Unilateral pedicle screw fixation of lumber spine: A safe internal fixation method

Abstract

Background

Unilateral pedicle screw fixation several advantages, including reduced trauma and low cost. However, its stability and safety have not been widely recognized. In this study, the biomechanical differences in the vertebral body and screw-rod system after unilateral and bilateral pedicle screw fixation were compared using both the finite element model and calf lumbar model.

Method

We used the verified finite element model to establish unilateral and bilateral posterior lumbar surgery models. The biomechanical data of different parts of the models were recorded under different working states. Then, three calf lumbar models were selected to simulate different working states with the help of a universal testing machine and other instruments. Finally, the biomechanical data of the screw-rod system were obtained from a static strain test and analysis system.

Results

By analyzing and comparing biomechanical data obtained using two different methods, this study found that unilateral pedicle screw fixation does not bring excessive loads to the lumbar spine and screw-rod system.

Conclusion

From the perspective of biomechanics, unilateral pedicle screw fixation is considered a safe and reliable implantation technique.

Unilateral pedicle screw fixation; Finite element analysis; Spinal biomechanics; Screw-rod system; Stress.

1. Introduction

Lumbar spine diseases often plague the normal life of elderly individuals for a long time [1]. Surgery is recommended for patients with severe symptoms and ineffective repeated conservative treatment. Currently, lumbar decompression and fusion is one of the most important treatment methods for lumbar spine diseases. Internal fixation is an important auxiliary fusion method and has also been widely used in the clinic [2]. Pedicle screws and connecting rods are vital appliances to implement this operation, which is characterized by restoring intervertebral disc height and reconstructing lumbar stability [3]. The posterior screw-rod system includes pedicle screws and connecting rods, which can help the injured lumbar segment maintain an appropriate intervertebral disc height and play a vital role in the treatment process.

In 1959, Boucher proposed a pedicle screw fixation technique that is of great significance for the establishment of a screw-rod system [4]. With the popularization of the pedicle screw fixation technique in the clinic, bilateral pedicle screw fixation has become the first choice for posterior lumbar fixation, which can provide both strong fixation and good conditions for interbody fusion [5]. Bilateral pedicle screw fixation can promote joint fusion, prevent bone nonunion and improve stability [6, 7]. Nevertheless, some postoperative adverse reactions following bilateral fixation cannot be neglected. First, excess internal fixation will lead to adjacent segment degeneration and instrument-related osteoporosis. Next, bilateral fixation also brings more damage to tissue and medical burden to patients [8, 9, 10, 11, 12, 13]. Therefore, unilateral pedicle screw fixation was proposed in 1991 [14]. Unilateral pedicle screw fixation can reduce the operation time and cost. Moreover, some studies have also proven that it may achieve stability and fusion rates similar to those of bilateral fixation [15, 16, 17, 18]. However, the safety and biomechanical properties of unilateral fixation remain controversial [7, 19, 20]. Previous studies often used a single method to assess the reliability of unilateral fixation, which may lead to unreliable experimental results. Therefore, the combination of finite element (FE) analysis and experimental measurements is of great value for analyzing the biomechanical properties of unilateral fixation. Notable comparative studies of unilateral and bilateral internal fixation for other fracture sites have begun, such as mandibular condylar fracture and humeral condylar fracture [21, 22]. These studies all provide an important reference for comparing unilateral and bilateral fixation in spine surgery.

This study mainly explores the biomechanical characteristics of the lumbar posterior screw-rod system. By simulating six different working states, we obtained the stress distribution of models under the two fixation methods and then compared the bilateral and unilateral pedicle screw fixation methods (Figure 1). This study will verify the safety of the unilateral fixation method to avoid fixation breakage in the reconstruction of lumbar stability.

Figure 1.

The methods of two pedicle screw insertion techniques: unilateral pedicle screw fixation (A) and bilateral pedicle screw fixation (B).

2. Methods

2.1. Reconstruction and validation of the finite element model

We established a model of L3-S1 based on a previous FE model of the total lumbar spine [23]. The CT scanning layer thickness was 0.625 mm. This new model included different bone components, endplates, intervertebral discs and ligaments. The main reconstruction process is outlined below: 1) The 3D geometric model of L3-S1 was preliminarily established by 3D reconstruction software. 2) Young's modulus and Poisson's ratio were adopted and added to this model. The material properties are shown in Table 1 [24,25]. 3) FE analysis software was then used to set the load and constraints. Movements at the bottom of the sacrum were constrained. The joint friction coefficient was set to 0.2. A compressive preload of 500 N combined with a pure moment of 10 Nm was applied at the central node on the top side of the third lumbar vertebra to simulate six different working states, including flexion, extension, left lateral bending, right lateral bending, left axial rotation and right axial rotation. 4) The range of motion (ROM) of the FE model was used as the verification standard. The new L3-S1 FE model was verified by comparison with the data of reported models. Then, this FE model (L3-S1) was adopted for further research. The following software was used for this process: Mimics 10.01 software (Materialise Inc., Leuven, Belgium), Geomagic Studio 12.0 software (Raindrop Geomagic Inc., Morrisville, NC, USA), Solidworks 2012 (SolidWorks Corp., Waltham, MA, USA), Hypermesh 13.0 (Altair Engineering Inc., Troy, MI, USA), and MSC NASTRAN solver and PATRAN postprocessing program (MSC Software Corp., Newport Beach, CA, USA).

Table 1.

Material properties of the finite element model components and implants.

Element set	Young's modulus (MPa)	Poisson's ratio
Cortical bone	12000.00	0.30
Cancellous bone	100.00	0.20
Posterior bone	3500.00	0.25
Annulus	4.20	0.45
Titanium screw and rod	113000.00	0.30
Nucleus pulposus	1.00	0.50
Ligaments
Anterior ligament	20.00	0.40
Posterior ligament	20.00	0.40
Interspinous ligament	10.00	0.30
Supraspinous ligament	10.00	0.30
Ligamentum flavum	10.00	0.30
Capsular ligament	10.00	0.30
Transverse ligament	10.00	0.30

2.2. Establishment of surgical model and loading condition

We designed the pedicle screw fixation model following a previous FE study. The diameter of the pedicle screw was 6.5 mm, and the length of threaded portion was 45 mm. The diameter of the connecting rod was 5.5 mm, and the effective length was approximately 35 mm. L4 and L5 were selected for surgery simulation. Then, the screw-rod system was implanted into the complete L3-S1 FE model according to the method of posterior lumbar surgery. In this model, pedicle screws were inserted into the pedicle in a forward and upward manner, and the right side of the FE model was selected for the unilateral fixation group. Subsequently, two different postoperative models, a unilateral fixation model and bilateral fixation model, were established (Figure 2). The postoperative model also required further processing, such as rmeshing, setting boundaries and loading conditions. Referring to previous research results and considering safety, we chose 300 N as the load under the working states of the model. The interaction between the model and the internal fixation is defined as surface-to-surface contact without relative sliding, which can be regarded as fixed connections. Other settings and conditions were the same as those of the complete model. Finally, under the condition of an L3 vertical body vertical compression preload of 300 N and pure torque of 10 Nm, the computer was used to simulate six different working states of the two models. Two types of numerical results were observed. The stress distribution of the lumbar vertebra, intervertebral disc and internal fixation were used to identify the stress concentration areas, and the corresponding stress values were recorded. The ROMs of the models in different working states were recorded to calculate intervertebral relative motion.

Figure 2.

Establishment of postoperative models: unilateral pedicle screw fixation (A) and bilateral pedicle screw fixation (B).

2.3. Preparation of the surgical model for measurement experiment

Bone disease of three calf spines was ruled out by imaging. Then, the muscle was removed, and the remaining soft tissue structure was preserved. The final L4-S2 spine models consisted of the vertebrae, intervertebral disc, main ligaments and joint capsule. The head and tail of the model were trimmed to ensure the close connection between the model and the fixture. In the horizontal direction, four Kirschner wires were inserted into L5 and L6 along the horizontal direction of the vertebral body, and the directions of the two Kirschner wires in the same segment were perpendicular to each other. The relative activity of this lumbar segment was observed through the displacement of Kirschner wires. The model was preserved at low temperature (−80 °C) for further experiments. Then, L5 and L6 of the models used the same insertion method as in the screw-rod system of the FE study. The design of the new pedicle test screw and connecting rod was based on the parameters of pedicle screws and connecting rods commonly used in clinical surgery. The parameters of compressive preload (300 N) and pure moment (10 Nm) were consistent with the FE model. To build a unilateral internal fixation model, the left connecting rod was removed from the bilateral internal fixation model (Figure 3). A universal testing machine, torque wrench and special fixture were used to drive the model to generate six different working states.

Figure 3.

The processing of calf lumbar model: fluoroscopy (A) and surgery simulation (B).

2.4. Design of the screw-rod system and strain measurement

We studied and analyzed the established FE models and obtained the area where the stress of the screw-rod system was relatively concentrated under normal working states, which laid a foundation for the follow-up test. To measure the change in the strain of the screw-rod system, we specifically designed pedicle screws and connecting rods that can be installed with strain gauges. The new pedicle test screw (diameter: 6.5 mm) had the similar characteristics with the previously used screw. The connecting rod was changed into a cuboid structure to facilitate the attachment of the strain gauge. And it was designed to a special specification (length of side of cross section: 5.5 mm). According to the characteristics and requirements of biomechanical tests, we designed a fixture to fix the model and connect the test instrument [23]. The fixture and model were specially designed and processed to ensure that they could be stably connected. Moreover, the special fixture could also help the model to complete different working states driven by the testing machine and torque wrench. When the load gradually increased to the maximum value, the measuring system recorded the change in the strain value induced by the strain gauge. The following instruments were used for this experiment: a torque wrench (WEC2-030BN, WIZTANK, Eclatorq Technology Co., Ltd. Taiwan, China), universal testing machine (SANS CMT4204, MTS System (CHINA) Co., Ltd., Shenzhen, China), and static strain test and analysis system (DH3821, Donghua test, Jiangsu, China).

2.5. Data analysis

The experimental data were statistically analyzed with the SPSS 17.0 software (SPSS, Inc., Chicago, IL, USA) and a repeated measures analysis of variance. The experimental data were tested by the Shapiro‒Wilk test, and the results are expressed as the mean ± standard deviation. P < 0.05 was regarded as statistically significant.

3. Results

3.1. Reconstruction and validation of the finite element model

The FE model established in this study included different bone structures, end plates, intervertebral discs and ligaments, which were regarded as uniformly distributed linear elastic materials. The cartilaginous endplate element was retained at the top and bottom of the model, which also truly simulated the connection between the vertebra and intervertebral disc. The cortical bone was completed by shell extraction and was approximately 2 mm thick. After defining the model characteristics and working states, we began to measure the angular displacement and greater stress concentration area of the L1-S1 FE model. For six working states (flexion, extension, left lateral bending, right lateral bending, left axial rotation, and right axial rotation), the results of ROM were validated and proved to be acceptable by comparison with those of previous in vitro studies (Table 2) [26, 27, 28, 29, 30]. Compared with the maximum stress of other screw-rod systems of the same type, the maximum stress data obtained by this FE model are in the reliable range [31, 32]. The intact FE model established in this study under different working states yielded results close to the reported values, and the model could be used for further studies. With 368233 elements and 79722 nodes (4-node tetrahedral elements; element edge length: 0.1 mm-0.8 mm), the L3-S1 model was selected and determined from the total L1-S1 model.

Table 2.

Comparison of ROM between this study and the results reported by the previous literature (Degrees).

Segment	Data sources	Flexion and extension	Left and right lateral bending	Left and right rotation
L1-L2	In vitro	10.47	10.26	4.08
	Yamamoto et al.	5.05	2.30	4.95
	Huang et al.	7.66	8.43	2.34
	Biswas et al.	11.63	10.09	6.91
	This study	9.04	9.91	4.52
L2-L3	In vitro	11.57	13.03	5.07
	Yamamoto et al.	4.85	2.60	7.00
	Kim et al.	8.37	9.48	4.69
	Huang et al.	10.71	10.46	2.44
	Biswas et al.	11.98	12.51	6.71
	This study	9.77	11.22	4.66
L3-L4	In vitro	11.37	12.24	5.16
	Yamamoto et al.	4.90	2.60	5.75
	Kim et al.	7.93	8.31	4.29
	Huang et al.	9.54	7.92	3.20
	Biswas et al.	12.22	10.70	4.87
	This study	9.77	11.34	5.39
L4-L5	In vitro	14.87	12.24	3.79
	Yamamoto et al.	6.45	2.20	5.70
	Kim et al.	10.90	7.11	4.69
	Huang et al.	10.25	8.12	3.71
	Biswas et al.	15.31	8.66	4.49
	This study	12.30	11.66	5.36
L5-S1	In vitro	16.97	11.37	2.59
	Yamamoto et al.	8.90	1.40	5.50
	Huang et al.	13.81	8.12	3.40
	Biswas et al.	15.95	7.11	3.91
	This study	14.72	13.18	3.18

3.2. ROMs and stress analysis in two different finite element models

The ROMs of the two FE models are shown in Figure 4. After internal fixation implantation, the two FE models were simulated and tested. The ROM of L4-L5 intervertebral discs in two different postoperative FE models decreased compared with the complete FE model. The ROM of unilateral internal fixation increased slightly compared with that of bilateral internal fixation in the intact L3-S1 model, but this increase was not significant. By analysizing the data from each group, the change in flexion state ROM of the L4-L5 intervertebral discs was the most pronounced. The ROM of unilateral internal fixation was 4.91°, while the ROM of bilateral internal fixation was 4.84°. In the right rotation state, the ROM difference between unilateral and bilateral internal fixation of L4-L5 was the largest, with a difference of 0.15°. In addition, the ROM comparison results of L3-L4 and L5-S1 were similar to those of L4-L5.

Figure 4.

Comparison of ROM, between postoperative FE models and the intact FE model.

Based on all the FE analysis data, we found that the stress distribution and maximum stress in different parts of the two postoperative models were different under different working states (Figure 5). However, in the two FE models, the stress concentration area of the L3-L4 and L5-S1 intervertebral discs was more pronounced than that in L4-L5. Notably, the screw-rod system of all models experiences stress concentration during movement, especially during flexion, extension and rotation (Figure 6).

Figure 5.

The maximum stress of different parts in FE models: the stress values of adjacent-segment intervertebral discs (A) and the lumbosacral vertebrae (B).

Figure 6.

The stress distribution of screw-rod systems in two postoperative FE models: unilateral pedicle screw fixation model and bilateral pedicle screw fixation model. According to the stress distribution diagram, red indicates the stress concentration area, while blue shows the stress dispersion area.

Compared with unilateral internal fixation, bilateral internal fixation under every working state could not significantly reduce the stress of the vertebral body and screw-rod system. Among these stresses, the maximum stress value of the unilateral group was approximately 79.95% of that of the bilateral group, 114.11 MPa for the unilateral group and 142.73 MPa for the bilateral group. The maximum stress value of the L4-L5 intervertebral discs in the unilateral group was approximately 112.26% of that in the bilateral group, 1.19 MPa for the unilateral group and 1.06 MPa for the bilateral group. The maximum stress value of the screw-rod system in the unilateral group was approximately 105.16% of that in the right screw-rod system in the bilateral group, 113.46 MPa for the unilateral group and 108.93 MPa for the bilateral group. Independent observations of different components showed that the maximum stress values of the L4 screws in the unilateral group and bilateral group were 113.46 MPa and 108.93 MPa, respectively (Figure 7A). Little difference was observed in connecting rods in the different groups (Figure 7B). The maximum stress value of screws in the L5 vertebral body was relatively low, 99.53 MPa and 95.64 MPa for unilateral and bilateral fixation, respectively (Figure 7C). Through a comparative analysis, we found that the data of the two groups of models did not significantly differ (Figure 8).

Figure 7.

The maximum stress of postoperative FE model: screw of L4 vertabral body (A), connecting rods (B), and screw of L5 vertabral body (C).

Figure 8.

The maximum stress value of screw-rod systems in two postoperative FE models under different conditions.

3.3. Establishment of the surgical model for biomechanical testing

We compared the experimental results of the two groups of FE models and determined the stress concentration areas of the screw-rod system. Then, the strain gauge was adhered to the surface of these positions in the postoperative calf lumbar models. Finally, the static strain test and analysis system were used to record the strain value change process in these areas. Specific stress testing devices and fixtures were derived from previous studies [23]. The special fixture stably connected the model with the universal testing machine and drove the calf lumbar model to produce corresponding activities in different directions.

3.4. Strain data in the measurement experiment

After the fixation device was assembled with the postoperative calf spine, 12 strain gauges were fixed on the right screw-rod system. The strain gauges were connected to the static strain test and analysis system, which was used to capture and record the strain value data at the corresponding position. Then, the strain value was converted into the stress value with the formula σ = E ×ε (σ: stress, E: modulus elasticity, ε: strain). During the experiment, the test machine drove the model to produce six different working states and simulate the normal physiological activities of the human spine (Figure 9). The intervertebral activity between the two vertebral bodies was judged based on the relative movement between the Kirschner wires of L5 and L6.

Figure 9.

The calf lumbar model is driven to simulate six different working states: flexion, extension, left lateral bending, right lateral bending, left axial rotation and right axial rotation. The strain values are obtained by the strain gauge.

Finally, the maximum stress value of each point was selected. By analyzing and comparing these data, we found that the maximum stress of the screw-rod system showed the same trend in different motion directions. This trend existed in the test results of the unilateral and bilateral internal fixation groups. The screw-rod system of the two groups of models suffered the largest stress value when bending forward, and the parts with the largest stress value were located on the connecting rod (Figure 10).

Figure 10.

The maximum stress of postoperative calf lumbar model: screw of L5 vertabral body (A), connecting rods (B), and screw of L6 vertabral body (C).

When the model was flexed forward, the maximum stress of the unilateral group was 64.33 MPa, which was approximately 100.66% of that of the bilateral group. In addition, during extension, left lateral bending and right lateral bending, the change in the maximum stress in the unilateral group was not obvious, 46.37 MPa, 42.66 MPa and 37.68 MPa, respectively, and similar manifestations were observed in the other group, which were 41.39 MPa, 44.24 MPa and 46.66 MPa, respectively. The maximum stress of the two groups of screw-rod systems during rotation was far less than the data of other working states. However, we also found that the maximum stress values of the unilateral group during flexion, extension and rotation were higher than those of the bilateral group, but opposite results were obtained during lateral bending (Figure 11). To better compare the stress concentration areas of the two screw-rod systems, we divided the internal fixation system into upper screws, lower screws and connecting rods. Therefore, we also compared the maximum stress values of the screws and connecting rods of the two groups. In the test, we found that the stress was mainly concentrated in the connecting rod and L6 vertebral body screws. The maximum stress of the L5 vertebral body screws of the two groups was 8.95 MPa and 9.04 MPa for unilateral and bilateral fixation, respectively. The maximum stresses of the L6 vertebral screws were 26.55 MPa and 37.11 MPa for unilateral and bilateral fixation, respectively. The maximum stress value of the screw of the L6 vertebral body was often greater than that of the screw of the L5 vertebral body, in both the unilateral and bilateral groups (Figure 10). Overall, the stress distribution did not significantly differ between the two screw-rod systems based on comparisons between screw, connecting rod and maximum stress data. This finding also showed that unilateral internal fixation can ensure the safety of the screw-rod system.

Figure 11.

The maximum stress value of screw-rod systems in two postoperative calf lumbar models under different conditions (NS = No statistical significance). (P > 0.05).

4. Discussion

Lumbar spine disease, especially lumbar degenerative disease, is a common disease perplexing the elderly populations [33]. Bilateral pedicle screw fixation is one of the most commonly used treatment methods for this type of disease and can reconstruct the stability of damaged lumbar segments [34, 35, 36, 37]. Although bilateral internal fixation yields more reliable fixation to the lumbar vertebral body, it also increases the risk of secondary vertebral degeneration, aging and osteoporosis. Surgically removing the intervertebral disc and other soft tissue will remove most of the tensile bearing capacity of the moving segment, and stiffness during flexion may increase when the vertebrae are in rigid contact with the intervertebral implant. To address this problem, Goel et al. described unilateral pedicle screw fixation in detail [14], which could provide the same lumbar stability and reduce excessive rigid fixation caused by bilateral pedicle screw fixation. Subsequently, researchers gradually recognized this surgical method and improved it.

The application of unilateral pedicle screw internal fixation has been reported and achieved good results, showing that unilateral pedicle screw internal fixation can effectively reconstruct the stability of the lumbar spine and protect the important function of the lumbar spine from being affected [38, 39, 40, 41]. Its advantages include better sagittal balance, shorter operation time and fewer complications than bilateral fixation [42, 43, 44]. In previous studies, the incidence of adjacent segment degeneration in the unilateral internal fixation group was lower than that in the bilateral fixation group [14, 45, 46]. Unilateral pedicle screw fixation can also reduce the stiffness of the fusion segment and the stress of small joints in the adjacent upper segment [30, 47]. In a retrospective study by Kim et al., unilateral pedicle screw fixation played a role in preventing postoperative adjacent segment degeneration to a certain extent and obtained better clinical results in at least 10 years of follow-up [48]. In addition, Liu et al. found that functional indices, such as the visual analog scale, Japanese Orthopedic Association score and Oswestry Disability Index (ODI), were significantly improved in the unilateral and bilateral groups, which is effective for spinal cord decompression and neurological function improvement. Unilateral fixation may be better than bilateral fixation in terms of the final follow-up ODI score and the length of hospital stay [49]. However, unilateral fixation also has some limitations. At present, because the fixation strength of unilateral pedicle screw fixation is weaker than that of bilateral fixation, it is often used for single segment posterior lumbar interbody fusion. For multilevel fusion, bilateral pedicle screw fixation remains recommended [50]. Notably, the stability of unilateral lumbar pedicle fixation is similar to that of bilateral pedicle fixation [40, 50, 51]. In addition, unilateral pedicle internal fixation not only reduces the wound area and bleeding during the operation but also shortens anesthesia time and decreases implant protrusion. Finally, the use of unilateral pedicle screw fixation can reduce the treatment cost for patients because unilateral internal fixation avoids contralateral surgical exposure and uses a much less invasive approach. Therefore, it can promote the early rehabilitation of patients [52, 53]. However, the difference between unilateral internal fixation and bilateral internal fixation has not been studied in depth, and biomechanical evidence is consequently insufficient.

This study was completed to explore the biomechanical differences between bilateral and unilateral screw-rod systems and determine whether unilateral internal fixation can provide reliable lumbar stability. The biomechanical properties of the screw-rod system were discussed based on two aspects of the FE model and calf lumbar model. According to the data of this study, the change in the stress value and intervertebral disc motion angle of the unilateral screw-rod system were similar to those of the bilateral screw-rod system, which could also support a previous report that unilateral fixation can obtain a joint fusion rate similar to that of bilateral fixation [15]. In the FE analysis, the overall mean stress of the unilateral group was smaller than that of the bilateral group, indicating that unilateral pedicle screw fixation reduces the load pressure on the lumbar spine. However, because bilateral pedicle screws can provide a firmer fixation effect, the maximum stresses of the L4-L5 intervertebral disc and screw-rod system in the unilateral group were slightly higher than those in the bilateral group, but this difference was not pronounced. Therefore, unilateral pedicle screw internal fixation can still provide reliable stability for postoperative lumbar segments.

The maximum stress of the screw-rod system of the postoperative model measured in this study was less than those reported in previous studies [31, 32], indicating that the stress the screw-rod system measured in this study was within the reliable range. Moreover, this conclusion has also been confirmed in the calf lumbar model. During flexion, extension and rotation, the maximum stress of the screw-rod system in the unilateral group was slightly greater than that in the bilateral group, but this difference was not significant. The performance of the unilateral internal fixation group in general was little different from that of the bilateral group. In a previous FE study, the stress of the upper and lower screws in the screw-rod system often showed little difference. However, we found that in the actual measurement, the screw at the lower position will bear greater stress, which may be related to the effect of gravity. This finding also proves the importance of the experimental measurements.

Few studies have adopted this special method that combines FE analysis with measured experiments. The outstanding advantage of this study is that it proves the reliability of unilateral lumbar pedicle internal fixation based on biomechanical properties obtained from a combination of two different research methods. In addition, the improvement and innovation of experimental methods and instruments not only allows the calf lumbar model to reproduce a working state but can also be applied to other lumbar biomechanical studies. However, this research is also subject to inevitable limitations, such as a small sample size and differences between the calf spine model and human spine model. We found that the stress values in the FE analysis and biomechanical test exhibited different trends under different working states. This phenomenon may be related to the anatomical structure of the calf lumbar model and the experimental environment. And the measurement and calculation undeniably differ between the two different stress measuring methods. Moreover, the model is subject to both interindividual differences and change of tissue freshness. Therefore, large clinical samples, long-term follow-up or more accurate measurements of the screw-rod system remain necessary to further confirm our results.

5. Conclusion

This study found that the maximum stress values of the whole vertebral body and intervertebral disc did not significantly differ between the unilateral and bilateral screw-rod systems in both the FE model or calf lumbar model, which showed that unilateral pedicle screw internal fixation would not bring excessive load to the screw-rod system and lumbar spine. Moreover, the ROMs of each segment of the lumbar spine in the unilateral group were also similar to those in the bilateral group, which shows that unilateral internal fixation can also effectively limit the movement of lumbar surgical segments and provide reliable stability for the lumbar spine. Generally, the present results suggest that unilateral pedicle screw fixation is a safe and effective method for maintaining lumbar stability.

Declarations

Author contribution statement

Simengge Yang: Performed the experiments; Wrote the paper.

Honggang Xia: Analyzed and interpreted the data.

Menglin Cong: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data.

Anyun Guo: Performed the experiments; Analyzed and interpreted the data.

Kai Ma: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Mingzhi Song: Contributed reagents, materials; Performed the experiments; Analysis tools or data; Wrote the paper.

Funding statement

Menglin Cong was supported by National Natural Science Foundation of China [82002844].

Anyun Guo was supported by Young Innovative Talents Project of Guangdong General Universities [2019KQNCX124], Natural Science Foundation of Shenzhen University General Hospital [SUGH2018QD017].

Data availability statement

The authors do not have permission to share data.

Declaration of interest’s statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.