Abstract
Introduction
The modified pedicle screw fixation (PSF) was designed to simulate an integrated framework structure to ameliorate the resistance to vertical and shearing forces of the disrupted sacroiliac complex, and the aim of this study was to compare the biomechanical characteristics of PSF and traditional lumbopelvic fixation (LPF) for the treatment of sacroiliac joint disruption.
Methods
The digital computer simulation model of an intact spine-pelvis-femur complex with main ligaments was built from clinical images. A left sacroiliac joint disruption model was mimicked by removing the concerned ligaments. After model validation, the two fixation models (modified PSF and traditional LPF) were established, and assembled with the disruption model. Under five loading scenarios (compression, flexion, extension, right bending, and left twisting), the finite element simulation was implemented. The maximum von Mises stress (VMS) of internal fixations and pelvises, maximum deformations on the Z-, Y-, X-axes and overall deformation of the sacrum were evaluated and compared.
Results
Under all loading conditions, the maximum VMS of internal fixations and pelvises in the modified PSF model were lower than those in the traditional LPF model. Under flexion, right bending, and left twisting, the maximum Z-axis deformation of the sacrum for the modified PSF model was smaller than that of the traditional LPF model. For compression, the maximum Y-axis deformation of the sacrum was smaller than that of the traditional LPF model. During various loading modes, the maximum X-axis, and overall deformations of the sacrum for the modified PSF model were smaller than those in the traditional LPF model.
Conclusions
Compared with the traditional LPF, the modified PSF shows superior biomechanical stability, with satisfied resistance to vertical and shearing forces, which might be potentially suitable for treating sacroiliac joint disruption.
Keywords: Sacroiliac joint disruption, Pedicle screw fixation, Lumbopelvic fixation, Internal fixation, Finite element analysis
Introduction
The sacroiliac joint, which is between the sacrum and the ilium, transmits the upper body loads to the lower extremities [1], and the integrity of the sacroiliac joint plays a pivotal part in the mechanical stability of the pelvic ring [2]. The disruption of the sacroiliac joint is frequently resulted from high-energy injuries such as motor vehicle accidents [3], and often accompanied by severe bleeding and high mortality [4]. As a result, the disrupted sacroiliac complex is vertically rotationally unstable. In recent years, operative management for dealing with this kind of injury has been considered as a preferred method [5]. However, there is still controversy regarding the best internal fixation treatment.
At present, various management attempts have been introduced to handle these injuries, including sacroiliac screw fixation, anterior locked plating, posterior locking plates, and so on [6–8]. The sacroiliac screw fixation technique is one of the most widely used managements in the treatment of posterior pelvic ring disruption [6]. However, this technique requires high experience because the pelvis is irregular and complex, and vascular and neural injuries have been reported at a high rate with this method [9]. In addition, the sacroiliac screw fixation cannot provide adequate stability for posterior pelvic ring disruption in biomechanics [10], complication such as screw loosening was discovered as high as 17.3% [11]. Lumbopelvic fixation (LPF), which was initially introduced by Käch and Trentz [12], can achieve satisfied fixation stability for posterior pelvic ring disruption [13]. However, the method of traditional LPF is unable to provide mechanical stability in the rotational direction because the device can only resist vertical forces of the pelvis. Triangular fixation, which was initially described by Schildhauer [14], combines traditional LPF and transverse fixation, has been demonstrated to offer multiplanar stability for vertically unstable posterior pelvic ring disruption clinically. A biomechanical study of Peng et al. [15] also proved that bilateral triangular fixation had superior stability in the vertical and rotational directions compared with traditional LPF for vertically unstable posterior pelvic ring disruption. However, triangular fixation is not an integrated framework structure.
To address the abovementioned limitation, our research group introduced an innovative internal fixation device for posterior pelvic ring disruption on the basis of transiliac internal fixator (TIFI). The critical invention of the novel technique consists in the vertical and integrated framework structure to greatly ameliorate the resistance to vertical and shearing forces of the disrupted sacroiliac complex after osteosynthesis of posterior pelvic ring disruption.
Finite element analysis was a favored used tool in orthopedic research [16]. This method can allow researchers to evaluate designing interventions to simulate different conditions [17], and help in understanding the biomechanics of orthopedic implants. Thus, the aim of this study was to compare the mechanical performance of modified pedicle screw fixation (PSF) and traditional LPF in the treatment of sacroiliac joint disruption with digital computer simulation method. The maximum von Mises stress (VMS), and maximum deformations of the sacrum would be evaluated and compared. Our hypothesis was that the modified PSF would have superior mechanical stability, and might be potentially suitable for treating sacroiliac joint disruption.
Materials and methods
Volunteer and ethical approval
In this study, we recruited a healthy Chinese adult male (176 cm, 31 years, 70 kg) as the volunteer. The volunteer fully understood the objective of this study and signed the informed consent before the initiation of the study. The X-ray appearance of the lumbar spine, pelvis, and femur was normal, with no signs of fractures, deformities, or other bone diseases. This study was reviewed and approved by the Ethics Committee of Pudong New Area People’s Hospital (Approval No: 2022-K70).
Sacroiliac joint disruption model establishment
After confirming that the volunteer was eligible for this research, a 128-row spiral computed tomography machine was used to scan from the first lumbar vertebra to the proximal parts of the femurs. The slice distance was 1 mm. The data of the scanning results were recorded in Digital Imaging and Communications in Medicine (DICOM) format for computer simulation. The DICOM data were input to Mimics 21.0 software (Materialise, Belgium) to reconstruct the three-dimensional spine-pelvis-femur model on the basis of different gray values of the tissue and segmentation of the clinical scanned images, and the model was recorded in stereolithography (STL) format. Then, the STL format model was loaded into the Geomagic Studio 2017 software (Geomagic, USA) for smoothing, noise reduction, and surface reconstruction, and the model was recorded in a standard for the exchange of product model data (STP) format. Subsequently, the STP format model was input to the SolidWorks 2017 software (Dassault, USA) to construct the solid spine-pelvis-femur model.
The following ligaments included anterior and posterior sacroiliac ligaments, interosseous sacroiliac ligament, sacrotuberous and sacrospinous ligaments, superior pubic and arcuate pubic ligaments, inguinal and iliolumbar ligaments, anterior and posterior longitudinal ligaments, intertransverse and supraspinous ligaments, all of which were modeled as tension-only spring elements in the ANSYS Workbench 17 software (ANSYS, USA) for finite element analysis. The properties of main pelvic and lumbar ligaments are expressed in stiffness (N/mm), and the stiffness coefficient is shown in Table 1. The material properties of cortical bone, cancellous bone, intervertebral discs, posterior elements, pubic symphysis, articular cartilage, and internal fixation devices were assigned according to previous studies [18–20] and are shown in Table 2. In this study, all models were considered continuous, isotropic, and linearly elastic materials. All bony parts and internal fixations were meshed with ten-node tetrahedron elements, and the mesh size was set to 4 mm. The intact spine-pelvis-femur model had 189,169 elements and 350,769 nodes. Sacroiliac joint disruption was mimicked according to the model described by Lee et al. [18], the left anterior and posterior sacroiliac ligaments, interosseous sacroiliac ligament and articular cartilage of the intact sacroiliac joint were deleted, as shown in Fig. 1.
Table 1.
Material properties of the main ligaments in this study
| K (N/mm) | Number of springs | |
|---|---|---|
|
Anterior sacroiliac ligament Posterior sacroiliac ligament Interosseous sacroiliac ligament Sacrotuberous ligament Sacrospinous ligament Superior pubic ligament Arcuate pubic ligament Inguinal ligament Iliolumbar ligament Anterior longitudinal ligament Posterior longitudinal ligament Intertransverse ligament Supraspinous ligament |
700 1400 2800 1500 1400 500 500 250 1000 23.75 26.15 9.8 9.8 |
4 × 2 4 × 2 4 × 2 1 × 2 1 × 2 1 × 1 1 × 1 1 × 2 2 × 2 5 × 1 5 × 1 5 × 2 5 × 1 |
Table 2.
Material properties used in the finite element models
| Young’s modulus (Mpa) | Poisson’s ratio | |
|---|---|---|
|
Lumbar cortical bone Lumbar cancellous bone Pelvis cortical bone Pelvis cancellous bone Femur cortical bone Femur cancellous bone Annulus fibrosus Nucleus pulposus Posterior elements Pubic symphysis Sacroiliac cartilage Articular cartilage Titanium alloy |
12,000 100 12,000 100 15,000 100 1 1 3500 5 54 100 110,000 |
0.3 0.2 0.3 0.2 0.3 0.2 0.499 0.499 0.25 0.45 0.4 0.3 0.3 |
Fig. 1.
Posterior view of of the left sacroiliac joint disruption model
Internal fixation and assembled model establishment
The three-dimensional geometric models of modified PSF and traditional LPF were developed in SolidWorks 2017 software. The modified PSF was formed by connecting TIFI and L4-L5 pedicle screws bilaterally, as shown in Fig. 2. TIFI was realized by using iliac screws (7.0 mm × 55 mm) on both sides connected to a 6-mm diameter rod. The L4-L5 pedicle screws (6.5 mm × 45 mm) were connected by a 6-mm diameter longitudinal rod, and the bilateral L4-L5 pedicle screws were connected by a 3-mm diameter transverse rod. TIFI and bilateral L4-L5 pedicle screws were connected by two 4-mm diameter rods. The traditional LPF was formed by connecting iliac screws and L4-L5 pedicle screws bilaterally, as shown in Fig. 2. The iliac screws and pedicle screws had exactly the same direction, position and depth for the two models. The internal fixations were positioned and assembled with the disruption model according to our clinical experience, as presented in Fig. 3.
Fig. 2.
Two types of internal fixation devices: (a) modified PSF, (b) traditional LPF
Fig. 3.
Finite element model of the injured pelvis fixed by two types of internal fixation devices: (a) modified PSF model, (b) traditional LPF model
Boundary conditions and loading settings
The proximal ends of the femur were totally constrained in all degrees of freedom to simulate double stance standing. The bone-bone interface, the internal fixation-internal fixation interface, and the bone-internal fixation interface were defined as binding contacts. Additionally, the contact condition between left sacroiliac joint surfaces was defined as frictionless contact in the disruption model. The loading force in this study mimicked the weight of upper body in a standing state, and a compressive follower load of 400 N was applied to the top surface of the first lumbar vertebra [18]. In addition, a 10 Nm moment in combination with a 400 N compressive follower load was applied during flexion, extension, right bending, and left twisting [19].
Evaluation criteria
In the finite element simulation, the VMS distributions and maximum values of internal fixations and pelvises were examined. Furthermore, the vertical (Z-axis), posterior (Y-axis), horizontal (X-axis), and overall deformation distributions and maximum amounts of the sacrum were measured and compared.
Results
Model validation
To validate the finite element model, we reconstructed an intact spine-pelvis-femur model and a 7.5 Nm moment was applied under one-leg stance condition. Figure 4 showed the comparison of our predicted data with the cadaveric study by Lindsey et al. [21]. The results of the sacroiliac joint (right side) range of motion (ROM) in our finite element model were agreeable with the experimental findings.
Fig. 4.
The intact sacroiliac joint (right side) ROM at 7.5 Nm moment under one-leg stance condition compared with the reported experimental data
VMS distribution
The VMS distributions of the internal fixations are depicted in Fig. 5. Under various loading conditions, the VMS of the internal fixation was concentrated on the connecting rod between TIFI and lumbar pedicle screws in the modified PSF model, and the VMS of the internal fixation was concentrated on the longitudinal rod at the upper edge of the iliac screws in the traditional LPF model. The maximum VMS value of the modified PSF construct was 340.84 MPa, 495.76 MPa, 197.7 MPa, 398.55 MPa, and 419.76 MPa respectively (Fig. 6) under compression, flexion, extension, right bending, and left twisting, lower than in the traditional LPF construct (489.77 MPa, 588.03 MPa, 392.9 MPa, 599.65 MPa, and 521.83 MPa, respectively), as shown in Fig. 7.
Fig. 5.
The maximum VMS of the internal fixations under various loading conditions in the two models
Fig. 6.
The VMS distributions of the modified PSF construct under various loading conditions (a compression, b flexion, c extension, d right bending, e left twisting)
Fig. 7.
The VMS distributions of the traditional LPF construct under various loading conditions (a compression, b flexion, c extension, d right bending, e left twisting)
The VMS distributions of the pelvises are presented in Fig. 8. For all loading conditions, the VMS of the pelvis consistently occurred around the contact area between the iliac screw and the cortical bone for both models. The maximum VMS value of the pelvis was 19.21 MPa, 54.93 MPa, 36.73 MPa, 36.17 MPa, and 49.83 MPa respectively for the modified PSF model under compression, flexion, extension, right bending, and left twisting, lower than in the traditional LPF model (87.59 MPa, 120.15 MPa, 55.07 MPa, 86.56 MPa, and 86.16 MPa, respectively).
Fig. 8.
The maximum VMS of the pelvises under various loading conditions in the two models
Deformation of the sacrum
The deformation distributions of the sacrum are presented in Figs. 9, 10, 11 and 12. For all loading scenarios, the maximum overall deformation was located at the top of the sacrum for both models. During the loading conditions of compression, flexion, extension, right bending, and left twisting, the maximum overall deformation of the sacrum was 0.68 mm, 1.24 mm, 0.35 mm, 0.70 mm, and 0.76 mm respectively for the modified PSF model, smaller than that of the traditional LPF model (1.41 mm, 1.93 mm, 0.89 mm, 1.40 mm, and 1.51 mm, respectively), as shown in Fig. 9. For the loading scenarios of compression and extension, the maximum Z-axis deformation of the sacrum in the two models was similar. However, under the loading conditions of flexion, right bending, and left twisting, the maximum Z-axis deformation of the sacrum was 0.15 mm, 0.03 mm, and 0.03 mm respectively for the modified PSF model, smaller than that of the traditional LPF model (0.55 mm, 0.36 mm, and 0.36 mm, respectively), as presented in Fig. 10. For the compressive loading, the maximum Y-axis deformation of the sacrum was 0.57 mm, smaller than that of the traditional LPF model (1.38 mm). However, under the loading conditions of flexion, extension, right bending, and left twisting, the maximum Y-axis deformation of the sacrum was 0.24 mm, 0.20 mm, 0.21 mm, and 0.22 mm respectively for the modified PSF model, higher than that of the traditional LPF model (0.02 mm, 0.003 mm, 0.01 mm, and 0.02 mm, respectively), as shown in Fig. 11. During compression, flexion, extension, right bending, and left twisting, the maximum X-axis deformation of the sacrum was 0.19 mm, 0.28 mm, 0.09 mm, 0.23 mm, and 0.28 mm respectively for the modified PSF model, smaller than that of the traditional LPF model (0.26 mm, 0.36 mm, 0.16 mm, 0.31 mm, and 0.41 mm, respectively), as shown in Fig. 12.
Fig. 9.
The maximum overall deformations of the sacrum under various loading conditions in the two models
Fig. 10.
The maximum Z-axis deformations of the sacrum under various loading conditions in the two models
Fig. 11.
The maximum Y-axis deformations of the sacrum under various loading conditions in the two models
Fig. 12.
The maximum X-axis deformations of the sacrum under various loading conditions in the two models
Discussion
Currently, LPF plays an important role in the treatment of vertically unstable posterior pelvic ring disruption. LPF connects the ilium and lumbar spine using a double or single pedicle screw-rod construct, which usually effectively prevents the vertical deformation of the pelvis. However, this technique cannot provide rotational stability of the posterior pelvic ring disruption because vertical two-point fixation still allows rotation of the posterior ring [22]. In recent years, the pedicle screw-rod construct has evolved into various types of fixation methods, including unilateral triangular fixation, bilateral triangular fixation, etc [23, 24]. Triangular fixation is a combination of the pedicle screw-rod construct and sacroiliac screw fixation. The pedicle screw-rod construct provides vertical stability, and the sacroiliac screw provides lateral stability, thus providing multiplanar stability for posterior pelvic ring disruption [25]. From the perspective of internal fixation design, there is still room for improvement in the pedicle screw-rod construct because triangular fixation is not an integrated framework structure. Guided by the concept of enhancing the resistance to vertical and shearing forces of the disrupted sacroiliac complex, our research group proposed a multi-dimensional fixation device. The innovation of the novel device lies in the vertical structure of TIFI and lumbar pedicle screw-rod, forming an integrated framework structure. The results of this study demonstrated that the modified PSF model had excellent biomechanical performance in the resistance to vertical and shearing forces of sacroiliac joint disruption, which might be important to improve the prognosis of vertically rotationally unstable posterior pelvic ring disruption.
It is beneficial for surgeons to evaluate the mechanical characteristics of internal fixations because it is conducive to perfect the therapeutic outcome of sacroiliac joint disruption. In the present study, we constructed the three-dimensional model of sacroiliac joint disruption treated with the modified PSF or traditional LPF to assess the mechanical differences. The results of this study found that the two pelvic fixation techniques, i.e., modified PSF, and traditional LPF, had a great difference on the deformation of the sacrum under various loading conditions. In terms of the maximum Z-axis deformation of the sacrum during compression and extension, the modified PSF model was similar to that of the traditional LPF model. The results demonstrated that the traditional LPF model had a good ability of resisting for vertical forces in the treatment of vertically unstable pelvic posterior ring disruption as expected. However, under flexion, right bending, and left twisting, the maximum Z-axis deformation of the sacrum for the modified PSF model was smaller than that of the traditional LPF model. For compression, the maximum Y-axis deformation of the sacrum was smaller than that of the traditional LPF model. However, under the loading conditions of flexion, extension, right bending, and left twisting, the maximum Y-axis deformation of the sacrum for the modified PSF model was higher than that of the traditional LPF model, anyway, the micromotion was relatively low. In addition, in terms of the maximum X-axis deformation of the sacrum, the percentage decreases for the modified PSF model under compression, flexion, extension, right bending, and left twisting were 26.9%, 22.2%, 43.8%, 25.8% and 31.7%, respectively. Furthermore, in terms of the maximum overall deformation of the sacrum, the percentage decreases for the modified PSF model under compression, flexion, extension, right bending, and left twisting were 51.8%, 35.8%, 60.7%, 50% and 49.7%, respectively. The results indicated that the modified PSF model increased the lateral stability and multiplanar stability of the posterior pelvic ring disruption. Gierig et al. [26] biomechanically compared the traditional LPF model with or without a transverse connector in a sacral fracture based on the computer simulation method and observed that ROM in the movement from the fifth vertebra to the sacrum and in the sacroiliac joint was substantially decreased in the fixation device model with a transverse connector compared to that without, and thus, the author concluded that the additional connector apparently increased rotational stability, and the fixation device might provide the basis for a quicker fracture healing of the bony area. Our study further confirmed that the rotational stability was enhanced in the fixation model with a transverse connecting rod compared to that without in the treatment of vertically rotationally unstable pelvic posterior ring disruption. We believe that although the modified PSF and traditional LPF both belong to the double pedicle screw-rod construct extending from the lumbar spine to the pelvis, the transverse connecting rod between the two iliac screws in the modified PSF constitutes a whole internal fixation device, which can provide sufficient lateral stability. However, the traditional LPF has limited ability to maintain lateral stability. Therefore, the maximum X-axis, and overall deformations of the sacrum in the modified PSF model were smaller than those in the traditional LPF model.
The lower maximum VMS of the internal fixation indicates that the model has a lower danger of internal fixation failure [18]. In this study, the maximum VMS values of the internal fixations were evaluated. Our results showed that the maximum VMS in the traditional LPF model was 489.77 MPa, 588.03 MPa, 392.9 MPa, 599.65 MPa, and 521.83 MPa respectively under compression, flexion, extension, right bending, and left twisting, while the value in the modified PSF model decreased by 30.4%, 15.7%, 49.7%, 33.5%, and 19.6%, respectively. Our results are compatible with the biomechanical study of Gierig et al. [26] who demonstrated that the addition of a transverse connector reduced the maximum VMS in the fracture area of approximately 10%. In the present study, the addition of a transverse connecting rod between the two iliac screws in the modified PSF model not only achieved mechanical stability but also reduced internal fixation VMS due to its excellent configuration distribution. In the viewpoint of biomechanics, the modified PSF model had lower danger of fatigue failure than the traditional LPF model. Nevertheless, it is worth noting that the maximum VMS in the modified PSF and traditional LPF models did not surpass the ultimate strength of the used material (1024 MPa) [27], therefore suggesting that the two internal fixation devices for the treatment of vertically rotationally unstable pelvic posterior ring disruption could be secure. In this study, we also evaluated the VMS distributions of the internal fixations. The VMS of the internal fixation was concentrated on the connecting rod between TIFI and the lumbar pedicle screwrod in the modified PSF model, but it was mainly concentrated on the connecting rod at the upper edge of the iliac screw in the traditional LPF model, which was in concordance with the results of Peng et al. [15]. Although the connecting rod withstood a high shear force, the ultimate strength of titanium alloy was much higher than the maximum VMS of the connecting rod.
The lower maximum VMS of the pelvis represents that the model has a lower danger of broken pelvis [18]. In the current study, the VMS of the pelvis consistently occurred around the contact area between the iliac screw and the cortical bone for both models. The VMS around the iliac screw was the concentration point of the pelvis-internal fixation assembly, which can be used as an important predictor of fixation failure. Our results found that the maximum VMS of the pelvis in the traditional LPF model was 87.59 MPa, 120.15 MPa, 55.07 MPa, 86.56 MPa, and 86.16 MPa respectively under compression, flexion, extension, right bending, and left twisting, while the value in the modified PSF model decreased by 78.1%, 54.3%, 33.3%, 58.2%, and 42.2%, respectively. For the two models, the VMS value was less than the reported yield strength of the cortical bone (150 MPa) [28]. However, the VMS around the iliac screw in the traditional LPF model was much higher than that of the modified PSF model. The results indicated that the cortical bone around the iliac screw has a higher danger of pelvic breakage and a higher rate of screw loosening. Therefore, the traditional LPF model appeared to have a higher danger of screw loosening than the modified PSF model. Through the finite element analysis, it was found that the modified PSF model characterized with the integrated frame structure can demonstrate less VMS distribution in the treatment of sacroiliac joint disruption, making it more reasonable, which is beneficial for improving the prognosis and reducing postoperative complications.
However, the present study had some limitations. Firstly, the muscles were not included in this study. Additionally, the properties of the bones and internal fixations were simplified into continuous, linear and isotropic materials. Finally, only sacroiliac joint dislocation was mimicked in this simulation, without considering posterior pelvic fractures and superior pubic ramus fractures. Despite this, the simulation model was generally agreeable with previous in vitro studies. In future research, we should create a finite element model that is more approximate to the real conditions.
Conclusion
In summary, the present study demonstrated that the modified PSF could decrease the stresses in the internal fixation and pelvis, and reduce the deformations of the sacrum. Therefore, the modified PSF has superior biomechanical stability compared with the traditional LPF, and might be potentially suitable for treating sacroiliac joint disruption.
Acknowledgements
None.
Author contributions
Jun Zhang and Yan Wei contributed equally to this work. JZ, YW, RGA and BQY contributed to the study concept and design. JZ and YW performed the finite element analysis, and data collection. JZ wrote the manuscript, WZY and JW contributed to the interpretation of the results, BQY edited the first draft of manuscript, BLL and RGA edited the final draft of manuscript. All authors reviewed the manuscript, and approved the final manuscript.
Funding
This study was funded by the Characteristic Program of Specific Diseases of Pudong New Area Health Commission (PWZzb2022-15); Peak Discipline Program of Traditional Chinese Medicine of Pudong New Area (YC-2023-0601); New Quality Specialty Program of Peak Plateau Clinical Medicine of Pudong New Area Health Commission (2024-PWXZ-21); Program of Key Medicine of Shanghai Municipal Health Commission (2024ZDXK0038).
Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate
This study was approved by the Ethics Committee of Pudong New Area People’s Hospital (Approval No: 2022-K70). All procedures performed in this study were in accordance with the ethical standards of the 1964 Helsinki Declaration. Informed consent was obtained from the individual participant included in this study.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Jun Zhang and Yan Wei are co-first authors.
Contributor Information
Rongguang Ao, Email: yxt-2008@163.com.
Baoqing Yu, Email: doctorybq@163.com.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.