positioned a cross-link at the L1/2 level in the model 2, at the L2/3 level in the model 3, and at the L3/5 level (PSO level) in the model 4 (Fig. 2).
Please cite this article as: J. B. Hong, D. M. Son, T. H. Park et al., Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2020.01.053
3
J.B. Hong et al. / Journal of Clinical Neuroscience xxx (xxxx) xxx
Fig. 4. Comparison of the peak von Mises stress (PVMS) values for each model at the pedicle subtraction osteotomy (PSO) site. Table 1 The peak von Mises stress (PVMS) values at Pedicle Subtraction Osteotomy site in each type model.
Type Type Type Type
1 2 3 4
Flexion
Extension
Axial rotation
Lateral bending
135.8 135.9 208.9 384.7
180.0 180.1 210.1 445.7
29.0 29.0 29.1 29.1
67.8 67.8 72.6 68.1
Surface-to-surface contact was defined along the PSO site after osteotomy with a friction coefficient of 0.46 [11]. The lower endplate of L5 was fixed and a compressive follower load of 400 N was applied (Fig. 3.) [16]. Pure moment of 10Nm was applied to the upper endplate of L1 in the direction of flexion, extension, axial rotation, and lateral bending for a physiological load [11]. A tie contact was applied assuming that the bony structure and the parts of the implant were in a fully fused state. For each loading condition, we measured the peak von Mises stress (PVMS) applied to the rods around the PSO site using Abaqus (version 6.14, Dassault Systèmes, France) software. 3. Results The PVMS outcomes at the L3/5 (PSO site) are shown in Fig. 4 and Table 1. During flexion, the PVMS values were 135.8, 135.9, 208.9, and 384.7 MPa for the model 1, 2, 3, and 4 models, respectively. During extension, the PVMS values were 180.0, 180.1, 210.1, and 445.7 MPa for the model 1, 2, 3, and 4, respectively. During axial rotation, the PVMS values were 29.0, 29.0, 29.1, and 29.1 for the model 1, 2, 3, and 4, respectively. During lateral bending, the PVMS values were 67.8, 67.8, 72.6, and 68.1 MPa for the corresponding models. With regard to model 4, where cross-links were located at the PSO site, the largest PVMS increase of the PSO site rod was found during flexion and extension loading. In model 2 and model 3, during flexion and extension, the PVMS values of the rod at the PSO site decreased when the cross-link was located away from the PSO site. In particular, we found that the rod stress in the model 2 was nearly identical to that in the model 1. Besides, during axial rotation and lateral bending, the position of the cross-link did not affect the PVMS of the rod stress significantly. 4. Discussion Surgeons often use a cross-link as a supplement to increase the stability of the rods during PSO. A cross-link is known to increase the stiffness of the pedicle screw system during axial rotation and to provide stability to surgical constructs [17–19]. However,
other studies have reported that cross-links do not make a difference in clinical outcomes [20,21]. In our previous study, we showed that the risk of a rod fracture is increased when a cross-link is applied to the PSO site [22]. Similarly, in the current study, the PVMS values increased by 283.3% (from 135.8 to 384.7 MPa) in flexion and 247.6% (from 180.0 to 445.7 MPa) in extension when cross-links were placed at the PSO site. Furthermore, here we used three different cross-link positions. As the position of the cross-link moved away from the PSO site, the increment in the PVMS of the rod at the PSO site decreased during flexion and extension. In addition, the cross-link when located two levels away from the PSO site had little effect on the rod stress. Through this study, we can suggest that the location of the crosslink is safe when it is two segments away from the PSO site. We also considered how meaningful the range of stress values in this study would be in actual clinical situations. When the material used in rods is titanium alloy, the yield stress is known to be 795 MPa [23]. This value is larger than the range of PVMS values for each model in this study. However, because stress can be repeatedly exerted for extended amounts of time in clinical situations, the results of this study are meaningful. Several limitations of this study should be noted. First, we used a single FEM based on the computed tomography of one patient. Therefore, our model may not be applicable to every sagittal imbalance situation. Second, our model only represented the anatomy and biomechanical properties of bones. Nonetheless, to the best of our knowledge, this is the first biomechanical study to elucidate the effect of the cross-link position during lumbar PSO surgery. 5. Conclusions A cross-link two levels away from the PSO site does not increase the risk of rod fracture. Funding This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded
Please cite this article as: J. B. Hong, D. M. Son, T. H. Park et al., Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2020.01.053
4
J.B. Hong et al. / Journal of Clinical Neuroscience xxx (xxxx) xxx
by the Ministry of Education (NRF-2018R1D1A1B07048252) and 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 (grant number: HI16C1559). 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. Declaration of Competing Interest The authors have no personal financial or institutional interest in any of the drugs, materials, or devices described in this article. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jocn.2020.01.053. References [1] Thomasen E. Vertebral osteotomy for correction of kyphosis in ankylosing spondylitis. Clin Orthop Relat Res 1985;142–52. [2] Kim YJ, Bridwell KH, Lenke LG, Cheh G, Baldus C. Results of lumbar pedicle subtraction osteotomies for fixed sagittal imbalance: a minimum 5-year follow-up study. Spine (Phila Pa 1976) 2007(32):2189–97. [3] Berjano P, Bassani R, Casero G, Sinigaglia A, Cecchinato R, Lamartina C. Failures and revisions in surgery for sagittal imbalance: analysis of factors influencing failure. Eur Spine J 2013;22(Suppl 6):S853–8. [4] Bridwell KH, Lewis SJ, Lenke LG, Baldus C, Blanke K. Pedicle subtraction osteotomy for the treatment of fixed sagittal imbalance. J Bone Joint Surg Am 2003:454–63. [5] Gupta S, Gupta MC. The nuances of pedicle subtraction osteotomies. Neurosurg Clin N Am 2018;29:355–63. [6] Charles YP, Yu B, Steib JP. Sacroiliac joint luxation after pedicle subtraction osteotomy: report of two cases and analysis of failure mechanism. Eur Spine J 2016;25(Suppl 1):63–74. [7] Smith JS, Sansur CA, Donaldson 3rd WF, Perra JH, Mudiyam R, Choma TJ, et al. Short-term morbidity and mortality associated with correction of thoracolumbar fixed sagittal plane deformity: a report from the Scoliosis Research Society Morbidity and Mortality Committee. Spine (Phila Pa 1976) 2011(36):958–64.
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Please cite this article as: J. B. Hong, D. M. Son, T. H. Park et al., Risk of rod fracture according to cross-link position in pedicle subtraction osteotomy (PSO): A finite element study, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2020.01.053