An upper bound computational model for investigation of fusion effects on adjacent segment biomechanics of the lumbar spine

An OLIF Cage Integrated with a Low-Profile Plate and Cross Screws Could Reduce the Risk of Postoperative Complications Biomechanically

BACKGROUND

Stand-alone oblique lumbar interbody fusion (OLIF) cannot provide credible postoperative stability; additional fixation devices (AFDs) have been used to optimize surgical segment stability. Anterior lateral single rod (ALSR) screw fixation can be performed without intraoperative body position changes and additional surgical incisions, but its biomechanical defect may trigger complications. Inspired by the cross screw in other fixation devices, we designed an OLIF cage integrated with a low-profile plate and cross screw (LPCS).

METHODS

This study was designed to investigate whether the biomechanical performance of the LPCS OLIF cage is better than that of OLIF with ALSR fixation. The pullout and bending strength of the newly designed conical screw were tested by comparing it with a clinically used cylindrical screw. Different directional fixation strengths of the LPCS OLIF cage were tested by comparing the failure moment and stiffness with the ALSR fixation model. To test the fixation stability and potential risk for screw loosening in models with LPCS OLIF, we also compared the surgical segment's range of motions (ROMs) and stress distributions on OLIF models without and with different AFD fixation under physiological loading conditions.

RESULTS

The bending and pullout strength of the conical screw was better than that of the clinically used screw, and the failure moment and stiffness of the LPCS OLIF model were higher than those of the ALSR model, especially under the extension loading conditions. In which, the maximum failure moment of ALSR fixed OLIF model was 0.88 Nm and 0.76 Nm, while that of the LPCS OLIF model was 9.79 Nm and 7.48 Nm in models with normal and osteoporotic BMD, respectively. Compared to the ALSR fixed OLIF model, failure moment of LPCS models increased by 1012.5% and 884.21% in normal and osteoporotic models, respectively. Moreover, under most physiological loading conditions, the ROM and stress values of the LPCS OLIF model were lower than those of the ALSR model and even slightly lower than those of the OLIF model with bilateral pedicle screw fixation under limited loading conditions.

CONCLUSIONS

Compared to OLIF with ALSR fixation, the newly developed LPCS OLIF cage demonstrates inherent biomechanical advantages in establishing immediate postoperative stability and reducing complications related to stress concentration. However, the conclusions of current research should still be validated through in vitro mechanical tests and clinical trials.

The cranial vertebral body suffers a higher risk of adjacent vertebral fracture due to the poor biomechanical environment in patients with percutaneous vertebralplasty

BACKGROUND CONTEXT

Adjacent vertebral fracture (AVF), a frequent complication of PVP, is influenced by factors such as osteoporosis progression, increased intervertebral cement leakage (ICL), and biomechanical deterioration. Notably, the risk of AVF is notably elevated in the cranial vertebral body compared with the caudal counterpart. Despite this knowledge, the underlying pathological mechanism remains elusive.

PURPOSE

This study delves into the role of biomechanical deterioration as a pivotal factor in the heightened risk of AVF in the cranial vertebral body following PVP. By isolating this variable, we aim to unravel its prominence relative to other potential risk factors.

STUDY DESIGN

A retrospective study and corresponding numerical mechanical simulations.

PATIENT SAMPLE

Clinical data from 101 patients treated by PVP were reviewed in this study.

OUTCOME MEASURES

Clinical assessments involved measuring Hounsfield unit (HU) values of adjacent vertebral bodies as a representation of patients' bone mineral density (BMD). Additionally, the rates of ICL were compared among these patients. Numerical simulations were conducted to compute stress values in the cranial and caudal vertebral bodies under various body positions.

METHODS

In a retrospective analysis of PVP patients spanning July 2016 to August 2019, we scrutinized the HU values of adjacent vertebral bodies to discern disparities in BMD between cranial and caudal regions. Additionally, we compared ICL rates on both cranial and caudal sides. To augment our investigation, well-validated numerical models simulated the PVP procedure, enabling the computation of maximum stress values in cranial and caudal vertebral bodies across varying body positions.

RESULTS

The incidence rate of cranial AVF was significantly higher than the caudal side. No notable distinctions in HU values or ICL rates were observed between the cranial and caudal sides. The incidence of AVF showed no significant elevation in patients with ICL in either region. However, numerical simulations unveiled heightened stress values in the cranial vertebral body.

CONCLUSIONS

In patients postPVP, the cranial vertebral body faces a heightened risk of AVF, primarily attributed to biomechanical deterioration rather than lower BMD or an elevated ICL rate.

Pedicle Screw Fixation With Cement Augmentation Versus Without in the Treatment of Spinal Stenosis Following Posterior Spinal Fusion Surgery, Superiority According to Bone Mineral Density: A Three-Arm Randomized Clinical Trial

OBJECTIVE

To investigate whether cement augmentation has an impact on clinical and radiologic outcomes following posterior spinal fusion (PSF) in low-density bones and whether its performance is comparable to regular bone density.

METHODS

Between 2017 and 2021, 114 patients with spinal canal stenosis were enrolled to undergo PSF. They were initially stratified based on their bone mineral density: T-score ≥ -1.5: Group I (n = 34), and T-score < -1.5: Group II (n = 80). Furthermore, group II was randomly subdivided: II-A (unaugmented, n = 35) and II-B (bone cement augmented, n = 45). The primary and secondary endpoints of the study were evaluated using visual analog scales (VAS) and radiologic parameters, including screw loosening, screw or rod fractures, proximal junctional kyphosis (PJK), proximal junctional vertebral fracture (PJVF), and adjacent segment disease (ASD), at 1 year postoperatively.

RESULTS

All 3 groups showed improvement in VAS scores, but the most significant improvements were seen in groups I and II-B. Group II-B had lower incidence rates of ASD, PJVF, PJK, rod, and screw fracture compared to group II-A, but only screw loosening was statistically significant (P < 0.0001). Contrary, there were statistically significant differences in all variables except for ASD and screw fracture (P = 0.0576 and 0.670, respectively) when comparing groups I and II-A. In both groups I and II-A, screw loosening was the most common complication following PSF, while only 5.41% of patients in group II-B experienced it.

CONCLUSIONS

The efficacy of cement augmentation in mitigating pain and screw loosening following PSF surgery in low-density bones is comparable to that of normal-density bone.

Development of a novel FE model for investigation of interactions of multi-motion segments of the lumbar spine

The spinal anatomy is composed of a series of motion segments (MSs). Although finite element (FE) analysis has been extensively used to investigate the spinal biomechanics with various simplifications of the spinal structures, it is still a challenge to investigate the interactions of different MSs. Anatomical studies have shown that there are major spine ligaments connecting not only single-MS (i.e., two consecutive vertebrae) but also spanning multi-vertebral bones or multi-MSs. However, the effects of the multi-MS spanning ligaments on the spine biomechanics have not been investigated previously. This study developed an FE model of the lumbar spine by simulating the anterior longitudinal ligaments (ALLs) in two portions, one connecting a single-MS and the other spanning two MSs, with varying physiological cross-sectional area (PCSA) ratios of the two portions. The spine biomechanics during extension motion were investigated. The results showed that on average, the constraining forces by the two-MS spanning elements were ∼18% of those of the single-MS ALL elements when the PCSA ratio was 50%, but the two-MS ALL elements also applied compressive forces on the anterior surfaces of the vertebrae. Decreases in intradiscal pressure were also calculated when the two-MS spanning ALL elements were included in the spine model. The multi-MS spanning ligaments were shown to synergistically function with the single-MS elements in spine biomechanics, especially in the interactions of different MSs. The novel lumbar FE model could therefore provide a useful analysis tool for investigation of physiological functions of the spine.

Annulus Calibration Increases the Computational Accuracy of the Lumbar Finite Element Model

STUDY DESIGN

Mechanical simulations.

OBJECTIVE

Inadequate calibration of annuli negatively affects the computational accuracy of finite element (FE) models. Specifically, the definition of annulus average radius (AR) does not have uniformity standards. Differences between the elastic moduli in the different layers and parts of the annulus were not fully calibrated when a linear elastic material is used to define its material properties. This study aims to optimize the computational accuracy of the FE model by calibrating the annulus.

METHODS

We calibrated the annulus AR and elastic modulus in our anterior-constructed lumbar model by eliminating the difference between the computed range of motion and that measured by in vitro studies under a flexion-extension loading condition. Multi-indicator validation was performed by comparing the computed indicators with those measured in in vitro studies. The computation time required for the different models has also been recorded to evaluate the computational efficiency.

RESULTS

The difference between computed and measured ROMs was less than 1% when the annulus AR and elastic modulus were calibrated. In the model validation process, all the indicators computed by the calibrated FE model were within ±1 standard deviation of the average values obtained from in vitro studies. The maximum difference between the computed and measured values was less than 10% under nearly all loading conditions. There is no apparent variation tendency for the computational time associated with different models.

CONCLUSION

The FE model with calibrated annulus AR and regional elastic modulus has higher computational accuracy and can be used in subsequent mechanical studies.

Fixation-induced surgical segment's high stiffness and the damage of posterior structures together trigger a higher risk of adjacent segment disease in patients with lumbar interbody fusion operations

BACKGROUND

Adjacent segment disease (ASD) is a commonly reported complication after lumbar interbody fusion (LIF); changes in the mechanical environment play an essential role in the generation of ASD. Traditionally, fixation-induced high stiffness in the surgical segment was the main reason for ASD. However, with more attention paid to the biomechanical significance of posterior bony and soft structures, surgeons hypothesize that this factor may also play an important role in ASD.

METHODS

Oblique and posterior LIF operations have been simulated in this study. The stand-alone OLIF and OLIF fixed by bilateral pedicle screw (BPS) system have been simulated. The spinal process (the attachment point of cranial ligamentum complex) was excised in the PLIF model; the BPS system has also been used in the PLIF model. Stress values related to ASD have been computed under physiological body positions, including flexion, extension, bending, and axial rotations.

RESULTS

Compared to the stand-alone OLIF model, the OLIF model with BPS fixation suffers higher stress values under extension body position. However, there are no apparent differences under other loading conditions. Moreover, significant increases in stress values can be recorded in flexion and extension loading conditions in the PLIF model with posterior structures damage.

CONCLUSIONS

Fixation-induced surgical segment's high stiffness and the damage of posterior soft tissues together trigger a higher risk of ASD in patients with LIF operations. Optimizing BPS fixation methods and pedicle screw designs and reducing the range of posterior structures excision may be an effective method to reduce the risk of ASD.

Will the adjustment of insertional pedicle screw positions affect the risk of adjacent segment diseases biomechanically? An in-silico study

Background

The fixation-induced biomechanical deterioration will increase the risk of adjacent segment diseases (ASD) after lumbar interbody fusion with Bilateral pedicle screw (BPS) fixation. The accurate adjustment of insertional pedicle screw positions is possible, and published studies have reported its mechanical effects. However, no studies clarified that adjusting insertional screw positions would affect the postoperative biomechanical environment and the risk of ASD. The objective of this study was to identify this issue and provide theoretical references for the optimization of insertional pedicle screw position selections.

Methods

The oblique lumbar interbody fusion fixed by BPS with different insertional positions has been simulated in the L4-L5 segment of our previously constructed and validated lumbosacral model. Biomechanical indicators related to ASD have been computed and recorded under flexion, extension, bending, and axial rotation loading conditions.

Results

The change of screw insertional positions has more apparent biomechanical effects on the cranial than the caudal segment. Positive collections can be observed between the reduction of the fixation length and the alleviation of motility compensation and stress concentration on facet cartilages. By contrast, no pronounced tendency of stress distribution on the intervertebral discs can be observed with the change of screw positions.

Conclusions

Reducing the fixation stiffness by adjusting the insertional screw positions could alleviate the biomechanical deterioration and be an effective method to reduce the risk of ASD caused by BPS.

Analysis of complications and unsatisfactory results of surgical treatment of degenerative lumbar spinal stenosis in the elderly patients

Background. The results of treatment of the elderly patients operated for spinal stenosis allow us to suggest that a cascade of degenerative changes in the spinal motion segments causes the formation of an adjacent level syndrome, pseudarthrosis, and in some cases – the instability in the fixing structure.The aim of the study. To determine the prognostic factors for the adjacent level syndrome in patients after decompressive and stabilizing spinal surgeries.Methods. We carried out a retrospective cohort study of the surgical treatment of 129 elderly patients (over 60 years of age) for the period from January 2018 to March 2022, who underwent surgery at the lumbosacral level of spine for degenerative spinal stenosis.Results. The outcomes of surgical treatment of 129 patients and the results of discriminant analysis of morphometric studies of computed tomography data indicate that the most significant indicators for the development of the adjacent level syndrome are the lordosis angle in the segment adjacent to the operated one (the mean value in the analyzed group is 12.87 ± 2.22°; in the control group – 11.92 ± 2.97°); the anterior height of the adjacent intervertebral disc (the mean value in the analyzed group is 12.70 ± 2.44 mm; in the control group – 11.46 ± 3.58 mm) and the difference of anterior and posterior disc heights at the adjacent level (the mean value in the analyzed group is 5.48 ± 2.84 mm; in the control group – 6.27 ± 2.71 mm).Conclusion. When analyzing the treatment outcomes of 129 elderly patients operated for degenerative spinal stenosis using instrumented spinal fusion, we revealed that in 16 patients, the adjacent level syndrome developed with an increase in the lordosis angle at the level adjacent to the operated segment. An increase in the anterior height of the adjacent intervertebral disc and the decrease in the difference of anterior and posterior disc heights at the adjacent level can be considered as unfavorable prognostic factors (p = 0.83).

Subsidence and fusion performance of a 3D-printed porous interbody cage with stress-optimized body lattice and microporous endplates - a comprehensive mechanical and biological analysis

BACKGROUND CONTEXT

Cage subsidence remains a serious complication after spinal fusion surgery. Novel porous designs in the cage body or endplate offer attractive options to improve subsidence and osseointegration performance.

PURPOSE

To elucidate the relative contribution of a porous design in each of the two major domains (body and endplates) to cage stiffness and subsidence performance, using standardized mechanical testing methods, and to analyze the fusion progression via an established ovine interbody fusion model to support the mechanical testing findings.

STUDY DESIGN/SETTING

A comparative preclinical study using standardized mechanical testing and established animal model.

METHODS

To isolate the subsidence performance contributed by each porous cage design feature, namely the stress-optimized body lattice (vs. a solid body) and microporous endplates (vs. smooth endplates), four groups of cages (two-by-two combination of these two features) were tested in: (1) static axial compression of the cage (per ASTM F2077) and (2) static subsidence (per ASTM F2267). To evaluate the progression of fusion, titanium cages were created with a microporous endplate and internal lattice architecture analogous to commercial implants used in subsidence testing and implanted in an endplate-sparing, ovine intervertebral body fusion model.

RESULTS

The cage stiffness was reduced by 16.7% by the porous body lattice, and by 16.6% by the microporous endplates. The porous titanium cage with both porous features showed the lowest stiffness with a value of 40.4±0.3 kN/mm (Mean±SEM) and a block stiffness of 1976.8±27.4 N/mm for subsidence. The body lattice showed no significant impact on the block stiffness (1.4% reduction), while the microporous endplates decreased the block stiffness significantly by 24.9% (p<.0001). All segments implanted with porous titanium cages were deemed rigidly fused by manual palpation, except one at 12 weeks, consistent with robotic ROM testing and radiographic and histologic observations. A reduction in ROM was noted from 12 to 26 weeks (4.1±1.6° to 2.2±1.4° in lateral bending, p<.05; 2.1±0.6° to 1.5±0.3° in axial rotation, p<.05); and 3.3±1.6° to 1.9±1.2° in flexion extension, p=.07). Bone in the available void improved with time in the central aperture (54±35% to 83±13%, p<.05) and porous cage structure (19±26% to 37±21%, p=.15).

CONCLUSIONS

Body lattice and microporous endplates features can effectively reduce the cage stiffness, therefore reducing the risk of stress shielding and promoting early fusion. While body lattice showed no impact on block stiffness and the microporous endplates reduced the block stiffness, a titanium cage with microporous endplates and internal lattice supported bone ingrowth and segmental mechanical stability as early as 12 weeks in ovine interbody fusion.

CLINICAL SIGNIFICANCE

Porous titanium cage architecture can offer an attractive solution to increase the available space for bone ingrowth and bridging to support successful spinal fusion while mitigating risks of increased subsidence.

FINITE ELEMENT SPINE MODELS AND SPINAL INSTRUMENTS: A REVIEW

There is considerable biomechanics literature on finite element modeling and analysis of the spine. To accurately mimic the biomechanical behavior of the vertebral column, a generated computational model has to include anatomical structures that are consistent with physiological reality. In this review article, we focused on the finite element spine models that have been developed by various approaches in the literature. Firstly, the anatomical features of the spine and the spinal components have been briefly explained. We then focused on the modeling stages of vertebrae, ligaments, facet joints, intervertebral discs, and spinal instruments. With this paper, we expect to provide a comprehensive resource regarding the modeling preferences used in spine modeling.

Cervical non-fusion using biomimetic artificial disc and vertebra complex: technical innovation and biomechanics analysis

Background

Changes in spinal mobility after vertebral fusion are important factors contributing to adjacent vertebral disease (ASD). As an implant for spinal non-fusion, the motion-preserving prosthesis is an effective method to reduce the incidence of ASD, but its deficiencies hamper the application in clinical. This study designs a novel motion-preserving artificial cervical disc and vertebra complex with an anti-dislocation mechanism (MACDVC-AM) and verifies its effect on the cervical spine.

Methods

The MACDVC-AM was designed on the data of healthy volunteers. The finite element intact model, fusion model, and MACDVC-AM model were constructed, and the range of motion (ROM) and stress of adjacent discs were compared. The biomechanical tests were performed on fifteen cervical specimens, and the stability index ROM (SI-ROM) were calculated.

Results

Compared with the intervertebral ROMs of the intact model, the MACDVC-AM model reduced by 28–70% in adjacent segments and increased by 26–54% in operated segments, but the fusion model showed the opposite result. In contrast to the fusion model, the MACDVC-AM model diminished the stress of adjacent intervertebral discs. In biomechanical tests, the MACDVC-AM group showed no significant difference with the ROMs of the intact group (p > 0.05). The SI-ROM of the MACDVC-AM group is negative but close to zero and showed no significant difference with the intact group (p > 0.05).

Conclusions

The MACDVC-AM was successfully designed. The results indicate that the MACDVC-AM can provide physiological mobility and stability, reduce adjacent intervertebral compensatory motion, and alleviate the stress change of adjacent discs, which contributes to protect adjacent discs and reduce the occurrence of ASD.

Disc measurement and nucleus calibration in a smoothened lumbar model increases the accuracy and efficiency of in-silico study

Backgrounds

Finite element analysis (FEA) is an important tool during the spinal biomechanical study. Irregular surfaces in FEA models directly reconstructed based on imaging data may increase the computational burden and decrease the computational credibility. Definitions of the relative nucleus position and its cross-sectional area ratio do not conform to a uniform standard in FEA.

Methods

To increase the accuracy and efficiency of FEA, nucleus position and cross-sectional area ratio were measured from imaging data. A FEA model with smoothened surfaces was constructed using measured values. Nucleus position was calibrated by estimating the differences in the range of motion (RoM) between the FEA model and that of an in-vitro study. Then, the differences were re-estimated by comparing the RoM, the intradiscal pressure, the facet contact force, and the disc compression to validate the measured and calibrated indicators. The computational time in different models was also recorded to evaluate the efficiency.

Results

Computational results indicated that 99% of accuracy was attained when measured and calibrated indicators were set in the FEA model, with a model validation of greater than 90% attained under almost all of the loading conditions. Computational time decreased by around 70% in the fitted model with smoothened surfaces compared with that of the reconstructed model.

Conclusions

The computational accuracy and efficiency of in-silico study can be improved in the lumbar FEA model constructed using smoothened surfaces with measured and calibrated relative nucleus position and its cross-sectional area ratio.

Investigation of Alterations in the Lumbar Disc Biomechanics at the Adjacent Segments After Spinal Fusion Using a Combined In Vivo and In Silico Approach

The development of adjacent segment degeneration (ASD) is a major concern after lumbar spinal fusion surgery, but the causative mechanisms remain unclear. This study used a combined in vivo and in silico method to investigate the changes of anatomical dimensions and biomechanical responses of the adjacent segment (L3-4) after spinal fusion (L4-S1) in five patients under weight-bearing upright standing conditions. The in vivo adjacent disc height changes before and after fusion were measured using a dual fluoroscopic imaging system (DFIS), and the measured in vivo intervertebral positions and orientations were used as displacement boundary conditions of the patient-specific three-dimensional (3D) finite element (FE) disc models to simulate the biomechanical responses of adjacent discs to fusion of the diseased segments. Our data (represented by medians and 95% confidence intervals) showed that a significant decrease by - 0.8 (- 1.2, - 0.4) mm (p < 0.05) in the adjacent disc heights occurred at the posterior region after fusion. The significant increases in disc tissue strains and stresses, 0.32 (0.21, 0.43) mm/mm (p < 0.05) and 1.70 (1.07, 3.60) MPa (p < 0.05), respectively, after fusion were found in the posterolateral portions of the outermost annular lamella. The intradiscal pressure of the adjacent disc was significantly increased by 0.29 (0.13, 0.47) MPa after fusion (p < 0.05). This study demonstrated that fusion could cause alterations in adjacent disc biomechanics, and the combined in vivo and in silico method could be a valuable tool for the quantitative assessment of ASD after fusion.

Influence of cement-augmented pedicle screw instrumentation in an osteoporotic lumbosacral spine over the adjacent segments: a 3D finite element study

Abstract

Purpose

To compare the effect of conventional pedicle screw (CPS) and cement-augmented pedicle screw instrumentation (CAPSI) on adjacent segment degeneration (ASD).

Methods

A normal male volunteer without a history of spinal disease was selected, lumbar CT data was collected, an intact L3-S1 three-dimensional finite element model was created by software including Mimics, Geomagic, and SolidWorks, and the fixation methods were performed accordingly. A common pedicle screw model and a cement-augmented pedicle screw model of L4–L5 with fusion and internal fixation were constructed. With ANSYS Workbench 17.0, a 500 N load was applied to the upper surface of L3 to simulate the weight of a human body, and a 7.5 N m moment was applied at the neutral point to simulate flexion, extension, left/right bending, left/right rotation of the spine. The peak von Mises stress of intervertebral disc and the range of motion (ROM) on the adjacent segments (L3–4 and L5–S1) were compared.

Results

The validity of the intact model shows that the ROM of the model is similar to that of a cadaveric study. Compared with the intact model, CPS model and CAPSI model in all motion patterns increased the ROM of adjacent segments. The intervertebral disc stress and the ROM of adjacent segments were found to be higher in the CAPSI model than in the CPS model, especially in L3–4.

Conclusion

In general, the biomechanical analysis of an osteoporotic lumbar spine showed that both CPS and CAPSI can increase the ROM and disc stresses of osteoporotic lumbar models, and compared with CPS, CAPSI is more likely to increase the potential risk of adjacent segment degeneration.