Evaluation of the porosity and structural stability of 3D-printed porous titanium pedicle screws using finite element analysis

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To enhance bone integration and fixation stability, considerable efforts have been made to improve the success rate of spinal fusion. However, no research has been conducted on the spinal biomechanics of 3D-printed porous titanium pedicle screws (3DPS). This study evaluates the structural performance of 3DPS under physiological loading conditions using finite element analysis (FEA) and analyzes the effects of varying porosity levels on their mechanical behavior. Method A validated FE model of the lumbar spine was used to simulate one-, two-, and three-level fusion scenarios with 3DPS and transforaminal lumbar interbody fusion (TLIF) cages. Physiological loads, including flexion, extension, lateral bending, and axial rotation, were applied. Peak von Mises stress (PVMS), stress distribution, and structural stability were assessed across the different porosity configurations (0%, 60%, 70%, and 80%). Result The PVMS value in the core increases as the porosity increases. the stress distribution of posterior fixations in a 3-level fusion. when the porosity of the porous layer was 80%, the stress was concentrated in the core. At 70% and 80% porosity, where the risk of structural instability exceeded safe thresholds under a conservative safety factor of 3. The 60% porosity demonstrated an optimal balance between mechanical stability and stress distribution. Conclusion 3DPS, particularly those with 60% porosity, offer promising potential for enhancing fixation stability. Further studies are needed to confirm their long-term clinical efficacy. The results of this study can serve as a starting point for preclinical and clinical studies to ensure the stability of 3DPS, and can serve as basic evidence that 3DPS are an alternative option to reduce pseudarthrosis and screw loosening. Biological sciences/Biophysics Health sciences/Health care 3D printing porous pedicle screws finite element analysis spinal fusion structural stability Screw loosening Pseudarthrosis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction As the global population continues to age, degenerative spine diseases, such as lumbar disc degeneration and spinal stenosis, are becoming more prevalent 1 . This has led to a significant increase in the number of spine fusion surgeries performed each year 2 . Spine fusion surgery aim to ensure spinal stability and alleviate symptoms in patients with spinal instability, deformity or spine fracture 3 . Pedicle screw fixation has been widely used to maintain spinal stability until solid fusion is achieved, in order to accomplish the goals of spine fusion surgery 4 , 5 . However, the challenges associated with spine fusion surgeries remain, particularly in achieving long-term stability and avoiding complications 6 , 7 . Despite advancements in spinal surgery, complications such as pseudarthrosis and screw loosening remain common and are often the cause of failure in spinal fusion procedures 7 . Pseudarthrosis can lead to persistent pain and requires additional surgical intervention, while screw loosening compromises the mechanical stability of the fixation, increasing the risk of revision surgery 7 . These issues are often associated with the mismatch between the spine implant materials and the bone's biological response, which can result in inadequate integration and fixation over time 8 . In response to these challenges, there has been considerable effort to improve the success rate of spinal fusion. One of the main areas of research is the development of new fusion device materials that enhance the biological response of bone to implants. As a method for this, the use of 3D printed technology for spine devices has emerged. In particular, 3D printed porous titanium cages have been introduced as a promising option for enhancing bone in-growth and improving fusion outcomes 9 – 13 . 3D printed porous titanium cages have gained attention for their ability to mimic the structure of bone, thereby promoting better integration with the surrounding tissue and facilitating faster, more effective fusion 14 . Additionally, the porous structure may provide more effective load distribution, further supporting interbody fusion 15 . Fogel et al. investigated the impact of porous design features on the stiffness and subsidence performance of 3D printed porous titanium cages used in spinal fusion 15 . Results showed that both the porous body lattice and microporous endplates reduced cage stiffness by around 16%, with the microporous endplates causing a significant decrease in block stiffness (24.9%). Furthermore, a cage with porous characteristics has been shown to support early bone growth and segmental stability, achieving solid fusion within 12 weeks. Therefore, incorporating 3D printed porous titanium pedicle screws (3DPS) could also be a key strategy in addressing the challenges of screw loosening and achieving better fusion outcomes. To date, 3D printing technology has advanced further, allowing for the creation of customized implants tailored to the patient's specific anatomical needs 14 , 16 , 17 . It has been applied to 3D printed titanium interbody fusion cages, which have shown clinically favorable outcomes 14 . However, regarding the application of 3D printing technology to screws, studies have focused on 3D printed pedicle screw-guided templates aimed at enhancing the precision of screw placement during surgery 18 . In contrast, research on 3DPS has been scarcely reported. This study aims to be the first to evaluate the structural stability of 3D printed porous titanium pedicle screws in the context of spine surgery. Using finite element analysis (FEA), we assessed the mechanical performance of these screws under typical physiological loading conditions, comparing them with stress for one, two, and three-level fusion in the flexion (FX), extension (EX), right lateral bending (RLB), left lateral bending (LLB), right axial rotation (RAR), and left axial rotation (LAR). Materials and methods Finite element models of native lumbar spine and validation A comprehensive FE model of the native lumbar spine was created, expanding on previous validated models of the lumbar spine (Fig. 1 ) 19 , 20 . The model included the vertebral bodies from the first (L1) to the fifth (L5) lumbar vertebrae, posterior elements, intervertebral discs, facet joints, pelvis, sacrum, and seven spinal ligaments: the anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), ligamentum flavum (LF), capsular ligament (CL), intertransverse ligament (ITL), interspinous ligament (ISL), and supraspinous ligament (SSL) (Fig. 1 A). Most components were modeled as deformable, while the pelvis and sacrum were treated as rigid. The model was designed to reflect an upright standing position with an initial lordotic curvature of 46° between the superior endplates of the first lumbar vertebra (L1) and the first sacral vertebra (S1) (Fig. 1 B). The material properties of all components were assumed to be homogeneous and isotropic, with values sourced from existing literature (Table 1 ) 21 – 23 . To validate the FE model of the native lumbar spine, the ranges of motion (ROMs) under simulated pure moments of 7.5 Nm in flexion-extension, lateral bending, and axial rotation were calculated and compared with ROMs reported in similar cadaveric studies 24 – 26 . Table 1 Modeling properties of components, including their Young’s modulus with linear and nonlinear representation, stress or load conditions, and Poisson's ratio. Component Young's modulus (MPa) [Range of stress condition (%) for nonlinearity] Poisson ratio (ν) References Cortical bone 12,000 0.3 21 , 22 Cancellous bone 100 0.2 21 , 22 Posterior elements 3,500 0.25 21 , 22 Endplate 25 0.25 22 Annulus ground 4.2 0.45 21 , 22 Annulus fibers 450 0.3 21 Nucleus pulposus 1 0.499 23 ALL 7.8 ( 12%) 0.3 21 , 23 PLL 10 ( 11%) 0.3 21 , 23 LF 15 ( 6.2%) 0.3 21 , 23 CL 7.5 ( 25%) 0.3 21 , 23 ITL 10 ( 18%) 0.3 21 , 23 ISL 10 ( 14%) 0.3 21 , 23 SSL 8 ( 20%) 0.3 21 , 23 Ti64ELI 110,000 0.3 28 Ti64ELI 60% Porous 24,400 0.3 28 Ti64ELI 70% Porous 9,700 0.3 28 Ti64ELI 80% Porous 2,420 0.3 28 *Note: the anterior longitudinal ligament, ALL; posterior longitudinal ligament, PLL; ligamentum flavum, LF; capsular ligament, CL; intertransverse ligament, ITL; interspinous ligament, ISL; supraspinous ligament, SSL Spine fixation model with 3DPS and transforaminal lumbar interbody fusion (TLIF) cages A spinal fixation model comprising 3DPS and a TLIF cage was created, with FE models used to assess its biomechanical performance (Fig. 1 C, D). The pedicle screws were based on a standard design (RX-MIS, RNX, South Korea), with a diameter of 5.5 mm and a length of 45 mm. Each screw consisted of a threaded layer, a cylindrical core, and a screw head. A 3D printed porous treatment was applied to the threaded layer to enhance insertion and pull-out strength. Specifically, the screw featured a solid cylindrical core with a surface treated by an irregular bone-like porous layer, 500 microns thick (Fig. 1 C). The porous structure was designed with four variations: 0% porosity (solid), 60% porosity, 70% porosity, and 80% porosity, to investigate the effects of different porosity levels. The screws were positioned according to standard surgical guidelines (Magerl insertion techniques) (Fig. 2 A) 27 . 3DPS were meshed with four-node tetrahedral elements (C3D4), with material properties of titanium alloy (Ti6Al4V ELI) assigned. The material properties of the porous structures were adjusted based on porosity-dependent variations reported by Pan et al. and other authors (Table 1 ) 28 . The TLIF cage was based on a generic curved design (RX-LC Curved TLIF Cage, RNX, South Korea), with dimensions of 33 mm in length, 10 mm in width, 14 mm in height, and an 8° angle (Fig. 1 D). The TLIF cage was meshed using C3D4 elements, assigned Ti6Al4V ELI material properties (Table 1 ), and positioned symmetrically in an anterior location based on surgical techniques (Fig. 2 B) 29 . It was aligned with the superior endplate of the vertebrae to avoid interference with the endplate geometry, with a tie constraint applied at the interfaces. To assess the impact of the fixation system on spinal stability, FE models of the lumbar spine were created for three configurations: one level (L4-5) with the fixation system, two levels (L3-4-5) with the fixation system, and three levels (L2-3-4-5) with the fixation system (Fig. 2 C, D, E). Loading Conditions and Simulations for FE Analysis Physiological loading conditions were applied to the FE models to replicate realistic spinal behavior. The body weight was represented as a 400 N follower load acting along the curvature of the lumbar spine, while an additional 7.5 Nm functional moment was applied to simulate six primary movements: FL, EX, RLB, LLB, RAR, and LAR. The inferior endplate of the L5 vertebra was fully constrained in all six degrees of freedom to act as the boundary condition. All simulations were performed under quasi-static conditions using Abaqus v6.24 FE analysis software (Dassault Systèmes, France) 30 . Analysis of structural stability in 3DPS The von Mises stresses were analyzed to quantitatively assess the structural stability of the 3DPS for one, two, and three-levels fusion with the fixation system. The stability of the 3DPS was initially assessed by comparing the peak von Mises stresses (PVMS) from the FEA to the yield strength of Ti6Al4V ELI for the 3DPS (990 MPa for 0% porosity, 230 MPa for 60% porosity, 60 MPa for 70% porosity, and 20 MPa for 80% porosity) 28 . Furthermore, to ensure the screws remain within safe margins under physiological loading conditions, a safety factor of 3 was applied to the yield strength, offering a conservative evaluation of structural stability 31 . Results Von Mises stress of 3DPS Figure 3 . presents the results of the PVMS for the core and the porous layer of the 3DPS at each level under working conditions. The porosity and motion with the highest PVMS in the core were 80% porosity across all levels, with motion being LLB at one level and RAR at two and three levels. In the porous layer, the porosity and motion with the highest PVMS were 0% porosity across all levels, with motion being RLB at one level, LAR at two levels, and RAR at three levels. The PVMS value in the core increases as the porosity increases, indicating that the risk of failure rises with higher porosity. On the other hand, in the porous layer, the PVMS decreases as the porosity increases. Additionally, the stress distribution of posterior fixations in a 3-level fusion was compared when the porosity of the porous layer was 0% and 80%. Figure 4 A shows the stress distribution including the porous layer, while Fig. 4 B presents the stress distribution with the porous layer removed to see the stress distribution of the core. It was observed that when the porosity of the porous layer was 0%, the porous layer also acted as a solid, resulting in stress occurring in both the core and the porous layer. However, when the porosity of the porous layer was 80%, the stress was concentrated in the core. Structural instability of 3DPS Structural instability is defined as the PVMS result divided by the yield strength, which indicates how stable it is, and expressed as a percentage. The yield strength used corresponded to the degree of porosity mentioned above. For the core, the yield strength of solid titanium alloy (porosity 0%) was applied, whereas for the porous layer, the yield strength specific to each degree of porosity was used. At a safety factor of 1, the structural instability (%) increased with higher porosity across all fusion levels. However, it did not exceed 100% in any part of the screw (Fig. 5 ). At a safety factor of 3, the structural instability (%) showed a similar trend to that observed at a safety factor of 1. The core remained stable across all fusion levels and motions. However, in the porous layer, structural instability exceeded 100% at 80% porosity for all fusion levels and motions. At 70% porosity, it exceeded 100% for all fusion levels during RLB, LLB, RAR, and LAR motions (Fig. 6 ). Validation of FE Model The ROMs obtained from validating the native lumbar spine FE model were 36.3° for flexion-extension, 20.3° for lateral bending, and 9.2° for axial rotation. the degree values are consistent with the ROM ranges found in the validation of the native lumbar spine FE model studies reported in the literature 25 , 32 – 34 . This indicates that the FE model used in the current study is highly valid. Discussion Despite significant advancements in research and technology aimed at achieving successful spine fusion, a considerable number of cases still experience pseudarthrosis and screw loosening 7 , 35 . In particular, coated pedicle screws have been introduced as a method to reduce screw loosening. Ohe et al. suggested that the use of pedicle screws with a thin hydroxyapatite coating provided strong fixation in an animal experiment of an osteoporosis model 36 . Another animal experiment using tantalum-coated pedicle screws reported that the coated screws had the advantage of strong coating-surface adhesion 37 . However, these coated pedicle screws have not yet been developed for clinical use due to the risk of detachment of the implant from the host bone by breakage of the coated layer 38 . Therefore, rather than a method of coating a new material on titanium, a component of the pedicle screw, a method of changing titanium itself was considered, and the application of 3D printing technology to spine implants was suggested. Application to interbody cages has been studied primarily. Many studies have demonstrated the potential of 3D printed porous titanium cages to enhance bone ingrowth and improve fusion outcomes 10 , 12 . From the perspective of porosity, Farber et al. reported a study on the appropriate design of 3D-printed titanium interbody cages. They found that increasing the surface contact area of the cage, rather than simply its porosity, reduces subsidence, a finding confirmed in both in vitro and in vivo studies 39 , 40 . Based on this, we have been exploring methods to increase the surface contact area of pedicle screws. To decrease pseudarthrosis and screw loosening, unlike previous studies focusing primarily on interbody fusion cages, this study developed 3DPS as a complementary solution and evaluated their stability using a FE model. The thickness of the porous layer was determined by referencing other studies on coated pedicle screws, where coating thicknesses varied widely depending on the coating material and method, ranging from 0.48–1.38 µm to 386.6 µm 36 , 37 , 41 . Considering the screw thread thickness (0.75mm − 1.5mm) 42 , 43 . the porous layer thickness was set to 500 microns. In addition, a screw diameter of 5.5mm and a length of 45mm were used, as these represent the smallest diameter and length commonly used in the lumbar spine in clinical practice, except in special cases. If this screw demonstrates stability in the FE model, it is expected that thicker and longer screws will naturally provide even greater stability. Our FE model also uses an interbody fusion model using a TLIF cage, as has been used in many other studies, to better reflect the fusion situation 44 , 45 . In the Von Mises stress results of 3DPS, as porosity increased, the PVMS of the core increased, while it decreased in the porous layer. However, this should not be interpreted directly, as the yield strength of the porous layer changes with varying porosity. As mentioned earlier, the yield strength (break strength) of the porous layer is 990 MPa at 0% porosity, 230 MPa at 60% porosity, 60 MPa at 70% porosity, and 20 MPa at 80% porosity. 28 Reflecting this, PVMS did not decrease with increasing porosity in the porous layer, and did not exceed the yield strength at any level or at any motion. In the stress distribution of posterior fixations in a 3-level fusion, the stress was concentrated in the core when the porosity was 80%. From a mechanical engineering perspective, this can be attributed to the fact that the stress generally decreases as the elastic modulus of the material decreases 46 . However, it also emphasizes the need for careful optimization of porosity to ensure the screws’ mechanical integrity while maximizing their biological performance 15 . The results of this study underscore the significant influence of porosity on the structural stability and mechanical performance of 3DPS. Specifically, the findings revealed that at 60% porosity, there were no instances of structural instability exceeding 100% across all levels and motions. In contrast, screws with 70% and 80% porosity exhibited high PVMS values exceeding 100% structural instability in the porous layer across all fusion levels, indicating an increased risk of structural failure when a conservative safety factor of 3, commonly used in surgical implants, was applied 31 , 47 . Based on studies on the porosity of 3D-printed titanium interbody cages, the porosity of these cages can range from 69–80% 39,48–51 . However, the optimal porosity of the 3DPS found in this study is 60%, which falls outside this range. This is thought to be due to the structural vulnerability of the pedicle screw, which is structurally simpler than the cage, and thus has difficulty maintaining stability compare to the cage when porosity is applied, resulting in a lower porosity measurement than the cage. These results, in particular, indicate that higher porosity with structural stability may be beneficial for enhancing bone fusion and alleviating stress shielding but poses significant mechanical risks that need to be addressed during the design stage. As a result, 60% porous screws remained within the safety margins across all loading movement, indicating their suitability for clinical use in both single and multi-level fusions. Screws with 60% porosity appear to offer the best compromise, providing sufficient strength to withstand physiological loads while potentially enhancing bone ingrowth and integration. This balance is critical for improving surgical outcomes and reducing the risk of complications such as screw loosening and pseudarthrosis. In particular, it suggests that this approach could be a viable option for decreasing screw loosening and pseudarthrosis in patients with osteoporosis. Therefore, these data support the importance of patient optimization before surgery. Our findings emphasize the importance of rigorous preclinical testing to validate the safety and efficacy of novel implant designs. While the FEA approach used in this study provides valuable insights into the mechanical behavior of 3DPS, in vivo studies and clinical trials are necessary to confirm these results and evaluate their impact on patient outcomes. While this study provides valuable insights into the mechanical performance of 3DPS, several limitations must be acknowledged. First, the FEA simulations were conducted under quasi-static conditions, which may not fully capture the dynamic loading patterns experienced during daily activities. Future studies should incorporate dynamic loading simulations to better replicate real-world conditions 23 , 27 , 33 . Second, the material properties used in the FEA model were assumed to be homogeneous and isotropic, which may not accurately reflect the anisotropic behavior of bone and porous titanium. Incorporating more realistic material models could enhance the accuracy of the simulations. Third, the study did not account for the biological factors influencing bone ingrowth and osseointegration, such as the effects of patient age, bone quality, and local biomechanical environment. Future research should explore these factors through in vivo studies and clinical trials to validate the findings and assess their clinical relevance. Finally, the focus on porosity levels within a specific range (0–80%) leaves room for exploring other design parameters, such as pore size, shape, and distribution. These factors could have a significant impact on both mechanical and biological performance and warrant further investigation. Conclusion In this study, we developed FE models of lumbar interbody fusion and found that the optimal porosity of the porous layer for ensuring the structural stability of 3DPS at multiple fusion levels is 60%. Based on these results, 3DPS with 60% porosity can be applied to cadaveric and animal specimen studies prior to clinical trials. Declarations Author contributions statement Kwang Hyeon Kim: Writing – original draft, Methodology, Formal analysis, Junsu Bae: Writing – original draft, Methodology, Data curation, Formal analysis, Hyeon Su Bae: Writing – review & editing, Data curation, Kyeong-Joo Yoo: Writing – review & editing, Data curation, Seonghoon Jeong: Writing – review & editing, Supervision, Dohyung Lim: Writing – review & editing, Supervision, Resource, Methodology, Supervision, Formal analysis, Conceptualization, Byung-Jou Lee: Writing – original draft, Supervision, Methodology, Formal analysis, Conceptualization, Project administration. Ethics Declarations Not applicable. Consent for Publication All authors consent to the publication of this manuscript. Author Contribution Kwang Hyeon Kim and Junsu Bae contributed equally to this work as first author.Kwang Hyeon Kim: Writing – original draft, Methodology, Formal analysis, Junsu Bae: Writing – original draft, Methodology, Data curation, Formal analysis, Hyeon Su Bae: Writing – review & editing, Data curation, Kyeong-Joo Yoo: Writing – review & editing, Data curation, Seonghoon Jeong: Writing – review & editing, Supervision, Dohyung Lim: Writing – review & editing, Supervision, Resource, Methodology, Supervision, Formal analysis, Conceptualization, Byung-Jou Lee: Writing – original draft, Supervision, Methodology, Formal analysis, Conceptualization, Project administration. Acknowledgement This work was supported by the Development of demand-linked technology based on hospital-company cooperation and commercialization [RS-2023-00233521, Development of manufacture technology of intervertebral fusion (Conventional, Expandable) device] funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea). No benefits in any form have been or will be received from a commercial party directly or indirectly related to the subject of this manuscript. The manuscript submitted does not contain information about medical device(s)/drug(s). Data Availability The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request. References Ravindra, V. M. et al. Degenerative lumbar spine disease: estimating global incidence and worldwide volume. Global spine J. 8 , 784–794 (2018). Reisener, M-J., Pumberger, M., Shue, J., Girardi, F. P. & Hughes, A. P. Trends in lumbar spinal fusion—a literature review. J. 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Spine . 28 , 679–687. 10.3171/2017.10.SPINE17702 (2018). Shi, L. Y. et al. Tantalum-coated pedicle screws enhance implant integration. Colloids Surf. B Biointerfaces . 160 , 22–32. 10.1016/j.colsurfb.2017.08.059 (2017). Lai, K. A. et al. Failure of hydroxyapatite-coated acetabular cups. Ten-year follow-up of 85 Landos Atoll arthroplasties. J. Bone Joint Surg. Br. 84 , 641–646. 10.1302/0301-620x.84b5.12384 (2002). Farber, S. H. et al. Optimization of 3D-printed titanium interbody cage design. Part 1: in vitro biomechanical study of subsidence. Spine J. doi . 10.1016/j.spinee.2024.12.002 (2024). Lombardo, J. A. et al. Optimization of 3D-printed titanium interbody cage design. Part 2: An in vivo study of spinal fusion in sheep. Spine J. 10.1016/j.spinee.2024.12.014 (2024). Kim, D. Y., Kim, J. R., Jang, K. Y., Kim, M. G. & Lee, K. B. Evaluation of Titanium-Coated Pedicle Screws: In Vivo Porcine Lumbar Spine Model. World Neurosurg. 91 , 163–171. 10.1016/j.wneu.2016.03.089 (2016). Liu, M. Y. et al. Biomechanical comparison of pedicle screw fixation strength in synthetic bones: Effects of screw shape, core/thread profile and cement augmentation. PLoS One . 15 , e0229328. 10.1371/journal.pone.0229328 (2020). Li, Y. D. et al. Biomechanical evaluation of pedicle screw stability after 360-degree turnback from full insertion: effects of screw shape, pilot hole profile and bone density. Front. Bioeng. Biotechnol. 11 , 1151627. 10.3389/fbioe.2023.1151627 (2023). Sin, D. A. & Heo, D. H. Comparative Finite Element Analysis of Lumbar Cortical Screws and Pedicle Screws in Transforaminal and Posterior Lumbar Interbody Fusion. Neurospine 16 , 298–304. 10.14245/ns.1836030.015 (2019). Lee, H. J. et al. Biomechanical Evaluation of Lateral Lumbar Interbody Fusion with Various Fixation Options for Adjacent Segment Degeneration: A Finite Element Analysis. World Neurosurg. 173 , e156–e167. 10.1016/j.wneu.2023.02.023 (2023). Zak, M. & Pezowicz, C. Effect of overload on changes in mechanical and structural properties of the annulus fibrosus of the intervertebral disc. Biomech. Model. Mechanobiol. 20 , 2259–2267. 10.1007/s10237-021-01505-w (2021). Amjad, M. et al. Finite element modeling of stress distribution and safety factors in a Ti-27Nb alloy hip implant under real-world physiological loading scenarios. PLoS One . 19 , e0300270. 10.1371/journal.pone.0300270 (2024). Yang, J. J., Kim, D. M. & Park, S. Comparison of Fusion, Subsidence, and Clinical Results Between 3D-Printed Porous Titanium Cage and Polyetheretherketone Cage in Posterior Lumbar Interbody Fusion: A Minimum of 2 Years Follow-Up. World Neurosurg. (2023). 10.1016/j.wneu.2023.06.132 Zhang, Z. et al. Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: A Finite Element Analysis. World Neurosurg. 111 , e581–e591. 10.1016/j.wneu.2017.12.127 (2018). Talukdar, R. G., Saviour, C. M., Dhara, S. & Gupta, S. Biomechanical analysis of functionally graded porous interbody cage for lumbar spinal fusion. Comput. Biol. Med. 164 , 107281. 10.1016/j.compbiomed.2023.107281 (2023). Kim, D. Y., Kwon, O. H. & Park, J. Y. Comparison Between 3-Dimensional-Printed Titanium and Polyetheretherketone Cages: 1-Year Outcome After Minimally Invasive Transforaminal Interbody Fusion. Neurospine 19 , 524–532. 10.14245/ns.2244140.070 (2022). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6215165","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":441606451,"identity":"c84efca5-0b64-4b2f-ad89-cbbedf827dc6","order_by":0,"name":"Kwang Hyeon Kim","email":"","orcid":"","institution":"Inje University Ilsan Paik Hospital","correspondingAuthor":false,"prefix":"","firstName":"Kwang","middleName":"Hyeon","lastName":"Kim","suffix":""},{"id":441606453,"identity":"3e0f82e9-efe6-4fc3-9213-3d7cf1fbf98c","order_by":1,"name":"Junsu Bae","email":"","orcid":"","institution":"Sejong University","correspondingAuthor":false,"prefix":"","firstName":"Junsu","middleName":"","lastName":"Bae","suffix":""},{"id":441606454,"identity":"227a7739-29c5-404b-87da-7b0443a4b467","order_by":2,"name":"Hyeon Su Bae","email":"","orcid":"","institution":"Sejong University","correspondingAuthor":false,"prefix":"","firstName":"Hyeon","middleName":"Su","lastName":"Bae","suffix":""},{"id":441606455,"identity":"5f831308-c856-4c1e-a0bb-e3faee4fe1aa","order_by":3,"name":"Kyeong-Joo Yoo","email":"","orcid":"","institution":"Corporate Research Institute, RNX Co., Ltd, Bucheon, Republic of Korea","correspondingAuthor":false,"prefix":"","firstName":"Kyeong-Joo","middleName":"","lastName":"Yoo","suffix":""},{"id":441606457,"identity":"b7e77579-d525-4cb9-9213-75e9c22a4e6f","order_by":4,"name":"Seonghoon Jeong","email":"","orcid":"","institution":"Inje University Ilsan Paik Hospital","correspondingAuthor":false,"prefix":"","firstName":"Seonghoon","middleName":"","lastName":"Jeong","suffix":""},{"id":441606459,"identity":"28df5a12-3e26-49df-b75b-b144484ec79a","order_by":5,"name":"Dohyung Lim","email":"","orcid":"","institution":"Sejong University","correspondingAuthor":false,"prefix":"","firstName":"Dohyung","middleName":"","lastName":"Lim","suffix":""},{"id":441606460,"identity":"7f21786c-55e4-4a37-a8a6-b5e91350bc13","order_by":6,"name":"Byung-Jou Lee","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA20lEQVRIiWNgGAWjYBACPgmGBCAlIWcA5hpYENbCBtViDNUiASSYCWoBg8QNEJoYLdINj1/z7rBI385+9uiGHwUSDObs/Qfwa5E5kGbNe0Yid2dPXtrNHqDDLHsOE3JYQpoxb5tE7oYDOWY3eIBaDG4kE6cl3eD8G7Obf0Ba7j8mqCX5MVBLgsGNHLPbEFsIeR/oF8a5bRKGG268MbstYyDBY3Am2QCvFn7pnuQPb9vq5A3O55jdfPPHRs7g+MEH+K1h4EmTQOESUA4C7Ic/EKFqFIyCUTAKRjIAAAfTQcwI0++BAAAAAElFTkSuQmCC","orcid":"","institution":"Inje University Ilsan Paik Hospital","correspondingAuthor":true,"prefix":"","firstName":"Byung-Jou","middleName":"","lastName":"Lee","suffix":""}],"badges":[],"createdAt":"2025-03-12 23:08:11","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6215165/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6215165/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81940664,"identity":"3386fe02-8805-4fff-b40f-011db66a316e","added_by":"auto","created_at":"2025-05-05 06:57:48","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":257466,"visible":true,"origin":"","legend":"\u003cp\u003eA comprehensive finite element (FE) model of the native lumbar spine and spine implant. (A) The model included the vertebral bodies from the first (L1) to the fifth (L5) lumbar vertebrae, posterior elements, intervertebral discs, facet joints, pelvis, sacrum, and seven spinal ligaments. (B) An upright standing position with an initial lordotic curvature of 46° between the superior endplates of the first lumbar vertebra (L1) and the first sacral vertebra (S1). (C) A spinal fixation system comprising 3D printed porous pedicle screw. (D) A transforaminal lumbar interbody fusion TLIF cage for finite element modeling.\u003c/p\u003e\n\u003cp\u003e*Note: The anterior longitudinal ligament, ALL; posterior longitudinal ligament, PLL; ligamentum flavum, LF; capsular ligament, CL; intertransverse ligament, ITL; interspinous ligament, ISL; supraspinous ligament, SSL\u003c/p\u003e","description":"","filename":"Figure1..png","url":"https://assets-eu.researchsquare.com/files/rs-6215165/v1/0b7f6c81b33d0f92df1f843b.png"},{"id":81940666,"identity":"fac34fee-0188-48ab-8b9b-a0b4bf1ead1b","added_by":"auto","created_at":"2025-05-05 06:57:48","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":379583,"visible":true,"origin":"","legend":"\u003cp\u003ePositioning of screw and cage in FE modal and multiple level fusion analysis. (A) Screw position, (B) Cage position, (C) One level fusion, (D) Two level fusion, (E) Three level fusion\u003c/p\u003e","description":"","filename":"Figure2..png","url":"https://assets-eu.researchsquare.com/files/rs-6215165/v1/f7f0713c67ebf166a7e5bf12.png"},{"id":81940663,"identity":"3d0c65c0-c551-4a3d-8c65-2c3d914fb696","added_by":"auto","created_at":"2025-05-05 06:57:48","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":142202,"visible":true,"origin":"","legend":"\u003cp\u003ePeak von Mises stress (PVMS) by porosity conditions. (A) One-level fusion. (B) Two-level fusion. (C) Three-level fusion.\u003c/p\u003e\n\u003cp\u003e*Note: FL: Flexion, EX: Extension, RLB: Right lateral bending, LLB: Left lateral bending, RAR: Right axial rotation, LAR: Left axial rotation\u003c/p\u003e","description":"","filename":"Figure3..png","url":"https://assets-eu.researchsquare.com/files/rs-6215165/v1/e5048df08407d1aa55b16757.png"},{"id":81941179,"identity":"36a6c6a3-3118-42fc-b913-5bdb58795698","added_by":"auto","created_at":"2025-05-05 07:05:48","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":662044,"visible":true,"origin":"","legend":"\u003cp\u003eStress distribution of 3D printed porous pedicle screw in three level fusion (0% vs 80%). (A) Stress distribution of 3D printed porous pedicle screw including porous layer (B) Stress distribution of core of 3D printed porous pedicle screw excluding porous layer\u003c/p\u003e\n\u003cp\u003e*Note: FL: Flexion, EX: Extension, RLB: Right lateral bending, LLB: Left lateral bending, RAR: Right axial rotation, LAR: Left axial rotation\u003c/p\u003e","description":"","filename":"Figure4..png","url":"https://assets-eu.researchsquare.com/files/rs-6215165/v1/553aa9ef8916e98544bc135b.png"},{"id":81941180,"identity":"fb6b8b63-e983-49ea-b281-2ccf61b0d92e","added_by":"auto","created_at":"2025-05-05 07:05:48","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":129159,"visible":true,"origin":"","legend":"\u003cp\u003eStructural instability of 3D printed porous pedicle screw at safety factor 1. (A) One-level fusion. (B) Two-level fusion. (C) Three-level fusion.\u003c/p\u003e\n\u003cp\u003e*Note: FL: Flexion, EX: Extension, RLB: Right lateral bending, LLB: Left lateral bending, RAR: Right axial rotation, LAR: Left axial rotation\u003c/p\u003e","description":"","filename":"Figure5..png","url":"https://assets-eu.researchsquare.com/files/rs-6215165/v1/c80bac9ff2f5c574c0045400.png"},{"id":81940668,"identity":"36f16842-f3b4-47f9-b3be-71beec813c59","added_by":"auto","created_at":"2025-05-05 06:57:48","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":165448,"visible":true,"origin":"","legend":"\u003cp\u003eStructural instability of 3D printed porous pedicle screw at safety factor 3. (A) One-level fusion. (B) Two-level fusion. (C) Three-level fusion.\u003c/p\u003e","description":"","filename":"Figure6..png","url":"https://assets-eu.researchsquare.com/files/rs-6215165/v1/0f717a861308b74cea7bb018.png"},{"id":81942783,"identity":"064f35cd-e48c-4c33-969b-d0770be6c50a","added_by":"auto","created_at":"2025-05-05 07:29:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2295752,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6215165/v1/1cd06e8f-dfd6-4ac7-a5d9-003bf1960543.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Evaluation of the porosity and structural stability of 3D-printed porous titanium pedicle screws using finite element analysis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAs the global population continues to age, degenerative spine diseases, such as lumbar disc degeneration and spinal stenosis, are becoming more prevalent \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. This has led to a significant increase in the number of spine fusion surgeries performed each year \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Spine fusion surgery aim to ensure spinal stability and alleviate symptoms in patients with spinal instability, deformity or spine fracture \u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Pedicle screw fixation has been widely used to maintain spinal stability until solid fusion is achieved, in order to accomplish the goals of spine fusion surgery \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. However, the challenges associated with spine fusion surgeries remain, particularly in achieving long-term stability and avoiding complications \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Despite advancements in spinal surgery, complications such as pseudarthrosis and screw loosening remain common and are often the cause of failure in spinal fusion procedures \u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Pseudarthrosis can lead to persistent pain and requires additional surgical intervention, while screw loosening compromises the mechanical stability of the fixation, increasing the risk of revision surgery \u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. These issues are often associated with the mismatch between the spine implant materials and the bone's biological response, which can result in inadequate integration and fixation over time \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. In response to these challenges, there has been considerable effort to improve the success rate of spinal fusion. One of the main areas of research is the development of new fusion device materials that enhance the biological response of bone to implants. As a method for this, the use of 3D printed technology for spine devices has emerged. In particular, 3D printed porous titanium cages have been introduced as a promising option for enhancing bone in-growth and improving fusion outcomes \u003csup\u003e\u003cspan additionalcitationids=\"CR10 CR11 CR12\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. 3D printed porous titanium cages have gained attention for their ability to mimic the structure of bone, thereby promoting better integration with the surrounding tissue and facilitating faster, more effective fusion \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Additionally, the porous structure may provide more effective load distribution, further supporting interbody fusion \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. Fogel et al. investigated the impact of porous design features on the stiffness and subsidence performance of 3D printed porous titanium cages used in spinal fusion \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. Results showed that both the porous body lattice and microporous endplates reduced cage stiffness by around 16%, with the microporous endplates causing a significant decrease in block stiffness (24.9%). Furthermore, a cage with porous characteristics has been shown to support early bone growth and segmental stability, achieving solid fusion within 12 weeks. Therefore, incorporating 3D printed porous titanium pedicle screws (3DPS) could also be a key strategy in addressing the challenges of screw loosening and achieving better fusion outcomes. To date, 3D printing technology has advanced further, allowing for the creation of customized implants tailored to the patient's specific anatomical needs \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. It has been applied to 3D printed titanium interbody fusion cages, which have shown clinically favorable outcomes \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. However, regarding the application of 3D printing technology to screws, studies have focused on 3D printed pedicle screw-guided templates aimed at enhancing the precision of screw placement during surgery \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. In contrast, research on 3DPS has been scarcely reported. This study aims to be the first to evaluate the structural stability of 3D printed porous titanium pedicle screws in the context of spine surgery. Using finite element analysis (FEA), we assessed the mechanical performance of these screws under typical physiological loading conditions, comparing them with stress for one, two, and three-level fusion in the flexion (FX), extension (EX), right lateral bending (RLB), left lateral bending (LLB), right axial rotation (RAR), and left axial rotation (LAR).\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eFinite element models of native lumbar spine and validation\u003c/h2\u003e \u003cp\u003eA comprehensive FE model of the native lumbar spine was created, expanding on previous validated models of the lumbar spine (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. The model included the vertebral bodies from the first (L1) to the fifth (L5) lumbar vertebrae, posterior elements, intervertebral discs, facet joints, pelvis, sacrum, and seven spinal ligaments: the anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), ligamentum flavum (LF), capsular ligament (CL), intertransverse ligament (ITL), interspinous ligament (ISL), and supraspinous ligament (SSL) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). Most components were modeled as deformable, while the pelvis and sacrum were treated as rigid. The model was designed to reflect an upright standing position with an initial lordotic curvature of 46\u0026deg; between the superior endplates of the first lumbar vertebra (L1) and the first sacral vertebra (S1) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB). The material properties of all components were assumed to be homogeneous and isotropic, with values sourced from existing literature (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) \u003csup\u003e\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. To validate the FE model of the native lumbar spine, the ranges of motion (ROMs) under simulated pure moments of 7.5 Nm in flexion-extension, lateral bending, and axial rotation were calculated and compared with ROMs reported in similar cadaveric studies \u003csup\u003e\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e.\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\u003eModeling properties of components, including their Young\u0026rsquo;s modulus with linear and nonlinear representation, stress or load conditions, and Poisson's ratio.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComponent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eYoung's modulus (MPa) [Range of stress condition (%) for nonlinearity]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePoisson ratio (ν)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eReferences\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCortical bone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e12,000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCancellous bone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePosterior elements\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e3,500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEndplate\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAnnulus ground\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e4.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAnnulus fibers\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e450\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNucleus pulposus\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.499\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eALL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.8 (\u0026lt;\u0026thinsp;12%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20 (\u0026gt;\u0026thinsp;12%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePLL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10 (\u0026lt;\u0026thinsp;11%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20 (\u0026gt;\u0026thinsp;11%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e15 (\u0026lt;\u0026thinsp;6.2%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e19 (\u0026gt;\u0026thinsp;6.2%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.5 (\u0026lt;\u0026thinsp;25%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e33 (\u0026gt;\u0026thinsp;25%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eITL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10 (\u0026lt;\u0026thinsp;18%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e59 (\u0026gt;\u0026thinsp;18%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eISL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10 (\u0026lt;\u0026thinsp;14%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12 (\u0026gt;\u0026thinsp;14%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSSL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8 (\u0026lt;\u0026thinsp;20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e15 (\u0026gt;\u0026thinsp;20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi64ELI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e110,000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi64ELI 60% Porous\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e24,400\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi64ELI 70% Porous\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e9,700\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi64ELI 80% Porous\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e2,420\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e*Note: the anterior longitudinal ligament, ALL; posterior longitudinal ligament, PLL; ligamentum flavum, LF; capsular ligament, CL; intertransverse ligament, ITL; interspinous ligament, ISL; supraspinous ligament, SSL\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSpine fixation model with 3DPS and transforaminal lumbar interbody fusion (TLIF) cages\u003c/h3\u003e\n\u003cp\u003eA spinal fixation model comprising 3DPS and a TLIF cage was created, with FE models used to assess its biomechanical performance (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC, D). The pedicle screws were based on a standard design (RX-MIS, RNX, South Korea), with a diameter of 5.5 mm and a length of 45 mm. Each screw consisted of a threaded layer, a cylindrical core, and a screw head. A 3D printed porous treatment was applied to the threaded layer to enhance insertion and pull-out strength. Specifically, the screw featured a solid cylindrical core with a surface treated by an irregular bone-like porous layer, 500 microns thick (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC). The porous structure was designed with four variations: 0% porosity (solid), 60% porosity, 70% porosity, and 80% porosity, to investigate the effects of different porosity levels. The screws were positioned according to standard surgical guidelines (Magerl insertion techniques) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA) \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. 3DPS were meshed with four-node tetrahedral elements (C3D4), with material properties of titanium alloy (Ti6Al4V ELI) assigned. The material properties of the porous structures were adjusted based on porosity-dependent variations reported by Pan et al. and other authors (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. The TLIF cage was based on a generic curved design (RX-LC Curved TLIF Cage, RNX, South Korea), with dimensions of 33 mm in length, 10 mm in width, 14 mm in height, and an 8\u0026deg; angle (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eD). The TLIF cage was meshed using C3D4 elements, assigned Ti6Al4V ELI material properties (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), and positioned symmetrically in an anterior location based on surgical techniques (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB) \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. It was aligned with the superior endplate of the vertebrae to avoid interference with the endplate geometry, with a tie constraint applied at the interfaces. To assess the impact of the fixation system on spinal stability, FE models of the lumbar spine were created for three configurations: one level (L4-5) with the fixation system, two levels (L3-4-5) with the fixation system, and three levels (L2-3-4-5) with the fixation system (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC, D, E).\u003c/p\u003e\n\u003ch3\u003eLoading Conditions and Simulations for FE Analysis\u003c/h3\u003e\n\u003cp\u003ePhysiological loading conditions were applied to the FE models to replicate realistic spinal behavior. The body weight was represented as a 400 N follower load acting along the curvature of the lumbar spine, while an additional 7.5 Nm functional moment was applied to simulate six primary movements: FL, EX, RLB, LLB, RAR, and LAR. The inferior endplate of the L5 vertebra was fully constrained in all six degrees of freedom to act as the boundary condition. All simulations were performed under quasi-static conditions using Abaqus v6.24 FE analysis software (Dassault Syst\u0026egrave;mes, France) \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eAnalysis of structural stability in 3DPS\u003c/h3\u003e\n\u003cp\u003eThe von Mises stresses were analyzed to quantitatively assess the structural stability of the 3DPS for one, two, and three-levels fusion with the fixation system. The stability of the 3DPS was initially assessed by comparing the peak von Mises stresses (PVMS) from the FEA to the yield strength of Ti6Al4V ELI for the 3DPS (990 MPa for 0% porosity, 230 MPa for 60% porosity, 60 MPa for 70% porosity, and 20 MPa for 80% porosity) \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. Furthermore, to ensure the screws remain within safe margins under physiological loading conditions, a safety factor of 3 was applied to the yield strength, offering a conservative evaluation of structural stability \u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eVon Mises stress of 3DPS\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. presents the results of the PVMS for the core and the porous layer of the 3DPS at each level under working conditions. The porosity and motion with the highest PVMS in the core were 80% porosity across all levels, with motion being LLB at one level and RAR at two and three levels. In the porous layer, the porosity and motion with the highest PVMS were 0% porosity across all levels, with motion being RLB at one level, LAR at two levels, and RAR at three levels. The PVMS value in the core increases as the porosity increases, indicating that the risk of failure rises with higher porosity. On the other hand, in the porous layer, the PVMS decreases as the porosity increases. Additionally, the stress distribution of posterior fixations in a 3-level fusion was compared when the porosity of the porous layer was 0% and 80%. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA shows the stress distribution including the porous layer, while Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB presents the stress distribution with the porous layer removed to see the stress distribution of the core. It was observed that when the porosity of the porous layer was 0%, the porous layer also acted as a solid, resulting in stress occurring in both the core and the porous layer. However, when the porosity of the porous layer was 80%, the stress was concentrated in the core.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eStructural instability of 3DPS\u003c/h3\u003e\n\u003cp\u003eStructural instability is defined as the PVMS result divided by the yield strength, which indicates how stable it is, and expressed as a percentage. The yield strength used corresponded to the degree of porosity mentioned above. For the core, the yield strength of solid titanium alloy (porosity 0%) was applied, whereas for the porous layer, the yield strength specific to each degree of porosity was used. At a safety factor of 1, the structural instability (%) increased with higher porosity across all fusion levels. However, it did not exceed 100% in any part of the screw (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). At a safety factor of 3, the structural instability (%) showed a similar trend to that observed at a safety factor of 1. The core remained stable across all fusion levels and motions. However, in the porous layer, structural instability exceeded 100% at 80% porosity for all fusion levels and motions. At 70% porosity, it exceeded 100% for all fusion levels during RLB, LLB, RAR, and LAR motions (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e\n\u003ch3\u003eValidation of FE Model\u003c/h3\u003e\n\u003cp\u003eThe ROMs obtained from validating the native lumbar spine FE model were 36.3\u0026deg; for flexion-extension, 20.3\u0026deg; for lateral bending, and 9.2\u0026deg; for axial rotation. the degree values are consistent with the ROM ranges found in the validation of the native lumbar spine FE model studies reported in the literature \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. This indicates that the FE model used in the current study is highly valid.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eDespite significant advancements in research and technology aimed at achieving successful spine fusion, a considerable number of cases still experience pseudarthrosis and screw loosening \u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. In particular, coated pedicle screws have been introduced as a method to reduce screw loosening. Ohe et al. suggested that the use of pedicle screws with a thin hydroxyapatite coating provided strong fixation in an animal experiment of an osteoporosis model \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Another animal experiment using tantalum-coated pedicle screws reported that the coated screws had the advantage of strong coating-surface adhesion \u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. However, these coated pedicle screws have not yet been developed for clinical use due to the risk of detachment of the implant from the host bone by breakage of the coated layer \u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. Therefore, rather than a method of coating a new material on titanium, a component of the pedicle screw, a method of changing titanium itself was considered, and the application of 3D printing technology to spine implants was suggested. Application to interbody cages has been studied primarily. Many studies have demonstrated the potential of 3D printed porous titanium cages to enhance bone ingrowth and improve fusion outcomes \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. From the perspective of porosity, Farber et al. reported a study on the appropriate design of 3D-printed titanium interbody cages. They found that increasing the surface contact area of the cage, rather than simply its porosity, reduces subsidence, a finding confirmed in both in vitro and in vivo studies \u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. Based on this, we have been exploring methods to increase the surface contact area of pedicle screws. To decrease pseudarthrosis and screw loosening, unlike previous studies focusing primarily on interbody fusion cages, this study developed 3DPS as a complementary solution and evaluated their stability using a FE model. The thickness of the porous layer was determined by referencing other studies on coated pedicle screws, where coating thicknesses varied widely depending on the coating material and method, ranging from 0.48\u0026ndash;1.38 \u0026micro;m to 386.6 \u0026micro;m \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. Considering the screw thread thickness (0.75mm \u0026minus;\u0026thinsp;1.5mm) \u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. the porous layer thickness was set to 500 microns. In addition, a screw diameter of 5.5mm and a length of 45mm were used, as these represent the smallest diameter and length commonly used in the lumbar spine in clinical practice, except in special cases. If this screw demonstrates stability in the FE model, it is expected that thicker and longer screws will naturally provide even greater stability. Our FE model also uses an interbody fusion model using a TLIF cage, as has been used in many other studies, to better reflect the fusion situation \u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e,\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e. In the Von Mises stress results of 3DPS, as porosity increased, the PVMS of the core increased, while it decreased in the porous layer. However, this should not be interpreted directly, as the yield strength of the porous layer changes with varying porosity. As mentioned earlier, the yield strength (break strength) of the porous layer is 990 MPa at 0% porosity, 230 MPa at 60% porosity, 60 MPa at 70% porosity, and 20 MPa at 80% porosity.\u003csup\u003e28\u003c/sup\u003e Reflecting this, PVMS did not decrease with increasing porosity in the porous layer, and did not exceed the yield strength at any level or at any motion. In the stress distribution of posterior fixations in a 3-level fusion, the stress was concentrated in the core when the porosity was 80%. From a mechanical engineering perspective, this can be attributed to the fact that the stress generally decreases as the elastic modulus of the material decreases \u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. However, it also emphasizes the need for careful optimization of porosity to ensure the screws\u0026rsquo; mechanical integrity while maximizing their biological performance \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. The results of this study underscore the significant influence of porosity on the structural stability and mechanical performance of 3DPS. Specifically, the findings revealed that at 60% porosity, there were no instances of structural instability exceeding 100% across all levels and motions. In contrast, screws with 70% and 80% porosity exhibited high PVMS values exceeding 100% structural instability in the porous layer across all fusion levels, indicating an increased risk of structural failure when a conservative safety factor of 3, commonly used in surgical implants, was applied \u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. Based on studies on the porosity of 3D-printed titanium interbody cages, the porosity of these cages can range from 69\u0026ndash;80% \u003csup\u003e39,48\u0026ndash;51\u003c/sup\u003e. However, the optimal porosity of the 3DPS found in this study is 60%, which falls outside this range. This is thought to be due to the structural vulnerability of the pedicle screw, which is structurally simpler than the cage, and thus has difficulty maintaining stability compare to the cage when porosity is applied, resulting in a lower porosity measurement than the cage. These results, in particular, indicate that higher porosity with structural stability may be beneficial for enhancing bone fusion and alleviating stress shielding but poses significant mechanical risks that need to be addressed during the design stage. As a result, 60% porous screws remained within the safety margins across all loading movement, indicating their suitability for clinical use in both single and multi-level fusions. Screws with 60% porosity appear to offer the best compromise, providing sufficient strength to withstand physiological loads while potentially enhancing bone ingrowth and integration. This balance is critical for improving surgical outcomes and reducing the risk of complications such as screw loosening and pseudarthrosis. In particular, it suggests that this approach could be a viable option for decreasing screw loosening and pseudarthrosis in patients with osteoporosis. Therefore, these data support the importance of patient optimization before surgery. Our findings emphasize the importance of rigorous preclinical testing to validate the safety and efficacy of novel implant designs. While the FEA approach used in this study provides valuable insights into the mechanical behavior of 3DPS, in vivo studies and clinical trials are necessary to confirm these results and evaluate their impact on patient outcomes. While this study provides valuable insights into the mechanical performance of 3DPS, several limitations must be acknowledged. First, the FEA simulations were conducted under quasi-static conditions, which may not fully capture the dynamic loading patterns experienced during daily activities. Future studies should incorporate dynamic loading simulations to better replicate real-world conditions \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Second, the material properties used in the FEA model were assumed to be homogeneous and isotropic, which may not accurately reflect the anisotropic behavior of bone and porous titanium. Incorporating more realistic material models could enhance the accuracy of the simulations. Third, the study did not account for the biological factors influencing bone ingrowth and osseointegration, such as the effects of patient age, bone quality, and local biomechanical environment. Future research should explore these factors through in vivo studies and clinical trials to validate the findings and assess their clinical relevance. Finally, the focus on porosity levels within a specific range (0\u0026ndash;80%) leaves room for exploring other design parameters, such as pore size, shape, and distribution. These factors could have a significant impact on both mechanical and biological performance and warrant further investigation.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, we developed FE models of lumbar interbody fusion and found that the optimal porosity of the porous layer for ensuring the structural stability of 3DPS at multiple fusion levels is 60%. Based on these results, 3DPS with 60% porosity can be applied to cadaveric and animal specimen studies prior to clinical trials.\u003c/p\u003e"},{"header":"Declarations","content":" \u003ch2\u003eAuthor contributions statement\u003c/h2\u003e \u003cp\u003eKwang Hyeon Kim: Writing \u0026ndash; original draft, Methodology, Formal analysis, Junsu Bae: Writing \u0026ndash; original draft, Methodology, Data curation, Formal analysis, Hyeon Su Bae: Writing \u0026ndash; review \u0026amp; editing, Data curation, Kyeong-Joo Yoo: Writing \u0026ndash; review \u0026amp; editing, Data curation, Seonghoon Jeong: Writing \u0026ndash; review \u0026amp; editing, Supervision, Dohyung Lim: Writing \u0026ndash; review \u0026amp; editing, Supervision, Resource, Methodology, Supervision, Formal analysis, Conceptualization, Byung-Jou Lee: Writing \u0026ndash; original draft, Supervision, Methodology, Formal analysis, Conceptualization, Project administration.\u003c/p\u003e \u003ch2\u003eEthics Declarations\u003c/h2\u003e \u003cp\u003eNot applicable.\u003c/p\u003e \u003ch2\u003eConsent for Publication\u003c/strong\u003e \u003cp\u003eAll authors consent to the publication of this manuscript.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eKwang Hyeon Kim and Junsu Bae contributed equally to this work as first author.Kwang Hyeon Kim: Writing \u0026ndash; original draft, Methodology, Formal analysis, Junsu Bae: Writing \u0026ndash; original draft, Methodology, Data curation, Formal analysis, Hyeon Su Bae: Writing \u0026ndash; review \u0026amp; editing, Data curation, Kyeong-Joo Yoo: Writing \u0026ndash; review \u0026amp; editing, Data curation, Seonghoon Jeong: Writing \u0026ndash; review \u0026amp; editing, Supervision, Dohyung Lim: Writing \u0026ndash; review \u0026amp; editing, Supervision, Resource, Methodology, Supervision, Formal analysis, Conceptualization, Byung-Jou Lee: Writing \u0026ndash; original draft, Supervision, Methodology, Formal analysis, Conceptualization, Project administration.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was supported by the Development of demand-linked technology based on hospital-company cooperation and commercialization [RS-2023-00233521, Development of manufacture technology of intervertebral fusion (Conventional, Expandable) device] funded By the Ministry of Trade, Industry \u0026amp; Energy (MOTIE, Korea). No benefits in any form have been or will be received from a commercial party directly or indirectly related to the subject of this manuscript. The manuscript submitted does not contain information about medical device(s)/drug(s).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eRavindra, V. M. et al. Degenerative lumbar spine disease: estimating global incidence and worldwide volume. \u003cem\u003eGlobal spine J.\u003c/em\u003e \u003cb\u003e8\u003c/b\u003e, 784\u0026ndash;794 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eReisener, M-J., Pumberger, M., Shue, J., Girardi, F. P. \u0026amp; Hughes, A. P. Trends in lumbar spinal fusion\u0026mdash;a literature review. \u003cem\u003eJ. 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Comparison Between 3-Dimensional-Printed Titanium and Polyetheretherketone Cages: 1-Year Outcome After Minimally Invasive Transforaminal Interbody Fusion. \u003cem\u003eNeurospine\u003c/em\u003e \u003cb\u003e19\u003c/b\u003e, 524\u0026ndash;532. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.14245/ns.2244140.070\u003c/span\u003e\u003cspan address=\"10.14245/ns.2244140.070\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2022).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"3D printing, porous pedicle screws, finite element analysis, spinal fusion, structural stability, Screw loosening, Pseudarthrosis","lastPublishedDoi":"10.21203/rs.3.rs-6215165/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6215165/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003ePurpose\u003c/h2\u003e \u003cp\u003eComplications in spinal fusion surgeries, such as pseudarthrosis and screw loosening, often compromise clinical outcomes. To enhance bone integration and fixation stability, considerable efforts have been made to improve the success rate of spinal fusion. However, no research has been conducted on the spinal biomechanics of 3D-printed porous titanium pedicle screws (3DPS). This study evaluates the structural performance of 3DPS under physiological loading conditions using finite element analysis (FEA) and analyzes the effects of varying porosity levels on their mechanical behavior.\u003c/p\u003e\u003ch2\u003eMethod\u003c/h2\u003e \u003cp\u003eA validated FE model of the lumbar spine was used to simulate one-, two-, and three-level fusion scenarios with 3DPS and transforaminal lumbar interbody fusion (TLIF) cages. Physiological loads, including flexion, extension, lateral bending, and axial rotation, were applied. Peak von Mises stress (PVMS), stress distribution, and structural stability were assessed across the different porosity configurations (0%, 60%, 70%, and 80%).\u003c/p\u003e\u003ch2\u003eResult\u003c/h2\u003e \u003cp\u003eThe PVMS value in the core increases as the porosity increases. the stress distribution of posterior fixations in a 3-level fusion. when the porosity of the porous layer was 80%, the stress was concentrated in the core. At 70% and 80% porosity, where the risk of structural instability exceeded safe thresholds under a conservative safety factor of 3. The 60% porosity demonstrated an optimal balance between mechanical stability and stress distribution.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003e3DPS, particularly those with 60% porosity, offer promising potential for enhancing fixation stability. Further studies are needed to confirm their long-term clinical efficacy. The results of this study can serve as a starting point for preclinical and clinical studies to ensure the stability of 3DPS, and can serve as basic evidence that 3DPS are an alternative option to reduce pseudarthrosis and screw loosening.\u003c/p\u003e","manuscriptTitle":"Evaluation of the porosity and structural stability of 3D-printed porous titanium pedicle screws using finite element analysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-05 06:57:43","doi":"10.21203/rs.3.rs-6215165/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"22b4beca-dc6d-4b79-b045-549e07c3cb68","owner":[],"postedDate":"May 5th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":47022394,"name":"Biological sciences/Biophysics"},{"id":47022395,"name":"Health sciences/Health care"}],"tags":[],"updatedAt":"2025-05-05T06:57:46+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-05 06:57:43","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6215165","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6215165","identity":"rs-6215165","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}