Changing rod stiffness to moderate stress of adjacent disc in oblique lumbar interbody fusion - A finite element analysis

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However, clinical studies indicated the early degeneration of adjacent segments after surgery. The rod stiffness of OLIF was associated with change at adjacent segments. Therefore, the study aimed to compare the biomechanical effects of OLIF with different rod material properties using the finite element (FE) method. Methods A validated L1-L5 lumbar spine was conducted in the biomechanical analysis using FE software ANSYS. The FE model of OLIF with a rod was created. Current biocompatible materials for the rod of the OLIF model were changed, including titanium alloy (OLIF_Ti), nickel-titanium alloy (OLIF_NiTi), and polycarbonate urethane (OLIF_PCU) rod. Four FE models, consisting of the intact model (INT) and implant models, were created. Hybrid control loads, such as flexion, extension, rotation, and lateral bending, were subjected to four models on the L1 vertebral body. The bottom of the L5 vertebral body was fixed. Results At the surgical level, while compared to the INT model, the OLIF_Ti and OLIF_NiTi model resulted in a ROM reduction of over 40% at least, but the OLIF_PCU changed about 10% in flexion and extension. At adjacent level L2-L3, the FE results indicated that the OLIF_Ti and OLIF_NiTi model increased more stress by about 12% at least than the INT model at the adjacent segment, but it demonstrated that the OLIF_PCU would not result in stress rise at the adjacent level L2-L3 in flexion and extension. Conclusion The study concluded that rod stiffness was associated with change at the adjacent segments. The use of OLIF surgery with PCU rods can minimize the impact of the adjacent segment after lumbar fusion. OLIF adjacent segment lumbar spine finite element analysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Low back pain is one of the most critical problems in decreasing the quality of life as a result of lumbar disc degeneration [1]. Low back pain is thought to result from degenerative intervertebral disc and facet joints [2]. Disc degeneration may lead to segmental instability, associated lateral recess, and foraminal stenosis, significantly contributing to symptomatic mechanical back pain, sciatica (leg radiation pain from buttock to calf), and neurogenic claudication [3, 4]. Conservative treatment such as rehabilitation programs and medication of NSAID (non-steroid anti-inflammatory drug) are first recommended for these symptomatic patients with sciatica and neurogenic claudication. Surgical goals are aimed to restore spinal stability with pedicle screws fixation and fusion and neural foramens completely decompressed. A spinal interbody fusion cage is effective in achieving fusion and restoring the original height of the intervertebral disc. The oblique lumbar interbody fusion (OLIF) is the current trend in minimally invasive spine surgery among existing fusion procedures. The OLIF can provide a larger support area to increase disc height and foraminal height to alleviate nerve compression pain. A review of the clinical literature [5–8] showed that the patient satisfaction rate for OLIF surgery ranged from 87.5–100%, indicating good satisfaction. However, a primary concern after posterior lumbar spine arthrodesis is the potential for adjacent segment degeneration (ASD) cephalad or caudad to the fusion segment due to stress concentration at the adjacent levels. Park et al. reported that the incidence of radiographic ASD ranges from 7.1–100% with retrolisthesis and disc height loss [9]. However, the incidence of symptomatic adjacent segment disease ranges from 5.2 to 18.5% [10]. The etiology is biomechanical alterations affecting the levels adjacent to a fused segment, as well as progressive spinal degeneration with age. The risk factors are instrumentation, fusion length, sagittal malalignment, facet injury, age, and degenerative changes. In a meta-analysis and systemic review [9], the potential risk factors for ASD are posterior lumbar interbody fusion, injury to the facet joint of the adjacent segment fusion length, sagittal alignment, and degenerated disc at the adjacent level, lumbar stenosis, age, osteoporosis, female gender, and post-menopausal state. Using only radiographic criteria, the incidence of ASD was generally higher, with rates varying from 8 to 100%. In contrast, studies involving symptomatic ASD reported incidence ranging from 5.2 to 18.5%. Previous studies using the finite element (FE) method have also developed the lumbar spine with an OLIF model to analyze the stress at the adjacent disc, and the results have shown that OLIF surgery does cause an increase in disc stress at the adjacent level [11–15]. Zhang et al. reported that OLIF with bilateral pedicle screw fixation could obtain good spinal stability [12]. However, Du et al. addressed OLIF raised a risk of accelerating the degeneration of segments adjacent to the fusion site [14]. From the above studies, it can be confirmed that OLIF with metal pedicle screw and rod fixation caused an increase in ROM and disc stress at the adjacent segment. The main reason for this may be the rod's strength, as most of the current cage materials have been changed to PEEK, reducing its rigidity. However, the rods are still made of titanium alloy, so the titanium alloy rods still cause the stress to be concentrated in the operated segment, which in turn causes ROM compensation at the adjacent end and increases the stress on the adjacent disc. Therefore, this study aims to change the rod stiffness with the current surgical materials to see if reducing the disc stress at the adjacent segment is possible using a validated FE lumbar spine model. Materials And Methods In this study, a validated three-dimensional FE lumbar spine model was conducted using ANSYS 2021(Swanson Analysis System Inc., Houston,TX, USA), included osseoligamentous L1–L5 vertebrae, intervertebral discs, endplates, posterior bony elements, and seven ligaments. The intervertebral disc comprised an annulus fibrosus and nucleus pulposus, with 12 double cross-linked fiber layers embedded in the ground substance. The annulus ground substance was modeled based on a Mooney-Rivlin formulation, while the nucleus pulposus was modeled as an incompressible fluid. A non-linear hyperelastic, two-parameter Mooney-Rivlin solid model simulated the elastic modulus of the ground substance in a spinal disc. This model employs the constants C10 and C01, used in FE analysis to describe hyperelastic deformation based on the material constants C1 and C2, as defined by the Mooney-Rivlin model [16–17]. A previous study conducted a convergence test with three different mesh densities: the finest model included 112,174 elements and 94,162 nodes; the standard model had 27,244 elements and 30,630 nodes; the coarse model comprised 4,750 elements and 4,960 nodes [17]. The study assessed the variability in range of motion (ROM) and ultimately opted for the finest mesh density. This choice was made because its variations compared to the standard model were minimal: within 1.03% for flexion (< 0.2°), 4.39% for extension (< 0.5°), 0.01% for torsion (< 0.2°), and 0.001% for lateral bending (< 0.1°) [23]. Consequently, this validated model was chosen as the FE model for the current study (Fig. 1 ). The OLIF surgery models were simulated with pedicle screws and rod systems consisting of conical titanium alloy screws with different materials of the rod. Some current implant materials consisting of titanium alloy rods (Ti-alloy), nickel-titanium alloy rods (Ni-Ti alloy), and polycarbonate urethane (PCU) rods were added to the lumbar spine model, respectively [17]. These three materials were chosen because they are all implantable in the human body and are already in clinical spinal surgery. The interbody fusion device model with oblique cage, made from polyetheretherketone (PEEK), featured as CLYDESDALE Spinal System (Medtronic, Minneapolis, MN, USA)(Table 1 and Fig. 2 ). The INT model consisted of 112,174 element and 94,162 nodes, and the OLIF model consisted of 294,619 elements and 126,021 nodes. In the OLIF model, the 6.4 mm diameter pedicle screws were implanted in L3 and L4, and the 5.5 mm diameter rod was used as the connection between the two pedicle screws, both of which were made of titanium alloy. The bonding setting using contact elements was used in the screw-bone and cage-bone interfaces to examine the biomechanical change of adjacent segments after the bony union. Table 1 Material parameters of the rod Material Young’s Modulus (MPa) Poisson’s Ratio Ti-alloy 110000 0.28 Ni-Ti alloy 47000 0.3 PCU 68.4 0.4 Boundary and Loading Conditions In this study, we fully constrained the inferior surfaces of the L5 vertebrae. The loading conditions were set as follows: An axial load of 150 N was applied to the superior surface of the L1 vertebrae. A bending moment was applied to the superior surface of the L1 vertebrae with these parameters: flexion at 21°, extension at 12°, lateral bending at 11°, and rotation at 15°. The ROM included the maximum ranges of all finite element models. Biomechanical Evaluation Fusion surgeries can excessively restrict ROM. Thus, this study focused on analyzing the ROM at the affected segment. Lumbar fusion with OLIF might lead to the problem of ASD, so the FE study aimed to concentrate on intervertebral disc stress and the facet contact force (FCF) at the adjacent segment. The surgery levels L3-L4 and adjacent levels L2-L3 were investigated in terms of ROM, disc stress, and FCF. Result 3 − 1 Comparison of ROM The implantation of titanium rods (OLIF_Ti and OLIF_NiTi) resulted in a remarkable reduction in ROM at the surgical levels as listed in Table 2 . This also increased from 7 to 20% in ROM of adjacent level L2-L3 in the OLIF_Ti and OLIF_NiTi models. However, after implantation of the PCU rod, the increase in ROM at the adjacent level L2-L3 was less. Especially in flexion and extension, the OLIF_PCU model was comparable to the INT model at the L2-L3 level. Table 2 ROM of the lumbar spine in different groups Unit: degree Model L1-L2 L2-L3 L3-L4 L4-L5 Total Flexion INT 4.86 5.15 5.17 6.66 21.8 OLIF_Ti 5.71 (+ 18%) 6.03 (+ 17%) 3 (-42%) 6.94 (+ 4%) 21.7 OLIF_NiTi 5.53 (+ 14%) 5.84 (+ 13%) 3.01 (-41%) 6.87 (+ 3%) 21.3 OLIF_PCU 4.86 (0%) 5.12 (0%) 5.39 (+ 4%) 6.18 (-7%) 21.6 Extension INT 3.38 3.23 2.9 3.27 12.8 OLIF_Ti 3.9 (+ 15%) 3.72 (+ 15%) 1.39 (-52%) 3.67 (+ 12%) 12.7 OLIF_NiTi 3.85 (+ 14%) 3.66 (+ 13%) 1.41 (-51%) 3.65 (+ 11%) 12.6 OLIF_PCU 3.36 (-1%) 3.23 (0%) 2.64 (-9%) 3.23 (-1%) 12.5 Rotation INT 3.19 3.39 3.88 5.3 15.8 OLIF_Ti 3.89 (+ 22%) 3.92 (+ 16%) 1.62 (-58%) 6.37(+ 20%) 15.8 OLIF_NiTi 3.83 (+ 20%) 3.88 (+ 15%) 1.74 (-55%) 6.37 (+ 20%) 15.8 OLIF_PCU 3.48 (+ 9%) 3.64 (+ 7%) 2.68 (-31%) 5.97 (+ 13%) 15.8 Bending INT 2.66 2.79 2.83 3.17 11.5 OLIF_Ti 3.29 (+ 24%) 3.36 (+ 20%) 0.62 (-78%) 3.83 (+ 21%) 11.1 OLIF_NiTi 3.09 (+ 16%) 3.19 (+ 14%) 0.63 (-78%) 3.67 (+ 16%) 10.6 OLIF_PCU 3.08 (+ 16%) 3.2 (+ 15%) 1.37 (-52%) 3.66 (+ 16%) 11.3 Note: percentage = ((OLIF groups-INT)/(INT))*100% 3 − 2 Comparison of adjacent disc stress The OLIF_PCU model is almost identical to the INT model in flexion and extension, as listed in Table 3 . After implanting the titanium rod, the OLIF_Ti and OLIF_NiTi model increased stress by about 12%, at least than the INT model. It demonstrated that the OLIF_PCU would not result in stress raise at adjacent level L2-L3 in flexion and extension as shown in Fig. 3 and Fig. 4 . Higher stress at adjacent disc L2-L3 was concentrated on posterolateral region in extension and anterior region in flexion. In torsion, the adjacent disc stress of the OLIF_PCU model was higher than that of the INT model but was much smaller than that of the OLIF_Ti and OLIF_NiTi models. Table 3 Adjacent disc stress in different groups Unit: KPa Model L2-L3 L3-L4 Flexion INT 791 723 OLIF_Ti 964 (+ 22%) - OLIF_NiTi 927 (+ 17%) - OLIF_PCU 793 (0%) - Extension INT 495 415 OLIF_Ti 567 (+ 15%) - OLIF_NiTi 559 (+ 13%) - OLIF_PCU 498 (1%) - Rotation INT 551 602 OLIF_Ti 678 (+ 23%) - OLIF_NiTi 666 (+ 21%) - OLIF_PCU 600 (+ 9%) - Bending INT 555 543 OLIF_Ti 651 (+ 17%) - OLIF_NiTi 619 (+ 12%) - OLIF_PCU 621 (+ 12%) - Note: percentage = ((OLIF groups-INT)/(INT))*100% 3–3 Comparison of facet joint force in adjacent level L2-L3 In extension and rotation, the OLIF_PCU model almost had the same contact force as the INT model, while OLIF_Ti and OLIF_NiTi models were increased by 22%, at least as listed in Table 4 . Table 4 The contact force of adjacent facet joints in different models in extension and rotation Unit: N Model Level Extension Rotation Left Right Left Right INT L2-L3 97 97 0 258 OLIF_Ti L2-L3 121 (+ 25%) 121 (+ 25%) 0 354 (+ 37%) OLIF_NiTi L2-L3 118 (+ 22%) 118 (+ 22%) 0 346 (+ 34%) OLIF_PCU L2-L3 97 (+ 0%) 97 (+ 0%) 0 293 (+ 14%) Note: percentage = ((OLIF groups-INT)/(INT))*100% 3–4 Stress distribution of the entire implanted lumbar spine In flexion and extension, after implantation of posterior rods with different materials in the lumbar spine, it could be seen that the Ti alloy model absorbed more stress, as shown in Fig. 5 . However, after implantation of PCU rods, it can be seen that the stress distribution of the posterior bone elements decreased and was closer to the INT model. Discussion Among lumbar fusion surgeries, the OLIF approach can effectively stabilize the lumbar spine structure, shorten the operation time, reduce blood loss, and improve postoperative recovery. Moreover, it causes less damage to the lumbar spine tissues during the operation, which makes it a mainstream minimally invasive lumbar spine surgery in recent years. However, from the clinical literature [18], there is still a problem of adjacent segment disease after OLIF surgery consisting of early adjacent disc degeneration and adjacent facet joint hypertrophy. Therefore, the study implemented the FE model to change different rod materials to realize whether it's possible to alleviate the change of adjacent segments after OLIF surgery. Our FE result had a similar trend to a previous study in which OLIF with bilateral pedicle screw fixation could stabilize the lumbar spine [12]. Wang et al. [19] used the FE model to investigate the condition of the adjacent segments implanted in the OLIF fusion model. They reported that adjacent ROM of the OLIF with posterior fixation increased from 13–23% compared to the INT model. The adjacent segment mobility of Wang et al. did not differ significantly from that of this study in all four loading modes. The adjacent ROM increased from 15–20% in our FE analysis. Compared to past studies, the overall trend was consistent with an increase in adjacent segment mobility after implantation of the OLIF with posterior fixation. Comparison of the adjacent facet joint contact forces between the Du et al. [14] OLIF fusion model and the present study showed a 79% difference in flexion and 53% in torsion, and the facet joint contact forces were greater than those in the present study. It is attributed to the reason for such differences is that the force of preload applied by Du et al. (500 N) was different from that of the present study (150 N), which resulted in greater force transfer to the facet joints in the Du et al. However, the same trend was found between the two studies regarding the increase of adjacent facet joints. Implanting a conventional posterior titanium alloy rod increased adjacent disc stress and the facet joint. However, replacing a modified Ni-Ti alloy with Young's modulus half that of the traditional Ti alloy does not change the problem of increased disc stress and facet joint force cephalic to the fusion site. Only by directly replacing the rods with PCU material can the ROM and adjacent disc forces be returned to a state close to the normal lumbar spine during the flexion and extension of the OLIF surgery. This is mainly because PCU is a soft material, which can help provide less displacement at the surgical end and allow the peek material cage to fuse with the bone tissue. Once fused, the PCU material does not absorb as much load as a conventional Ti alloy rod, which in turn causes compensatory behavior in the adjacent segment. The only difference is that the adjacent disc stresses and facet joint forces in extension and torsion are still elevated, whereas the human body moves more frequently in flexion. As a result, it should be possible to minimize the accelerated degeneration of the adjacent discs after OLIF surgery. Therefore, the use of an implant material that is sufficiently rigid to provide spinal stability and that does not differ too much from the original lumbar tissue material will allow for a reasonable ROM of the lumbar spine. After all, lumbar spine surgery is about achieving stability and preserving some mobility, both of which are important. Clinically, the decision to use a softer rod depends on the stability of the lumbar spine. If the patient has better lumbar spine stability after implantation of the fusion device, it is recommended that a less rigid rod be given. However, suppose the patient's lumbar spine stability is poorer. In that case, a rod with a higher rigidity is still needed to provide immediate better stability to assist with the bony fusion. This is to avoid the inability to complete the bony fusion before the adjacent segment problem has occurred. However, further research is needed to determine how clinical lumbar stability should be determined. The main assumptions and limitations of this study are as follows. The model used in this study did not build up muscles and soft tissues, so the value of the model's loading was lower than the range of normal physiological movements in the human body. Regarding the implant model, the pedicle screws in this study were not threaded, and the teeth of the fusion device were not considered. The study assumed complete fusion and, therefore, simulated the biomechanical effects of the implant in the postoperative period, and the results focused on the stiffness of the overall structure and the influence of the adjacent segment. Conclusion The rod material was associated with the change of adjacent segments in OLIF surgery. Adding Ti or NiTi alloy rods can result in a greater reduction of ROM at the surgical level, which causes an increase in disc stress and facet joint contact force at the adjacent segment. After replacing the PCU rod, the cephalic disc stress and facet joint contact force generated in flexion and extension were close to the intact spine. Therefore, compared to titanium alloy rods, the use of OLIF surgery with PCU rods can minimize the impact of the adjacent segment after lumbar fusion. Declarations Consent for publication Not applicable. Ethics approval and consent to participate Not applicable. Data Availability I do not have any research data outside the submitted manuscript file. Competing interests The authors declare that they have no competing interests. Funding The study was supported by National Science and Technology Council (NSTC 112-2221-E-A49 -017 -MY3) Authors' contributions Chou PC: conception AND interpretation of data Chen JJ: acquisition AND analysis Chen CS: design of work Shih SL: draft the work Wang ST: conception Liu CL: conception References Brinjikji W, Diehn F, Jarvik J, Carr C, Kallmes DF, Murad MH, Luetmer PH. MRI findings of disc degeneration are more prevalent in adults with low back pain than in asymptomatic controls: a systematic review and meta-analysis. AJNR Am J Neuroradiol. 2015;36(12):2394-9. van den Eerenbeemt KD, Ostelo RW, van Royen BJ, Peul WC, van Tulder MW. Total disc replacement surgery for symptomatic degenerative lumbar disc disease: a systematic review of the literature. Eur Spine J. 2010;19(8):1262–1280. Mijiyawa M, Oniankitan O, Kolani B, Koriko T. Low back pain in hospital outpatients in Lomé (Togo). Joint Bone Spine. 2000;67(6):533-8. Riihimäki H, Tola S, Videman T, Hänninen K. 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Radiographic and clinical outcomes following combined oblique lumbar interbody fusion and lateral instrumentation for the treatment of degenerative spine deformity: a preliminary retrospective study. Biomed Res Int. 2019;5672162. doi: 10.1155/2019/5672162. PMID: 30729127; PMCID: PMC6341239. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 17 Mar, 2025 Read the published version in BMC Musculoskeletal Disorders → Version 1 posted Editorial decision: Revision requested 11 Nov, 2024 Editor assigned by journal 07 Nov, 2024 Submission checks completed at journal 07 Nov, 2024 First submitted to journal 05 Nov, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Shih","email":"","orcid":"","institution":"National Yang Ming Chiao Tung University","correspondingAuthor":false,"prefix":"","firstName":"Shih-Liang","middleName":"","lastName":"Shih","suffix":""},{"id":376806703,"identity":"110abd2b-6669-4463-a3fe-e08c2469a905","order_by":4,"name":"Shih-Tien Wang","email":"","orcid":"","institution":"Taipei Veterans General Hospital","correspondingAuthor":false,"prefix":"","firstName":"Shih-Tien","middleName":"","lastName":"Wang","suffix":""},{"id":376806704,"identity":"f7e98d49-4fc0-43de-ab63-8ef0af85d378","order_by":5,"name":"Chien-Lin Liu","email":"","orcid":"","institution":"Taipei Veterans General Hospital","correspondingAuthor":false,"prefix":"","firstName":"Chien-Lin","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2024-11-06 02:53:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5398913/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5398913/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s12891-025-08504-3","type":"published","date":"2025-03-17T15:57:30+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":71053694,"identity":"42eea350-5715-4e0c-a2cd-485b46a8feb2","added_by":"auto","created_at":"2024-12-10 15:57:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":229730,"visible":true,"origin":"","legend":"\u003cp\u003eFE model of the intact lumbar spine.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-5398913/v1/a3a9d82ad8fa2975dad75563.png"},{"id":71051682,"identity":"cb670216-d500-41e9-906d-23876e3e4cc1","added_by":"auto","created_at":"2024-12-10 15:49:23","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":999622,"visible":true,"origin":"","legend":"\u003cp\u003eFE model of the OLIF surgery with different materials of rods\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5398913/v1/49b1b627ffcddaec4884d553.jpeg"},{"id":71051679,"identity":"79992e1d-7f31-4d85-a9bc-35a8443cde21","added_by":"auto","created_at":"2024-12-10 15:49:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":124518,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of stress distribution at adjacent disc L2-L3 in flexion\u003c/p\u003e\n\u003cp\u003eNote: arrow indicates the region of higher stress\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-5398913/v1/47e940a7712658320a10acbb.png"},{"id":71051680,"identity":"e1caa2b1-6e29-4bca-9691-d435a914de38","added_by":"auto","created_at":"2024-12-10 15:49:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":154148,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of stress distribution at adjacent disc L2-L3 in extension\u003c/p\u003e\n\u003cp\u003eNote: arrow indicates the region of higher stress\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-5398913/v1/bfd76f4dfe81dc554d17d211.png"},{"id":71051681,"identity":"0afb3955-f1d8-43a8-bdcd-c9889c1d6b92","added_by":"auto","created_at":"2024-12-10 15:49:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":324946,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of stress distribution in the lumbar spine with different materials of rods (A) flexion (B) extension\u003c/p\u003e\n\u003cp\u003eNote: The greater arrow indicates higher stress on the rod\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-5398913/v1/554f184bd2c8d88df2682253.png"},{"id":79120618,"identity":"35e21de9-5bb1-4dfa-9eb9-ac3f03678033","added_by":"auto","created_at":"2025-03-24 16:10:26","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2443455,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5398913/v1/a1ec675a-7503-46e2-8eea-a35883080535.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Changing rod stiffness to moderate stress of adjacent disc in oblique lumbar interbody fusion - A finite element analysis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eLow back pain is one of the most critical problems in decreasing the quality of life as a result of lumbar disc degeneration [1]. Low back pain is thought to result from degenerative intervertebral disc and facet joints [2]. Disc degeneration may lead to segmental instability, associated lateral recess, and foraminal stenosis, significantly contributing to symptomatic mechanical back pain, sciatica (leg radiation pain from buttock to calf), and neurogenic claudication [3, 4]. Conservative treatment such as rehabilitation programs and medication of NSAID (non-steroid anti-inflammatory drug) are first recommended for these symptomatic patients with sciatica and neurogenic claudication. Surgical goals are aimed to restore spinal stability with pedicle screws fixation and fusion and neural foramens completely decompressed. A spinal interbody fusion cage is effective in achieving fusion and restoring the original height of the intervertebral disc. The oblique lumbar interbody fusion (OLIF) is the current trend in minimally invasive spine surgery among existing fusion procedures.\u003c/p\u003e \u003cp\u003eThe OLIF can provide a larger support area to increase disc height and foraminal height to alleviate nerve compression pain. A review of the clinical literature [5\u0026ndash;8] showed that the patient satisfaction rate for OLIF surgery ranged from 87.5\u0026ndash;100%, indicating good satisfaction. However, a primary concern after posterior lumbar spine arthrodesis is the potential for adjacent segment degeneration (ASD) cephalad or caudad to the fusion segment due to stress concentration at the adjacent levels. Park et al. reported that the incidence of radiographic ASD ranges from 7.1\u0026ndash;100% with retrolisthesis and disc height loss [9]. However, the incidence of symptomatic adjacent segment disease ranges from 5.2 to 18.5% [10]. The etiology is biomechanical alterations affecting the levels adjacent to a fused segment, as well as progressive spinal degeneration with age. The risk factors are instrumentation, fusion length, sagittal malalignment, facet injury, age, and degenerative changes.\u003c/p\u003e \u003cp\u003eIn a meta-analysis and systemic review [9], the potential risk factors for ASD are posterior lumbar interbody fusion, injury to the facet joint of the adjacent segment fusion length, sagittal alignment, and degenerated disc at the adjacent level, lumbar stenosis, age, osteoporosis, female gender, and post-menopausal state. Using only radiographic criteria, the incidence of ASD was generally higher, with rates varying from 8 to 100%. In contrast, studies involving symptomatic ASD reported incidence ranging from 5.2 to 18.5%. Previous studies using the finite element (FE) method have also developed the lumbar spine with an OLIF model to analyze the stress at the adjacent disc, and the results have shown that OLIF surgery does cause an increase in disc stress at the adjacent level [11\u0026ndash;15]. Zhang et al. reported that OLIF with bilateral pedicle screw fixation could obtain good spinal stability [12]. However, Du et al. addressed OLIF raised a risk of accelerating the degeneration of segments adjacent to the fusion site [14].\u003c/p\u003e \u003cp\u003eFrom the above studies, it can be confirmed that OLIF with metal pedicle screw and rod fixation caused an increase in ROM and disc stress at the adjacent segment. The main reason for this may be the rod's strength, as most of the current cage materials have been changed to PEEK, reducing its rigidity. However, the rods are still made of titanium alloy, so the titanium alloy rods still cause the stress to be concentrated in the operated segment, which in turn causes ROM compensation at the adjacent end and increases the stress on the adjacent disc. Therefore, this study aims to change the rod stiffness with the current surgical materials to see if reducing the disc stress at the adjacent segment is possible using a validated FE lumbar spine model.\u003c/p\u003e "},{"header":"Materials And Methods","content":"\u003cp\u003eIn this study, a validated three-dimensional FE lumbar spine model was conducted using ANSYS 2021(Swanson Analysis System Inc., Houston,TX, USA), included osseoligamentous L1\u0026ndash;L5 vertebrae, intervertebral discs, endplates, posterior bony elements, and seven ligaments. The intervertebral disc comprised an annulus fibrosus and nucleus pulposus, with 12 double cross-linked fiber layers embedded in the ground substance. The annulus ground substance was modeled based on a Mooney-Rivlin formulation, while the nucleus pulposus was modeled as an incompressible fluid. A non-linear hyperelastic, two-parameter Mooney-Rivlin solid model simulated the elastic modulus of the ground substance in a spinal disc. This model employs the constants C10 and C01, used in FE analysis to describe hyperelastic deformation based on the material constants C1 and C2, as defined by the Mooney-Rivlin model [16\u0026ndash;17]. A previous study conducted a convergence test with three different mesh densities: the finest model included 112,174 elements and 94,162 nodes; the standard model had 27,244 elements and 30,630 nodes; the coarse model comprised 4,750 elements and 4,960 nodes [17]. The study assessed the variability in range of motion (ROM) and ultimately opted for the finest mesh density. This choice was made because its variations compared to the standard model were minimal: within 1.03% for flexion (\u0026lt;\u0026thinsp;0.2\u0026deg;), 4.39% for extension (\u0026lt;\u0026thinsp;0.5\u0026deg;), 0.01% for torsion (\u0026lt;\u0026thinsp;0.2\u0026deg;), and 0.001% for lateral bending (\u0026lt;\u0026thinsp;0.1\u0026deg;) [23]. Consequently, this validated model was chosen as the FE model for the current study (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe OLIF surgery models were simulated with pedicle screws and rod systems consisting of conical titanium alloy screws with different materials of the rod. Some current implant materials consisting of titanium alloy rods (Ti-alloy), nickel-titanium alloy rods (Ni-Ti alloy), and polycarbonate urethane (PCU) rods were added to the lumbar spine model, respectively [17]. These three materials were chosen because they are all implantable in the human body and are already in clinical spinal surgery. The interbody fusion device model with oblique cage, made from polyetheretherketone (PEEK), featured as CLYDESDALE Spinal System (Medtronic, Minneapolis, MN, USA)(Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The INT model consisted of 112,174 element and 94,162 nodes, and the OLIF model consisted of 294,619 elements and 126,021 nodes. In the OLIF model, the 6.4 mm diameter pedicle screws were implanted in L3 and L4, and the 5.5 mm diameter rod was used as the connection between the two pedicle screws, both of which were made of titanium alloy. The bonding setting using contact elements was used in the screw-bone and cage-bone interfaces to examine the biomechanical change of adjacent segments after the bony union.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMaterial parameters of the rod\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMaterial\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eYoung\u0026rsquo;s Modulus (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePoisson\u0026rsquo;s Ratio\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi-alloy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e110000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.28\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi-Ti alloy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e47000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePCU\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e68.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003eBoundary and Loading Conditions\u003c/h3\u003e\n\u003cp\u003eIn this study, we fully constrained the inferior surfaces of the L5 vertebrae. The loading conditions were set as follows:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAn axial load of 150 N was applied to the superior surface of the L1 vertebrae.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eA bending moment was applied to the superior surface of the L1 vertebrae with these parameters: flexion at 21\u0026deg;, extension at 12\u0026deg;, lateral bending at 11\u0026deg;, and rotation at 15\u0026deg;. The ROM included the maximum ranges of all finite element models.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eBiomechanical Evaluation\u003c/h2\u003e \u003cp\u003eFusion surgeries can excessively restrict ROM. Thus, this study focused on analyzing the ROM at the affected segment. Lumbar fusion with OLIF might lead to the problem of ASD, so the FE study aimed to concentrate on intervertebral disc stress and the facet contact force (FCF) at the adjacent segment. The surgery levels L3-L4 and adjacent levels L2-L3 were investigated in terms of ROM, disc stress, and FCF.\u003c/p\u003e \u003c/div\u003e"},{"header":"Result","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3\u0026thinsp;\u0026minus;\u0026thinsp;1 Comparison of ROM\u003c/h2\u003e \u003cp\u003eThe implantation of titanium rods (OLIF_Ti and OLIF_NiTi) resulted in a remarkable reduction in ROM at the surgical levels as listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. This also increased from 7 to 20% in ROM of adjacent level L2-L3 in the OLIF_Ti and OLIF_NiTi models. However, after implantation of the PCU rod, the increase in ROM at the adjacent level L2-L3 was less. Especially in flexion and extension, the OLIF_PCU model was comparable to the INT model at the L2-L3 level.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eROM of the lumbar spine in different groups Unit: degree\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eModel\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eL1-L2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eL2-L3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eL3-L4\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eL4-L5\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eTotal\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eFlexion\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eINT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e6.66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e21.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_Ti\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.71 (+\u0026thinsp;18%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6.03 (+\u0026thinsp;17%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3 (-42%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e6.94 (+\u0026thinsp;4%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e21.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_NiTi\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.53 (+\u0026thinsp;14%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5.84 (+\u0026thinsp;13%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.01 (-41%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e6.87 (+\u0026thinsp;3%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e21.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_PCU\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4.86 (0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5.12 (0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.39 (+\u0026thinsp;4%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e6.18 (-7%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e21.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eExtension\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eINT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e12.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_Ti\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.9 (+\u0026thinsp;15%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.72 (+\u0026thinsp;15%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.39 (-52%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.67 (+\u0026thinsp;12%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e12.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_NiTi\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.85 (+\u0026thinsp;14%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.66 (+\u0026thinsp;13%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.41 (-51%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.65 (+\u0026thinsp;11%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e12.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_PCU\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.36 (-1%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.23 (0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.64 (-9%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.23 (-1%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e12.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eRotation\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eINT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e5.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e15.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_Ti\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.89 (+\u0026thinsp;22%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.92 (+\u0026thinsp;16%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.62 (-58%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e6.37(+\u0026thinsp;20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e15.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_NiTi\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.83 (+\u0026thinsp;20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.88 (+\u0026thinsp;15%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.74 (-55%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e6.37 (+\u0026thinsp;20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e15.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_PCU\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.48 (+\u0026thinsp;9%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.64 (+\u0026thinsp;7%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.68 (-31%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e5.97 (+\u0026thinsp;13%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e15.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eBending\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eINT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e11.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_Ti\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.29 (+\u0026thinsp;24%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.36 (+\u0026thinsp;20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.62 (-78%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.83 (+\u0026thinsp;21%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e11.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_NiTi\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.09 (+\u0026thinsp;16%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.19 (+\u0026thinsp;14%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.63 (-78%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.67 (+\u0026thinsp;16%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e10.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_PCU\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.08 (+\u0026thinsp;16%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.2 (+\u0026thinsp;15%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.37 (-52%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.66 (+\u0026thinsp;16%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e11.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"7\"\u003eNote:\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"7\"\u003epercentage = ((OLIF groups-INT)/(INT))*100%\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003e3 − 2 Comparison of adjacent disc stress\u003c/h3\u003e\n\u003cp\u003eThe OLIF_PCU model is almost identical to the INT model in flexion and extension, as listed in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. After implanting the titanium rod, the OLIF_Ti and OLIF_NiTi model increased stress by about 12%, at least than the INT model. It demonstrated that the OLIF_PCU would not result in stress raise at adjacent level L2-L3 in flexion and extension as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Higher stress at adjacent disc L2-L3 was concentrated on posterolateral region in extension and anterior region in flexion. In torsion, the adjacent disc stress of the OLIF_PCU model was higher than that of the INT model but was much smaller than that of the OLIF_Ti and OLIF_NiTi models.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAdjacent disc stress in different groups Unit: KPa\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eModel\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eL2-L3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eL3-L4\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eFlexion\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eINT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e791\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e723\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_Ti\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e964 (+\u0026thinsp;22%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_NiTi\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e927 (+\u0026thinsp;17%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_PCU\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e793 (0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eExtension\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eINT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e495\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e415\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_Ti\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e567 (+\u0026thinsp;15%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_NiTi\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e559 (+\u0026thinsp;13%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_PCU\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e498 (1%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eRotation\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eINT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e551\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e602\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_Ti\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e678 (+\u0026thinsp;23%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_NiTi\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e666 (+\u0026thinsp;21%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_PCU\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e600 (+\u0026thinsp;9%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eBending\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eINT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e555\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e543\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_Ti\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e651 (+\u0026thinsp;17%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_NiTi\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e619 (+\u0026thinsp;12%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eOLIF_PCU\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e621 (+\u0026thinsp;12%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eNote: percentage = ((OLIF groups-INT)/(INT))*100%\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003e3–3 Comparison of facet joint force in adjacent level L2-L3\u003c/h3\u003e\n\u003cp\u003eIn extension and rotation, the OLIF_PCU model almost had the same contact force as the INT model, while OLIF_Ti and OLIF_NiTi models were increased by 22%, at least as listed in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThe contact force of adjacent facet joints in different models in extension and rotation Unit: N\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eModel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eLevel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003eExtension\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e \u003cp\u003eRotation\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLeft\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRight\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eLeft\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eRight\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eINT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eL2-L3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e258\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eOLIF_Ti\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eL2-L3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e121 (+\u0026thinsp;25%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e121 (+\u0026thinsp;25%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e354 (+\u0026thinsp;37%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eOLIF_NiTi\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eL2-L3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e118 (+\u0026thinsp;22%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e118 (+\u0026thinsp;22%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e346 (+\u0026thinsp;34%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eOLIF_PCU\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eL2-L3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e97 (+\u0026thinsp;0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e97 (+\u0026thinsp;0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e293 (+\u0026thinsp;14%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003eNote: percentage = ((OLIF groups-INT)/(INT))*100%\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3\u0026ndash;4 Stress distribution of the entire implanted lumbar spine\u003c/h2\u003e \u003cp\u003eIn flexion and extension, after implantation of posterior rods with different materials in the lumbar spine, it could be seen that the Ti alloy model absorbed more stress, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. However, after implantation of PCU rods, it can be seen that the stress distribution of the posterior bone elements decreased and was closer to the INT model.\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eAmong lumbar fusion surgeries, the OLIF approach can effectively stabilize the lumbar spine structure, shorten the operation time, reduce blood loss, and improve postoperative recovery. Moreover, it causes less damage to the lumbar spine tissues during the operation, which makes it a mainstream minimally invasive lumbar spine surgery in recent years. However, from the clinical literature [18], there is still a problem of adjacent segment disease after OLIF surgery consisting of early adjacent disc degeneration and adjacent facet joint hypertrophy. Therefore, the study implemented the FE model to change different rod materials to realize whether it's possible to alleviate the change of adjacent segments after OLIF surgery.\u003c/p\u003e \u003cp\u003eOur FE result had a similar trend to a previous study in which OLIF with bilateral pedicle screw fixation could stabilize the lumbar spine [12]. Wang et al. [19] used the FE model to investigate the condition of the adjacent segments implanted in the OLIF fusion model. They reported that adjacent ROM of the OLIF with posterior fixation increased from 13\u0026ndash;23% compared to the INT model. The adjacent segment mobility of Wang et al. did not differ significantly from that of this study in all four loading modes. The adjacent ROM increased from 15\u0026ndash;20% in our FE analysis. Compared to past studies, the overall trend was consistent with an increase in adjacent segment mobility after implantation of the OLIF with posterior fixation. Comparison of the adjacent facet joint contact forces between the Du et al. [14] OLIF fusion model and the present study showed a 79% difference in flexion and 53% in torsion, and the facet joint contact forces were greater than those in the present study. It is attributed to the reason for such differences is that the force of preload applied by Du et al. (500 N) was different from that of the present study (150 N), which resulted in greater force transfer to the facet joints in the Du et al. However, the same trend was found between the two studies regarding the increase of adjacent facet joints.\u003c/p\u003e \u003cp\u003eImplanting a conventional posterior titanium alloy rod increased adjacent disc stress and the facet joint. However, replacing a modified Ni-Ti alloy with Young's modulus half that of the traditional Ti alloy does not change the problem of increased disc stress and facet joint force cephalic to the fusion site. Only by directly replacing the rods with PCU material can the ROM and adjacent disc forces be returned to a state close to the normal lumbar spine during the flexion and extension of the OLIF surgery. This is mainly because PCU is a soft material, which can help provide less displacement at the surgical end and allow the peek material cage to fuse with the bone tissue. Once fused, the PCU material does not absorb as much load as a conventional Ti alloy rod, which in turn causes compensatory behavior in the adjacent segment. The only difference is that the adjacent disc stresses and facet joint forces in extension and torsion are still elevated, whereas the human body moves more frequently in flexion. As a result, it should be possible to minimize the accelerated degeneration of the adjacent discs after OLIF surgery. Therefore, the use of an implant material that is sufficiently rigid to provide spinal stability and that does not differ too much from the original lumbar tissue material will allow for a reasonable ROM of the lumbar spine. After all, lumbar spine surgery is about achieving stability and preserving some mobility, both of which are important.\u003c/p\u003e \u003cp\u003eClinically, the decision to use a softer rod depends on the stability of the lumbar spine. If the patient has better lumbar spine stability after implantation of the fusion device, it is recommended that a less rigid rod be given. However, suppose the patient's lumbar spine stability is poorer. In that case, a rod with a higher rigidity is still needed to provide immediate better stability to assist with the bony fusion. This is to avoid the inability to complete the bony fusion before the adjacent segment problem has occurred. However, further research is needed to determine how clinical lumbar stability should be determined.\u003c/p\u003e \u003cp\u003eThe main assumptions and limitations of this study are as follows.\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe model used in this study did not build up muscles and soft tissues, so the value of the model's loading was lower than the range of normal physiological movements in the human body.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eRegarding the implant model, the pedicle screws in this study were not threaded, and the teeth of the fusion device were not considered. The study assumed complete fusion and, therefore, simulated the biomechanical effects of the implant in the postoperative period, and the results focused on the stiffness of the overall structure and the influence of the adjacent segment.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe rod material was associated with the change of adjacent segments in OLIF surgery. Adding Ti or NiTi alloy rods can result in a greater reduction of ROM at the surgical level, which causes an increase in disc stress and facet joint contact force at the adjacent segment. After replacing the PCU rod, the cephalic disc stress and facet joint contact force generated in flexion and extension were close to the intact spine. Therefore, compared to titanium alloy rods, the use of OLIF surgery with PCU rods can minimize the impact of the adjacent segment after lumbar fusion.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eConsent for publication\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003eEthics approval and consent to participate\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003eData Availability\u003c/p\u003e\n\u003cp\u003eI do not have any research data outside the submitted manuscript file.\u003c/p\u003e\n\u003cp\u003eCompeting interests\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eThe study was supported by National Science and Technology Council (NSTC 112-2221-E-A49 -017 -MY3)\u003c/p\u003e\n\u003cp\u003eAuthors\u0026apos; contributions\u003c/p\u003e\n\u003cp\u003eChou PC: conception AND interpretation of data\u003c/p\u003e\n\u003cp\u003eChen JJ: acquisition AND analysis\u003c/p\u003e\n\u003cp\u003eChen CS: design of work\u003c/p\u003e\n\u003cp\u003eShih SL: draft the work\u003c/p\u003e\n\u003cp\u003eWang ST: conception\u003c/p\u003e\n\u003cp\u003eLiu CL: conception\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBrinjikji W, Diehn F, Jarvik J, Carr C, Kallmes DF, Murad MH, Luetmer PH. MRI findings of disc degeneration are more prevalent in adults with low back pain than in asymptomatic controls: a systematic review and meta-analysis. AJNR Am J Neuroradiol. 2015;36(12):2394-9.\u003c/li\u003e\n\u003cli\u003evan den Eerenbeemt KD, Ostelo RW, van Royen BJ, Peul WC, van Tulder MW. Total disc replacement surgery for symptomatic degenerative lumbar disc disease: a systematic review of the literature. Eur Spine J. 2010;19(8):1262\u0026ndash;1280.\u003c/li\u003e\n\u003cli\u003eMijiyawa M, Oniankitan O, Kolani B, Koriko T. Low back pain in hospital outpatients in Lom\u0026eacute; (Togo). Joint Bone Spine. 2000;67(6):533-8.\u003c/li\u003e\n\u003cli\u003eRiihim\u0026auml;ki H, Tola S, Videman T, H\u0026auml;nninen K. Low-back pain and occupation. A cross-sectional questionnaire study of men in machine operating, dynamic physical work, and sedentary work. Spine. 1989;14(2):204-9.\u003c/li\u003e\n\u003cli\u003eSheng SR, Geng YB, Zhou KL, Wu AM, Wang XY, Ni WF. Minimally invasive surgery for degenerative spondylolisthesis: transforaminal or oblique lumbar interbody fusion. J Comp Eff Res. 2020;9(1):45\u0026ndash;51.\u003c/li\u003e\n\u003cli\u003eHe W, He D, Sun Y, Xing Y, Wen J, Wang W, Xi Y, Liu M, Tian W, Ye X. Standalone oblique lateral interbody fusion vs. combined with percutaneous pedicle screw in spondylolisthesis. BMC Musculoskelet Disord. 2020;21(1):1\u0026ndash;9.\u003c/li\u003e\n\u003cli\u003eLin GX, Akbary K, Kotheeranurak V, Quillo-Olvera J, Jo HJ, Yang XW, Mahatthanatrakul A, Kim JS. Clinical and radiologic outcomes of direct versus indirect decompression with lumbar interbody fusion: a matched-pair comparison analysis. World Neurosurg. 2018;119:e898-e909.\u003c/li\u003e\n\u003cli\u003eJin C, Jaiswal MS, Jeun SS, Ryu KS, Hur JW, Kim JS. Outcomes of oblique lateral interbody fusion for degenerative lumbar disease in patients under or over 65 years of age. Journal of Orthopaedic Surgery. 2018;13(1):1\u0026ndash;10.\u003c/li\u003e\n\u003cli\u003ePark P, Garton HJ, Gala VC, Hoff JT, McGillicuddy JE. Adjacent segment disease after lumbar or lumbosacral fusion: review of the literature. Spine. 2004;29(17):1938-44.\u003c/li\u003e\n\u003cli\u003eKaito T, Hosono N, Mukai Y, Makino T, Fuji T, Yonenobu K. Induction of early degeneration of the adjacent segment after posterior lumbar interbody fusion by excessive distraction of lumbar disc space. J Neurosurg Spine. 2010;12(6):671\u0026ndash;679.\u003c/li\u003e\n\u003cli\u003eLiu ZX, Gao ZW, Chen C, Liu ZY, Cai XY, Ren YN, Sun X, Ma XL, Du CF, Yang Q. Effects of osteoporosis on the biomechanics of various supplemental fixations co-applied with oblique lumbar interbody fusion (OLIF): a finite element analysis. BMC Musculoskelet Disord. 2022; 23(1):794. doi: 10.1186/s12891-022-05645-7. PMID: 35986271; PMCID: PMC9392247.\u003c/li\u003e\n\u003cli\u003eZhang S, Liu Z, Lu C, Zhao L, Feng C, Wang Y, Zhang Y. Oblique lateral interbody fusion combined with different internal fixations for the treatment of degenerative lumbar spine disease: a finite element analysis. BMC Musculoskelet Disord. 2022;23(1):206. doi: 10.1186/s12891-022-05150-x. PMID: 35246101; PMCID: PMC8897936.\u003c/li\u003e\n\u003cli\u003eHuang S, Min S, Wang S, Jin A. Biomechanical effects of an oblique lumbar interbody fusion combined with posterior augmentation: a finite element analysis. BMC Musculoskelet Disord. 2022;23(1):611.\u003c/li\u003e\n\u003cli\u003eDu CF, Cai XY, Gui W, Sun MS, Liu ZX, Liu CJ, Zhang CQ, Huang YP. Does oblique lumbar interbody fusion promote adjacent degeneration in degenerative disc disease: A finite element analysis. Comput Biol Med. 2021;128:104122. doi: 10.1016/j.compbiomed.2020.104122. Epub 2020 Nov 21. 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PMID: 36008782; PMCID: PMC9413940.\u003c/li\u003e\n\u003cli\u003eAbe K, Orita S, Mannoji C, Motegi H, Aramomi M, Ishikawa T, Kotani T, Akazawa T, Morinaga T, Fujiyoshi T. Perioperative complications in 155 patients who underwent oblique lateral interbody fusion surgery: perspectives and indications from a retrospective, multicenter survey. Spine. 2017;42(1):55\u0026ndash;62.\u003c/li\u003e\n\u003cli\u003eWang K, Zhang C, Cheng C, Jian F, Wu H. Radiographic and clinical outcomes following combined oblique lumbar interbody fusion and lateral instrumentation for the treatment of degenerative spine deformity: a preliminary retrospective study. Biomed Res Int. 2019;5672162. doi: 10.1155/2019/5672162. PMID: 30729127; PMCID: PMC6341239.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bmc-musculoskeletal-disorders","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"bmsd","sideBox":"Learn more about [BMC Musculoskeletal Disorders](http://bmcmusculoskeletdisord.biomedcentral.com/)","snPcode":"","submissionUrl":"https://author-welcome.nature.com/12891","title":"BMC Musculoskeletal Disorders","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"OLIF, adjacent segment, lumbar spine, finite element analysis ","lastPublishedDoi":"10.21203/rs.3.rs-5398913/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5398913/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cb\u003eBackground\u003c/b\u003e\u003c/p\u003e \u003cp\u003eOLIF (oblique lumbar interbody fusion) is a minimally invasive surgery to treat spinal instability. However, clinical studies indicated the early degeneration of adjacent segments after surgery. The rod stiffness of OLIF was associated with change at adjacent segments. Therefore, the study aimed to compare the biomechanical effects of OLIF with different rod material properties using the finite element (FE) method.\u003c/p\u003e\u003cp\u003e\u003cb\u003eMethods\u003c/b\u003e\u003c/p\u003e \u003cp\u003eA validated L1-L5 lumbar spine was conducted in the biomechanical analysis using FE software ANSYS. The FE model of OLIF with a rod was created. Current biocompatible materials for the rod of the OLIF model were changed, including titanium alloy (OLIF_Ti), nickel-titanium alloy (OLIF_NiTi), and polycarbonate urethane (OLIF_PCU) rod. Four FE models, consisting of the intact model (INT) and implant models, were created. Hybrid control loads, such as flexion, extension, rotation, and lateral bending, were subjected to four models on the L1 vertebral body. The bottom of the L5 vertebral body was fixed.\u003c/p\u003e\u003cp\u003e\u003cb\u003eResults\u003c/b\u003e\u003c/p\u003e \u003cp\u003eAt the surgical level, while compared to the INT model, the OLIF_Ti and OLIF_NiTi model resulted in a ROM reduction of over 40% at least, but the OLIF_PCU changed about 10% in flexion and extension. At adjacent level L2-L3, the FE results indicated that the OLIF_Ti and OLIF_NiTi model increased more stress by about 12% at least than the INT model at the adjacent segment, but it demonstrated that the OLIF_PCU would not result in stress rise at the adjacent level L2-L3 in flexion and extension.\u003c/p\u003e\u003cp\u003e\u003cb\u003eConclusion\u003c/b\u003e\u003c/p\u003e \u003cp\u003eThe study concluded that rod stiffness was associated with change at the adjacent segments. The use of OLIF surgery with PCU rods can minimize the impact of the adjacent segment after lumbar fusion.\u003c/p\u003e","manuscriptTitle":"Changing rod stiffness to moderate stress of adjacent disc in oblique lumbar interbody fusion - A finite element analysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-10 15:49:18","doi":"10.21203/rs.3.rs-5398913/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-11-11T18:11:22+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-11-07T11:49:19+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-11-07T11:46:19+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Musculoskeletal Disorders","date":"2024-11-06T02:41:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bmc-musculoskeletal-disorders","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"bmsd","sideBox":"Learn more about [BMC Musculoskeletal Disorders](http://bmcmusculoskeletdisord.biomedcentral.com/)","snPcode":"","submissionUrl":"https://author-welcome.nature.com/12891","title":"BMC Musculoskeletal Disorders","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b657339c-631a-45a9-8180-3bca33d46bf5","owner":[],"postedDate":"December 10th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-03-24T16:05:40+00:00","versionOfRecord":{"articleIdentity":"rs-5398913","link":"https://doi.org/10.1186/s12891-025-08504-3","journal":{"identity":"bmc-musculoskeletal-disorders","isVorOnly":false,"title":"BMC Musculoskeletal Disorders"},"publishedOn":"2025-03-17 15:57:30","publishedOnDateReadable":"March 17th, 2025"},"versionCreatedAt":"2024-12-10 15:49:18","video":"","vorDoi":"10.1186/s12891-025-08504-3","vorDoiUrl":"https://doi.org/10.1186/s12891-025-08504-3","workflowStages":[]},"version":"v1","identity":"rs-5398913","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5398913","identity":"rs-5398913","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}