Biomechanical comparison of stand-alone and bilateral pedicle screws fixation for oblique lumbar inter-body fusion surgery – a finite element analysis

Objectives: The aim of this study was to evaluate the biomechanical stability and safety in patients undergoing oblique lumbar inter-body fusion (OLIF) surgery with stand-alone (SA) and Bilateral pedicle screw fixation (BPSF). Methods: A finite element model of L4-L5 spinal unit was established and validated. Based on the validated model technique, function surgical models corresponding to SA, BPSF were created. Simulations employing the models were performed to investigate the OLIF surgery. A bending moment of 7.5 Nm and a 500 N follower load were applied to the models in flexion, extension, axial rotation and lateral bending. Finite element(FE) models were developed to compare the biomechanics of the intact group, SA, BPSF group. Results: Compared with the Range of motion (ROM) of the intact lumbar model, SA model decreased by 79.5% in flexion, 54.2% in extension, BPSF model decreased by 86.4% in flexion, 70.8% in extension. Compared with the BPSF, the maximum stresses of L4 inferior endplate (IEP) and L5 superior endplate (SEP) increased significantly in SA model, L4 IEP increased to 49.7MPa in extension, L5 SEP increased to 47.7MPa in flexion. Conclusions: OLIF surgery with BPSF could reduce the max stresses of the endplate which may reduce cage sedimentation incidence. However, OLIF surgery with SA could not provide enough rigidity for the fusion segment in osteoporosis patients which may increase the cage sedimentation incidence.

OLIF was introduced in 2012 by Silvestre [1]. The stand-alone procedure brings low risk of post-treatment trauma or bleeding and offers good stability and quick recovery. However, the complications associated with this technique have been reported frequently [2][3][4][5]. Abe reported 155 patients with OLIF surgery, 75 complications were reported (incidence rate, 48.3%). The most common complication was endplate fracture/subsidence (18.7%) [6].
Shun-wu Fan reviewed 235 patients with OLIF surgery and found 22 cases of endplate damage [7]. The mechanics of endplate fracture in OLIF surgery was still unclear. Avoiding such complications could be a major factor in deciding to use this procedure. Whether OLIF surgery with BPSF could provide enough stability and reduce the complication was still unknown.
Finite element analysis (FEA) in lumbar biomechanics has become popular in the recent decades, as a complement for the cadaver test [8,9]. FE models of cage and spine were used for the evaluation of surgery feasibility and the design of instruments. The purpose of this study was to evaluate safety of OLIF surgery with SA and BPSF.

Materials And Methods
The intact lumbar FE mode development were modeled by axial connectors (Figure 1). The mechanical properties of the model were also adopted from the literature (table 1) (9).

OLIF FE model's development
An OLIF cage was assembled to L4-L5 functional spinal unit (FSU) model ( Figure 2) to simulate the Stand-alone model. Four pedicle screws and two rods were assembled to both sides of L4-L5 FSU to simulate the BPSF model ( Figure 2). The properties were the same as the intact lumbar model. The bottom of the L5 vertebral body was fully constrained. A 500 N axial load and 7.5 NM moment were applied on the top of the L4 vertebral body.

Model validation
The overall ROM of intact model was compared with those for in vitro and in vivo kinematics ( Figure
Shun-wu Fan reviewed 235 patients with OLIF surgery and found 22 cases of endplate damage. The cage sedimentation incidence in the stand-alone group was higher than in the OLIF combined with posterior pedicle screw fixation [7]. Avoiding such complications could be a major factor in deciding to use this procedure. The mechanics of endplate fracture was unclear. Whether OLIF surgery with BPSF could provide enough stability and reduce the complication was still unknown.
In this study, the OLIF model was developed using published biomechanical assessment methods. A validated lumbar FE lumbar model enabled the accuracy and reliability of the simulation results. In validation, ROMs were compared with those in the literature [10][11][12].
The results were in good agreement with the pre-studies. The FE model was validated successfully, and it was considered reliable for lumbar biomechanical predictions. This indicated the OLIF with BPSF was safer than OLIF with SA in cage subsidence. Lumbar intervertebral fusion with BPSF are the standard for instrumentation, providing rigid fixation and increased fusion rates.
In all, the FEA revealed SA could not provide enough rigidity in OLIF surgery in osteoporosis patients. The maximum stresses of L4 IEP and L5 SED increased largely in SA model in flexion and extension moment, which may be a key risk factor of cage subsidence. Therefore, the OLIF surgery with SA is not favored for osteoporotic spine.
From the study, we also found additional BPSF could share the stresses of endplate, restrict the flexion and extension of lumbar, which may be an effective method to reduce the complication of cage subsidence. The Clinical study had proven that BPSF can decrease the ratio of cage displacement [16]. In conclusion, additional BPSF was essential for OLIF surgery in osteoporosis patients.

Limitations
The post-operative residual annular fibrous were not constructed in the stand-alone OLIF model. The risk factors of endplate fracture may be multiple, including endplate damage, obesity, high iliac crest, poor stability of lesion segments and so on [7].

Conclusion
The FEA indicated that OLIF procedure with SA could not stabilized the lumbar, especially in flexion and extension movement. The Maximum stresses of L4 IEP and L5SEP of SA model in flexion and extension increase significantly which may be a potential factor of cage subsidence. OLIF with additional pedicle screw-rod system was essential for osteoporosis patients. Availability of data and material 8 The data came from the demo file in Mimics 20.0 software.

Competing interests
None.

Funding
This study was supported by Sanming Project of Medicine in Shenzhen SZSM201612019 , Shenzhen key laboratory of digital surgical printing project(ZDSYS201707311542415) and Southern Medical University clinical start-up fund LC2016ZD036 .
Authors' contributions Dr. Fang Guofang and Dr. Lin yunzhi had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Dr. Sang Hongxun designed the study protocol.  The maximum von Mises stress of L4 inferior endplate in all models The maximum von Mises stress of L5 SEP in all models The maximum von Mises stress of cage in all models