Biomechanical and clinical studies on lumbar spine fusion surgery: a review

Analyses of proximal adjacent segment degeneration and prognostic factors after lumbar fusion surgery: study based on proximal facet joint angle

OBJECTIVE

Lumbar fusion surgery is a common procedure for treating various degenerative spinal conditions. However, the incidence of proximal adjacent segment degeneration (PASD) remains a concern. This study aimed to investigate the effect of proximal facet joint angle (FJA) on PASD and then identify factors that influence prognosis after lumbar fusion surgery.

METHODS

In this retrospective study, the cases of 192 patients who underwent lumbar fusion surgery between January 2020 and June 2022 were analysed. Patients were classified in accordance with their baseline proximal FJA into the high (≥ 40°) and low (< 40°) FJA groups. Prognosis was evaluated during the last follow-up by using clinical, imaging and functional recovery criteria. PASD was assessed using Weishaupt criteria, and imaging parameters were measured on postoperative computed tomography (CT) reconstructions. Statistical analyses, including univariate and multivariate logistic regression, were performed to identify prognostic factors. Receiver operating characteristic (ROC) curves were used to assess predictive value.

RESULTS

The high FJA group exhibited significantly higher rates of PASD compared with the low FJA group (P < 0.001). No significant differences were observed in sex, age, body mass index (BMI) or follow-up duration between the two groups. Poor prognosis was associated with higher BMI, larger FJA and wider facet joint diameter. Logistic regression analysis identified BMI (odds ratio [OR] = 1.801, P = 0.001), FJA (OR = 6.320, P < 0.001) and facet joint sagittal (OR = 1.888, P < 0.001) and coronal (OR = 1.462, P < 0.001) diameters as independent predictors of poor prognosis. A smaller screw inclination angle was associated with better outcomes (OR = 0.907, P = 0.017). Joint ROC analysis underscored the significant predictive power of these factors (area under the curve = 0.881).

CONCLUSION

This study demonstrates that a larger proximal FJA is associated with increased PASD. It also identifies several prognostic factors that influence outcomes after lumbar fusion surgery. Patients with higher BMI, larger FJA and wider sagittal and coronal diameters are at increased risk for poor prognosis. These findings highlight the importance of comprehensive preoperative assessments to optimise surgical planning and improve outcomes in lumbar fusion surgery.

Effect of cage surface geometry on load transfer and ranges of motion in a fused lumbar spine model: A comparative finite element analysis

Lumbar degenerative disc diseases (DDDs) are the common causes of low back pain, leading to non-conservative treatments like fusion and non-fusion surgery as a last resort. Fusion surgery is the gold standard for addressing DDDs, where implants such as cages, pedicle screws and rods are used for posterior stabilization. Various finite element (FE) studies have reported using corrugated cage surface textures; some others have used flat textures for virtual implantation. No comparative studies have been reported on the biomechanical effects of fusion surgery under implantation with cages of varying surface textures. The present biomechanical study compares the mechanical behaviour of an L4-L5 segment implanted with cages of different surface textures. The surgical techniques used for implantation are posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion. The virtual surgical models were developed from a previously validated intact lumbar spine FE model and simulated for physiological loading conditions. Compared to the flat cage implantation, a higher magnitude of stress was experienced by the cages and pedicle screw-rod systems under corrugated cage implantation. The maximum von Mises stress generated in the PLIF corrugated cage was 80.69% more than that observed in the flat cage. The maximum stresses in the corrugated cage were higher than those of the flat cage by 38.43%-80.69%, considering all the applied loading conditions. The findings of the study suggest that corrugated cage surface texture and suitable material selection may help in improving the long-term stability of cages.

Early functional training is not superior to routine rehabilitation in improving walking distance and multifidus atrophy after lumbar fusion: a randomized controlled trial with 6-month follow-up

PURPOSE

To investigate the effects of early functional training within a 12-week rehabilitation program on walking distance and lumbar multifidus atrophy following lumbar fusion surgery.

METHODS

A total of 52 patients who underwent lumbar fusion surgery were randomly assigned to either the early functional training group (n = 26) or the routine rehabilitation group (n = 26). The early functional training group (FT group) began a progressive functional training program within the first 12 weeks post-surgery, while the routine rehabilitation group (RH group) started the same training program 12 weeks after surgery. The primary outcome was the walking distance, measured as the 6-minute walk distance (6MWD) at the 3-month, and 6-month follow-up. Secondary outcomes included the cross-sectional area (CSA) of the multifidus muscles, the Visual Analogue Scale (VAS) score, the Oswestry Disability Index (ODI) score, the Short Form 36 Health Survey (SF-36) subscales, including the Physical Component Summary (PCS) and Mental Component Summary (MCS), and the incidence of adverse events. The primary aim was evaluated using a 2-way mixed-model analysis of covariance (ANCOVA), with treatment group as the between-subjects factor and time as the within-subjects factor, after adjusting for baseline measures.

RESULTS

Among the total patients, 38 (73.1%) left with at least one follow-up result, and 29 (55.8%) completed all treatment sessions and obtained an MRI assessment. The mixed-model ANCOVA revealed a significant group-by-time interaction for the 6MWD (F = 6.214, p = 0.003). Patients in the FT group demonstrated superior 6MWD compared to the GH group at 3 months (95% CI: 32.51 to 101.88, p < 0.001). No significant differences were found between the two groups at the 6-month follow-up (95% CI: -15.34 to 54.03, p = 0.271). Regarding secondary outcomes, no significant group-by-time effects were found for ODI (F = 1.221, p = 0.299), PCS (F = 0.166, p = 0.847), or MCS (F = 0.282, p = 0.755). No significant differences were found in the CSA of multifidus muscles between the groups. Nine patients (17.3%) experienced adverse events.

CONCLUSION

Early functional training after lumbar fusion was not superior to routine rehabilitation in improving walking distance, multifidus atrophy, pain, physical function, and quality of life during the 6-month follow-up. Trial registration number (TRN): ChiCTR2300068296.

An experimental methodology to measure the effects of intervertebral interventions on the facet biomechanics in situ

Whereas facet joints are recognised as being involved in back pain with high prevalence of early degeneration, the biomechanics of facet joints remain understudied and often overlooked in the evaluation of spinal interventions. This study aimed to develop a methodology for investigating the biomechanics of lumbar facet joints and applied it to a mock fusion model with posteriorly centred compressive loads. The proposed methodology involved measuring facet joint biomechanics through synchronized specimen load and displacement measurements, motion capture of the superior facets with 4 K webcams, and pressure mapping through the facet joints. The experimental method was developed using aged ovine lumbar functional units (N = 6). Results showed that the proposed methodology to measure facet joints biomechanics was accurate (displacement errors below 0.2 mm) and able to capture changes in biomechanics following a mock fusion (with significant differences in all measured displacements). Pressure measurement was challenging due to curvature changes in old ovine tissue which was used for method development but translated successfully to human lumbar tissue. It showed that an aged sheep model is not a good model for posterior spinal biomechanics. This work specifies a new, accurate, methodology to evaluate facet joint biomechanics in vitro and, uniquely, how they change following a spinal intervention.

An investigation of the mechanism of adjacent segment disease in a porcine spine model

BACKGROUND

Fusion changes the biomechanics of the spine leading to the potential development of adjacent segment disease. Despite many studies on adjacent segment disease, it is largely unknown how spinal fixation affects the mechanical properties of the adjacent disc. The purpose of this study was to assess whether axial compression causes mechanical disruption to the annulus when the caudal spinal level is immobilized or injured.

METHODS

Fifty-two porcine spines were assigned to one of four conditions: 1) control; 2) injured (18.5-gauge needle inserted into the nucleus of cervical 4/5); 3) immobilized (18-gauge steel wire wrapped around the transverse and spinous processes of cervical 4/5); and 4) injured+immobilized. Each specimen was then subjected to 0.5 Hz cyclic compression (300-1200N) for two hours. Post-compression, three annular samples were dissected from the cervical 3/4 disc (adjacent to immobilized and/or injured level) and mechanically tested. The same loading protocol and annular testing was also conducted in eight human cadaveric lumbar spines.

FINDINGS

Immobilization and injury resulted in a reduction in adjacent disc lamellar strength including toe region stress (p < 0.001), initial failure stress (p = 0.03), and ultimate stress (p = 0.004), with immobilization having the greatest impact. Similar findings were observed in the human cadaver samples with reduced toe region strength in the injured+ immobilized samples compared to the control (p = 0.049).

INTERPRETATION

The current study provides empirical evidence of decreased lamellar strength in the disc adjacent to an immobilized and/or injured level following prolonged cyclic axial loading, lending mechanistic insight into the development of adjacent segment disease.

Transforaminal lumbar interbody fusion (TLIF) surgery: A finite element analysis of open and minimally invasive approach on L4-L5 segment

Background

To compare the effect of minimally invasive and open transforaminal lumbar interbody fusion (TLIF) approaches in fusing the L4-L5 segment and predicting the potential risk of adjacent segment degeneration (ASD).

Methods

A computed tomography scan image was processed and the three-dimensional model of the L1-L5 spine was reconstructed. The minimally invasive and Open TLIF finite element models were constructed. The models were analyzed using an axial compressive load (500 N) and physiological movements like flexion, extension, and lateral bending with a combined load and moment (150 N and 10 Nm). The ranges of motion (ROM), stress-strain distributions on the L4-L5 implanted segment and cranial adjacent soft structures were compared with the intact model.

Results

A substantially comparable drop in ROM was observed for both models due to implantation. The stress and strain distributions on the implanted segment of both models were nearly identical. The peak strain on the L4-L5 was higher than 0.007 for both models. The maximum stress and strain observed on adjacent segment soft structures, except for the annulus fibrosus of both the implanted models, were substantially higher than the intact structures.

Conclusions

The open and MI-TLIF approaches are effective in reducing ROMs. However, the higher stress and strain on the L4-L5 segment indicate the chances of bone failure. The higher stress and strain on the adjacent segment soft structures indicate the potential risk of ASD in both models. However, considering the lower intrusive nature of the MI-TLIF technique, it might be favoured over Open TLIF.

Development and validation of a nomogram model for prolonged length of stay in spinal fusion patients: a retrospective analysis

OBJECTIVE

To develop a nomogram model for the prediction of the risk of prolonged length of hospital stay (LOS) in spinal fusion patients.

METHODS

A retrospective cohort study was carried out on 6272 patients who had undergone spinal fusion surgery. Least absolute shrinkage and selection operator (LASSO) regression was performed on the training sets to screen variables, and the importance of independent variables was ranked via random forest. In addition, various independent variables were used in the construction of models 1 and 2. A receiver operating characteristic curve was used to evaluate the models' predictive performance. We employed Delong tests to compare the area under the curve (AUC) of the different models. Assessment of the models' capability to improve classification efficiency was achieved using continuous net reclassification improvement (NRI) and integrated discrimination improvement (IDI). The Hosmer-Lemeshow method and calibration curve was utilised to assess the calibration degree, and decision curve to evaluate its clinical practicality. A bootstrap technique that involved 10 cross-validations and was performed 10,000 times was used to conduct internal and external validation. The were outcomes of the model exhibited in a nomogram graphics. The developed nomogram was validated both internally and externally.

RESULTS

Model 1 was identified as the optimal model. The risk factors for prolonged LOS comprised blood transfusion, operation type, use of tranexamic acid (TXA), diabetes, electrolyte disturbance, body mass index (BMI), surgical procedure performed, the number of preoperative diagnoses and operative time. The diagnostic performance of the nomogram model was satisfactory, with AUC values of 0.784 and 0.795 for the internal and external validation sets, respectively. Model discrimination was favourable in both the internal (C-statistic, 0.811) and external (C-statistic, 0.814) validation sets. Calibration curve and Hosmer-Lemeshow test showed acceptable agreement between predicted and actual results. The decision curve shows that the model provides net clinical benefit within a certain decision threshold range.

CONCLUSIONS

This study developed and validated a nomogram to identify the risk of prolonged LOS in spinal fusion patients, which may help clinicians to identify high-risk groups at an early stage. Predictors identified included blood transfusion, operation type, use of TXA, diabetes, electrolyte disturbance, BMI, surgical procedure performed, number of preoperative diagnoses and operative time.

Open laminectomy plus posterolateral fusion versus open laminectomy plus transforaminal lumbar interbody fusion surgical approaches for fusing degenerated L4-L5 segment: A comparative finite element study

Various finite element (FE) studies reported the biomechanical effects of fusion surgeries in the lumbar spine. However, a comparative study on Open laminectomy plus Posterolateral Fusion (OL-PLF) and Open Laminectomy plus Transforaminal Lumbar Interbody Fusion (OL-TLIF) for fusing an L4-L5 segment has not been reported in the literature. The present comparative FE study evaluates the biomechanical variations in an L4-L5 segment fused using OL-PLF and OL-TLIF surgical approaches. The three-dimensional implanted models were constructed from a computed-tomography scan dataset using image processing software. The models were simulated for the physiological movements such as lateral bending, flexion and extension. The OL-TLIF model had a considerably larger peak equivalent strain than the OL-PLF model under extension (126 %), lateral bending (88 %) and flexion (13 %). However, in both implanted models, a peak equivalent strain above the compressive yield strain limit of the vertebra (0.007) was observed over 60 % of the L4-L5 fused segment, indicating an imminent post-operative bone failure under the imposed loading conditions. The maximum equivalent strain observed in the disc and endplates of the L3-L4 segment was substantially larger to initiate the adjacent segment degeneration. No discernible biomechanical benefits were observed for the OL-TLIF or OL-PLF approaches in fusing the L4-L5 segment.

Finite element analysis of implanted lumbar spine: Effects of open laminectomy plus PLF and open laminectomy plus TLIF surgical approaches on L3-L4 FSU

Several finite element (FE) studies reported performances of various lumbar fusion surgical approaches. However, comparative studies on the performance of Open Laminectomy plus Posterolateral Fusion (OL-PLF) and Open Laminectomy plus Transforaminal Interbody Fusion (OL-TLIF) surgical approaches are rare. In the current FE study, the variation in ranges of motions (ROM), stress-strain distributions in an implanted functional spinal unit (FSU) and caudal adjacent soft structures between OL-PLF and OL-TLIF virtual models were investigated. The implanted lumbar spine FE models were developed from subject-specific computed tomography images of an intact spine and solved for physiological loadings such as compression, flexion, extension and lateral bending. Reductions in the ROMs of L1-L5 (49 % to 59 %) and L3-L4 implanted FSUs (91 % to 96 %) were observed for both models. Under all the loading cases, the maximum von Mises strain observed in the implanted segment of both models exceeds the mean compressive yield strain for the vertebra. The maximum von Mises stress and strain observed on the caudal adjacent soft structures of both the implanted models are at least 22 % higher than the natural spine model. The findings indicate the risk of failure in the implanted FSUs and higher chances of adjacent segment degeneration for both models.

Determining a relative total lumbar range of motion to alleviate adjacent segment degeneration after transforaminal lumbar interbody fusion: a finite element analysis

BACKGROUND

A reduction in total lumbar range of motion (ROM) after lumbar fusion may offset the increase in intradiscal pressure (IDP) and facet joint force (FJF) caused by the abnormally increased ROM at adjacent segments. This study aimed to determine a relative total lumbar ROM rather than an ideal adjacent segment ROM to guide postoperative waist activities and further delay adjacent segment degeneration (ASD).

METHODS

An intact L1-S1 finite element model was constructed and validated. Based on this, a surgical model was created to allow the simulation of L4/5 transforaminal lumbar interbody fusion (TLIF). Under the maximum total L1-S1 ROM, the ROM, IDP, and FJF of each adjacent segment between the intact and TLIF models were compared to explore the biomechanical influence of lumbar fusion on adjacent segments. Subsequently, the functional relationship between total L1-S1 ROM and IDP or total L1-S1 ROM and FJF was fitted in the TLIF model to calculate the relative total L1-S1 ROMs without an increase in IDP and FJF.

RESULTS

Compared with those of the intact model, the ROM, IDP, and FJF of the adjacent segments in the TLIF model increased by 12.6-28.9%, 0.1-6.8%, and 0-134.2%, respectively. As the total L1-S1 ROM increased, the IDP and FJF of each adjacent segment increased by varying degrees. The relative total L1-S1 ROMs in the TLIF model were 11.03°, 12.50°, 12.14°, and 9.82° in flexion, extension, lateral bending, and axial rotation, respectively.

CONCLUSIONS

The relative total L1-S1 ROMs after TLIF were determined, which decreased by 19.6-29.3% compared to the preoperative ones. Guiding the patients to perform postoperative waist activities within these specific ROMs, an increase in the IDP and FJF of adjacent segments may be effectively offset, thereby alleviating ASD.

A Biomechanical Comparison of 2 Different Topping-off Devices and Their Influence on the Sacroiliac Joint Following Lumbosacral Fusion Surgery

OBJECTIVE

Interspinous spacer (ISS)-based and pedicle screw-rod dynamic fixator (PDF)-based topping-off devices have been applied in lumbar/lumbosacral fusion surgeries for preventing the development of proximal adjacent segment degeneration. However, little attention has been paid to sacroiliac joint (SIJ), which belongs to the adjacent joints. Accordingly, the objective of this study was to compare how these 2 topping-off devices affect the SIJ biomechanics.

METHODS

A validated, normal finite-element lumbopelvic model (L3-pelvis) was initially adjusted to simulate interbody fusion with rigid fixation at the L5-S1 level, and then the DIAM or BioFlex system was instrumented at the L4-5 level to establish the ISS-based or PDF-based topping-off model, respectively. All the developed models were loaded with moments of 4 physiological motions using hybrid loading protocol.

RESULTS

Compared with the rigid fusion model (without topping-off devices), range of motion and von-Mises stress at the SIJs were increased by 23.1%-64.1% and 23.6%-62.8%, respectively, for the ISS-based model and by 51.2%-126.7% and 50.4%-108.7%, respectively, for the PDF-based model.

CONCLUSION

The obtained results suggest that the PDF-based topping-off device leads to higher increments in SIJ motion and stress than ISS-based topping-off device following lumbosacral fusion, implying topping-off technique could be linked to an increased risk of SIJ degeneration, especially when using PDF-based device.

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.

Effects of open and minimally invasive Transforaminal Lumbar Interbody Fusion (TLIF) surgical techniques on mechanical behaviour of fused L3-L4 FSU: A comparative finite element study

For predicting the biomechanical effects of the fusion procedure, finite element (FE) analysis is widely used as a preclinical tool. Although several FE studies examined the efficacies of various fusion surgical techniques, comparative studies on Open and minimally invasive (MIS) transforaminal lumbar interbody fusion (TLIF) procedures incorporating a follower coordinate system have not been investigated yet. The current FE study evaluates the ranges of motion (ROM) and load distributions of Open-TLIF and MIS-TLIF implanted models, under physiological loading such as compression, flexion, extension and lateral bending. The most noteworthy finding from the investigation is that both the fusion procedures significantly reduced the ROMs of the implanted segment (L3-L4) and full model (L1-L5) by at least 89 % and 44 %, respectively, compared to the intact model. For all loading situations, over 95 % of the implanted models' cancellous bone volume was subjected to von Mises strains ranging from 0.0003 to 0.005. The maximum von Mises strain was observed to be localized on a small amount of cancellous bone volume (<5 %). The likelihood of adjacent segment degeneration is higher in the case of MIS-TLIF due to the higher stress (22-53 MPa) and strain (0.018-0.087) noticed on the upper facet of the L3 vertebra.