Cell Biochem Biophys DOI 10.1007/s12013-014-9963-y
ORIGINAL PAPER
Finite Element Analysis of Minimal Invasive Transforaminal Lumbar Interbody Fusion Chuncheng Zhao • Xinhu Wang • Changchun Chen Yanzhong Kang
•
Ó Springer Science+Business Media New York 2014
Abstract The purpose of our study is to develop and validate three-dimensional finite element models of transforaminal lumbar interbody fusion, and explore the most appropriate method of fixation and fusion by comparing biomechanical characteristics of different fixation method. We developed four fusion models: bilateral pedicle screws fixation with a single cage insertion model (A), bilateral pedicle screws fixation with two cages insertion model (B), unilateral pedicle screws fixation with a single cage insertion model (C), and unilateral pedicle screws fixation with two cages insertion model (D); the models were subjected to different forces including anterior bending, posterior extension, left bending, right bending, rotation, and axial compressive. The von Mises stress of the fusion segments on the pedicle screw and cages was recorded. Angular variation and stress of pedicle screw and cage were compared. There were differences of Von Mises peak stress among four models, but were within the range of maximum force. The angular variation in A, B, C, and D decreased significantly compared with normal. There was no significant difference of angular variation between A and B, and C and D. Bilateral pedicle screws fixation had more superior biomechanics than unilateral pedicle screws fixation. In conclusion, the lumbar interbody fusion models were established using varying fixation methods, and the results verified that unilateral pedicle screws fixation with a
C. Zhao (&) C. Chen Y. Kang Second Department of Orthopedic, Baoji Centre Hospital, Baoji 721008, Shannxi, China e-mail: [email protected] X. Wang Third Department of Orthopedic, Baoji Centre Hospital, Baoji 721008, Shannxi, China
single cage could meet the stability demand in minimal invasive transforaminal interbody fusion. Keywords Finite element analysis Lumbar spine Lumbar fusion Biomechanics With the continuous development of an aging society, lumbar degenerative disease has become a critical problem which affects public health [1–3]. Transforaminal lumbar interbody fusion (TLIF) has been widely applied in clinical, in which only unilateral zygapophyseal joint is removed, causing less effect on dura mater and nerve root [4, 5]. It has more advantages than posterior lumbar interbody fusion [6, 7]. Furthermore, minimal invasive TLIF can avoid continuous pull force on paravertebral muscles in routine TLIF, having a quicker recovery of waist and back muscle strength and shorter postoperative time in bed [8, 9]. Our research developed finite element model of L4–L5 segment and minimal invasive TLIF pedicle screw posterior fixation system to observe angular variation and stress of pedicle screw and cage of different models under various operating conditions, which provided a theoretical basis for the selection and clinical application of minimal invasive intervertebral fusion and fixation method.
Materials and Methods Establishment of Three-Dimension Finite Element Analysis One Chinese male volunteer without history of lumbar disease was selected as modeling material. A 64-row spiral
123
Cell Biochem Biophys
CT was applied for continuous transverse scan on L4–L5 segment to obtain continuous cross-section image with thickness of 1 mm and saved in computer with DICOM format of cross-section image. 3 Dimensional reconstruction software mimics 10.0 were applied to establish 3 dimension computer model of L4–L5 segment. Pre-processing function of finite element software Ansys was used to add structures such as cortical bone, cancellous bone, intervertebral disk, anterior and posterior longitudinal ligament, ligamenta flava, interspinous ligament, and supraspinous ligament based on the segmental bony structure. Appropriate unit type and material property were used for finite element mesh of models, and handled joint surface of adjacent articular processes by contact element to ensure that zygapophysial joints can function properly in maintaining stability of spinal function structure. According to related references [10–12], appropriate unit type and material property were used for finite element mesh of models; material factors of each part material such as elasticity modulus, Poisson’s ratio, and their characteristic value were input in model to complete establishment of normal human L4–L5 segment finite element model (INT) (Tables 1 and 2). Establishment of Different Fusion and Fixation Models of Minimal Invasive TLIF Data of lumbar vertebra internal fixation system were input by our operators using AutoCAD2007 drawing software and were saved as STL format. According to minimal invasive TLIF clinical surgical method, in experimental simulation, pedicle screw diameter was 6.0 mm, length was 55 mm, elasticity modulus of screw was
Table 1 Material property of bone and intervertebral disk in lumber vertebra infinite element model Material
Elasticity modulus (MPa)
Vertebral body Cortical 12,000 bone
Poisson’s ratio
property
0.3
Homogeneous and isotropic
Main ligament
Anterior longitudinal ligament
Elasticity modulus (MPa) 7.8
Cross sectional area (mm2)
Mean length (mm)
22.4
20
8.74
Rigidity
Posterior longitudinal ligament
10
7.0
12
5.83
Ligamenta flavum
17
14.1
15
15.38
Intertransverse ligament
10
0.6
32
0.19
10.5
5
15.75
14.1
13
10.85
10.5
22
2.39
Capsular ligament Interspinous ligament Supraspinous ligament
7.5 10 8.0
Rigidity = elasticity modulus*cross sectional area/mean length
1,10,000 MPa, Poisson’s ratio was 0.3, the relation with vertebral body was designed as elastic fixation, screw, and connecting rod were integration; cage shape was designed as rectangle: single cage: length 24 mm, width 12 mm, height 11 mm; double cage: length 22 mm, width 8 mm, height 11 mm. Elasticity modulus was 3,700 MPa, Poisson’s ratio was 0.25, the contact with vertebral endplate was facet contact, and contact point was subchondral bone of end plate. Three-dimensional geometric modeling function of three-dimension reconstruction software Mimics 10.0 was used to establish the following fusion and fixation models: single cage?bilateral pedicle screws (Model A); two cage?bilateral pedicle screws (Model B); single cage?unilateral pedicle screw (Model C); and two cage?unilateral pedicle screw (Model D). Loading and Recording Method
Cancellous bone
100
0.2
Homogeneous and isotropic
Cartilage endplate Posterior structure
12,000
0.3
3,500
0.25
Homogeneous and isotropic Homogeneous and isotropic
Intervertebral disk Fibrous ring
6
0.4
Homogeneous and isotropic
Nucleus pulposus
0.0013
0.4999
Homogeneous and isotropic
123
Table 2 Material properties of main ligaments in lumber vertebra infinite element model
Surface of L5 inferior endplates was all fixed in different models, 500 N loading force was applied on L4 vertebral body upper endplate to simulate daily walking posture, meanwhile 15 Nm movement additional force was applied on upper surface of L4 vertebral body. Mimics software was used to calculate under axial compressive, anterior bending, posterior extension, left bending, right bending, left rotation, and right rotation. The main observational parameters included 1. range of motion, presented as angular variation of L4–L5 segment. The spatial coordinates of most anterior point, the most posterior point, the left-most point, and the right-most point of L4 and L5 were
Cell Biochem Biophys Table 3 The Von Mises peak stress of screw and cage in different models under different operating conditions (MPa) Axial compressive
Anterior bending
Posterior extension
Left bending
Right bending
Left rotation
Right rotation
Model A
534
417
1,005
1,218
485
784
397
Model B
531
401
988
1,189
469
774
378
Model C
673
524
1,488
1,379
576
836
471
Model D
641
497
1,339
1,166
505
799
412
Pedicle screw
Cage Model A
43.4
53.2
38.6
49.3
57.3
41.4
43.2
Model B
39.5
47.8
32.9
44.1
56.2
37.7
39.3
Model C
92.3
137.4
97.5
59.4
152.1
88.5
92.5
Model D
85.6
121.5
88.3
50.5
142.8
81.3
84.5
connected as lines, included angles between each line presented included angles of upper surfaces between adjacent vertebral bodies, the absolute value of included angle difference between before and after loading was angular variation of L4–L5. 2. Stress of pedicle screw and cage: The Von Mises peak stress of screw and cage under different operating conditions were directly recorded, and mean of two cages was taken.
Results L4–L5 Stress of Pedicle Screw and Cage in Different Models Under Different Operating Conditions is Shown in Table 3 Through comparison of Model A and Model B, Model C and Model D, it was found that stress of pedicle screw and cage in two cages implanted intervertebral model was less than that of single cage implanted; through comparison of Model A and Model C, Model B and Model D, it was found that stress of pedicle screw and cage in unilateral internal fixation was more than that of bilateral fixation. Von Mises peak stress of pedicle screw and cage was within the range of maximum force under different operating conditions, indicating that either unilateral/bilateral or single/double cage fusion had the same effect. Angular Variation of L4–L5 Segment in Different Models Under Different Operating Conditions is Shown in Table 4 Through comparison of Model A and Model B, Model C and Model D, two cages could maintain good angle stability; through comparison of Model A and Model C, Model B and Model D, it was found that angular variation of unilateral pedicle screw fixation was bigger. However, angular variation of all fixation and fusion models was
significantly decreased compared to INT, indicating that unilateral pedicle screw fixation could meet demand for spinal stability and provided a fixation method for minimal invasive TLIF.
Discussion Surgical treatment in treating lumbar vertebra degenerative disease can keep three columns strong and stable and recover physiological curvature of lumbar vertebra by pedicle screw system [3, 13, 14]. Intervertebral fusion can maintain the normal alignment of spine, preventing the reoccurrence of inability, which is the guarantee of postoperative long-term curative effects [15–18]. Along with the improvement of intervertebral cage and fusion method, two cages and single cage were gradually applied in intervertebral fusion. Schils et al. [19] considered that two carbon fiber cages based on combination with pedicle screw fixation system had great advantages on clinical curative effect, maintenance of intervertebral space height, and vertebra fusion rate compared to simple pedicle screw fixation system. Two cages implanted in intervertebral fusion not only required wide excision of bilateral zygopophysis and laminectomy, but also needed to over pull cauda equine and bilateral nerve root [15, 20, 21]. This can destroy the posterior structure and stability of spine and is also a potential risk factor of injuring nerve root of cauda equina. Morelan et al. [22] applied pedicle screw fixation combined with posterior single cage obliquely in early stage, which only needed to remove unilateral zygopophysis and half vertebral plate and met the demand of fusion by keeping completion of intervertebral structure. Tsuang et al. [23] established infinite element models including half vertebral plate incision, whole vertebral plate incision plus intervertebral joint incision and single or two cage implantation. Through comparison of maximum Von Mises stress under different loads, posterior fixation device
123
Cell Biochem Biophys Table 4 Angular variation of L4–L5 segment in different models under different operating conditions (degree) Anterior bending
Posterior extension
Left bending
Right bending
Left rotation
Right rotation
INT
3.41
3.61
1.74
1.74
1.79
1.79
Model A
0.14
0.11
0.19
0.32
0.17
0.15
Model B
0.12
0.09
0.18
0.29
0.16
0.14
Model C
0.39
0.61
0.29
0.81
0.40
0.35
Model D
0.36
0.57
0.23
0.77
0.32
0.29
combined with single cage implantation could provide better stability, and there was no stress difference between single and two cages intervertebral implantation. Our research established infinite element model of minimal invasive intervertebral foramen lumbar fusion, considered that model angular variation after intervertebral implantation of single or two cages was less than that of INT. Although in single cage implantation, stress of pedicle screw and cage was more, the stress was still within the maximum range, indicating that the single cage implantation could provide spinal stability and act as fusion method of minimal invasive intervertebral foramen lumbar fusion. Pedicle screw fusion assisted by spinal fusion can immediately provide stability of lumbar vertebra, patients has early ambulation, and the fusion rate is high [24–26]; but it also changes load of adjacent segment intervertebral disk and zygopophysis to accelerate degeneration of adjacent motion segment [27]. To decrease the strength problem of internal spinal fixation, some scholars reported that unilateral pedicle internal fixation system could be applied to decrease fixation strength to improve degeneration of adjacent joint [28–30]. Zang et al. [31] conducted the metaanalysis of unilateral and bilateral pedicle screw internal fixation on curative effects after short segmental lumbar fusion surgery; they reported that unilateral pedicle screw fixation could significantly reduce operative time, intraoperative blood loss, and hospitalization cost, and there was no difference on fusion rate, complication rate, and good rate compared with bilateral fixation. Srirekha and Bashetty [11] established different internal fixation models of L4–L5, by infinite element analysis to analyze angular variation, pedicle screw, and cage stress distribution of L4– L5 under different operating conditions, we considered that unilateral pedicle screw fixation combined with single cage implantation could provide enough stability; however, peak stress of cage was significantly higher than that of bilateral pedicle screw, indicating that possibility of cage sedimentation was higher in unilateral pedicle screw fixation than in bilateral pedicle screw fixation. Our research established infinite element analysis: Von Mises peak stress of screw and cage in unilateral and bilateral pedicle screw fixation was within the maximum stress range. Although angular variation of unilateral pedicle screw fixation was
123
bigger than that of bilateral pedicle screw fixation, angular variations in all fixation and fusion models were smaller than that in INT, indicating that either unilateral pedicle screw fixation or bilateral screw fixation could provide enough stability. Besides, for the patients only with unilateral nerve root symptoms, bilateral pedicle screw fixation destroyed the spinal stability because of exposure and screw placement on asymptomatic side, indicating that unilateral pedicle screw fixation can be applied as fixation method of minimal invasive TLIF. Infinite element analysis can simulate human body structure and gives biomechanical characteristics similar to real structure, which can be applied in the researches of spinal dynamics, and stress change of spine and internal intervertebral disk [4]. Compared to animal model and cadaver model, computer model simulation can collect the stress distribution condition of vertebral body, intervertebral disk, zygopophysis and fixation device; eliminate individual difference; and reduce experiment cost [2]. Infinite element analysis is repeatable, can analyze the physiological status that cannot be explored by biological experiment by simulation analysis, and is effective supplement of cadaveric experiment model of human body in vitro [3]. As a mathematical modeling method of theoretical mechanics, although infinite analysis has a lot of irreplaceable advantages, it is abstract of objective things. Simplify, hypothesis and material assignment are inevitably different from the real structure. Lumbar spine mechanics is complicated and related with body weight, muscles around lumbar vertebra, ligament, and abdominal pressure. Thus, the conclusion by infinite element analysis must have hypothesis and limitation, which has to be examined by experimental research.
References 1. Phillips, F. M., Slosar, P. J., Youssef, J. A., Andersson, G., & Papatheofanis, F. (2013). Lumbar spine fusion for chronic low back pain due to degenerative disc disease: A systematic review. Spine, 38(7), E409–E422. 2. Taher, F., Essig, D., Lebl, D. R., Hughes, A. P., Sama, A. A., Cammisa, F. P., et al. (2012). Lumbar degenerative disc disease:
Cell Biochem Biophys
3. 4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
current and future concepts of diagnosis and management. Advances in Orthopedics, 2012, 970752. Eliyas, J. K., & Karahalios, D. (2011). Surgery for degenerative lumbar spine disease. Disease-a-Month, 57(10), 592–606. Gu, G., Zhang, H., Fan, G., He, S., Cai, X., Shen, X., et al. (2013). Comparison of minimally invasive versus open transforaminal lumbar interbody fusion in two-level degenerative lumbar disease. International Orthopaedics, 38(4), 817–824. Brodano, G. B., Martikos, K., Lolli, F., Gasbarrini, A., Cioni, A., Bandiera, S., et al. (2013). Transforaminal lumbar interbody fusion in degenerative disc disease and spondylolisthesis grade i: Minimally invasive versus open surgery. Journal of Spinal Disorders & Techniques. doi:10.1097/BSD.0000000000000034. Kunze, B., Drasseck, T., & Kluba, T. (2011). Posterior and transforaminal lumbar interbody fusion (PLIF/TLIF) for the treatment of localised segment degeneration of lumbar spine. Zeitschrift fur Orthopadie und Unfallchirurgie, 149(3), 312–316. Cole, C. D., McCall, T. D., Schmidt, M. H., & Dailey, A. T. (2009). Comparison of low back fusion techniques: transforaminal lumbar interbody fusion (TLIF) or posterior lumbar interbody fusion (PLIF) approaches. Current Reviews in Musculoskeletal Medicine, 2(2), 118–126. Zairi, F., Allaoui, M., Thines, L., Arikat, A., & Assaker, R. (2013). Transforaminal lumbar interbody fusion: Goals of the minimal invasive approach. Neuro-Chirurgie, 59(4–5), 171–177. Rouben, D., Casnellie, M., & Ferguson, M. (2011). Long-term durability of minimal invasive posterior transforaminal lumbar interbody fusion: a clinical and radiographic follow-up. Journal of Spinal Disorders & Techniques, 24(5), 288–296. Remillieux, M. C., & Burdisso, R. A. (2012). Vibro-acoustic response of an infinite, rib-stiffened, thick-plate assembly using finite-element analysis. The Journal of the Acoustical Society of America, 132(1), EL36–EL42. Srirekha, A., & Bashetty, K. (2010). Infinite to finite: an overview of finite element analysis. Indian Journal of Dental Research, 21(3), 425–432. Xin, H., Ma, X., Ying, L., Zhang, S., & Qian, Z. (2002). The application of infinite element method to endodontic endosseous implant stress analysis. Zhonghua kou qiang yi xue za zhi = Zhonghua kouqiang yixue zazhi =. Chinese Journal of Stomatology, 37(3), 183–186. van den Eerenbeemt, K. D., Ostelo, R. W., van Royen, B. J., Peul, W. C., & van Tulder, M. W. (2010). Total disc replacement surgery for symptomatic degenerative lumbar disc disease: A systematic review of the literature. European Spine Journal, 19(8), 1262–1280. Liu, H. Y., Zhou, J., Wang, B., Wang, H. M., Jin, Z. H., Zhu, Z. Q., et al. (2012). Comparison of Topping-off and posterior lumbar interbody fusion surgery in lumbar degenerative disease: A retrospective study. Chinese Medical Journal, 125(22), 3942– 3946. Cho, C. B., Ryu, K. S., & Park, C. K. (2010). Anterior lumbar interbody fusion with stand-alone interbody cage in treatment of lumbar intervertebral foraminal stenosis : Comparative study of two different types of cages. Journal of Korean Neurosurgical Society, 47(5), 352–357. Bao, D., Ma, Y., Chen, X., Li, H., & Gao, T. (2009). Preliminary clinical study of anchoring cervical intervertebral fusion cage. Zhongguo xiu fu chong jian wai ke za zhi = Zhongguo xiufu chongjian waike zazhi =Chinese Journal of Reparative and Reconstructive Surgery, 23(4), 389–392. Tchako, A., & Sadegh, A. M. (2009). Stress changes in intervertebral discs of the cervical spine due to partial discectomies and fusion. Journal of Biomechanical Engineering, 131(5), 051013.
18. Goto, K., Tajima, N., Chosa, E., Totoribe, K., Kubo, S., Kuroki, H., et al. (2003). Effects of lumbar spinal fusion on the other lumbar intervertebral levels (three-dimensional finite element analysis). Journal of Orthopaedic Science, 8(4), 577–584. 19. Schils, F., Rilliet, B., & Payer, M. (2006). Implantation of an empty carbon fiber cage or a tricortical iliac crest autograft after cervical discectomy for single-level disc herniation: A prospective comparative study. Journal of Neurosurgery Spine, 4(4), 292–299. 20. Joo, Y. H., Lee, J. W., Kwon, K. Y., Rhee, J. J., & Lee, H. K. (2010). Comparison of fusion with cage alone and plate instrumentation in two-level cervical degenerative disease. Journal of Korean Neurosurgical Society, 48(4), 342–346. 21. Ren, H., Lai, Z., Biggs, J. D., Wang, J., & Mukamel, S. (2013). Two-dimensional stimulated resonance Raman spectroscopy study of the Trp-cage peptide folding. Physical Chemistry Chemical Physics, 15(44), 19457–19464. 22. Moreland, D. B., Asch, H. L., Czajka, G. A., Overkamp, J. A., & Sitzman, D. M. (2009). Posterior lumbar interbody fusion: comparison of single intervertebral cage and single side pedicle screw fixation versus bilateral cages and screw fixation. Minimally Invasive Neurosurgery, 52(3), 132–136. 23. Tsuang, Y. H., Chiang, Y. F., Hung, C. Y., Wei, H. W., Huang, C. H., & Cheng, C. K. (2009). Comparison of cage application modality in posterior lumbar interbody fusion with posterior instrumentation—a finite element study. Medical Engineering & Physics, 31(5), 565–570. 24. Tanaka, M., Nakanishi, K., Sugimoto, Y., Misawa, H., Takigawa, T., Nishida, K., et al. (2009). Computer navigation-assisted spinal fusion with segmental pedicle screw instrumentation for scoliosis with Rett syndrome: A case report. Acta Medica Okayama, 63(6), 373–377. 25. Wang, J., Wu, H., Ding, X., & Liu, Y. (2008). Treatment of thoracolumbar vertebrate fracture by transpedicular morselized bone grafting in vertebrae for spinal fusion and pedicle screw fixation. Journal of Huazhong University of Science and Technology Medical sciences = Hua zhong ke ji da xue xue bao Yi xue Ying De wen ban = Huazhong keji daxue xuebao Yixue Yingdewen ban, 28((3), 322–326. 26. Zhao, Y., Wang, Z., Zhu, X., Wang, C., He, S., & Li, M. (2011). Prediction of postoperative trunk imbalance after posterior spinal fusion with pedicle screw fixation for adolescent idiopathic scoliosis. Journal of Pediatric Orthopedics. Part B, 20(4), 199–208. 27. Bode, K. S., & Newton, P. O. (2007). Pediatric nonaccidental trauma thoracolumbar fracture-dislocation: posterior spinal fusion with pedicle screw fixation in an 8-month-old boy. Spine, 32(14), E388–E393. 28. Wang, J., & Zhang, C. (2010). Preliminary evaluation and clinical application of unilateral decompression, interbody fusion and pedicle screw fixation under endoscopic system. Zhongguo gu shang = China Journal of Orthopaedics and Traumatology, 23(5), 360–364. 29. Lin, B., Xu, Y., He, Y., Zhang, B., Lin, Q., & He, M. (2013). Minimally invasive unilateral pedicle screw fixation and lumbar interbody fusion for the treatment of lumbar degenerative disease. Orthopedics, 36(8), e1071–e1076. 30. Ding, W., Chen, Y., Liu, H., Wang, J., & Zheng, Z. (2013). Comparison of unilateral versus bilateral pedicle screw fixation in lumbar interbody fusion: a meta-analysis. European Spine Journal, 23(2), 395–403. 31. Zang, J. C., Ma, X. L., Wang, T., Ma, J. X., Tian, P., & Han, C. (2012). Unilateral versus bilateral pedicle screw fixation in lumbar spinal fusion: A meta-analysis of available evidence. Zhonghua wai ke za zhi [Chinese Journal of Surgery], 50(9), 848–853.
123
ORIGINAL PAPER
Finite Element Analysis of Minimal Invasive Transforaminal Lumbar Interbody Fusion Chuncheng Zhao • Xinhu Wang • Changchun Chen Yanzhong Kang
•
Ó Springer Science+Business Media New York 2014
Abstract The purpose of our study is to develop and validate three-dimensional finite element models of transforaminal lumbar interbody fusion, and explore the most appropriate method of fixation and fusion by comparing biomechanical characteristics of different fixation method. We developed four fusion models: bilateral pedicle screws fixation with a single cage insertion model (A), bilateral pedicle screws fixation with two cages insertion model (B), unilateral pedicle screws fixation with a single cage insertion model (C), and unilateral pedicle screws fixation with two cages insertion model (D); the models were subjected to different forces including anterior bending, posterior extension, left bending, right bending, rotation, and axial compressive. The von Mises stress of the fusion segments on the pedicle screw and cages was recorded. Angular variation and stress of pedicle screw and cage were compared. There were differences of Von Mises peak stress among four models, but were within the range of maximum force. The angular variation in A, B, C, and D decreased significantly compared with normal. There was no significant difference of angular variation between A and B, and C and D. Bilateral pedicle screws fixation had more superior biomechanics than unilateral pedicle screws fixation. In conclusion, the lumbar interbody fusion models were established using varying fixation methods, and the results verified that unilateral pedicle screws fixation with a
C. Zhao (&) C. Chen Y. Kang Second Department of Orthopedic, Baoji Centre Hospital, Baoji 721008, Shannxi, China e-mail: [email protected] X. Wang Third Department of Orthopedic, Baoji Centre Hospital, Baoji 721008, Shannxi, China
single cage could meet the stability demand in minimal invasive transforaminal interbody fusion. Keywords Finite element analysis Lumbar spine Lumbar fusion Biomechanics With the continuous development of an aging society, lumbar degenerative disease has become a critical problem which affects public health [1–3]. Transforaminal lumbar interbody fusion (TLIF) has been widely applied in clinical, in which only unilateral zygapophyseal joint is removed, causing less effect on dura mater and nerve root [4, 5]. It has more advantages than posterior lumbar interbody fusion [6, 7]. Furthermore, minimal invasive TLIF can avoid continuous pull force on paravertebral muscles in routine TLIF, having a quicker recovery of waist and back muscle strength and shorter postoperative time in bed [8, 9]. Our research developed finite element model of L4–L5 segment and minimal invasive TLIF pedicle screw posterior fixation system to observe angular variation and stress of pedicle screw and cage of different models under various operating conditions, which provided a theoretical basis for the selection and clinical application of minimal invasive intervertebral fusion and fixation method.
Materials and Methods Establishment of Three-Dimension Finite Element Analysis One Chinese male volunteer without history of lumbar disease was selected as modeling material. A 64-row spiral
123
Cell Biochem Biophys
CT was applied for continuous transverse scan on L4–L5 segment to obtain continuous cross-section image with thickness of 1 mm and saved in computer with DICOM format of cross-section image. 3 Dimensional reconstruction software mimics 10.0 were applied to establish 3 dimension computer model of L4–L5 segment. Pre-processing function of finite element software Ansys was used to add structures such as cortical bone, cancellous bone, intervertebral disk, anterior and posterior longitudinal ligament, ligamenta flava, interspinous ligament, and supraspinous ligament based on the segmental bony structure. Appropriate unit type and material property were used for finite element mesh of models, and handled joint surface of adjacent articular processes by contact element to ensure that zygapophysial joints can function properly in maintaining stability of spinal function structure. According to related references [10–12], appropriate unit type and material property were used for finite element mesh of models; material factors of each part material such as elasticity modulus, Poisson’s ratio, and their characteristic value were input in model to complete establishment of normal human L4–L5 segment finite element model (INT) (Tables 1 and 2). Establishment of Different Fusion and Fixation Models of Minimal Invasive TLIF Data of lumbar vertebra internal fixation system were input by our operators using AutoCAD2007 drawing software and were saved as STL format. According to minimal invasive TLIF clinical surgical method, in experimental simulation, pedicle screw diameter was 6.0 mm, length was 55 mm, elasticity modulus of screw was
Table 1 Material property of bone and intervertebral disk in lumber vertebra infinite element model Material
Elasticity modulus (MPa)
Vertebral body Cortical 12,000 bone
Poisson’s ratio
property
0.3
Homogeneous and isotropic
Main ligament
Anterior longitudinal ligament
Elasticity modulus (MPa) 7.8
Cross sectional area (mm2)
Mean length (mm)
22.4
20
8.74
Rigidity
Posterior longitudinal ligament
10
7.0
12
5.83
Ligamenta flavum
17
14.1
15
15.38
Intertransverse ligament
10
0.6
32
0.19
10.5
5
15.75
14.1
13
10.85
10.5
22
2.39
Capsular ligament Interspinous ligament Supraspinous ligament
7.5 10 8.0
Rigidity = elasticity modulus*cross sectional area/mean length
1,10,000 MPa, Poisson’s ratio was 0.3, the relation with vertebral body was designed as elastic fixation, screw, and connecting rod were integration; cage shape was designed as rectangle: single cage: length 24 mm, width 12 mm, height 11 mm; double cage: length 22 mm, width 8 mm, height 11 mm. Elasticity modulus was 3,700 MPa, Poisson’s ratio was 0.25, the contact with vertebral endplate was facet contact, and contact point was subchondral bone of end plate. Three-dimensional geometric modeling function of three-dimension reconstruction software Mimics 10.0 was used to establish the following fusion and fixation models: single cage?bilateral pedicle screws (Model A); two cage?bilateral pedicle screws (Model B); single cage?unilateral pedicle screw (Model C); and two cage?unilateral pedicle screw (Model D). Loading and Recording Method
Cancellous bone
100
0.2
Homogeneous and isotropic
Cartilage endplate Posterior structure
12,000
0.3
3,500
0.25
Homogeneous and isotropic Homogeneous and isotropic
Intervertebral disk Fibrous ring
6
0.4
Homogeneous and isotropic
Nucleus pulposus
0.0013
0.4999
Homogeneous and isotropic
123
Table 2 Material properties of main ligaments in lumber vertebra infinite element model
Surface of L5 inferior endplates was all fixed in different models, 500 N loading force was applied on L4 vertebral body upper endplate to simulate daily walking posture, meanwhile 15 Nm movement additional force was applied on upper surface of L4 vertebral body. Mimics software was used to calculate under axial compressive, anterior bending, posterior extension, left bending, right bending, left rotation, and right rotation. The main observational parameters included 1. range of motion, presented as angular variation of L4–L5 segment. The spatial coordinates of most anterior point, the most posterior point, the left-most point, and the right-most point of L4 and L5 were
Cell Biochem Biophys Table 3 The Von Mises peak stress of screw and cage in different models under different operating conditions (MPa) Axial compressive
Anterior bending
Posterior extension
Left bending
Right bending
Left rotation
Right rotation
Model A
534
417
1,005
1,218
485
784
397
Model B
531
401
988
1,189
469
774
378
Model C
673
524
1,488
1,379
576
836
471
Model D
641
497
1,339
1,166
505
799
412
Pedicle screw
Cage Model A
43.4
53.2
38.6
49.3
57.3
41.4
43.2
Model B
39.5
47.8
32.9
44.1
56.2
37.7
39.3
Model C
92.3
137.4
97.5
59.4
152.1
88.5
92.5
Model D
85.6
121.5
88.3
50.5
142.8
81.3
84.5
connected as lines, included angles between each line presented included angles of upper surfaces between adjacent vertebral bodies, the absolute value of included angle difference between before and after loading was angular variation of L4–L5. 2. Stress of pedicle screw and cage: The Von Mises peak stress of screw and cage under different operating conditions were directly recorded, and mean of two cages was taken.
Results L4–L5 Stress of Pedicle Screw and Cage in Different Models Under Different Operating Conditions is Shown in Table 3 Through comparison of Model A and Model B, Model C and Model D, it was found that stress of pedicle screw and cage in two cages implanted intervertebral model was less than that of single cage implanted; through comparison of Model A and Model C, Model B and Model D, it was found that stress of pedicle screw and cage in unilateral internal fixation was more than that of bilateral fixation. Von Mises peak stress of pedicle screw and cage was within the range of maximum force under different operating conditions, indicating that either unilateral/bilateral or single/double cage fusion had the same effect. Angular Variation of L4–L5 Segment in Different Models Under Different Operating Conditions is Shown in Table 4 Through comparison of Model A and Model B, Model C and Model D, two cages could maintain good angle stability; through comparison of Model A and Model C, Model B and Model D, it was found that angular variation of unilateral pedicle screw fixation was bigger. However, angular variation of all fixation and fusion models was
significantly decreased compared to INT, indicating that unilateral pedicle screw fixation could meet demand for spinal stability and provided a fixation method for minimal invasive TLIF.
Discussion Surgical treatment in treating lumbar vertebra degenerative disease can keep three columns strong and stable and recover physiological curvature of lumbar vertebra by pedicle screw system [3, 13, 14]. Intervertebral fusion can maintain the normal alignment of spine, preventing the reoccurrence of inability, which is the guarantee of postoperative long-term curative effects [15–18]. Along with the improvement of intervertebral cage and fusion method, two cages and single cage were gradually applied in intervertebral fusion. Schils et al. [19] considered that two carbon fiber cages based on combination with pedicle screw fixation system had great advantages on clinical curative effect, maintenance of intervertebral space height, and vertebra fusion rate compared to simple pedicle screw fixation system. Two cages implanted in intervertebral fusion not only required wide excision of bilateral zygopophysis and laminectomy, but also needed to over pull cauda equine and bilateral nerve root [15, 20, 21]. This can destroy the posterior structure and stability of spine and is also a potential risk factor of injuring nerve root of cauda equina. Morelan et al. [22] applied pedicle screw fixation combined with posterior single cage obliquely in early stage, which only needed to remove unilateral zygopophysis and half vertebral plate and met the demand of fusion by keeping completion of intervertebral structure. Tsuang et al. [23] established infinite element models including half vertebral plate incision, whole vertebral plate incision plus intervertebral joint incision and single or two cage implantation. Through comparison of maximum Von Mises stress under different loads, posterior fixation device
123
Cell Biochem Biophys Table 4 Angular variation of L4–L5 segment in different models under different operating conditions (degree) Anterior bending
Posterior extension
Left bending
Right bending
Left rotation
Right rotation
INT
3.41
3.61
1.74
1.74
1.79
1.79
Model A
0.14
0.11
0.19
0.32
0.17
0.15
Model B
0.12
0.09
0.18
0.29
0.16
0.14
Model C
0.39
0.61
0.29
0.81
0.40
0.35
Model D
0.36
0.57
0.23
0.77
0.32
0.29
combined with single cage implantation could provide better stability, and there was no stress difference between single and two cages intervertebral implantation. Our research established infinite element model of minimal invasive intervertebral foramen lumbar fusion, considered that model angular variation after intervertebral implantation of single or two cages was less than that of INT. Although in single cage implantation, stress of pedicle screw and cage was more, the stress was still within the maximum range, indicating that the single cage implantation could provide spinal stability and act as fusion method of minimal invasive intervertebral foramen lumbar fusion. Pedicle screw fusion assisted by spinal fusion can immediately provide stability of lumbar vertebra, patients has early ambulation, and the fusion rate is high [24–26]; but it also changes load of adjacent segment intervertebral disk and zygopophysis to accelerate degeneration of adjacent motion segment [27]. To decrease the strength problem of internal spinal fixation, some scholars reported that unilateral pedicle internal fixation system could be applied to decrease fixation strength to improve degeneration of adjacent joint [28–30]. Zang et al. [31] conducted the metaanalysis of unilateral and bilateral pedicle screw internal fixation on curative effects after short segmental lumbar fusion surgery; they reported that unilateral pedicle screw fixation could significantly reduce operative time, intraoperative blood loss, and hospitalization cost, and there was no difference on fusion rate, complication rate, and good rate compared with bilateral fixation. Srirekha and Bashetty [11] established different internal fixation models of L4–L5, by infinite element analysis to analyze angular variation, pedicle screw, and cage stress distribution of L4– L5 under different operating conditions, we considered that unilateral pedicle screw fixation combined with single cage implantation could provide enough stability; however, peak stress of cage was significantly higher than that of bilateral pedicle screw, indicating that possibility of cage sedimentation was higher in unilateral pedicle screw fixation than in bilateral pedicle screw fixation. Our research established infinite element analysis: Von Mises peak stress of screw and cage in unilateral and bilateral pedicle screw fixation was within the maximum stress range. Although angular variation of unilateral pedicle screw fixation was
123
bigger than that of bilateral pedicle screw fixation, angular variations in all fixation and fusion models were smaller than that in INT, indicating that either unilateral pedicle screw fixation or bilateral screw fixation could provide enough stability. Besides, for the patients only with unilateral nerve root symptoms, bilateral pedicle screw fixation destroyed the spinal stability because of exposure and screw placement on asymptomatic side, indicating that unilateral pedicle screw fixation can be applied as fixation method of minimal invasive TLIF. Infinite element analysis can simulate human body structure and gives biomechanical characteristics similar to real structure, which can be applied in the researches of spinal dynamics, and stress change of spine and internal intervertebral disk [4]. Compared to animal model and cadaver model, computer model simulation can collect the stress distribution condition of vertebral body, intervertebral disk, zygopophysis and fixation device; eliminate individual difference; and reduce experiment cost [2]. Infinite element analysis is repeatable, can analyze the physiological status that cannot be explored by biological experiment by simulation analysis, and is effective supplement of cadaveric experiment model of human body in vitro [3]. As a mathematical modeling method of theoretical mechanics, although infinite analysis has a lot of irreplaceable advantages, it is abstract of objective things. Simplify, hypothesis and material assignment are inevitably different from the real structure. Lumbar spine mechanics is complicated and related with body weight, muscles around lumbar vertebra, ligament, and abdominal pressure. Thus, the conclusion by infinite element analysis must have hypothesis and limitation, which has to be examined by experimental research.
References 1. Phillips, F. M., Slosar, P. J., Youssef, J. A., Andersson, G., & Papatheofanis, F. (2013). Lumbar spine fusion for chronic low back pain due to degenerative disc disease: A systematic review. Spine, 38(7), E409–E422. 2. Taher, F., Essig, D., Lebl, D. R., Hughes, A. P., Sama, A. A., Cammisa, F. P., et al. (2012). Lumbar degenerative disc disease:
Cell Biochem Biophys
3. 4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
current and future concepts of diagnosis and management. Advances in Orthopedics, 2012, 970752. Eliyas, J. K., & Karahalios, D. (2011). Surgery for degenerative lumbar spine disease. Disease-a-Month, 57(10), 592–606. Gu, G., Zhang, H., Fan, G., He, S., Cai, X., Shen, X., et al. (2013). Comparison of minimally invasive versus open transforaminal lumbar interbody fusion in two-level degenerative lumbar disease. International Orthopaedics, 38(4), 817–824. Brodano, G. B., Martikos, K., Lolli, F., Gasbarrini, A., Cioni, A., Bandiera, S., et al. (2013). Transforaminal lumbar interbody fusion in degenerative disc disease and spondylolisthesis grade i: Minimally invasive versus open surgery. Journal of Spinal Disorders & Techniques. doi:10.1097/BSD.0000000000000034. Kunze, B., Drasseck, T., & Kluba, T. (2011). Posterior and transforaminal lumbar interbody fusion (PLIF/TLIF) for the treatment of localised segment degeneration of lumbar spine. Zeitschrift fur Orthopadie und Unfallchirurgie, 149(3), 312–316. Cole, C. D., McCall, T. D., Schmidt, M. H., & Dailey, A. T. (2009). Comparison of low back fusion techniques: transforaminal lumbar interbody fusion (TLIF) or posterior lumbar interbody fusion (PLIF) approaches. Current Reviews in Musculoskeletal Medicine, 2(2), 118–126. Zairi, F., Allaoui, M., Thines, L., Arikat, A., & Assaker, R. (2013). Transforaminal lumbar interbody fusion: Goals of the minimal invasive approach. Neuro-Chirurgie, 59(4–5), 171–177. Rouben, D., Casnellie, M., & Ferguson, M. (2011). Long-term durability of minimal invasive posterior transforaminal lumbar interbody fusion: a clinical and radiographic follow-up. Journal of Spinal Disorders & Techniques, 24(5), 288–296. Remillieux, M. C., & Burdisso, R. A. (2012). Vibro-acoustic response of an infinite, rib-stiffened, thick-plate assembly using finite-element analysis. The Journal of the Acoustical Society of America, 132(1), EL36–EL42. Srirekha, A., & Bashetty, K. (2010). Infinite to finite: an overview of finite element analysis. Indian Journal of Dental Research, 21(3), 425–432. Xin, H., Ma, X., Ying, L., Zhang, S., & Qian, Z. (2002). The application of infinite element method to endodontic endosseous implant stress analysis. Zhonghua kou qiang yi xue za zhi = Zhonghua kouqiang yixue zazhi =. Chinese Journal of Stomatology, 37(3), 183–186. van den Eerenbeemt, K. D., Ostelo, R. W., van Royen, B. J., Peul, W. C., & van Tulder, M. W. (2010). Total disc replacement surgery for symptomatic degenerative lumbar disc disease: A systematic review of the literature. European Spine Journal, 19(8), 1262–1280. Liu, H. Y., Zhou, J., Wang, B., Wang, H. M., Jin, Z. H., Zhu, Z. Q., et al. (2012). Comparison of Topping-off and posterior lumbar interbody fusion surgery in lumbar degenerative disease: A retrospective study. Chinese Medical Journal, 125(22), 3942– 3946. Cho, C. B., Ryu, K. S., & Park, C. K. (2010). Anterior lumbar interbody fusion with stand-alone interbody cage in treatment of lumbar intervertebral foraminal stenosis : Comparative study of two different types of cages. Journal of Korean Neurosurgical Society, 47(5), 352–357. Bao, D., Ma, Y., Chen, X., Li, H., & Gao, T. (2009). Preliminary clinical study of anchoring cervical intervertebral fusion cage. Zhongguo xiu fu chong jian wai ke za zhi = Zhongguo xiufu chongjian waike zazhi =Chinese Journal of Reparative and Reconstructive Surgery, 23(4), 389–392. Tchako, A., & Sadegh, A. M. (2009). Stress changes in intervertebral discs of the cervical spine due to partial discectomies and fusion. Journal of Biomechanical Engineering, 131(5), 051013.
18. Goto, K., Tajima, N., Chosa, E., Totoribe, K., Kubo, S., Kuroki, H., et al. (2003). Effects of lumbar spinal fusion on the other lumbar intervertebral levels (three-dimensional finite element analysis). Journal of Orthopaedic Science, 8(4), 577–584. 19. Schils, F., Rilliet, B., & Payer, M. (2006). Implantation of an empty carbon fiber cage or a tricortical iliac crest autograft after cervical discectomy for single-level disc herniation: A prospective comparative study. Journal of Neurosurgery Spine, 4(4), 292–299. 20. Joo, Y. H., Lee, J. W., Kwon, K. Y., Rhee, J. J., & Lee, H. K. (2010). Comparison of fusion with cage alone and plate instrumentation in two-level cervical degenerative disease. Journal of Korean Neurosurgical Society, 48(4), 342–346. 21. Ren, H., Lai, Z., Biggs, J. D., Wang, J., & Mukamel, S. (2013). Two-dimensional stimulated resonance Raman spectroscopy study of the Trp-cage peptide folding. Physical Chemistry Chemical Physics, 15(44), 19457–19464. 22. Moreland, D. B., Asch, H. L., Czajka, G. A., Overkamp, J. A., & Sitzman, D. M. (2009). Posterior lumbar interbody fusion: comparison of single intervertebral cage and single side pedicle screw fixation versus bilateral cages and screw fixation. Minimally Invasive Neurosurgery, 52(3), 132–136. 23. Tsuang, Y. H., Chiang, Y. F., Hung, C. Y., Wei, H. W., Huang, C. H., & Cheng, C. K. (2009). Comparison of cage application modality in posterior lumbar interbody fusion with posterior instrumentation—a finite element study. Medical Engineering & Physics, 31(5), 565–570. 24. Tanaka, M., Nakanishi, K., Sugimoto, Y., Misawa, H., Takigawa, T., Nishida, K., et al. (2009). Computer navigation-assisted spinal fusion with segmental pedicle screw instrumentation for scoliosis with Rett syndrome: A case report. Acta Medica Okayama, 63(6), 373–377. 25. Wang, J., Wu, H., Ding, X., & Liu, Y. (2008). Treatment of thoracolumbar vertebrate fracture by transpedicular morselized bone grafting in vertebrae for spinal fusion and pedicle screw fixation. Journal of Huazhong University of Science and Technology Medical sciences = Hua zhong ke ji da xue xue bao Yi xue Ying De wen ban = Huazhong keji daxue xuebao Yixue Yingdewen ban, 28((3), 322–326. 26. Zhao, Y., Wang, Z., Zhu, X., Wang, C., He, S., & Li, M. (2011). Prediction of postoperative trunk imbalance after posterior spinal fusion with pedicle screw fixation for adolescent idiopathic scoliosis. Journal of Pediatric Orthopedics. Part B, 20(4), 199–208. 27. Bode, K. S., & Newton, P. O. (2007). Pediatric nonaccidental trauma thoracolumbar fracture-dislocation: posterior spinal fusion with pedicle screw fixation in an 8-month-old boy. Spine, 32(14), E388–E393. 28. Wang, J., & Zhang, C. (2010). Preliminary evaluation and clinical application of unilateral decompression, interbody fusion and pedicle screw fixation under endoscopic system. Zhongguo gu shang = China Journal of Orthopaedics and Traumatology, 23(5), 360–364. 29. Lin, B., Xu, Y., He, Y., Zhang, B., Lin, Q., & He, M. (2013). Minimally invasive unilateral pedicle screw fixation and lumbar interbody fusion for the treatment of lumbar degenerative disease. Orthopedics, 36(8), e1071–e1076. 30. Ding, W., Chen, Y., Liu, H., Wang, J., & Zheng, Z. (2013). Comparison of unilateral versus bilateral pedicle screw fixation in lumbar interbody fusion: a meta-analysis. European Spine Journal, 23(2), 395–403. 31. Zang, J. C., Ma, X. L., Wang, T., Ma, J. X., Tian, P., & Han, C. (2012). Unilateral versus bilateral pedicle screw fixation in lumbar spinal fusion: A meta-analysis of available evidence. Zhonghua wai ke za zhi [Chinese Journal of Surgery], 50(9), 848–853.
123
