Biomechanical evaluation of strategies for adjacent segment disease after lateral lumbar interbody fusion: is the extension of pedicle screws necessary?

Biomechanical Effects of Different Prosthesis Types and Fixation Ranges in Multisegmental Total En Bloc Spondylectomy: A Finite Element Study

OBJECTIVE

Multi-segmental total en bloc spondylectomy (TES) gradually became more commonly used by clinicians. However, the choice of surgical strategy is unclear. This study aims to investigate the biomechanical performance of different prosthesis types and fixation ranges in multisegmental TES.

METHODS

In this study, a validated finite element model of T12-L2 post-spondylectomy operations were carried out. The prostheses of these models used either 3D-printed artificial vertebrae or titanium mesh cages. The fixed range was two or three segment levels. Range of motion, stress distribution of the endplate and internal fixation system, intervertebral disc pressure, and facet joint surface force of four postoperative models and intact model in flexion and extension, as well as lateral bending and rotation were analyzed and compared.

RESULTS

The type of prosthesis used in the anterior column reconstruction mainly affected the stress of the adjacent endplate and the prosthesis itself. The posterior fixation range had a greater influence on the overall range of motion (ROM), the ROM of the adjacent segment, the stress of the screw-rod system, and adjacent facet joint surface force. For the model of the same prosthesis, the increase of fixed length resulted in an obvious reduction of ROM. The maximal decrease was 70.23% during extension, and the minimal decrease was 30.19% during rotation.

CONCLUSION

In three-segment TES, the surgical strategy of using 3D-printed artificial prosthesis for anterior column support and pedicle screws for posterior fixation at both two upper and lower levels respectively can reduce the stress on internal fixation system, endplates, and adjacent intervertebral discs, resulting in a reduced risk of internal fixation failure, and ASD development.

Biomechanical Comparison of Different Surgical Strategies for Skip-level Cervical Degenerative Disc Disease: A Finite Element Study

STUDY DESIGN

We constructed finite element (FE) models of the cervical spine consisting of C2-C7 and predicted the biomechanical effects of different surgical procedures and instruments on adjacent segments, internal fixation systems, and the overall cervical spine through FE analysis.

OBJECTIVE

To compare the biomechanical effects between the zero-profile device and cage-plate device in skip-level multistage anterior cervical discectomy and fusion (ACDF).

SUMMARY OF BACKGROUND DATA

ACDF is often considered the standard treatment for degenerative cervical spondylosis. However, the selection of surgical methods and instruments in cases of skip-level cervical degenerative disk disease is still controversial.

MATERIALS AND METHODS

Three FE models were constructed, which used noncontiguous 2-level Zero-P (NCZP) devices for C3/4 and C5/6, a noncontiguous 2-level cage-plate (NCCP) for C3/4 and C5/6, and a contiguous 3-level cage-plate (CCP) for C3/6. Simulate daily activities in ABAQUS. The range of motion (ROM), von Mises stress distribution of the endplate and internal fixation system, and intervertebral disk pressure (IDP) of each model were recorded and compared.

RESULTS

Similar to the stress of the cortical bone, the maximum stress of the Zero-P device was higher than that of the CP device for most activities. The ROM increments of the superior, inferior, and intermediate segments of the NCZP model were lower than those of the NCCP and CCP models in many actions. In terms of the IDP, the increment value of stress for the NCZP model was the smallest, whereas those of the NCCP and CCP models were larger. Similarly, the increment value of stress on the endplate also shows the minimum in the NCZP model.

CONCLUSIONS

Noncontiguous ACDF with zero profile can reduce the stress on adjacent intervertebral disks and endplates, resulting in a reduced risk of adjacent segment disease development. However, the high cortical bone stress caused by the Zero-P device may influence the risk of fractures.

Advancing Prone-Transpsoas Spine Surgery: A Narrative Review and Evolution of Indications with Representative Cases

The Prone Transpsoas (PTP) approach to lumbar spine surgery, emerging as an evolution of lateral lumbar interbody fusion (LLIF), offers significant advantages over traditional methods. PTP has demonstrated increased lumbar lordosis gains compared to LLIF, owing to the natural increase in lordosis afforded by prone positioning. Additionally, the prone position offers anatomical advantages, with shifts in the psoas muscle and lumbar plexus, reducing the likelihood of postoperative femoral plexopathy and moving critical peritoneal contents away from the approach. Furthermore, operative efficiency is a notable benefit of PTP. By eliminating the need for intraoperative position changes, PTP reduces surgical time, which in turn decreases the risk of complications and operative costs. Finally, its versatility extends to various lumbar pathologies, including degeneration, adjacent segment disease, and deformities. The growing body of evidence indicates that PTP is at least as safe as traditional approaches, with a potentially better complication profile. In this narrative review, we review the historical evolution of lateral interbody fusion, culminating in the prone transpsoas approach. We also describe several adjuncts of PTP, including robotics and radiation-reduction methods. Finally, we illustrate the versatility of PTP and its uses, ranging from 'simple' degenerative cases to complex deformity surgeries.

Finite element biomechanical analysis of 3D printed intervertebral fusion cage in osteoporotic population

OBJECTIVE

To study the biomechanical characteristics of each tissue structure when using different 3D printing Cage in osteoporotic patients undergoing interbody fusion.

METHODS

A finite element model of the lumbar spine was reconstructed and validated with regarding a range of motion and intervertebral disc pressure from previous in vitro studies. Cage and pedicle screws were implanted and part of the lamina, spinous process, and facet joints were removed in the L4/5 segment of the validated mode to simulate interbody fusion. A 280 N follower load and 7.5 N·m moment were applied to different postoperative models and intact osteoporotic model to simulate lumbar motion. The biomechanical characteristics of different models were evaluated by calculating and analyzing the range of motion of the fixed and cephalic adjacent segment, the stress of the screw-rod system, the stress at the interface between cage and L5 endplate, and intervertebral disc pressure of the adjacent segment.

RESULTS

After rigid fixation, the range of motion of the fixed segment of model A-C decreased significantly, which was much smaller than that of the osteoporotic model. And with the increase of the axial area of the interbody fusion cages, the fixed segment of model A-C tended to be more stable. The range of motion and intradiscal pressure of the spinal models with different interbody fusion cages were higher than those of the complete osteoporosis model, but there was no significant difference between the postoperative models. On the other hand, the L5 upper endplate stress and screw-rod system stress of model A-C show a decreasing trend in different directions of motion. The stress of the endplate is the highest during flexion, which can reach 40.5 MPa (model A). The difference in endplate stress between models A-C was the largest during lateral bending. The endplate stress of models A and B was 150.5% and 140.9% of that of model C, respectively. The stress of the screw-rod system was the highest during lateral bending (model A, 102.0 MPa), which was 108.4%, 102.4%, 110.4%, 114.2% of model B and 158.5%, 110.1%, 115.8%, 125.4% of model C in flexion, extension, lateral bending, and rotation, respectively.

CONCLUSIONS

For people with osteoporosis, no matter what type of cage is used, good immediate stability can be achieved after surgery. Larger cage sizes provide better fixation without significantly increasing ROM and IDP in adjacent segments, which may contribute to the development of ASD. In addition, larger cage sizes can disperse endplate stress and reduce stress concentration, which is of positive significance in preventing cage subsidence after operation. The cage and screw rod system establish a stress conduction pathway on the spine, and a larger cage greatly enhances the stress-bearing capacity of the front column, which can better distribute the stress of the posterior spine structure and the stress borne by the posterior screw rod system, reduce the stress concentration phenomenon of the nail rod system, and avoid exceeding the yield strength of the material, resulting in the risk of future instrument failure.

Biomechanical behaviour of a novel bone cement screw in the minimally invasive treatment of Kummell's disease: a finite element study

OBJECTIVE

To investigate and evaluate the biomechanical behaviour of a novel bone cement screw in the minimally invasive treatment of Kummell's disease (KD) by finite element (FE) analysis.

METHODS

A validated finite element model of healthy adult thoracolumbar vertebrae T12-L2 was given the osteoporotic material properties and the part of the middle bone tissue of the L1 vertebral body was removed to make it wedge-shaped. Based on these, FE model of KD was established. The FE model of KD was repaired and treated with three options: pure percutaneous vertebroplasty (Model A), novel unilateral cement screw placement (Model B), novel bilateral cement screw placement (Model C). Range of motion (ROM), maximum Von-Mises stress of T12 inferior endplate and bone cement, relative displacement of bone cement, and stress distribution of bone cement screws of three postoperative models and intact model in flexion and extension, as well as lateral bending and rotation were analyzed and compared.

RESULTS

The relative displacements of bone cement of Model B and C were similar in all actions studied, and both were smaller than that of Model A. The minimum value of relative displacement of bone cement is 0.0733 mm in the right axial rotation of Model B. The maximum Von-Mises stress in T12 lower endplate and bone cement was in Model C. The maximum Von-Mises stress of bone cement screws in Model C was less than that in Model B, and it was the most substantial in right axial rotation, which is 34%. There was no substantial difference in ROM of the three models.

CONCLUSION

The novel bone cement screw can effectively reduce the relative displacement of bone cement by improving the stability of local cement. Among them, novel unilateral cement screw placement can obtain better fixation effect, and the impact on the biomechanical environment of vertebral body is less than that of novel bilateral cement screw placement, which provides a reference for minimally invasive treatment of KD in clinical practice.

A comparative analysis of using cage acrossing the vertebral ring apophysis in normal and osteoporotic models under endplate injury: a finite element analysis

Background: Lateral lumbar fusion is an advanced, minimally invasive treatment for degenerative lumbar diseases. It involves different cage designs, primarily varying in size. This study aims to investigate the biomechanics of the long cage spanning the ring apophysis in both normal and osteoporotic models, considering endplate damage, using finite element analysis. Methods: Model 1 was an intact endplate with a long cage spanning the ring apophysis. Model 2 was an endplate decortication with a long cage spanning the ring apophysis. Model 3 was an intact endplate with a short cage. Model 4 was an endplate decortication with a short cage. On the basis of the four original models, further osteoporosis models were created, yielding a total of eight finite element models. The provided passage delineates a study that elucidates the utilization of finite element analysis as a methodology to simulate and analyze the biomechanical repercussions ensuing from the adoption of two distinct types of intervertebral fusion devices (cages) within the physiological framework of a human body. Results: The investigation found no appreciable changes between Models 1 and 2 in the range of motion at the fixed and neighboring segments, the L3-4 IDP, screw-rod stress, endplate stress, or stress on the trabecular bone of the L5. Increases in these stresses were seen in models 3 and 4 in the ranges of 0.4%-676.1%, 252.9%-526.9%, 27.3%-516.6%, and 11.4%-109.3%, respectively. The osteoporotic models for scenarios 3 and 4 exhibit a similar trend to their respective normal bone density models, but these osteoporotic models consistently have higher numerical values. In particular, except for L3-4 IDP, the maximum values of these parameters in osteoporotic Models 3 and 4 were much higher than those in normal bone quality Models 1 and 2, rising by 385.3%, 116%, 435.1%, 758.3%, and 786.1%, respectively. Conclusion: Regardless of endplate injury or osteoporosis, it is advised to utilize a long cage that is 5 mm longer on each side than the bilateral pedicles because it has good biomechanical features and may lower the likelihood of problems after surgery. Additionally, using Long cages in individuals with osteoporosis may help avoid adjacent segment disease.

Biomechanical Comparison of Different Surgical Approaches for the Treatment of Adjacent Segment Diseases after Primary Transforaminal Lumbar Interbody Fusion: A Finite Element Analysis

BACKGROUND AND OBJECTIVE

Adjacent segment disease (ASD) is a well-known complication after interbody fusion. Revision surgery is necessary for symptomatic ASD to further decompress and fix the affected segment. However, no optimal construct is accepted as a standard in treating ASD. The purpose of this study was to compare the biomechanical effects of different surgical approaches for the treatment of ASD after primary transforaminal lumbar interbody fusion (TLIF).

METHODS

A finite element model of the L1-S1 was conducted based on computed tomography scan images. The primary surgery model was developed with a single-level TLIF at L4-L5 segment. The revision surgical models were developed with anterior lumbar interbody fusion (ALIF), lateral lumbar interbody fusion (LLIF), or TLIF at L3-L4 segment. The range of motion (ROM), intradiscal pressure (IDP), and the stress in cages were compared to investigate the biomechanical influences of different surgical approaches.

RESULTS

The results indicated that all the three surgical approaches can stabilize the spinal segment by reducing the ROM at revision level. The ROM and IDP at adjacent segments of revision model of TLIF was greater than those of other revision models. While revision surgery with ALIF and LLIF had similar effects on the ROM and IDP of adjacent segments. Compared among all the surgical models, cage stress in revision model of TLIF was the maximum in extension and axial rotation.

CONCLUSION

The IDP at adjacent segments and stress in cages of revision model of TLIF was greater than those of ALIF and LLIF. This may be that direct extension of the surgical segment in the same direction results in stress concentration.

Biomechanical properties of lumbar vertebral ring apophysis cage under endplate injury: a finite element analysis

OBJECTIVE

This study aimed to compare the biomechanical properties of lumbar interbody fusion involving two types of cages. The study evaluated the effectiveness of the cage spanning the ring apophysis, regardless of the endplate's integrity.

METHODS

A finite element model of the normal spine was established and validated in this study. The validated model was then utilized to simulate Lateral Lumbar Interbody Fusion (LLIF) with posterior pedicle screw fixation without posterior osteotomy. Two models of interbody fusion cage were placed at the L4/5 level, and the destruction of the bony endplate caused by curetting the cartilaginous endplate during surgery was simulated. Four models were established, including Model 1 with an intact endplate and long cage spanning the ring apophysis, Model 2 with endplate decortication and long cage spanning the ring apophysis, Model 3 with an intact endplate and short cage, and Model 4 with endplate decortication and short cage. Analyzed were the ROM of the fixed and adjacent segments, screw rod system stress, interface stress between cage and L5 endplate, trabecular bone stress on the upper surface of L5, and intervertebral disc pressure (IDP) of adjacent segments.

RESULTS

There were no significant differences in ROM and IDP between adjacent segments in each postoperative model. In the short cage model, the range of motion (ROM), contact pressure between the cage and endplate, stress in L5 cancellous bone, and stress in the screw-rod system all exhibited an increase ranging from 0.4% to 79.9%, 252.9% to 526.9%, 27.3% to 133.3%, and 11.4% to 107%, respectively. This trend was further amplified when the endplate was damaged, resulting in a maximum increase of 88.6%, 676.1%, 516.6%, and 109.3%, respectively. Regardless of the integrity of the endplate, the long cage provided greater support strength compared to the short cage.

CONCLUSIONS

Caution should be exercised during endplate preparation and cage placement to maintain the endplate's integrity. Based on preoperative X-ray evaluation, the selection of a cage that exceeds the width of the pedicle by at least 5 mm (ensuring complete coverage of the vertebral ring) has demonstrated remarkable biomechanical performance in lateral lumbar interbody fusion procedures. By opting for such a cage, we expect a reduced occurrence of complications, including cage subsidence, internal fixation system failure, and rod fracture.

Finite element analysis of lattice designed lumbar interbody cage based on the additive manufacturing

Additive manufacturing (AM) methods, which facilitate the production of complex structures with different geometries, have been used in producing interbody cages in recent years. In this study, the effects of Ti6Al4V alloy interbody lattice designed fusion cages between the third and fourth lumbar vertebrae where degenerative disc diseases occur were investigated using the finite element method. Face centered cubic (FCC), body centered cubic (BCC), and diamond structures were selected as the lattice structure suitable for the interbody cage. A kidney shaped interbody lumbar cage was designed. The designated lattice structures were selected by adjusting the cell sizes suitable for the designed geometry, and the mesh configuration was made by the lumbar lattice structure. 400 N Axial force and 7.5 N.m moments were applied to the spine according to lateral bending, flexion, and torsion. 400 N axial force and 7.5 N.m flexion moment is shown high strain and total deformation then lateral bending and torsion on BCC FCC and diamond lattice structured interbody cages. In addition, the effects of lattice structures under high compression forces were investigated by applying 1000 N force to the lattice structures. When von Mises stresses were examined, lower von Mises stress and strains were observed in the BCC structure. However, a lower total deformation was observed in the FCC. Due to the design of the BCC and the diamond structure, it is assumed that bone implant adhesion will increase. In the finite element analysis (FEA), the best results were shown in BCC structures.

Biomechanical effects of transverse connectors on total en bloc spondylectomy of the lumbar spine: a finite element analysis

BACKGROUND

The influence of total en bloc spondylectomy (TES) on spinal stability is substantial, necessitating strong fixation to restore spinal stability. The transverse connector (TC) serves as a posterior spinal instrumentation that connects the left and right sides of the pedicle screw-rod system. Several studies have highlighted the potential of a TC in enhancing the stability of the fixed segments. However, contradictory results have suggested that a TC not only fails to improve the stability of the fixed segments but also might promote stress associated with internal fixation. To date, there is a lack of previous research investigating the biomechanical effects of a TC on TES. This study aimed to investigate the biomechanical effects of a TC on internal fixation during TES of the lumbar (L) spine.

METHODS

A single-segment (L3 segment) TES was simulated using a comprehensive L spine finite element model. Five models were constructed based on the various positions of the TC, namely the intact model (L1-sacrum), the TES model without a TC, the TES model with a TC at L1-2, the TES model with a TC at L2-4, and the TES model with a TC at L4-5. Mechanical analysis of these distinct models was conducted using the Abaqus software to assess the variations in the biomechanics of the pedicle screw-rod system, titanium cage, and adjacent endplates.

RESULTS

The stability of the surgical segments was found to be satisfactory across all models. Compared with the complete model, the internal fixation device exhibited the greatest constraint on overextension (95.2-95.6%), while showing the least limitation on left/right rotation (53.62-55.64%). The application of the TC had minimal effect on the stability of the fixed segments, resulting in a maximum reduction in segment mobility of 0.11° and a variation range of 3.29%. Regardless of the use of a TC, no significant changes in stress were observed for the titanium cage. In the model without the TC, the maximum von Mises stress (VMS) for the pedicle screw-rod system reached 136.9 MPa during anterior flexion. Upon the addition of a TC, the maximum VMS of the pedicle screw-rod system increased to varying degrees. The highest recorded VMS was 459.3 MPa, indicating a stress increase of 335.5%. Following the TC implantation, the stress on the adjacent endplate exhibited a partial reduction, with the maximum stress reduced by 27.6%.

CONCLUSION

The use of a TC in TES does not improve the stability of the fixed segments and instead might result in increased stress concentration within the internal fixation devices. Based on these findings, the routine utilisation of TC in TES is deemed unnecessary.

Postoperative clinical outcomes in patients undergoing MIS-TLIF versus LLIF for adjacent segment disease

PURPOSE

Few studies examine the clinical outcomes in patients undergoing minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) versus lateral lumbar interbody fusion (LLIF) for adjacent segment disease (ASD). We aim to compare the postoperative clinical trajectory through patient-reported outcome measures (PROMs) and minimum clinically important difference (MCID) in patients undergoing MIS-TLIF versus LLIF for ASD.

METHODS

Patients were stratified into two cohorts based on surgical technique for ASD: MIS-TLIF versus LLIF. PROMs of 12-Item Short Form Physical Component Score (SF-12 PCS), visual analog scale (VAS) back, VAS leg, and Oswestry Disability Index (ODI) were collected at preoperative and postoperative 6-week/12-week/6-month/1-year time points. MCID attainment was calculated through comparison to established thresholds. Cohorts were compared through nonparametric inferential statistics.

RESULTS

Fifty-four patients were identified, with 22 patients undergoing MIS-TLIF after propensity score matching. Patients undergoing MIS-TLIF for ASD demonstrated significant postoperative improvement up to 1-year VAS back, up to 1-year VAS leg, and 6-month through 1-year ODI (p ≤ 0.035, all). Patients undergoing LLIF demonstrated significant postoperative improvement in 6-month SF-12 PCS, 6-month through 1-year VAS back, 12-week through 6-month VAS leg, and 6-month to 1-year ODI (p ≤ 0.035, all). No significant differences were calculated between surgical techniques for PROMs or MCID achievement rates.

CONCLUSION

Patients undergoing either MIS-TLIF or LLIF for adjacent segment disease demonstrated significant postoperative improvement in pain and disability outcomes. Additionally, patients undergoing LLIF reported significant improvement in physical function. Both MIS-TLIF and LLIF are effective for the treatment of adjacent segment disease.

Quantitatively biomechanical response analysis of posterior musculature reconstruction in cervical single-door laminoplasty

BACKGROUND AND OBJECTIVE

The current trend of laminoplasty is developing toward the goal of muscle preservation and minimum tissue damage. Given this, muscle-preserving techniques in cervical single-door laminoplasty have been modified with protecting the spinous processes at the sites of C2 and/or C7 muscle attachment and reconstruct the posterior musculature in recent years. To date, no study has reported the effect of preserving the posterior musculature during the reconstruction. The purpose of this study is to quantitatively evaluate the biomechanical effect of multiple modified single-door laminoplasty procedures for restoring stability and reducing response level on the cervical spine.

METHODS

Different cervical laminoplasty models were established for evaluating kinematics and response simulations based on a detailed finite element (FE) head-neck active model (HNAM), including ① C3 - C7 laminoplasty (LP_C37), ② C3 - C6 laminoplasty with C7 spinous process preservation (LP_C36), ③ C3 laminectomy hybrid decompression with C4 - C6 laminoplasty (LT_C3 + LP_C46) and ④ C3 - C7 laminoplasty with unilateral musculature preservation (LP_C37 + UMP). The laminoplasty model was validated by the global range of motion (ROM) and percentage changes relative to the intact state. The C2 - T1 ROM, axial muscle tensile force, and stress/strain levels of functional spinal units were compared among the different laminoplasty groups. The obtained effects were further analysed by comparison with a review of clinical data on cervical laminoplasty scenarios.

RESULTS

Analysis of the locations of concentration of muscle load showed that the C2 muscle attachment sustained more tensile loading than the C7 muscle attachment, primarily in flexion-extension (FE) and in lateral bending (LB) and axial rotation (AR), respectively. Simulated results further quantified that LP_C36 primarily produced 10% decreases in LB and AR modes relative to LP_C37. Compared with LP_C36, LT_C3 + LP_C46 resulted in approximately 30% decreases in FE motion; LP C37 + UMP also showed a similar trend. Additionally, when compared to LP_C37, LT_C3 + LP_C46 and LP C37 + UMP reduced the peak stress level at the intervertebral disc by at most 2-fold as well as the peak strain level of the facet joint capsule by 2-3-fold. All these findings were well correlated with the result of clinical studies comparing modified laminoplasty and classic laminoplasty.

CONCLUSIONS

Modified muscle-preserving laminoplasty is superior to classic laminoplasty due to the biomechanical effect of the posterior musculature reconstruction, with a retained postoperative ROM and loading response levels of the functional spinal units. More motion-sparing is beneficial for increasing cervical stability, which probably accelerates the recovery of postoperative neck movement and reduces the risk of the complication for eventual kyphosis and axial pain. Surgeons are encouraged to make every effort to preserve the attachment of the C2 whenever feasible in laminoplasty.

A biomechanical comparison of posterior fixation approaches in lumbar fusion using computed tomography based lumbosacral spine modelling

Extreme lateral interbody fusion (XLIF) may be performed with a standalone interbody cage, or with the addition of unilateral or bilateral pedicle screws; however, decisions regarding supplemental fixation are predominantly based on clinical indicators. This study examines the impact of posterior supplemental fixation on facet micromotions, cage loads and load-patterns at adjacent levels in a L4-L5 XLIF at early and late fusion stages. CT data from an asymptomatic subject were segmented into anatomical regions and digitally stitched into a surface mesh of the lumbosacral spine (L1-S1). The interbody cage and posterior instrumentation (unilateral and bilateral) were inserted at L4-L5. The volumetric mesh was imported into finite element software for pre-processing, running nonlinear static solves and post-processing. Loads and micromotions at the index-level facets reduced commensurately with the extent of posterior fixation accompanying the XLIF, while load-pattern changes observed at adjacent facets may be anatomically dependent. In flexion at partial fusion, compressive stress on the cage reduced by 54% and 72% in unilateral and bilateral models respectively; in extension the reductions were 58% and 75% compared to standalone XLIF. A similar pattern was observed at full fusion. Unilateral fixation provided similar stability compared to bilateral, however there was a reduction in cage stress-risers with the bilateral instrumentation. No changes were found at adjacent discs. Posterior supplemental fixation alters biomechanics at the index and adjacent levels in a manner that warrants consideration alongside clinical information. Unilateral instrumentation is a more efficient option where the stability requirements and subsidence risk are not excessive.

Biomechanical evaluation of different sizes of 3D printed cage in lumbar interbody fusion-a finite element analysis

OBJECTIVE

To study the biomechanical characteristics of various tissue structures of different sizes of 3D printed Cage in lumbar interbody fusion.

METHODS

A finite element model of normal spine was reconstructed and verified. Pedicle screws and Cage of different sizes were implanted in the L4/5 segment to simulate lumbar interbody fusion. The range of motion of the fixed and cephalic adjacent segment, the stress of the screw-rod system, the stress at the interface between cage and L5 endplate, and intervertebral disc pressure of the adjacent segment were calculated and analyzed.

RESULTS

The range of motion and intervertebral disc pressure of the adjacent segment of each postoperative model were larger than those of the intact model, but there was not much difference between them. The stress of cage-endplate interface was also larger than that of the intact model. However, the difference is that the stress of the endplate and the screw-rod system has a tendency to decrease with the increase of the axial area of cage.

CONCLUSIONS

Cage with larger axial area in lumbar interbody fusion can reduce the stress of internal fixation system and endplate, but will not increase the range of motion and intervertebral disc pressure of adjacent segment. It has a certain effect in preventing the cage subsidence, internal fixation system failure and screw rod fracture.

Biomechanical comparative analysis of conventional pedicle screws and cortical bone trajectory fixation in the lumbar spine: An in vitro and finite element study

Numerous screw fixation systems have evolved in clinical practice as a result of advances in screw insertion technology. Currently, pedicle screw (PS) fixation technology is recognized as the gold standard of posterior lumbar fusion, but it can also have some negative complications, such as screw loosening, pullout, and breakage. To address these concerns, cortical bone trajectory (CBT) has been proposed and gradually developed. However, it is still unclear whether cortical bone trajectory can achieve similar mechanical stability to pedicle screw and whether the combination of pedicle screw + cortical bone trajectory fixation can provide a suitable mechanical environment in the intervertebral space. The present study aimed to investigate the biomechanical responses of the lumbar spine with pedicle screw and cortical bone trajectory fixation. Accordingly, finite element analysis (FEA) and in vitro specimen biomechanical experiment (IVE) were performed to analyze the stiffness, range of motion (ROM), and stress distribution of the lumbar spine with various combinations of pedicle screw and cortical bone trajectory screws under single-segment and dual-segment fixation. The results show that dual-segment fixation and hybrid screw placement can provide greater stiffness, which is beneficial for maintaining the biomechanical stability of the spine. Meanwhile, each segment's range of motion is reduced after fusion, and the loss of adjacent segments' range of motion is more obvious with longer fusion segments, thereby leading to adjacent-segment disease (ASD). Long-segment internal fixation can equalize total spinal stresses. Additionally, cortical bone trajectory screws perform better in terms of the rotation resistance of fusion segments, while pedicle screw screws perform better in terms of flexion-extension resistance, as well as lateral bending. Moreover, the maximum screw stress of L4 cortical bone trajectory/L5 pedicle screw is the highest, followed by L45 cortical bone trajectory. This biomechanical analysis can accordingly provide inspiration for the choice of intervertebral fusion strategy.

Recent advancement in finite element analysis of spinal interbody cages: A review

Finite element analysis (FEA) is a widely used tool in a variety of industries and research endeavors. With its application to spine biomechanics, FEA has contributed to a better understanding of the spine, its components, and its behavior in physiological and pathological conditions, as well as assisting in the design and application of spinal instrumentation, particularly spinal interbody cages (ICs). IC is a highly effective instrumentation for achieving spinal fusion that has been used to treat a variety of spinal disorders, including degenerative disc disease, trauma, tumor reconstruction, and scoliosis. The application of FEA lets new designs be thoroughly "tested" before a cage is even manufactured, allowing bio-mechanical responses and spinal fusion processes that cannot easily be experimented upon in vivo to be examined and "diagnosis" to be performed, which is an important addition to clinical and in vitro experimental studies. This paper reviews the recent progress of FEA in spinal ICs over the last six years. It demonstrates how modeling can aid in evaluating the biomechanical response of cage materials, cage design, and fixation devices, understanding bone formation mechanisms, comparing the benefits of various fusion techniques, and investigating the impact of pathological structures. It also summarizes the various limitations brought about by modeling simplification and looks forward to the significant advancement of spine FEA research as computing efficiency and software capabilities increase. In conclusion, in such a fast-paced field, the FEA is critical for spinal IC studies. It helps in quantitatively and visually demonstrating the cage characteristics after implanting, lowering surgeons' learning costs for new cage products, and probably assisting them in determining the best IC for patients.

Biomechanical comparison of spinal column shortening - a finite element study

BACKGROUND

At present, research on spinal shortening is mainly focused on the safe distance of spinal shortening and the mechanism of spinal cord injury, but there is no research on the biomechanical characteristics of different shortening distances. The purpose of this study was to study the biomechanical characteristics of spine and internal fixation instruments at different shortening distances by the finite element (FE) method.

METHODS

An FE model of lumbar L1-S was established and referred to the previous in vitro experiments to verify the rationality of the model by verifying the Intradiscal pressure (IDP) and the range of motion (ROM) of the motion segment. Five element models of spinal shortening were designed under the safe distance of spinal shortening, and the entire L3 vertebra and both the upper and lower intervertebral discs were resected. Model A was not shortened, while models B-E were shortened by 10%, 20%, 30% and 50% of the vertebral body, respectively. Constraining the ROM of the sacrum in all directions, a 7.5 N ·m moment and 280 N follower load were applied on the L1 vertebra to simulate the motion of the lumbar vertebrae in three planes. The ROM of the operated segments, the Von Mises stress (VMS) of the screw-rod system, the VMS of the upper endplate at the interface between the titanium cage and the L4 vertebral body, and the ROM and the IDP of the adjacent segment (L5/S) were recorded and analysed.

RESULTS

All surgical models showed good stability at the operated segments (L1-5), with the greatest constraint in posterior extension (99.3-99.7%), followed by left-right bending (97.9-98.7%), and the least constraint in left-right rotation (84.9-86.3%) compared with the intact model. The VMS of the screw-rod system and the ROM and IDP of the distal adjacent segments of models A-E showed an increasing trend, in which the VMS of the screw-rod system of model E was the highest under flexion (172.5 MPa). The VMS of the endplate at the interface between the cage and L4 upper endplate of models A-E decreased gradually, and these trend were the most obvious in flexion, which were 3.03, 2.95, 2.83, 2.78, and 2.61 times that of the intact model, respectively.

CONCLUSION

When performing total vertebrae resection and correcting the spinal deformity, if the corrected spine has met our needs, the distance of spinal shortening should be minimized to prevent spinal cord injury, fracture of internal fixations and adjacent segment disease (ASD).

Application of dual-trajectory screws in revision surgery for lumbar adjacent segment disease: a finite element study

BACKGROUND

Advancements in medicine and the popularity of lumbar fusion surgery have made lumbar adjacent segment disease (ASDz) increasingly common, but there is no mature plan for guiding its surgical treatment. Therefore, in this study, four different finite element (FE) ASDz models were designed and their biomechanical characteristics were analysed to provide a theoretical basis for clinical workers to choose the most appropriate revision scheme for ASDz.

METHODS

According to whether internal fixation was retained, different FE models were created to simulate ASDz revision surgery, and flexion, extension, axial rotation and lateral bending were simulated by loading. The biomechanical characteristics of the adjacent segments of the intervertebral disc and the internal fixation system and the range of motion (ROM) of the lumbar vertebrae were analysed.

RESULTS

The difference in the ROM of the fixed segment between FE models that did or did not retain the original internal fixation was less than 0.1°, and the difference was not significant. However, the stress of the screw-rod system when the original internal fixation was retained and prolonged fixation was performed with dual-trajectory screws was less than that when the original internal fixation was removed and prolonged fixation was performed with a long bar. Especially in axial rotation, the difference between models A and B is the largest, and the difference in peak stress reached 30 MPa. However, for the ASDz revision surgery segment, the endplate stress between the two models was the lowest, and the intradiscal pressure (IDP) of the adjacent segment was not significantly different between different models.

CONCLUSION

Although ASDz revision surgery by retaining the original internal fixation and prolonging fixation with dual-trajectory screws led to an increase in stress in the fusion segment endplate, it provides stability similar to ASDz revision surgery by removing the original internal fixation and prolonging fixation with a long bar and does not lead to a significant change in the IDP of the adjacent segment while avoiding a greater risk of rod fracture.

Biomechanical study of rod stress in lumbopelvic fixation with lateral interbody fusion: an in vitro experimental study using synthetic bone models

OBJECTIVE

Despite improvements in surgical techniques and instruments, high rates of rod fracture following a long spinal fusion in the treatment of adult spinal deformity (ASD) remain a concern. Thus, an improved understanding of rod fracture may be valuable for better surgical planning. The authors aimed to investigate mechanical stress on posterior rods in lumbopelvic fixation for the treatment of ASD.

METHODS

Synthetic lumbopelvic bone models were instrumented with intervertebral cages, pedicle screws, S2-alar-iliac screws, and rods. The construct was then placed in a testing device, and compressive loads were applied. Subsequently, the strain on the rods was measured using strain gauges on the dorsal aspect of each rod.

RESULTS

When the models were instrumented using titanium alloy rods at 30° lumbar lordosis and with lateral interbody fusion cages, posterior rod strain was highest at the lowest segment (L5-S1) and significantly higher than that at the upper segment (L2-3) (p = 0.002). Changing the rod contour from 30° to 50° caused a 36% increase in strain at L5-S1 (p = 0.009). Changing the rod material from titanium alloy to cobalt-chromium caused a 140% increase in strain at L2-3 (p = 0.009) and a 28% decrease in strain at L5-S1 (p = 0.016). The rod strain at L5-S1 using a flat bender for contouring was 23% less than that obtained using a French bender (p = 0.016).

CONCLUSIONS

In lumbopelvic fixation in which currently available surgical techniques for ASD are used, the posterior rod strain was highest at the lumbosacral junction, and depended on the contour and material of the rods.

Novel force-displacement control passive finite element models of the spine to simulate intact and pathological conditions; comparisons with traditional passive and detailed musculoskeletal models

Passive finite element (FE) models of the spine are commonly used to simulate intact and various pre- and postoperative pathological conditions. Being devoid of muscles, these traditional models are driven by simplistic loading scenarios, e.g., a constant moment and compressive follower load (FL) that do not properly mimic the complex in vivo loading condition under muscle exertions. We aim to develop novel passive FE models that are driven by more realistic yet simple loading scenarios, i.e., in vivo vertebral rotations and pathological-condition dependent FLs (estimated based on detailed musculoskeletal finite element (MS-FE) models). In these novel force-displacement control FE models, unlike the traditional passive FE models, FLs vary not only at different spine segments (T12-S1) but between intact, pre- and postoperative conditions. Intact, preoperative degenerated, and postoperative fused conditions at the L4-L5 segment for five static in vivo activities in upright and flexed postures were simulated by the traditional passive FE, novel force-displacement control FE, and gold-standard detailed MS-FE spine models. Our findings indicate that, when compared to the MS-FE models, the force-displacement control passive FE models could accurately predict the magnitude of disc compression force, intradiscal pressure, annulus maximal von Mises stress, and vector sum of all ligament forces at adjacent segments (L3-L4 and L5-S1) but failed to predict disc shear and facet joint forces. In this regard, the force-displacement control passive FE models were much more accurate than the traditional passive FE models. Clinical recommendations made based on traditional passive FE models should, therefore, be interpreted with caution.

Effect of the In Situ Screw Implantation Region and Angle on the Stability of Lateral Lumbar Interbody Fusion: A Finite Element Study

Objective

To investigate the effect of the in situ screw implantation region and angle on the stability of lateral lumbar interbody fusion (LLIF) from a biomechanical perspective.

Methods

A validated L2‐4 finite element (FE) model was modified for simulation. The L3‐4 fused segment undergoing LLIF surgery was modeled. The area between the superior and inferior edges and the anterior and posterior edges of the vertebral body (VB) is divided into four zones by three parallel lines in coronal and horizontal planes. In situ screw implantation methods with different angles based on the three parallel lines in coronal plane were applied in Models A, B, and C (A: parallel to inferior line; B: from inferior line to midline; C: from inferior line to superior line). In addition, four implantation methods with different regions based on the three parallel lines in horizontal plane were simulated as types 1–2, 1–3, 2–2, and 2–3 (1–2: from anterior line to midline; 1–3: from anterior line to posterior line; 2–2: parallel to midline; 2–3: from midline to posterior line). L3‐4 ROM, interbody cage stress, screw‐bone interface stress, and L4 superior endplate stress were tracked and calculated for comparisons among these models.

Results

The L3‐4 ROM of Models A, B, and C decreased with the extent ranging from 47.9% (flexion‐extension) to 62.4% (lateral bending) with no significant differences under any loading condition. Types 2–2 and 2–3 had 45% restriction, while types 1–2 and 1–3 had 51% restriction in ROM under flexion‐extension conditions. Under lateral bending, types 2–2 and 2–3 had 70.6% restriction, while types 1–2 and 1–3 had 61.2% restriction in ROM. Under axial rotation, types 2–2 and 2–3 had 65.2% restriction, while types 1–2 and 1–3 had 59.3% restriction in ROM. The stress of the cage in types 2–2 and 2–3 was approximately 20% lower than that in types 1–2 and 1–3 under all loading conditions in all models. The peak stresses at the screw‐bone interface in types 2–2 and 2–3 were much lower (approximately 35%) than those in types 1–2 and 1–3 under lateral bending, while no significant differences were observed under flexion‐extension and axial rotation. The peak stress on the L4 superior endplate was approximately 30 MPa and was not significantly different in all models under any loading condition.

Conclusions

Different regions of entry‐exit screws induced multiple screw trajectories and influenced the stability and mechanical responses. However, different implantation angles did not. Considering the difficulty of implantation, the ipsilateral‐contralateral trajectory in the lateral middle region of the VB can be optimal for in situ screw implantation in LLIF surgery.

Single-position prone transpsoas fusion for the treatment of lumbar adjacent segment disease: early experience of twenty-four cases across three tertiary medical centers

PURPOSE

Prone transpsoas fusion (PTP) is a minimally invasive technique that maximizes the benefit of lateral access interbody surgery and the prone positioning for surgically significant adjacent segment disease. The authors describe the feasibility, reproducibility and radiographic efficacy of PTP when performed for cases of lumbar ASD.

METHODS

Adult patients undergoing PTP for treatment of lumbar ASD at three institutions were retrospectively enrolled. Demographic information was recorded, as was operative data such as adjacent segment levels, operative time, blood loss, laterality of approach, open versus percutaneous pedicle screw instrumentation and need for primary decompression. Radiographic measurements including segmental and global lumbar lordosis, pelvic incidence, pelvic tilt, sacral slope and sagittal vertical axis were recorded both pre- and immediately post-operatively.

RESULTS

Twenty-four patients met criteria for inclusion. Average age was 60.4 ± 10.4 years and average BMI was 31.6 ± 5.0 kg/m². Total operative time was 204.7 ± 83.3 min with blood loss of 187.9 ± 211 mL. Twenty-one patients had pedicle screw instrumentation exchanged percutaneously and 3 patients had open pedicle screw exchange. Two patients suffered pulmonary embolism that was treated medically with no long-term sequelae. One patient had transient lumbar radicular pain and all patients were discharged home with an average length of stay of 3.0 days (range 1-6). Radiographically, global lumbar lordosis improved by an average of 10.3 ± 9.0 degrees, segmental lordosis by 10.1 ± 13.3 degrees and sagittal vertical axis by 3.2 ± 3.2 cm.

CONCLUSION

Single-position prone transpsoas lumbar interbody fusion is a clinically reproducible minimally invasive technique that can effectively treat lumbar adjacent segment disease.

When to Consider Stand-Alone Lateral Lumbar Interbody Fusion: Is There a Role for a Comeback With New Implants?

OBJECTIVE

To perform a comprehensive review of the literature about the role of stand-alone lateral lumbar interbody fusion (LLIF).

METHODS

A MEDLINE review was conducted including studies about stand-alone LLIF for any condition. The opinions of the authors were also considered. Studies that included biomechanical, cadaveric, or clinical aspects of stand-alone cages were revised to obtain data about the pros, cons, and limitations of the technique. Comparative studies with 360° (lateral + posterior) fusions were also analyzed.

RESULTS

A total of 36 studies were identified. After reviewing the abstracts, 18 full articles of interest for the objective of this review were analyzed. Recommendations based on the literature were made. Although most of the recommendations in the literature were about augmentation with pedicle screws, there may be a role for stand-alone LLIF in some particular cases. Specific technical aspects should be considered to reduce the failure rate.

CONCLUSION

Although there might be some specific indications for stand-alone LLIF, it should be considered an exception rather than the rule.

LEVEL OF EVIDENCE: 4

Early Outcomes of 3D-printed Porous Titanium versus Polyetheretherketone (PEEK) Cage Implantation for Standalone Lateral Lumbar Interbody Fusion in the Treatment of Symptomatic Adjacent Segment Degeneration

OBJECTIVE

This study compared outcomes of 3D-printed porous titanium (Ti) versus polyetheretherketone (PEEK) cage implantation for standalone lateral lumbar interbody fusion (SA-LLIF) in the treatment of symptomatic adjacent segment degeneration (ASD).

METHODS

44 patients (59 levels) underwent SA-LLIF with Ti or PEEK cages between 10/2016 and 07/2020. The primary outcome was cage subsidence according to Marchi et al. Secondary outcomes included revision/recommendations for revision surgery, back/leg pain severity, changes in disc/foraminal height and global/segmental lumbar lordosis.

RESULTS

44 patients (21 female) were included with a mean age at surgery of 61.8±11.5 years, average radiological follow-up of 12.5±8.2 and clinical follow-up of 11.0±7.1 months. The overall subsidence rate was significantly less in the Ti versus PEEK group (20% vs. 58.8%; p=0.004). Revision was recommended to none of the patients in the Ti and 3 in the PEEK group (p=0.239). Furthermore, patients in the Ti group showed significantly better improvement in back pain NRS score (p=0.001). Disc height (p<0.001) and foraminal height restoration (p=0.011) were statistically significant in the Ti group, whereas only disc height restoration was significant in the PEEK group (p=0.003).

CONCLUSION

In patients undergoing SA-LLIF for ASD treatment, 3D-printed Ti cages had significantly lower overall subsidence rate compared to PEEK cages. Furthermore, Ti cages resulted in fewer recommendations for revision surgery. Whether greater pain reduction in the Ti group is associated with earlier or higher fusion rates needs to be further elucidated.

Development of a decision-making pathway for utilizing standalone lateral lumbar interbody fusion

PURPOSE

To develop a decision-making pathway for primary SA-LLIF. Furthermore, we analyzed the agreement of this pathway and compared outcomes of patients undergoing either SA-LLIF or 360-LLIF.

METHOD

A decision-making pathway for SA-LLIF was created based on the results of interviews/surveys of senior spine surgeons with over 10 years of experience. Internal validity was retrospectively evaluated using consecutive patients undergoing either SA-LLIF or 360-LLIF between 01/2018 and 07/2020 with 3D-printed Titanium cages. An outcome assessment looking primarily at revision surgery and secondary at cage subsidence, changes in disk and foraminal height, global and segmental lumbar lordosis, duration of surgery, estimated blood loss, and length of stay was carried out.

RESULTS

78 patients with 124 treated levels (37 SA-LLIF, 41 360-LLIF) were retrospectively analyzed. The pathway showed a direct agreement (SA-LLIF) of 100.0% and an indirect agreement (360-LLIF) of 95.1%. Clinical follow-up averaged 13.5 ± 6.5 months including 4 revision surgeries in the 360-LLIF group and none in the SA-LLIF group (p = 0.117). Radiographic follow-up averaged 9.5 ± 4.3 months, with no statistically significant difference in cage subsidence rate between the groups (p = 0.440). Compared to preoperative images, patients in both groups showed statistically significant changes in disk height (p < 0.001), foraminal height (p < 0.001), as well as restoration of segmental lordosis (p < 0.001 and p = 0.018). The SA-LLIF group showed shorter duration of surgery, less estimated blood loss and shorter LOS (p < 0.001).

CONCLUSION

The proposed decision-making pathway provides a guide to adequately select patients for SA-LLIF. Further studies are needed to assess the external applicability and validity.

LEVEL OF EVIDENCE III

Diagnostic: individual cross-sectional studies with consistently applied reference standard and blinding.

Influence of posterior pedicle screw fixation at L4-L5 level on biomechanics of the lumbar spine with and without fusion: a finite element method

Background

Posterior pedicle screw (PS) fixation, a common treatment method for widespread low-back pain problems, has many uncertain aspects including stress concentration levels, effects on adjacent segments, and relationships with physiological motions. A better understanding of how posterior PS fixation affects the biomechanics of the lumbar spine is needed. For this purpose, a finite element (FE) model of a lumbar spine with posterior PS fixation at the L4–L5 segment level was developed by partially removing facet joints (FJs) to imitate an actual surgical procedure. This FE study aimed to investigate the influence of the posterior PS fixation system on the biomechanics of the lumbar spine before and after fusion by determining which physiological motions have the most increase in posterior instrumentation (PI) stresses and FJ loading.

Results

It was determined that posterior PS fixation increased FJ loading by approximately 35% and 23% at the L3–L4 adjacent level with extension and lateral bending motion, respectively. This increase in FJ loading at the adjacent level could point to the possibility that adjacent segment disease has developed or progressed after posterior lumbar interbody fusion. Furthermore, analyses of peak von Mises stresses on PI showed that the maximum PI stresses of 272.1 MPa and 263.7 MPa occurred in lateral bending and flexion motion before fusion, respectively.

Conclusions

The effects of a posterior PS fixation system on the biomechanics of the lumbar spine before and after fusion were investigated for all physiological motions. This model could be used as a fundamental tool for further studies, providing a better understanding of the effects of posterior PS fixation by clearing up uncertain aspects.

Selection of the fusion and fixation range in the intervertebral surgery to correct thoracolumbar and lumbar tuberculosis: a retrospective clinical study

Background

To compare the diseased verses the non-diseased intervertebral surgery used in the treatment of thoracolumbar and lumbar spinal tuberculosis and to explore the best choice of fusion of fixation range.

Methods

Two hundred twenty-one patients with thoracolumbar and lumbar tuberculosis were categorized into two groups. One hundred eighteen patients underwent the diseased intervertebral surgery (lesion vertebral pedicle fixation, Group A) and 103 patients underwent the non-diseased intervertebral surgery (1 or 2 vertebral fixation above and below the affected vertebra, group B). Spinal tuberculosis diagnosis was confirmed in both groups of patients before lesion removal, bone graft fusion, and internal fixation. Clinical data and efficacy of the two surgical methods were then evaluated.

Results

The mean follow-up duration for both procedures was 65 months (50–68 months range). There were no significant differences in laboratory examinations, VAS scores, and the Cobb angle correction rate and the angle loss. However, significant differences existed in the operation time, blood loss, serosanguineous drainage volume, and blood transfusion requirement between the two groups. The diseased intervertebral surgery group performed significantly better than the non-diseased intervertebral surgery group in all of these areas. In both cases, the bone graft fused completely with the normal bone by the last follow-up, occuring at 50–86 months post surgery.

Conclusion

The diseased intervertebral surgery is a safe and feasible option for the treatment of thoracolumbar and lumbar tuberculosis. It effectively restores the physiological curvature of the spine and reduces the degeneration of adjacent vertebral bodies in the spinal column.