A finite element biomechanical study of anterior transpedicular root screw plate fixation

doi:10.21203/rs.3.rs-5323742/v1

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A finite element biomechanical study of anterior transpedicular root screw plate fixation

https://doi.org/10.21203/rs.3.rs-5323742/v1

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Purpose

To compare the biomechanical properties of anterior transpedicular root screw (ATPRS), anterior transpedicular screw (ATPS) and anterior cervical locked-plate (ACLP) in the lower cervical spine by finite element method.

Methods

We collect CT data of the cervical spine from a healthy 34-year-old adult male volunteer. Use Mimics 10.01 software to build a nonlinear complete model of the lower cervical spine. The model was subjected to 75N axial force and 1.0 N·M to induce various movements. The range of motion (ROM) and stress distribution of each model under different working conditions were compared.

Results

Compared with the intact model, the ROM in the ACLP, ATPS and ATPRS groups decreased to 0.65, 0.58 and 0.62 during flexion and extension. In terms of titanium mesh graft stress, the ATPS and ATPRS groups had the largest load during extension and the smallest load during flexion. In terms of bone-screw interface stress, the peak stress around screw C7 was higher than that around screw C4 during extension in ACLP, ATPS and ATPRS groups, respectively.

Conclusion

Biomechanical characteristics of anterior transpedicular root screw system are favorable.

Lower cervical spine

Anterior transpedicular root screw

Anterior transpedicular screw

biomechanics

Finite element study

The degeneration, trauma, or infection of the lower cervical spine often occurs in the anterior column of the vertebral body, with spinal damage commonly affecting both the anterior and middle columns[1][1–3]. The traditional anterior cervical approach with vertebral screw fixation is one of the preferred clinical treatments. However, for multi-segment cervical instability caused by cervical spine injury, using solely the anterior cervical approach with vertebral screw fixation can lead to complications such as loosening of internal fixation, failed bone fusion, and formation of pseudoarthrosis, which can affect surgical outcomes[4, 5]. In 1957, Boucher[6] first proposed the use of transpedicular root screw fixation to treat spinal instability, citing its superior biomechanical properties. However, posterior approaches using transpedicular root screw fixation alone may not adequately reconstruct the anterior and middle columns of the cervical spine, potentially leading to later problems such as collapse of the anterior and middle columns. Therefore, in 2008, Koller et al.[1] introduced the concept of anterior transpedicular screw (ATPS) fixation, cleverly integrating ATPS fixation with anterior approaches. They found that ATPS has 2.5 times the pullout strength of conventional anterior vertebral body screws (VBS). However, due to the demanding nature of ATPS placement and the potential for damaging the vertebral artery, spinal cord, and nerve roots. Zhao et al.[7] proposed the anterior transpedicular root screw (ATPRS) (Fig. 1). Their goal was to not only simplify screw placement and mitigate the risk of neurovascular injury but also achieve satisfactory biomechanical performance.

Therefore, using finite element analysis, we established a two-segment vertebral body semi-sect model and based on it, respectively established ATPRS fixation model, ATPS fixation model, and ACLP fixation model. We tested and compared the biomechanical characteristics of the three internal fixation models, providing data support for future internal fixation methods.

1. Subject

We select a 34-year-old healthy adult male volunteer with a height of 175 cm and weight of 64 kg as study subject. Inclusion criteria were as follows: (1) aged between 18 and 75 years; (2) no history of cervical spine surgery; (3) no congenital deformities of the cervical spine. Through strict inclusion and exclusion criteria, we selected a representative healthy adult male as the study subject for this research. All methods were carried out in accordance with guidelines. All protocols were reviewed and approved by the Ethics Committee of the Medical School of Ningbo University, approval number: NBU-2021-021.

The informed consent was obtained before research from the volunteer.

2. Establishment of Lower Cervical Spine Model

2.1 The acquisition and retrieval of imaging data

Instruct the volunteer to lie quietly and supine on the 64-slice spiral CT scanning bed, move the bed so that the cervical spine scanning area is centered for scanning. Utilize the 64-slice spiral CT scanner to perform continuous scanning starting from the head side, covering the range from C0 to T1. Extract the scanning data and save the Dicom format data on a hard disk

2.2 Establishment of Lower Cervical Spine Tricolumnar Model

Import the collected CT raw data into the computer, extract the CT data of C3 to C7, import it into the medical 3D image processing software Mimics 10.01. Utilize Mimics 10.01 to convert the CT data of C3-C7 into STL format data. Use Geomagic 2012 reverse engineering software to repair, denoise, mesh, and convert the data images into surface models. Import into Solidworks/UGS computer-aided CAD software, perform surface repair and stitching to obtain a complete 3D solid model of the lower cervical spine.

2.3 Ligaments and material properties

The ligaments established include the anterior longitudinal ligament, posterior longitudinal ligament, capsular ligament, interspinous ligament, and supraspinous ligament. Attachment points and pathways for each ligament are defined based on relevant published literature[8, 9]. Parameters for these ligaments are primarily determined from previous finite element analyses of the lower cervical spine, as cited in the literature. To meet the characteristics and requirements derived from multiple relevant studies, load-deformation curves for all ligaments are divided into neutral, elastic, plastic, and failure regions. Plasticity and failure regions are disregarded based on literature references, and load-deformation curves for each ligament of the lower cervical spine are identified post-fitting. Material assignment is then performed based on these load-deformation curve relationships (Table 1).

Table 1

Material properties
Tissue	Unit Type	Elastic Modulus(MPa)	Poisson’s ratio
Cortical Bone	C3D4	12000	0.3
Trabecular Bone	C3D4	100	0.2
Vertebral Arch	C3D4	600	0.3
Annulus Fibrosus	Solid	4.7	0.45
Collagen Fibers	Rebar	500	0.3
Nucleus Pulposus	Fluid	1666.7*	-
Ligament	Combine	Nonlinear Elastic Curve
*: Bulk Modulus(MPa)

2.4 Establishment of a C3-C7 two-segment vertebral partial corpectomy mode

In the three-dimensional model of the lower cervical spine, perform surgical simulation by sequentially removing the anterior longitudinal ligament, C4-5, C5-6, and C6-7 intervertebral discs. Then, perform partial vertebrectomies of C5 and C6 from anterior to posterior, including the posterior longitudinal ligament. Preserve the lower endplate of C4 and the upper endplate of C7, as well as the decompression width of the vertebral bodies of C5 and C6, set at 15mm based on the coronal CT images of this case.

2.5 Establishment of an internal fixation model

The article is based on the novel cervical anterior titanium mesh cage reported by Sun Maji et al. Using Solidworks/UGS computer-aided CAD software, models of ACLP, ATPS and ATPRS were established. The diameter of all three screws is 3.5mm, with lengths of 16mm, 30mm, and 22mm respectively. Assembly of internal fixation and titanium mesh cages was completed using Solidworks/UGS (computer-aided CAD software). The vertebral screws were placed at an angle of 15° in the transverse and sagittal planes, following the method described by Xu Rongming et al.[10] based on measurements from cadaver specimens. The placement of transpedicular screws followed the method reported by Zhao Liujun et al.[11]

2.6 Biomechanical testing

Based on the Clausen et al. reference, axial pressure of 75N was applied at C3, along with a 1.0 N·m pure moment to simulate head weight and muscle forces. This was done to calculate the range of motion (ROM) of cervical spine segments in six directions: flexion, extension, left and right lateral bending, and left and right rotation. Von Mises stress contour maps were used to visualize stress distribution under different conditions for the ACLP, ATPS, and ATPRS models of the lower cervical spine anterior fixation systems.

1. Model validation

The three-dimensional nonlinear finite element model of the cervical spine (C0 ~ T1) established in this experiment accurately represents the anatomy of normal individuals. The three-dimensional finite element model of the entire cervical spine of a healthy person includes 423,956 elements and 189,197 nodes (Fig. 2). The intervertebral mobility under six loading conditions—flexion, extension, lateral bending to the left and right, and rotation to the left and right—corresponds well with the results of in vitro biomechanical experiments by Panjabi et al. and Ito S et al., aligning with the intervertebral mobility in a normal physiological state. The three-dimensional finite element model of the vertebral body screws consists of 813,755 elements and 238,303 nodes. The three-dimensional finite element model of the pedicle screws comprises 453,706 elements and 204,669 nodes. The three-dimensional finite element model of the base screws includes 554,795 elements and 227,601 nodes.

2.Comparison of the mobility of three internal fixation models

The range of motion (ROM) of the C4-C7 segment in the complete cervical spine model during flexion-extension, left-right lateral bending, and left-right rotation is 13.6 ± 0.8°, 12.3 ± 0.9°, and 11.3 ± 0.3°, respectively. In contrast, the ACLP, ATPS, and ATPRS model groups significantly reduced the ROM of the C4-C7 segment under these movements: 0.65 (-95.2%), 0.58 (-95.7%), and 0.62 (-95.4%) during flexion-extension; 0.58 (-95.2%), 0.51 (-95.8%), and 0.6 (-95.1%) during left-right lateral bending; and 1.17 (-89.6%), 1.26 (-88.8%), and 1.27 (-88.7%) during left-right rotation. This indicates that the three internal fixation models can significantly decrease the mobility of the cervical spine in multiple directions after fixation of the C4-C7 segment(Table 2 and Fig. 2).

Table 2

ROM of intervertebral range of motion between different models
Conditions (°)	Segment	Complete Model Group	ACLP model group	ATPS model group	ATPRS model group
Flexion	C3-C4	4.7	4.8	4.7	4.8
Flexion	C4-C7	14.4	0.60	0.54	0.58
Extension	C3-C4	4.4	4.5	4.3	4.6
Extension	C4-C7	12.8	0.70	0.62	0.66
Left lateral bending	C3-C4	12.3	11.6	11.9	11.7
Left lateral bending	C4-C7	11.2	0.56	0.44	0.62
Right lateral bending	C3-C4	12.2	11.4	11.9	11.7
Right lateral bending	C4-C7	13.4	0.60	0.58	0.58
Left rotation	C3-C4	12.7	10.2	10.8	10.9
Left rotation	C4-C7	11.0	1.18	1.28	1.28
Right rotation	C3-C4	12.5	10.5	11.1	10.6
Right rotation	C4-C7	11.6	1.16	1.24	1.26

3.Comparison of stress between three types of internal fixation models: titanium mesh cage-bone grafts.

For the ATPS and ATPRS groups, the stress on the titanium mesh cage-bone graft is highest during extension loading and lowest during flexion loading. Compared to the ACLP group, stress levels in ATPS and ATPRS are reduced by: flexion (-33.7% and − 15.8%), extension (-29.4% and − 13.2%), lateral bending (-26.2% and − 23.4%), and axial rotation (-18.8% and − 5.4%) (Fig. 3).

4.Comparison of bone-screw interface stress among three internal fixation models

During extension activities, the peak bone stress near the C7 screw is increased by + 49.2%, + 45%, and + 47.6% in the ACLP group, ATPS group, and ATPRS group, respectively, compared to the peak bone stress near the C4 screw. This indicates that the bone near the C7 screw bears greater stress. However, during flexion and lateral bending activities, there is no significant difference in peak bone stress near the C4 and C7 screws. During rotational activities, there are significant differences in peak bone stress near the C4 and C7 screws: in the ACLP group, the left rotation (37.6%) and right rotation (36.7%) are similar, while in the ATPS group, the left rotation (32.1%) and right rotation (52.2%) show significant differences. In the ATPRS group, the left rotation (35.4%) and right rotation (45.3%) also exhibit significant differences (Figs. 4 and 5).

The cervical spine is one of the smallest and most complex joints in the human body. Numerous techniques have been employed to understand knowledge about the cervical spine. Finite element analysis is crucial for predicting spinal mechanics when in vivo and in vitro models are inadequate. Numerical models have been used to determine internal loads, stresses, and strains in spinal tissues[12]. Simulation results from numerical spine models can provide insights into the internal workings of the cervical spine. Additionally, virtual models can display information previously unattainable through physical models, such as stress distribution within intervertebral discs. Gradually, more precise cervical spine modeling has enhanced the accuracy of spine surgery guidance and the design of new intervertebral fusion devices. This study measured the ROM of various intervertebral movements in flexion, extension, lateral bending, and rotation with a complete model subjected to 75N axial pressure at C3 and 1.0N·M pure moment, matching the data obtained Ito S.[13]. The model aligns with intervertebral mobility under normal physiological conditions, confirming the effectiveness of this comprehensive model.

In this study, we analyzed and compared the biomechanical performance of fixation models: anterior cervical plate (ACLP), anterior transpedicular screw (ATPS), and anterior transpedicular screw with plate (ATPRS) systems established after partial corpectomy of C5 and C6 vertebral bodies in the lower cervical spine. We found that compared to the intact model group, all three internal fixation model groups showed significantly reduced range of motion (ROM) from C4 to C7 after fusion. Among them, the ATPS group exhibited the smallest ROM during flexion, extension, and lateral bending activities, followed by the ATPRS group, with the ACLP group showing the largest ROM. Wu et al.[14] conducted stability comparisons of ATPS and three traditional cervical spine internal fixation techniques on six adult fresh-frozen cervical spine specimens, demonstrating biomechanical stability superior to anterior plate fixation, consistent with our findings. However, in rotational activities, both ATPS and ATPRS groups showed larger ROM compared to the ACLP group. This could be attributed to greater stiffness and stability of fixation in the ATPS and ATPRS groups, albeit with uneven distribution of stiffness leading to uneven stress distribution during rotation. Nevertheless, these groups still performed better than the ACLP group, indicating that ATPRS, like ATPS, provides good primary stability. Li Jie et al.[15], through finite element analysis of ATPS after partial corpectomy of C3-C7 in the lower cervical spine, found biomechanical advantages in flexion, extension, and lateral bending, albeit with potential limitations in rotational stability, consistent with our results.

The ATPS and ATPRS groups exhibit stress offset in the C4 and C7 vertebral bodies, with the highest stress concentrations at the screw locations. In contrast, the ACLP group shows a more uniform stress distribution. This difference is attributed to varying resistance to pull-out forces among the vertebral body screws, pedicle screws, and subpedicular screws. The vertebral body screws in the ATPS group are fixed in the anterior column of the cervical spine, unable to transfer forces to the middle and posterior columns, whereas the pedicle screws span across all three columns, providing a larger contact area with the bone and concentrating in the cortical bone, thus increasing the bone-screw interface strength and resistance to pull-out forces. While the subpedicular screws are not as superior as pedicle screws, they are still better than vertebral body screws. The uneven stress distribution on the side where vertebral body screws and pedicle screws are fixed in the plate system and the side where vertebral body screws and subpedicular screws are fixed in the plate system may lead to differences in lateral bending and rotational movements. Therefore, in asymmetric fixation, postoperative rotational movements should be strictly limited to prevent vertebral body screw fracture or loosening. We observed that during extension, the stress peak near the screws at C7 is higher than in other motion states, making the screws more susceptible to fracture. This is consistent with Ning et al.[16], who observed in 2,233 patients using ACLP that excessive neck extension was a major cause of screw damage and cervical plate failure. This may be due to all three groups undergoing anterior surgeries, with less ligamentous tissue in the anterior part of the cervical spine, making the neck extension state more likely to increase the load on screws, cervical plates, and fusion devices.

Through finite element research, we found that the biomechanical characteristics of the anterior transpedicular sub-base screw-plate system for the lower cervical spine are good, and overall, it is superior to the anterior transpedicular screw-plate system and the anterior vertebral body screw-plate system. However, finite element studies also have certain limitations. First, since the cervical spine finite element model lacks muscles and can only reconstruct structures such as intervertebral discs and important ligaments, it cannot fully simulate the natural state of the complete cervical spine. Second, the finite element model is based on the imaging data of a single healthy individual and is reconstructed virtually through digital technology, which does not represent the majority of the population. However, overall, this study still has certain significance and provides a certain reference for the choice of future cervical surgery methods.

Author Contribution

All authors contributed to the study conception and design. The first draft of the manuscript was written by Xiaoping Xu and all authors commented on previous versions of the manuscript. Polish of language was performed by Wenzhao Shang. Zhipeng Hou and Liujun Zhao prepared figures and tables. All authors read and approved the final manuscript. We certify that the submission is original work and the content is not submitted to or under review by any other journals. All contents in the manuscript have never been published before.

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No competing interests reported.

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You are reading this latest preprint version

A finite element biomechanical study of anterior transpedicular root screw plate fixation

Status:

Version 1

Abstract

Purpose

Methods

Results

Conclusion

Figures

Introduction

Materials and Methods

1. Subject

2. Establishment of Lower Cervical Spine Model

2.1 The acquisition and retrieval of imaging data

2.2 Establishment of Lower Cervical Spine Tricolumnar Model

2.3 Ligaments and material properties

2.4 Establishment of a C3-C7 two-segment vertebral partial corpectomy mode

2.5 Establishment of an internal fixation model

2.6 Biomechanical testing

Results

1. Model validation

2.Comparison of the mobility of three internal fixation models

4.Comparison of bone-screw interface stress among three internal fixation models

Discussion

Conclusion and limiting

Declarations

Author Contribution

References

Additional Declarations

Supplementary Files

Status:

Version 1