Biomechanical study of oblique lumbar interbody fusion (OLIF) augmented with different types of instrumentation: a finite element analysis

Can zoledronic acid reduce the risk of cage subsidence after oblique lumbar interbody fusion combined with bilateral pedicle screw fixation in the elderly population? A retrospective study

BACKGROUND

The objective of this study was to evaluate the potential of zoledronic acid for reducing the incidence of cage subsidence and enhancing interbody fusion rates following oblique lumbar interbody fusion (OLIF) surgery, particularly as the first reported evidence of the role of zoledronic acid combined with OLIF.

METHODS

A retrospective analysis was conducted on data from 108 elderly patients treated for degenerative lumbar diseases using OLIF combined with bilateral pedicle screw fixation from January 2018 to December 2021. Patients were divided into the zoledronic acid (ZOL) group (43 patients, 67 surgical segments) and the control group (65 patients, 86 surgical segments). A comparative analysis of the radiographic and clinical outcomes between the groups was performed, employing univariate and multivariate regression analyses to explore the relationships between cage subsidence and the independent variables.

RESULTS

Radiographic outcomes, including anterior height, posterior height, disc height, coronal disc angle, foraminal height, and lumbar lordosis, were not significantly different between the two groups. Similarly, no statistically significant differences were noted in the back visual analog scale (VAS) scores and Oswestry Disability Index (ODI) scores between the groups. However, at the 1-year follow-up, the leg VAS score was lower in the ZOL group than in the control group (P = 0.028). The ZOL group demonstrated a notably lower cage subsidence rate (20.9%) than did the control group (43.0%) (P < 0.001). There was no significant difference in the interbody fusion rate between the ZOL group (93.0%) and the control group (90.8%). Non-use of zoledronic acid emerged as an independent risk factor for cage subsidence (OR = 6.047, P = 0.003), along with lower bone mineral density, lower postoperative anterior height, and concave endplate morphology. The model exhibited robust discriminative performance, with an area under the curve (AUC) of 0.872.

CONCLUSION

The administration of zoledronic acid mitigates the risk of cage subsidence following OLIF combined with bilateral pedicle screw fixation in elderly patients; however, it does not improve the interbody fusion rate.

The effect of different fixation systems on oblique lumbar interbody fusion under vibration conditions

Despite the fact that lower back pain caused by degenerative lumbar spine pathologies seriously affects the quality of life, however, there is a paucity of research on the biomechanical properties of different auxiliary fixation systems for its primary treatment (oblique lumbar interbody fusion) under vibratory environments. In order to study the effects of different fixation systems of OLIF surgery on the vibration characteristics of the human lumbar spine under whole-body vibration (WBV), a finite element (FE) model of OLIF surgery with five different fixation systems was established by modifying a previously established model of the normal lumbar spine (L1-S1). In this study, a compressive follower load of 500 N and a sinusoidal axial vertical load of ±40 N at the frequency of 5 Hz with a duration of 0.6 s was applied. The results showed that the bilateral pedicle screw fixation model had the highest resistance to cage subsidence and maintenance of disc height under WBV. In contrast, the lateral plate fixation model exerted very high stresses on important tissues, which would be detrimental to the patient's late recovery and reduction of complications. Therefore, this study suggests that drivers and related practitioners who are often in vibrating environments should have bilateral pedicle screws for OLIF surgery, and side plates are not recommended to be used as a separate immobilization system. Additionally, the lateral plate is not recommended to be used as a separate fixation system.

Patients with Hip Osteoarthritis Have a Higher Rate of Spinal Reoperation Following Lumbar Spinal Fusion

BACKGROUND

Lumbar spinal pathology is known to affect outcomes following total hip arthroplasty (THA). However, the effect of hip osteoarthritis (OA) on outcomes following lumbar fusion has not been well studied. The purpose of this study was to determine the association between hip OA and spinal reoperation following lumbar spinal fusion.

METHODS

The 5% Medicare Part B claims database was queried for all patients who underwent primary elective lumbar fusion from 2005 to 2019. Patients were divided into 2 groups: those who underwent elective THA within 1 year after primary lumbar fusion, indicating that they had severe hip OA at the time of lumbar fusion, and those who underwent lumbar fusion with no diagnosed hip OA and no THA during the study period. Exclusion criteria included THA as a result of trauma, revision THA or primary THA in the 5-year period before primary lumbar fusion, <65 years of age, and no enrollment in the database for 5 years before and 1 year after primary lumbar fusion. The primary outcome was spinal reoperation within 1, 3, and 5 years. Multivariable Cox regression was performed with age, sex, diabetes, heart disease, obesity, smoking status, osteoporosis, number of levels fused, use of posterior instrumentation, use of an interbody device, use of bone graft, and surgical approach as covariates.

RESULTS

Overall, 1,123 patients (63.4% female; 91.3% White; mean age, 76.8 ± 4.1 years) were included in the hip OA group and 8,893 patients (56.2% female; 91.3% White; mean age, 74.8 ± 4.9 years) were included in the control group. After multivariable analysis, patients with severe hip OA had significantly greater rates of revision surgery at 3 years (odds ratio [OR], 1.61; p < 0.001) and 5 years (OR, 1.87; p < 0.001) after the index lumbar fusion.

CONCLUSIONS

Patients with severe hip OA at the time of primary lumbar fusion had a significantly increased risk of spinal reoperation at 3 and 5 years postoperatively. These data provide further evidence to support performing THA prior to lumbar fusion in the unsettled debate regarding which surgery should be prioritized for patients with simultaneous degenerative diseases of the hip and lumbar spine.

LEVEL OF EVIDENCE

Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

The effect of polymethylmethacrylate augmentation on the primary stability of stand-alone implant construct versus posterior stabilization in oblique lumbar interbody fusion with osteoporotic bone quality- a finite element study

BACKGROUND CONTEXT

Oblique lumbar interbody fusion (OLIF) can provide an ideal minimally invasive solution for achieving spinal fusion in an older, more frail population where decreased bone quality can be a limiting factor. Stabilization can be achieved with bilateral pedicle screws (BPS), which require additional incisions and longer operative time. Alternatively, a novel self-anchoring stand-alone lateral plate system (SSA) can be used, where no additional incisions are required. Based on the relevant literature, BPS constructs provide greater primary biomechanical stability compared to lateral plate constructs, including SSA. This difference is further increased by osteoporosis. Screw augmentation in spinal fusion surgeries is commonly used; however, in the case of OLIF, it is a fairly new concept, lacking a consensus-based guideline.

PURPOSE

This comparative finite element (FE) study aimed to investigate the effect of PMMA screw augmentation on the primary stability of a stand-alone implant construct versus posterior stabilization in OLIF with osteoporotic bone quality.

STUDY DESIGN

The biomechanical effect of screw augmentation was studied inside an in-silico environment using computer-aided FE analysis.

METHODS

A previously validated and published L2-L4 FE model with normal and osteoporotic bone material properties was used. Geometries based on the OLIF implants (BPS, SSA) were created and placed inside the L3-L4 motion segment with increasing volumes (1-6 cm3) of PMMA augmentation. A follower load of 400 N and 10 Nm bending moment (in the three anatomical planes) were applied to the surgical FE models with different bone material properties. The operated L3-L4 segmental range of motion (ROM), the inserted cage's maximal caudal displacements, and L4 cranial bony endplate principal stress values were measured.

RESULTS

The non-augmented values for the BPS construct were generally lower compared to SSA, and the difference was increased by osteoporosis. In osteoporotic bone, PMMA augmentation gradually decreased the investigated parameters and the difference between the two constructs as well. Between 3 cm3 and 4 cm3 of injected PMMA volume per screw, the difference between augmented SSA and standard BPS became comparable.

CONCLUSIONS

Based on this study, augmentation can enhance the primary stability of the constructs and decrease the difference between them. Considering leakage as a possible complication, between 3 cm3 and 4 cm3 of injected PMMA per screw can be an adequate amount for SSA augmentation. However, further in silico, and possibly in vitro and clinical testing is required to thoroughly understand the investigated biomechanical aspects.

CLINICAL SIGNIFICANCE

This study sheds light on the possible biomechanical advantage offered by augmented OLIF implants and provides a theoretical augmentation amount for the SSA construct. Based on the findings, the concept of an SSA device with PMMA augmentation capability is desirable.

Biomechanical evaluation of different oblique lumbar interbody fusion constructs: a finite element analysis

BACKGROUND

Finite element analysis (FEA) was performed to investigate the biomechanical differences between different adjunct fixation methods for oblique lumbar interbody fusion (OLIF) and to further analyze its effect on adjacent segmental degeneration.

METHODS

We built a single-segment (Si-segment) finite element model (FEM) for L4-5 and a double-segment (Do-segment) FEM for L3-5. Each complete FEM was supplemented and modified, and both developed two surgical models of OLIF with assisted internal fixation. They were OLIF with posterior bilateral percutaneous pedicle screw (TINA system) fixation (OLIF + BPS) and OLIF with lateral plate system (OLIF + LPS). The range of motion (ROM) and displacement of the vertebral body, cage stress, adjacent segment disc stress, and spinal ligament tension were recorded for the four models during flexion/extension, right/left bending, and right/left rotation by applying follower load.

RESULTS

For the BPS and LPS systems in the six postures of flexion, extension, right/left bending, and right/left rotation, the ROM of L4 in the Si-segment FEM were 0.32°/1.83°, 0.33°/1.34°, 0.23°/0.47°, 0.24°/0.45°, 0.33°/0.79°, and 0.34°/0.62°; the ROM of L4 in the Do-segment FEM were 0.39°/2.00°, 0.37°/1.38°, 0.23°/0.47°, 0.21°/0.44°, 0.33°/0.57°, and 0.31°/0.62°, and the ROM of L3 in the Do-segment FEM were 6.03°/7.31°, 2.52°/3.50°, 4.21°/4.38°, 4.21°/4.42°, 2.09°/2.32°, and 2.07°/2.43°. BPS system had less vertebral displacement, less cage maximum stress, and less spinal ligament tension in Si/Do-segment FEM relative to the LPS system. BPS system had a smaller upper adjacent vertebral ROM, greater intervertebral disc stress in terms of left and right bending as well as left and right rotation compared to the LPS system in the L3-4 of the Do-segment FEM. There was little biomechanical difference between the same fixation system in the Si/Do-segment FEM.

CONCLUSIONS

Our finite element analysis showed that compared to OLIF + LPS, OLIF + BPS (TINA) is more effective in reducing interbody stress and spinal ligament tension, and it better maintains the stability of the target segment and provides a better fusion environment to resist cage subsidence. However, OLIF + BPS (TINA) may be more likely to cause adjacent segment degeneration than OLIF + LPS.

Biomechanical differences between two different shapes of oblique lumbar interbody fusion cages on whether to add posterior internal fixation system: a finite element analysis

BACKGROUND

Oblique lateral lumbar fusion (OLIF) is widely used in spinal degeneration, deformity and other diseases. The purpose of this study was to investigate the biomechanical differences between two different shapes of OLIF cages on whether to add posterior internal fixation system, using finite element analysis.

METHODS

A complete three-dimensional finite element model is established and verified for L3-L5. Surgical simulation was performed on the verified model, and the L4-L5 was the surgical segment. A total of the stand-alone group (Model A1, Model B1) and the BPSF group (Model A2, Model B2) were constructed. The four OLIF surgical models were: A1. Stand-alone OLIF with a kidney-shaped Cage; B1. Stand-alone OLIF with a straight cage; A2. OLIF with a kidney-shaped cage + BPSF; B2. Stand-alone OLIF with a straight cage + BPSF, respectively. The differences in the range of motion of the surgical segment (ROM), equivalent stress peak of the cage (ESPC), the maximum equivalent stress of the endplate (MESE) and the maximum stress of the internal fixation (MSIF) were compared between different models.

RESULTS

All OLIF surgical models showed that ROM declines between 74.87 and 96.77% at L4-L5 operative levels. The decreasing order of ROM was Model A2 > Model B2 > Model A1 > Model A2. In addition, the ESPC and MESE of Model A2 are smaller than those of other OLIF models. Except for the left-bending position, the MSIF of Model B2 increased by 1.51-16.69% compared with Model A2 in each position. The maximum value of MESE was 124.4 Mpa for Model B1 in the backward extension position, and the minimum value was 7.91 Mpa for Model A2 in the right rotation. Stand-alone group showed significantly higher ROMs and ESPCs than the BPSF group, with maximum values of 66.66% and 70.59%. For MESE, the BPSF group model can be reduced by 89.88% compared to the stand-alone group model.

CONCLUSIONS

Compared with the traditional straight OLIF cage, the kidney-shaped OLIF cage can further improve the stability of the surgical segment, reduce ESPC, MESE and MSIF, and help to reduce the risk of cage subsidence.

Reverse Lumbar Pedicle Screw in Oblique Lateral Interbody Fusion: A Novel Concept to Restrict Cage Subsidence

OBJECTIVE

Cage subsidence is a common morbidity after oblique lumbar interbody fusion (OLIF), with risk of compromising clinical and radiographic outcomes. The study aims to describe an innovative reverse lumbar pedicle screw (RLPS) technique in OLIF and compare its effect on restricting cage subsidence with classical lateral fixation (LF) in radiological and clinical evaluation.

METHOD

Consecutive patients having undergone single-level OLIF-LF/RLPS from 2018 to 2020 were retrospectively reviewed. In OLIF-RLPS, the upper entry point was determined at the intersection between one horizontal line (1 cm above inferior endplate) and one vertical line (dissecting anterior and middle thirds of the vertebra) while the inferior entry point between one horizontal line (5 mm below superior endplate) and the same vertical line. Trajectories were from vertebrae reverse into contralateral pedicle. Radiological evaluation included disc height (DH) and segmental lordosis (SL); cage subsidence was evaluated by DH loss. Clinical assessment included visual analogue scale (VAS) and Oswestry disability index (ODI). Student t or Mann-Whitney U test was used for continuous variation according to normality analysis while Chi-square test for category variation.

RESULTS

A total of 29 patients had been enrolled in the study including 14 cases in the RLPS group and 15 cases in the LF group. The DH in the OLIF-RLPS group had increased from the preoperative 9.07 ± 1.73 mm to 13.73 ± 1.83 mm postoperatively, without significant difference compared with the OLIF-LF group during the perioperative, but decreased to 12.53 ± 1.74 mm in 3 months and maintained at 12.00 ± 1.45 mm in 12 months, significantly higher than the OLIF-LF group (p < 0.05). At the last follow-up, 7.1% (1/14) cases in the OLIF-RLPS group had shown subsidence of grade I, significantly less than 46.7% (7/15) cases in the OLIF-LF group. Pain and disability had improved similarly in two groups, without significant difference detected between two groups at the last follow-up.

CONCLUSION

RLPS technique with modified entry points and prolonged trajectory could effectively restrict cage subsidence in OLIF postoperatively compared with traditional lateral fixation.

Oblique lateral internal fusion combined with percutaneous pedicle screw fixation in severe lumbar spinal stenosis: clinical and radiographic outcome

BACKGROUND

Oblique lumbar interbody fusion (OLIF) has been a popular technique for treating lumbar degenerative diseases. Previous studies have shown its efficiency in lumbar spinal stenosis; yet, only a few studies have investigated its application to severe lumbar spinal stenosis. Herein, we investigated the clinical and radiographic outcome of OLIF with percutaneous pedicle screws in the treatment of severe lumbar spinal stenosis.

METHODS

A total of 15 patients who underwent OLIF with percutaneous pedicle screws were retrospectively analysed. All patients were diagnosed with severe lumbar stenosis (Schizas grade C or D) through preoperative magnetic resonance image (MRI) and received OLIF combined with percutaneous pedicle screw surgery. Clinical outcomes, including visual analogue scale (VAS)-back and VAS-leg scores, and Oswestry Disability Index (ODI), as well as mean disc height (DH), mean foraminal height (FH), segmental lumbar lordosis (SLL) and cross-sectional area (CSA) of the spinal canal, were analysed before and after surgery and at the last follow-up. Intraoperative data, complications and fusion rate were also investigated.

RESULTS

OLIF combined with percutaneous pedicle screws was performed on 18 segments in 15 patients. Mean follow-up was 23.1 ± 4.6 months (range 15-29 months). VAS-back, VAS-leg, and ODI scores were significantly improved at the last follow-up. DH increased from 8.86 ± 3.06 mm before surgery to 13.31 ± 2.14 mm after; at the last follow-up, DH was 11.69 ± 1.87 mm. FH increased from 17.85 ± 2.26 mm before surgery to 22.09 ± 1.36 mm after; at the last follow-up, FH was 20.41 ± 0.99 mm. CSA of the spinal canal increased from 30.83 ± 21.15 mm2 before surgery to 74.99 ± 33.65 mm2 after the operation and 81.22 ± 35.53 mm2 at the last follow-up. The segmental LL before surgery, after surgery and at last follow-up was 20.27 ± 6.25 degrees, 20.83 ± 6.52 degrees and 19.75 ± 5.87 degrees, respectively. All patients have gained fusion at the last follow-up.

CONCLUSION

OLIF with percutaneous pedicle screws could achieve satisfactory clinical and radiographic effects through indirect compression by increasing DH, FH and CSA of the spinal canal in severe lumbar stenosis patients.

Comparative Biomechanical Analysis of Unilateral, Bilateral, and Lateral Pedicle Screw Implantation in Oblique Lumbar Interbody Fusion: A Finite Element Study

Oblique lumbar interbody fusion (OLIF) can be combined with different screw instrumentations. The standard screw instrumentation is bilateral pedicle screw fixation (BPSF). However, the operation is time consuming because a lateral recumbent position must be adopted for OLIF during surgery before a prone position is adopted for BPSF. This study aimed to employ a finite element analysis to investigate the biomechanical effects of OLIF combined with BPSF, unilateral pedicle screw fixation (UPSF), or lateral pedicle screw fixation (LPSF). In this study, three lumbar vertebra finite element models for OLIF surgery with three different fixation methods were developed. The finite element models were assigned six loading conditions (flexion, extension, right lateral bending, left lateral bending, right axial rotation, and left axial rotation), and the total deformation and von Mises stress distribution of the finite element models were observed. The study results showed unremarkable differences in total deformation among different groups (the maximum difference range is approximately 0.6248% to 1.3227%), and that flexion has larger total deformation (5.3604 mm to 5.4011 mm). The groups exhibited different endplate stress because of different movements, but these differences were not large (the maximum difference range between each group is approximately 0.455% to 5.0102%). Using UPSF fixation may lead to higher cage stress (411.08 MPa); however, the stress produced on the endplate was comparable to that in the other two groups. Therefore, the length of surgery can be shortened when unilateral back screws are used for UPSF. In addition, the total deformation and endplate stress of UPSF did not differ much from that of BPSF. Hence, combining OLIF with UPSF can save time and enhance stability, which is comparable to a standard BPSF surgery; thus, this method can be considered by spine surgeons.

Oblique lateral interbody fusion with internal fixations in the treatment for cross-segment degenerative lumbar spine disease (L2-3 and L4-5) finite element analysis

Multi-segmental lumbar degenerative disease, including intersegmental disc degeneration, is found in clinical practice. Controversy still exists regarding the treatment for cross-segment degeneration. Oblique Lateral Interbody Fusion (OLIF) with several internal fixations was used to treat cross-segment lumbar degenerative disease. A whole lumbar spine model was extracted from CT images of the whole lumbar spine of patients with lumbar degeneration. The L2-3 and L4-5 intervertebral spaces were fused with OLIF using modeling software, the Pedicle screws were performed on L2-3 and L4-5, and different internal fixations were performed on L3-4 in Finite Element (FE) software. Among the six 10 Nm moments of different directions, the L3-4 no surgery (NS) group had the relatively largest Range of Motion (ROM) in the whole lumbar spine, while the L2-5 Long segmental fixation (LSF)group had the smallest ROM and the other groups had similar ROM. The ROM in the L1-2 and L5-S1 was relatively close in the six group models, and the articular cartilage stress and disc stress on the L1-2 and L5-S1 were relatively close. In contrast, the L3-4 ROM differed relatively greatly, with the LSF ROM the smallest and the NS ROM the largest, and the L3-4 Coflex (Coflex) group more active than the L3-4 Bacfuse (Bacfuse) group and the L3-4 translaminar facet screw fixation (TFSF) group. The stress on the articular cartilage and disc at L3-4 was relatively greater in the NS disc and articular cartilage, and greater in the Coflex group than in the Bacfuse and TFSF groups, with the greatest stress on the internal fixation in the TFSF group, followed by the Coflex group, and relatively similar stress in the Bacfuse, LSF, and NS groups. In the TFSF group, the stress on the internal fixation was greater than the yield strength among different directional moments of 10 Nm, which means it is unsuitable to be an internal fixation. The LSF group had the greatest overall ROM, which may lead to postoperative low back discomfort. The NS group has the greatest overall ROM, but its increased stress on the L3-4 disc and articular cartilage may lead to accelerated degeneration of the L3-4 disc and articular cartilage. The Coflex and Bacfuse groups had a reduced L3-4 ROM but a greater stress on disc compared to the LSF group, which may lead to disc degeneration in the long term. However, their stress on the articular cartilage was relatively low. Coflex and Bacfuse can still be considered better surgical options.

Biomechanical stability of oblique lateral interbody fusion combined with four types of internal fixations: finite element analysis

Objective: Using finite element analysis to identify the optimal internal fixation method for oblique lateral lumbar interbody fusion (OLIF), providing guidance for clinical practice. Methods: A finite element model of the L4 - L5 segment was created. Five types of internal fixations were simulated in the generated L4-L5 finite element (FE) model. Then, six loading scenarios, i.e., flexion, extension, left-leaning, right-leaning, rotate left, and rotate right, were simulated in the FE models with different types of fixations. The biomechanical stability of the spinal segment after different fixations was investigated. Results: Regarding the range of motion (ROM) of the fused segment, OLIF + Bilateral Pedicle Screws (BPS) has a maximum ROM of 1.82° during backward bending and the smallest ROM in all directions of motion compared with other models. In terms of the von Mises stress distribution on the cage, the average stress on every motion direction of OLIF + BPS is about 17.08MPa, and of OLIF + Unilateral Vertebral Screw - Pedicle Screw (UVS-PS) is about 19.29 MPa. As for the von Mises stress distribution on the internal fixation, OLIF + BPS has the maximum internal fixator stress in left rotation (31.85 MPa) and OLIF + Unilateral Pedicle Screw (UPS) has the maximum internal fixator stress in posterior extension (76.59 MPa). The data of these two models were smaller than those of other models. Conclusion: OLIF + BPS provides the greatest biomechanical stability, OLIF + UPS has adequate biomechanical stability, OLIF + UVS-PS is inferior to OLIF + UPS synthetically, and OLIF + Double row vertical screw (DRVS) and Individual OLIF (IO) do not present significant obvious advantages.

Stand-Alone Oblique Lumbar Interbody Fusion (OLIF) for the Treatment of Adjacent Segment Disease (ASD) after Previous Posterior Lumbar Fusion: Clinical and Radiological Outcomes and Comparison with Posterior Revision Surgery

Background: Radiological evidence of adjacent segment disease (ASD) has been reported to have a prevalence of more than 30% and several risk factors have been reported. The aim of this study is to evaluate the clinical and radiological outcomes of patients with symptomatic ASD treated with stand-alone OLIF and compare results with a posterior revision surgery cohort. Methods: This is a retrospective case-control study. Clinical-patient-reported outcomes were obtained at preoperative, postoperative and final follow-up visits using the Short Form (SF-36) scale, the Oswestry Disability Index (ODI) and the visual analog scale (VAS). Radiological measures include lumbar lordosis (LL), segmental lordosis (SL), pelvic incidence-lumbar lordosis (PI-LL) mismatch, segmental coronal Cobb angle and intervertebral disc height (DH). The data are compared with a retrospective series of patients that underwent a posterior revision surgery for ASD. Results: Twenty-eight patients in the OLIF group and 25 patients in the posterior group meet inclusion criteria. The mean ages at the time of the surgery are 65.1 years and 67.5, respectively. The mean follow-up time is 36.1 months (range of 14-56). The clinical outcomes significantly improve from preoperative values from the surgery in both groups. The radiological parameters are significantly improved postoperatively and were maintained at the last follow-up in both groups. A statistically significant difference is observed between the two groups for minor complication rate, length of surgery, blood loss and DH restoration. Conclusions: Stand-alone OLIF is an effective and safe technique with low morbidity and complication rates for the treatment of selected patients with symptomatic ASD following a previous lumbar fusion.

Biomechanical characteristics of 2 different posterior fixation methods of bilateral pedicle screws: A finite element analysis

BACKGROUND

To explore the biomechanical characteristics of 2 posterior bilateral pedicle screw fixation methods using finite element analysis.

METHODS

A normal L3-5 finite element model was established. Based on the verification of its effectiveness, 2 different posterior internal fixation methods were simulated: bilateral pedicle screws (model A) were placed in the L3 and L5 vertebral bodies, and bilateral pedicle screws (model B) were placed in the L3, L4, and L5 vertebral bodies. The stability and stress differences of intervertebral discs, endplates, screws, and rods between models were compared.

RESULTS

Compared with the normal model, the maximum stress of the range of motion, intervertebral disc, and endplate of the 2 models decreased significantly. Under the 6 working conditions, the 2 internal fixation methods have similar effects on the stress of the endplate and intervertebral disc, but the maximum stress of the screws and rods of model B is smaller than that of model A.

CONCLUSIONS

Based on these results, it was found that bilateral pedicle screw fixation in 2 vertebrae L3 and L5 can achieve similar stability as bilateral pedicle screw fixation in 3 vertebrae L3, L4, and L5. However, the maximum stress of the screw and rod in model B is less than that in model A, so this internal fixation method can effectively reduce the risk of fracture. The 3-dimensional finite element model established in this study is in line with the biomechanical characteristics of the spine and can be used for further studies on spinal column biomechanics. This information can serve as a reference for clinicians for surgical selection.

Effects of osteoporosis on the biomechanics of various supplemental fixations co-applied with oblique lumbar interbody fusion (OLIF): a finite element analysis

Background

Oblique lumbar interbody fusion (OLIF) is an important surgical modality for the treatment of degenerative lumbar spine disease. Various supplemental fixations can be co-applied with OLIF, increasing OLIF stability and reducing complications. However, it is unclear whether osteoporosis affects the success of supplemental fixations; therefore, this study analyzed the effects of osteoporosis on various supplemental fixations co-applied with OLIF.

Methods

We developed and validated an L3-S1 finite element (FE) model; we assigned different material properties to each component and established models of the osteoporotic and normal bone lumbar spine. We explored the outcomes of OLIF combined with each of five supplemental fixations: standalone OLIF; OLIF with lateral plate fixation (OLIF + LPF); OLIF with translaminar facet joint fixation and unilateral pedicle screw fixation (OLIF + TFJF + UPSF); OLIF with unilateral pedicle screw fixation (OLIF + UPSF); and OLIF with bilateral pedicle screw fixation (OLIF + BPSF). Under the various working conditions, we calculated the ranges of motion (ROMs) of the normal bone and osteoporosis models, the maximum Mises stresses of the fixation instruments (MMSFIs), and the average Mises stresses on cancellous bone (AMSCBs).

Results

Compared with the normal bone OLIF model, no demonstrable change in any segmental ROM was apparent. The MMSFIs increased in all five osteoporotic OLIF models. In the OLIF + TFJF + UPSF model, the MMSFIs increased sharply in forward flexion and extension. The stress changes of the OLIF + UPSF, OLIF + BPSF, and OLIF + TFJF + UPSF models were similar; all stresses trended upward. The AMSCBs decreased in all five osteoporotic OLIF models during flexion, extension, lateral bending, and axial rotation. The average stress change of cancellous bone was most obvious under extension. The AMSCBs of the five OLIF models decreased by 14%, 23.44%, 21.97%, 40.56%, and 22.44% respectively.

Conclusions

For some supplemental fixations, the AMSCBs were all reduced and the MMSFIs were all increased in the osteoporotic model, compared with the OLIF model of normal bone. Therefore, the biomechanical performance of an osteoporotic model may be inferior to the biomechanical performance of a normal model for the same fixation method; in some instances, it may increase the risks of fracture and internal fixation failure.