The risk of disc prolapses with complex loading in different degrees of disc degeneration - a finite element analysis

Understanding the etiopathogenesis of lumbar intervertebral disc herniation: From clinical evidence to basic scientific research

Lumbar intervertebral disc herniation, as a leading cause of low back pain, productivity loss, and disability, is a common musculoskeletal disorder that results in significant socioeconomic burdens. Despite extensive clinical and basic scientific research efforts, herniation etiopathogenesis, particularly its initiation and progression, is not well understood. Understanding herniation etiopathogenesis is essential for developing effective preventive measures and therapeutic interventions. Thus, this review seeks to provide a thorough overview of the advances in herniation-oriented research, with a discussion on ongoing challenges and potential future directions for clinical, translational, and basic scientific investigations to facilitate innovative interdisciplinary research aimed at understanding herniation etiopathogenesis. Specifically, risk factors for herniation are identified and summarized, including familial predisposition, obesity, diabetes mellitus, smoking tobacco, selected cardiovascular diseases, disc degeneration, and occupational risks. Basic scientific experimental and computational research that aims to understand the link between excessive mechanical load, catabolic tissue remodeling due to inflammation or insufficient nutrient supply, and herniation, are also reviewed. Potential future directions to address the current challenges in herniation-oriented research are explored by combining known progressive development in existing research techniques with ongoing technological advances. More research on the relationship between occupational risk factors and herniation, as well as the relationship between degeneration and herniation, is needed to develop preventive measures for working-age individuals. Notably, researchers should explore using or modifying existing degeneration animal models to study herniation etiopathogenesis, as such models may allow for a better understanding of how to prevent mild-to-moderately degenerated discs from herniating.

Future of low back pain: unravelling IVD components and MSCs' potential

Low back pain (LBP) mainly emerges from intervertebral disc (IVD) degeneration. However, the failing mechanism of IVD ́s components, like the annulus fibrosus (AF) and nucleus pulposus (NP), leading to IVD degeneration/herniation is still poorly understood. Moreover, the specific role of cellular populations and molecular pathways involved in the inflammatory process associated with IVD herniation remains to be highlighted. The limited knowledge of inflammation associated with the initial steps of herniation and the lack of suitable models to mimic human IVD ́s complexity are some of the reasons for that. It has become essential to enhance the knowledge of cellular and molecular key players for AF and NP cells during inflammatory-driven degeneration. Due to unique properties of immunomodulation and pluripotency, mesenchymal stem cells (MSCs) have attained diverse recognition in this field of bone and cartilage regeneration. MSCs therapy has been particularly valuable in facilitating repair of damaged tissues and may benefit in mitigating inflammation' degenerative events. Therefore, this review article conducts comprehensive research to further understand the intertwine between the mechanisms of action of IVD components and therapeutic potential of MSCs, exploring their characteristics, how to optimize their use and establish them safely in distinct settings for LPB treatment.

Bibliometric and Visualization Analysis of Biomechanical Research on Lumbar Intervertebral Disc

Background

Biomechanical research on the lumbar intervertebral disc (IVD) provides valuable information for the diagnosis, treatment, and prevention of related diseases, and has received increasing attention. Using bibliometric methods and visualization techniques, this study investigates for the first time the research status and development trends in this field, with the aim of providing guidance and support for subsequent research.

Methods

The Science Citation Index Expanded (SCI-Expanded) within the Web of Science Core Collection (WoSCC) database was used as the data source to select literature published from 2003 to 2022 related to biomechanical research on lumbar IVD. VOSviewer 1.6.19 and CiteSpace 6.2.R2 visualization software, as well as the online analysis platform of literature metrology, were utilized to generate scientific knowledge maps for visual display and data analysis.

Results

The United States is the most productive country in this field, with the Ulm University making the largest contribution. Wilke HJ is both the most prolific author and one of the highly cited authors, while Adams MA is the most cited author. Spine, J Biomech, Eur Spine J, Spine J, and Clin Biomech are not only the journals with the highest number of publications, but also highly cited journals. The main research topics in this field include constructing and validating three-dimensional (3D) finite element model (FEM) of lumbar spine, measuring intradiscal pressure, exploring the biomechanical effects and related risk factors of lumbar disc degeneration, studying the mechanical responses to different torque load combinations, and classifying lumbar disc degeneration based on magnetic resonance images (MRI), which are also the hot research themes in recent years.

Conclusion

This study systematically reviews the knowledge system and development trends in the field of biomechanics of lumbar IVD, providing valuable references for further research.

Lumbar Disc Degeneration Affects the Risk of Rod Fracture Following PSO; A Finite Element Study

STUDY DESIGN

Finite element (FE) study.

OBJECTIVE

Pedicle subtraction osteotomy (PSO) is a surgical method to correct sagittal plane deformities. In this study, we aimed to investigate the biomechanical effects of lumbar disc degeneration on the instrumentation following PSO and assess the effects of using interbody spacers adjacent to the PSO level in a long instrumented spinal construct.

METHODS

A spinopelvic model (T10-pelvis) with PSO at the L3 level was used to generate 3 different simplified grades of degenerated lumbar discs (mild (Pfirrmann grade III), moderate (Pfirrmann grade IV), and severe (Pfirrmann grade V)). Instrumentation included eighteen pedicle screws and bilateral primary rods. To investigate the effect of interbody spacers, the model with normal disc height was modified to accommodate 2 interbody spacers adjacent to the PSO level through a lateral approach. For the models, the rods' stress distribution, PSO site force values, and the spine range of motion (ROM) were recorded.

RESULTS

The mildly, moderately, and severely degenerated models indicated approximately 10%, 26%, and 40% decrease in flexion/extension motion, respectively. Supplementing the instrumented spinopelvic PSO model using interbody spacers reduced the ROM by 22%, 21%, 4%, and 11% in flexion, extension, lateral bending, and axial rotation, respectively. The FE results illustrated lower von Mises stress on the rods and higher forces at the PSO site at higher degeneration grades and while using the interbody spacers.

CONCLUSIONS

Larger and less degenerated discs adjacent to the PSO site may warrant consideration for interbody cage instrumentation to decrease the risk of rod fracture and PSO site non-union.

Biomechanical evaluation of different semi-rigid junctional fixation techniques using finite element analysis

BACKGROUND

Proximal junctional failure is a common complication attributed to the rigidity of long pedicle screw fixation constructs used for surgical correction of adult spinal deformity. Semi-rigid junctional fixation achieves a gradual transition in range of motion at the ends of spinal instrumentation, which could lead to reduced junctional stresses, and ultimately reduce the incidence of proximal junctional failure. This study investigates the biomechanical effect of different semi-rigid junctional fixation techniques in a T8-L3 finite element spine segment model.

METHODS

First, degeneration of the intervertebral disc was successfully implemented by altering the height. Second, transverse process hooks, one- and two-level clamped tapes, and one- and two-level knotted tapes instrumented proximally to three-level pedicle screw fixation were validated against ex vivo range of motion data of a previous study. Finally, the posterior ligament complex forces and nucleus pulposus stresses were quantified.

FINDINGS

Simulated range of motions demonstrated the fidelity of the general model and modelling of semi-rigid junctional fixation techniques. All semi-rigid junctional fixation techniques reduced the posterior ligament complex forces at the junctional zone compared to pedicle screw fixation. Transverse process hooks and knotted tapes reduced nucleus pulposus stresses, whereas clamped tapes increased nucleus pulposus stresses at the junctional zone.

INTERPRETATION

The relationship between the range of motion transition and the reductions in posterior ligament complex and nucleus pulposus stresses was complex and dependent on the fixation techniques. Clinical trials are required to compare the effectiveness of semi-rigid junctional fixation techniques in terms of reducing proximal junctional failure incidence rates.

Biomechanical evaluation of a novel intervertebral disc repair technique for large box-shaped ruptures

Objective: The purpose of this study was to analyze the feasibility of repairing a ruptured intervertebral disc using a patch secured to the inner surface of the annulus fibrosus (AF). Different material properties and geometries for the patch were evaluated. Methods: Using finite element analysis, this study created a large box-shaped rupture in the posterior-lateral region of the AF and then repaired it with a circular and square inner patch. The elastic modulus of the patches ranged from 1 to 50 MPa to determine the effect on the nucleus pulposus (NP) pressure, vertical displacement, disc bulge, AF stress, segmental range of motion (ROM), patch stress, and suture stress. The results were compared against the intact spine to determine the most suitable shape and properties for the repair patch. Results: The intervertebral height and ROM of the repaired lumbar spine was similar to the intact spine and was independent of the patch material properties and geometry. The patches with a modulus of 2-3 MPa resulted in an NP pressure and AF stresses closest to the healthy disc, and produced minimal contact pressure on the cleft surfaces and minimal stress on the suture and patch of all models. Circular patches caused lower NP pressure, AF stress and patch stress than the square patch, but also caused greater stress on the suture. Conclusion: A circular patch with an elastic modulus of 2-3 MPa secured to the inner region of the ruptured annulus fibrosus was able to immediately close the rupture and maintain an NP pressure and AF stress similar to the intact intervertebral disc. This patch had the lowest risk of complications and produced the greatest restorative effect of all patches simulated in this study.

Biomechanical responses of the human lumbar spine to vertical whole-body vibration in normal and osteoporotic conditions

BACKGROUND

The prevalence of osteoporosis is continuing to escalate with an aging population. However, it remains unclear how biomechanical behavior of the lumbar spine is affected by osteoporosis under whole-body vibration, which is considered a significant risk factor for degenerative spinal disease and is typically present when driving a car. Accordingly, the objective of this study was to compare the spine biomechanical responses to vertical whole-body vibration between normal and osteoporotic conditions.

METHODS

A three-dimensional finite-element model of the normal human lumbar spine-pelvis segment was developed using computed tomographic scans and was validated against experimental data. Osteoporotic condition was simulated by modifying material properties of bone tissues in the normal model. Transient dynamic analyses were conducted on the normal and osteoporotic models to compute deformation and stress in all lumbar motion segments.

FINDINGS

When osteoporosis occurred, vibration amplitudes of the vertebral axial displacement, disc bulge, and disc stress were increased by 32.1-45.4%, 25.7-47.1% and 23.0-42.7%, respectively. In addition, it was found that for both the normal and osteoporotic models, the response values (disc bugle and disc stress) were higher in L4-L5 and L5-S1 intervertebral discs than in other discs.

INTERPRETATION

Osteoporosis deteriorates the effect of whole-body vibration on lumbar spine, and the lower lumbar segments might have a higher likelihood of disc degeneration under whole-body vibration.

Comparison of the biomechanical effects of lumbar disc degeneration on normal patients and osteoporotic patients: A finite element analysis

BACKGROUND

Some older patients who suffered from both conditions (disc degeneration and osteoporosis) have higher surgical risks and longer postoperative recovery times. Understanding the relation between disc degeneration and osteoporosis is fundamental to know the mechanisms of orthopedic disorders and improve clinical treatment. However, there is a lack of finite element (FE) studies to predict the combined effects of disc degeneration and osteoporosis. So the aim of the present study is to explore the differences of biomechanical effects of lumbar disc degeneration on normal patients and osteoporotic patients.

METHODS

A normal lumbar spine finite element model (FEM) was developed based on the geometric information of a healthy male subject (age 35 years; height 178 cm; weight 65 kg). This normal lumbar spine FEM was modified to build three lumbar spine degeneration models simulating mild, moderate and severe grades of disc degeneration at the L4-L5 segment. Then the degenerative lumbar spine models for osteoporotic patients were constructed on the basis of the above-mentioned degeneration models. Firstly, the normal model (flexion: 8 Nm; extension: 6 Nm; lateral bending: 6 Nm; torsion: 4 Nm) and degenerative models (10 Nm) were calibrated under pure moment load, respectively. Secondly, under a 400 N follower load, the 7.5 Nm moments of different directions were applied on all models to simulate different motion postures. Finally, under the above loading conditions, we calculated and analyzed the range of motion (ROM), Mises stress in cortical (MSC1), Mises stress in endplate (MSE), Mises stress in cancellous (MSC2), and Mises stress in post (MSP).

RESULTS

Compared with disc degeneration patients without osteoporosis, the ROM, MSC1, and MSE of osteoporosis patients with various disc degeneration decreased in all postures, while the MSC2 and MSP increased. With increase in the degree of disc degeneration, the reduction proportions of ROM and MSE in osteoporotic patients gradually increased, while the reduction percentages in MSC1 of osteoporotic patients gradually decreased. The increase percentages of MSC2 in osteoporotic patients gradually increased. Given the progressive changes of disc degeneration, the changes in MSP in osteoporosis patients were uneven.

CONCLUSION

In summary, the effect of disc degeneration on flexibility in the two kinds of patients (osteoporosis and non-osteoporosis patients) was nearly same. By comparing the remaining biomechanical parameters (MSC1, MSE, MSC2, and MSP), we found that degenerated intervertebral discs caused changes in loading patterns of osteoporosis patients. Disc degeneration reduced the Mises stress in the cortical and endplate, which increased the Mises stress in the cancellous and post. That is to say, in order to cope with the changes in bone stresses caused by disc degeneration and osteoporosis, clinicians should be more careful in choosing the surgical option for osteoporotic patients with disc degeneration.

Effect of different fixation methods on biomechanical property of cervical vertebral body replacement and fusion

BACKGROUND

The main purpose of this study was to examine the effect of different fixation methods (anterior fixation, self-stabilizing fixation and anterior-posterior fixation) on biomechanical property of vertebral body replacement and fusion.

METHODS

Three finite element models of cervical vertebral body replacement and fusion were established. The implanted models included artificial vertebral body and fixation system, and the loads imposed on the models included 75 N compression load and 1 Nm moment load.

FINDINGS

For anterior-posterior fixation, the cervical load was mainly transmitted by the posterior pedicle screw and rod (more than 50%), and the stress shielding problem was the most significant than the self-stabilizing and anterior fixation. Self-stabilizing fixation was more helpful to the fusion of implant and vertebrae, but the higher risk of vertebral body collapse was worthy of attention if the cervical spine with osteoporosis. The stress of bone was mainly concentrated around the screw hole. The maximum stress (20.03 MPa) was lower than the yield stress of cortical bone and the possibility of fracture around the fixation device of cervical spine was low. The anterior fixation could meet the requirement of vertebral body replacement and fusion, and the addition of posterior pedicle screws and rods might obtain better treatment in cases of severe spine injury or osteoporosis.

INTERPRETATION

The findings of this study may provide guidance on clinical treatments for choosing more appropriate fixation methods for different patients.

Biomechanical analysis of lumbar nonfusion dynamic stabilization using a pedicle screw-based dynamic stabilizer or an interspinous process spacer

This study aimed to investigate and compare the effects of two widely used nonfusion posterior dynamic stabilization (NPDS) devices, pedicle screw-based dynamic stabilizer (PSDS) and interspinous process spacer (IPS), on biomechanics of the implanted lumbar spine under static and vibration loadings. The finite element model of healthy human lumbosacral segment was modified to incorporate NPDS device insertion at L4-L5 segment. Bioflex and DIAM were used as PSDS-based and IPS-based NPDS devices, respectively. As a comparison, lumbar interbody fusion with rigid stabilization was also simulated at L4-L5. For static loading, segmental range of motion (ROM) of the models under moments of four physiological motions was computed using hybrid testing protocol. For vibration loading, resonant modes and dynamic stress of the models under vertical excitation were extracted through random response analysis. The results showed that compared with the rigid fusion model, ROM of the nonfusion models was higher at L4-L5 level but lower at adjacent levels (L1- L2, L2-L3, L3-L4, L5-S1). Compared with the Bioflex model, the DIAM model produced higher ROM at L4-L5 level but lower ROM at adjacent levels, especially under lateral bending and axial rotation; resonant frequency of the DIAM model was slightly lower; dynamic response of nucleus stress at L4-L5 level was slightly higher for the DIAM model, and the dynamic stress at adjacent levels was no obvious difference between the nonfusion models. This study reveals biomechanical differences between the Bioflex and DIAM systems, which may provide references for selecting surgical approaches in clinical practice.

Biomechanical effect of age-related structural changes on cervical intervertebral disc: A finite element study

Previous literature has investigated the biomechanical response of healthy and degenerative discs, but the biomechanical response of suboptimal healthy intervertebral discs received less attention. The purpose was to compare the biomechanical responses and risk of herniation of young healthy, suboptimal healthy, and degenerative intervertebral discs. A cervical spine model was established and validated using the finite element method. Suboptimal healthy, mildly, moderately, and severely degenerative disc models were developed. Disc height deformation, range of motion, intradiscal pressure, and von Mises stress in annulus fibrosus were analyzed by applying a moment of 4 Nm in flexion, extension, lateral bending, and axial rotation with 100 N compressive loads. Disc height deformation in young healthy, suboptimal healthy, mildly, moderately, and severely degenerative discs was 40%, 37%, 21%, 12%, and 8%, respectively. The decreasing order of the range of motion was young healthy spine > suboptimal healthy spine > mildly degenerative spine > moderately degenerative spine > severely degenerative spine. The mean stress of annulus ground substance in the suboptimal healthy disc was higher than in the young healthy disc. The mean stress of inter-lamellar matrix and annulus ground substance in moderately and severely degenerative discs was higher than in other discs. Age-related structural changes and degenerative changes increased the stiffness and reduced the elastic deformation of intervertebral discs. Decreased range of motion due to the effects of aging or degeneration on the intervertebral disc, may cause compensation of adjacent segments and lead to progressive degeneration of multiple segments. The effect of aging on the intervertebral disc increased the risk of annulus fibrosus damage from the biomechanical point of view. Moderately and severely degenerative discs may have a higher risk of herniation due to the higher risk of damage and layers separation of annulus fibrosus caused by increased stress in the annulus ground substance and inter-lamellar matrix.

Biomechanical effect of C5-C6 intervertebral disc degeneration on the human lower cervical spine (C3-C7): a finite element study

The biomechanical effects of intervertebral discs and facet joints degeneration on the cervical spine are essential to understanding the mechanisms of spinal disorders to improve pathological and clinical treatment. In this study, the biomechanical effects of a progressively degenerated C5-C6 segment on the human lower cervical spine are determined by a detailed simulation of intervertebral disc degeneration. A detailed asymmetric three-dimension intact finite element model was developed using computed tomography scan data of the human lower cervical spine (C3-C7). The intact finite element model was then modified at the C5-C6 segment to build three degenerated models, such as mild, moderate, and severe degeneration. The physiological compressive load 73.6 N, and moment 1 Nm were applied at the superior endplate of the vertebra C3, and the inferior endplate of the C7 vertebra was a constraint for all degrees of freedom. Range of motion, maximum von Mises stress in the annulus, intradiscal pressure, and facet joint force of the degenerated models were computed. With progressive degeneration in the C5-C6 segment, the range of motion of degenerated and normal segments decreases in all postures. Intradiscal pressure of the degenerated segment decreases but increases in normal segments of degenerated segment C5-C6, and facet joint forces increase at both degenerated and normal segments. This study emphasizes that the degenerated disc alters the degenerated and normal segments' motion and loading patterns. The abnormal increase in facet joint force in the degenerated models threatened to accelerate the degeneration in the normal segments.

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.

Adjacent segments biomechanics following lumbar fusion surgery: a musculoskeletal finite element model study

PURPOSE

This study exploits a novel musculoskeletal finite element (MS-FE) spine model to evaluate the post-fusion (L4-L5) alterations in adjacent segment kinetics.

METHODS

Unlike the existing MS models with idealized representation of spinal joints, this model predicts stress/strain distributions in all passive tissues while organically coupled to a MS model. This generic (in terms of musculature and material properties) model uses population-based in vivo vertebral sagittal rotations, gravity loads, and an optimization algorithm to calculate muscle forces. Simulations represent individuals with an intact L4-L5, a preoperative severely degenerated L4-L5 (by reducing the disc height by ~ 60% and removing the nucleus incompressibility), and a postoperative fused L4-L5 segment with either a fixed or an altered lumbopelvic rhythm with respect to the intact condition (based on clinical observations). Changes in spine kinematics and back muscle cross-sectional areas (due to intraoperative injuries) are considered based on in vivo data while simulating three activities in upright/flexed postures.

RESULTS

Postoperative changes in some adjacent segment kinetics were found considerable (i.e., larger than 25%) that depended on the postoperative lumbopelvic kinematics and preoperative L4-L5 disc condition. Postoperative alterations in adjacent disc shear, facet/ligament forces, and annulus stresses/strains were greater (> 25%) than those found in intradiscal pressure and compression (< 25%). Kinetics of the lower (L5-S1) and upper (L3-L4) adjacent segments were altered to different degrees.

CONCLUSION

Alterations in segmental rotations mainly affected adjacent disc shear forces, facet/ligament forces, and annulus/collagen fibers stresses/strains. An altered lumbopelvic rhythm (increased pelvis rotation) tends to mitigate some of these surgically induced changes.

Computational model predicts risk of spinal screw loosening in patients

PURPOSE

Pedicle screw loosening is a frequent complication in lumbar spine fixation, most commonly among patients with poor bone quality. Determining patients at high risk for insufficient implant stability would allow clinicians to adapt the treatment accordingly. The aim of this study was to develop a computational model for quantitative and reliable assessment of the risk of screw loosening.

METHODS

A cohort of patient vertebrae with diagnosed screw loosening was juxtaposed to a control group with stable fusion. Imaging data from the two cohorts were used to generate patient-specific biomechanical models of lumbar instrumented vertebral bodies. Single-level finite element models loading the screw in axial or caudo-cranial direction were generated. Further, multi-level models incorporating individualized joint loading were created.

RESULTS

The simulation results indicate that there is no association between screw pull-out strength and the manifestation of implant loosening (p = 0.8). For patient models incorporating multiple instrumented vertebrae, CT-values and stress in the bone were significantly different between loose screws and non-loose screws (p = 0.017 and p = 0.029, for CT-values and stress, respectively). However, very high distinction (p = 0.001) and predictability (R²_Pseudo = 0.358, AUC = 0.85) were achieved when considering the relationship between local bone strength and the predicted stress (loading factor). Screws surrounded by bone with a loading factor higher than 25% were likely to be loose, while the chances of screw loosening were close to 0 with a loading factor below 15%.

CONCLUSION

The use of a biomechanics-based score for risk assessment of implant fixation failure might represent a paradigm shift in addressing screw loosening after spondylodesis surgery.

Optimization of a lumbar interspinous fixation device for the lumbar spine with degenerative disc disease

Interspinous spacer devices used in interspinous fixation surgery remove soft tissues in the lumbar spine, such as ligaments and muscles and may cause degenerative diseases in adjacent segments its stiffness is higher than that of the lumbar spine. Therefore, this study aimed to structurally and kinematically optimize a lumbar interspinous fixation device (LIFD) using a full lumbar finite element model that allows for minimally invasive surgery, after which the normal behavior of the lumbar spine is not affected. The proposed healthy and degenerative lumbar spine models reflect the physiological characteristics of the lumbar spine in the human body. The optimum number of spring turns and spring wire diameter in the LIFD were selected as 3 mm and 2 turns, respectively—from a dynamic range of motion (ROM) perspective rather than a structural maximum stress perspective—by applying a 7.5 N∙m extension moment and 500 N follower load to the LIFD-inserted lumbar spine model. As the spring wire diameter in the LIFD increased, the maximum stress generated in the LIFD increased, and the ROM decreased. Further, as the number of spring turns decreased, both the maximum stress and ROM of the LIFD increased. When the optimized LIFD was inserted into a degenerative lumbar spine model with a degenerative disc, the facet joint force of the L3-L4 lumbar segment was reduced by 56%–98% in extension, lateral bending, and axial rotation. These results suggest that the optimized device can strengthen the stability of the lumbar spine that has undergone interspinous fixation surgery and reduce the risk of degenerative diseases at the adjacent lumbar segments.

Torque- and Muscle-Driven Flexion Induce Disparate Risks of In Vitro Herniation: A Multiscale and Multiphasic Structure-Based Finite Element Study

The intervertebral disc is a complex structure that experiences multiaxial stresses regularly. Disc failure through herniation is a common cause of lower back pain, which causes reduced mobility and debilitating pain, resulting in heavy socioeconomic burdens. Unfortunately, herniation etiology is not well understood, partially due to challenges in replicating herniation in vitro. Previous studies suggest that flexion elevated risks of herniation. Thus, the objective of this study was to use a multiscale and multiphasic finite element model to evaluate the risk of failure under torque- or muscle-driven flexion. Models were developed to represent torque-driven flexion with the instantaneous center of rotation (ICR) located on the disc, and the more physiologically representative muscle-driven flexion with the ICR located anterior of the disc. Model predictions highlighted disparate disc mechanics regarding bulk deformation, stress-bearing mechanisms, and intradiscal stress-strain distributions. Specifically, failure was predicted to initiate at the bone-disc boundary under torque-driven flexion, which may explain why endplate junction failure, instead of herniation, has been the more common failure mode observed in vitro. By contrast, failure was predicted to initiate in the posterolateral annulus fibrosus under muscle-driven flexion, resulting in consistent herniation. Our findings also suggested that muscle-driven flexion combined with axial compression could be sufficient for provoking herniation in vitro and in silico. In conclusion, this study provided a computational framework for designing in vitro testing protocols that can advance the assessment of disc failure behavior and the performance of engineered disc implants.

A novel approach for tetrahedral-element-based finite element simulations of anisotropic hyperelastic intervertebral disc behavior

Intervertebral discs are microstructurally complex spinal tissues that add greatly to the flexibility and mechanical strength of the human spine. Attempting to provide an adjustable basis for capturing a wide range of mechanical characteristics and to better address known challenges of numerical modeling of the disc, we present a robust finite-element-based model formulation for spinal segments in a hyperelastic framework using tetrahedral elements. We evaluate the model stability and accuracy using numerical simulations, with particular attention to the degenerated intervertebral discs and their likely skewed and narrowed geometry. To this end, 1) annulus fibrosus is modeled as a fiber-reinforced Mooney-Rivlin type solid for numerical analysis. 2) An adaptive state-variable dependent explicit time step is proposed and utilized here as a computationally efficient alternative to theoretical estimates. 3) Tetrahedral-element-based FE models for spinal segments under various loading conditions are evaluated for their use in robust numerical simulations. For flexion, extension, lateral bending, and axial rotation load cases, numerical simulations reveal that a suitable framework based on tetrahedral elements can provide greater stability and flexibility concerning geometrical meshing over commonly employed hexahedral-element-based ones for representation and study of spinal segments in various stages of degeneration.

Construction of biomimetic artificial intervertebral disc scaffold via 3D printing and electrospinning

Intervertebral disc (IVD) degeneration is a clinically disease that seriously endangers people's health. Tissue engineering provides a promising method to repair and regenerate the damaged IVD physiological function. Successfully tissue-engineered IVD scaffold should mimic the native IVD histological and macro structures. Here, 3D printing and electrospinning were combined to construct an artificial IVD composite scaffold. Poly lactide (PLA) was used to print the IVD frame structure, the oriented porous poly(l-lactide)/octa-armed polyhedral oligomeric silsesquioxanes (PLLA/POSS-(PLLA)₈) fiber bundles simulated the annulus fibrosus (AF), and the gellan gum/poly (ethylene glycol) diacrylate (GG/PEGDA) double network hydrogel loaded with bone marrow mesenchymal stem cells (BMSCs) simulated the nucleus pulposus (NP) structure. Morphological and mechanical tests showed that the structure and mechanical properties of the IVD scaffold were similar to that of the natural IVD. The compression modulus of the scaffold is about 10 MPa, which is comparable to natural IVD and provides good mechanical support for tissue repair and regeneration. At the same time, the porosity and mechanical properties of the scaffold can be regulated through the 3D model design. In the AF structure, the fiber bundles are oriented concentrically with each subsequent layer oriented 60° to the spinal column, and can withstand the tension generated during the deformation of the NP. In the NP structure, BMSCs were evenly distributed in the hydrogel and could maintain high cell viability. Animal experiment results demonstrated that the biomimetic artificial IVD scaffold could maintain the disc space and produce the new extracellular matrix. This engineered biomimetic IVD scaffold is a promising biomaterial for individualized IVD repair and regeneration.

Osteophytes on the zygapophyseal (facet) joints of the cervical spine (C3-C7): A skeletal study

Previous studies have reported that osteophytes in the cervical vertebrae may cause immobility, neck stiffness, osteoarthritis, headaches, nerve entrapment syndromes, and compression of the vertebral artery. Our objective was to explore the osteophytes' expression on zygapophyseal joints C3-C7. This is a cross-sectional observational skeletal study. The study sample comprised 273 human skeletons of both sexes, aged 20-93, housed at the Natural History Museum, OH, USA. A grading system assessed the presence and severity of osteophytosis on the zygapophyseal joints. The chi-square test (SPSS 25.0) examined the association between osteophytes and demographic factors. The level of significance (α) was set at .05. The highest prevalence of osteophytes was found on C5 vertebra, the lowest on C7. On vertebrae C3, C4, C6, the rate of moderate and severe osteophytes found on the superior and inferior facets were comparable. Moderate and severe degrees of osteophytes were observed more frequently on the superior facets, whereas, on vertebra C7, osteophytes were found on the inferior facet joints. Osteophytes' prevalence was significantly higher in the elderly compared to the younger population. Osteophytes in the C3-C7 zygapophyseal joints are age-dependent. No significant sex and ethnic differences were observed. Vertebra C5 was most prone to develop osteophytes, most probably due to its location in the cervical lordotic peak, C5 in the superior ZF; C7 in the inferior ZF are significant (p = .05). The zygapophyseal joints of C7 were least frequent overall, yet, the C7 inferior facets had significantly more moderate-severe osteophytes compared to other cervical vertebrae.

A microstructure-based model for a full lamellar-interlamellar displacement and shear strain mapping inside human intervertebral disc core

The determinant role of the annulus fibrosus interlamellar zones in the intervertebral disc transversal and volumetric responses and hence on their corresponding three-dimensional conducts have been only revealed and appreciated recently. Their consideration in disc modeling strategies has been proven to be essential for the reproduction of correct local strain and displacement fields inside the disc especially in the unconstrained directions of the disc. In addition, these zones are known to be the starting areas of annulus fibrosus circumferential tears and disc delamination failure mode, which is often judged as one of the most dangerous disc failure modes that could evolve with time leading to disc hernia. For this latter reason, the main goal of the current contribution is to incorporate physically for the first time, the interlamellar zones, at the scale of a complete human lumbar intervertebral disc, in order to allow a correct local vision and replication of the different lamellar-interlamellar interactions and an identification of the interlamellar critical zones. By means of a fully tridimensional chemo-viscoelastic constitutive model, which we implemented into a finite element code, the physical, mechanical and chemical contribution of the interlamellar zones is added to the disc. The chemical-induced volumetric response is accounted by the model for both the interlamellar zones and the lamellae using experimentally-based fluid kinetics. Computational simulations are performed and critically discussed upon different simple and complex physiological movements. The disc core and the interlamellar zones are numerically accessed, allowing the observation of the displacement and shear strain fields that are compared to direct MRI experiments from the literature. Important conclusions about the correct lamellar-interlamellar-nucleus interactions are provided thanks to the developed model. The critical interlamellar spots with the highest delamination potentials are defined, analyzed and related to the local kinetics and microstructure.

The effect of cervical intervertebral disc degeneration on the motion path of instantaneous center of rotation at degenerated and adjacent segments: A finite element analysis

BACKGROUND

The motion path of instantaneous center of rotation (ICR) is a crucial kinematic parameter to dynamically characterize cervical spine intervertebral patterns of motion; however, few studies have evaluated the effect of cervical disc degeneration (CDD) on ICR motion path. The purpose of this study was to investigate the effect of CDD on the ICR motion path of degenerated and adjacent segments.

METHOD

A validated nonlinear three-dimensional finite element (FE) model of a healthy adult cervical spine was used. Progressive degeneration was simulated with six FE models by modifying intervertebral disc height and material properties, anterior osteophyte size, and degree of endplate sclerosis at the C5-C6 level. All models were subjected to a pure moment of 1 Nm and a compressive follower load of 73.6 N to simulate physical motion. ICR motion paths were compared among different models.

RESULTS

The normal FE model results were consistent with those of previous studies. In degenerative models, average ICR motion paths shifted significantly anterior at the degenerated segment (β = 0.27 mm; 95% CI: 0.22, 0.32) and posterior at the proximal adjacent segment (β = -0.09 mm; 95% CI: -0.15, -0.02) than those of the normal model.

CONCLUSION

CDD significantly affected ICR motion paths at the degenerated and proximal adjacent segments. The changes at adjacent segments may be a result of compensatory mechanisms to maintain the balance of the cervical spine. Surgical treatment planning should take into account the restoration of ICR motion path to normal. These findings could provide a basis for prosthesis design and clinical practice.

Biomechanical Investigation Between Rigid and Semirigid Posterolateral Fixation During Daily Activities: Geometrically Parametric Poroelastic Finite Element Analyses

While spinal fusion using rigid rods remains the gold standard treatment modality for various lumbar degenerative conditions, its adverse effects, including accelerated adjacent segment disease (ASD), are well known. In order to better understand the performance of semirigid constructs using polyetheretherketone (PEEK) in fixation surgeries, the objective of this study was to analyze the biomechanical performance of PEEK versus Ti rods using a geometrically patient-specific poroelastic finite element (FE) analyses. Ten subject-specific preoperative models were developed, and the validity of the models was evaluated with previous studies. Furthermore, FE models of those lumbar spines were regenerated based on postoperation images for posterolateral fixation at the L4–L5 level. Biomechanical responses for instrumented and adjacent intervertebral discs (IVDs) were analyzed and compared subjected to static and cyclic loading. The preoperative model results were well comparable with previous FE studies. The PEEK construct demonstrated a slightly increased range of motion (ROM) at the instrumented level, but decreased ROM at adjacent levels, as compared with the Ti. However, no significant changes were detected during axial rotation. During cyclic loading, disc height loss, fluid loss, axial stress, and collagen fiber strain in the adjacent IVDs were higher for the Ti construct when compared with the intact and PEEK models. Increased ROM, experienced stress in AF, and fiber strain at adjacent levels were observed for the Ti rod group compared with the intact and PEEK rod group, which can indicate the risk of ASD for rigid fixation. Similar to the aforementioned pattern, disc height loss and fluid loss were significantly higher at adjacent levels in the Ti rod group after cycling loading which alter the fluid–solid interaction of the adjacent IVDs. This phenomenon debilitates the damping quality, which results in disc disability in absorbing stress. Such finding may suggest the advantage of using a semirigid fixation system to decrease the chance of ASD.

Biomechanical evaluation of a novel decompression surgery: Transforaminal full-endoscopic lateral recess decompression (TE-LRD)

Background

Transforaminal full endoscopic lateral recess decompression (TE-LRD) can decompress lateral recess stenosis transforaminally under the endoscopy procedure. However, the biomechanical effects of the TE-LRD compared to the conventional decompression techniques are not reported. The purpose of this study is to compare the biomechanical effects of TE-LRD with conventional decompression techniques using finite element method.

Methods

Three finite element models of lumbar functional spinal unit (FSU) of the L4-L5 levels were created: 1) normal disc 2) moderate grade disc degeneration 3) severe grade disc degeneration. For each of these three models, the following decompression techniques were simulated, 1) 50% TE-LRD, 2) 100% TE-LRD, 3) Unilateral laminectomy, 4) Bilateral laminectomy. The lower endplate of the fifth lumbar vertebra was fixed and 10Nm of moment in flexion/extension, left/right bending and axial rotation was applied to the upper endplate of the fourth lumbar vertebra, under a follower load of 400N. The range of motion, intervertebral disc stress, and facet joint stress were compared.

Results

50% TE-LRD was found to be the most stable decompression technique in all intervertebral disc models. Though the increase in the range of motion of 100% TE-LRD was higher than other decompression techniques in the normal disc model, it was not significantly different from 50% TE-LRD or unilateral laminectomy techniques in the degenerated disc models. The increase in the intervertebral disc stress was lowest for the 50% TE-LRD surgery in all intervertebral disc models. The increase in the facet stresses for 50% TE-LRD was lower than in the conventional decompression techniques for all intervertebral disc models.

Conclusions

50% TE-LRD was the decompression surgical technique with the least effect on spinal instability. 100% TE-LRD showed to be effective for cases with degenerative discs. 50% TE-LRD may decrease the risk of postoperative intervertebral disc and facet joint degeneration.

Does oblique lumbar interbody fusion promote adjacent degeneration in degenerative disc disease: A finite element analysis

BACKGROUND

The number of oblique lumbar interbody fusion (OLIF) procedures has continued to rise over recent years. Adjacent segment degeneration (ASD) is a common complication following vertebral body fusion. Although the precise mechanism remains uncertain, ASD has gradually become more common in OLIF. Therefore, the present study analyzed the association between disc degeneration and OLIF to explore whether adjacent degeneration was promoted by OLIF in degenerative disc disease.

METHODS

A three-dimensional nonlinear finite element (FE) model of the L3-S1 lumbar spine was developed and validated. Three lumbar spine degeneration models with different degrees of degeneration (mild, moderate and severe) and a model of OLIF surgery were constructed at the L4-L5 level. When subjected to a follower compressive load (500 N), hybrid moment loading was applied to all models of the lumbar spine and the range of motion (ROM), intradiscal pressure (IDP), facet joint force (FJF), average mises stress in the annulus (AMSA), average tresca stress in the annulus (ATSA) and average endplate stress (AES) were measured.

RESULTS

Compared with the healthy lumbar spine model, the ROM, IDP, FJF, AMSA, ATSA and AES of the segments adjacent to the degenerated segment increased in each posture as the degree of disc degeneration increased. In different directions of motion, the ROM, IDP, FJF, AMSA, ATSA and AES in the OLIF model in the L3-L4 and L5-S1 segments were higher than those of the healthy model and each degenerated model. Compared with the healthy model, the largest relative increase in biomechanical parameters above (ROM, IDP, FJF, AMSA, ATSA or AES) was observed in the L3-L4 segment in the OLIF model, of 77.13%, 32.63%, 237.19%, 45.36%, 110.92% and 80.28%, respectively. In the L5-S1 segment the corresponding values were 68.88%, 36.12%, 147.24%, 46.00%, 45.88% and 51.29%, respectively.

CONCLUSIONS

Both degenerated discs and OLIF surgery modified the pattern of motion and load distribution of adjacent segments (L3-L4 and L5-S1 segments). The increases in the biomechanical parameters of segments adjacent to the surgical segment in the OLIF model were more apparent than those of the degenerated models. In summary, OLIF risked accelerating the degeneration of segments adjacent to those of a surgical segment.

Numerical implementation of an osmo-poro-visco-hyperelastic finite element solver: application to the intervertebral disc

This work deals with the finite element (FE) implementation of a biphasic poroelastic formulation specifically developed to address the intricate behaviour of the Intervertebral Disc (IVD) and other highly hydrated soft tissues. This formulation is implemented in custom FE solver V-Biomech, being the validation performed with a lumbar IVD model, which was compared against the analogous FE model of Williams et al. and the experiments of Tyrrell et al. Good agreement with these benchmarks was achieved, meaning that V-Biomech and its novel poroelastic formulation are a viable alternative for simulation of biphasic soft tissues.

Finite element investigation on the dynamic mechanical properties of low-frequency vibrations on human L2-L3 spinal motion segments with different degrees of degeneration

Exposure to low-frequency vibration is harmful to human lumbar health. However, the dynamic mechanical properties of lumbar spines with varying degrees of degeneration during time-dependent vibration remain incompletely understood. In this study, four poroelastic finite element models of human L2-L3 spinal motion segments, including the non-degeneration and the mild, moderate, and serious degeneration, were established. One-hour low-frequency vibrations with different frequencies were applied. Then, the dynamic mechanical properties of different degenerated lumbar models under the same vibration and the same lumbar model under vibrations at different frequencies were investigated. The results indicated and implied that the negative influences of 1-h vibration on the dynamic mechanical properties of the non-degenerated and mildly degenerated models were similar, but became obvious for the moderately and seriously degenerated models with time. Therefore, the damage caused by low-frequency vibration on the degenerated spinal motion segments was more serious compared with that on the healthy one. Meanwhile, the dynamic mechanical properties of the same lumbar model under vibrations at different frequencies expressed the negligible differences when the vibration frequency was not close to the lumbar natural frequency. Thus, the effects of the 1-h vibrations at different frequencies on one spinal motion segment were similar. Vibration frequency sensitivity analysis on the dynamic characteristics of human L2-L3 spinal motion segments with different degrees of degeneration.

Biomechanics of the effect of subaxial cervical spine degeneration on atlantoaxial complex in idiopathic retro-odontoid pseudotumor development

OBJECTIVES

Retro-odontoid pseudotumor (ROP), with no rheumatoid arthritis, atlantoaxial instability, or other primary diseases, is defined as idiopathic retro-odontoid pseudotumor (IROP). Cervical spine degeneration is associated with IROP development. This study aims to evaluate the effect of cervical spine degeneration on the atlantoaxial complex and find the possible biomechanical mechanism of IROP development.

METHODS

This study was performed using a three-dimensional (3D) finite element (FE) analysis. A degenerated FE model (FEM) and five operation FEMs (C1-C2 fusion, C0-C2 fusion, C0-C3 fusion, C0-C4 fusion, and C1 posterior arch resection) were established based on a normal 3D FEM of the cervical spine including C0-T1 with the main ligaments and muscles. The parameters, including the C1-C2 range of motions (ROMs) and odontoid-related ligaments' stresses in degenerated and operation FEMs, were obtained and compared with those in normal FEM.

RESULTS

Compared to normal FEM, degenerated FEM had reduced C3-C7 ROMs and increased C1-C2 ROMs and odontoid-related ligaments' stresses. After internal fixation, C1-C2 ROMs and most odontoid-related ligaments' stresses were greatly decreased, but with no significant differences among C0-C2, C0-C3, C0-C4, and C1-C2 fusion models. For the C1 posterior arch resection model, C1-C2 ROMs and most odontoid-related ligaments' stresses increased, compared with normal FEM.

CONCLUSIONS

Cervical spine degeneration plays an important part in IROP development in biomechanics. Atlantoaxial complex compensates for cervical spine degeneration, with increased C1-C2 ROMs and odontoid-related ligaments' stresses. Atlantoaxial fusion or short segmental occipitocervical fusion can effectively reduce the stress and should be considered in IROP treatment.

The effect of non-fusion dynamic stabilization on biomechanical responses of the implanted lumbar spine during whole-body vibration

BACKGROUND AND OBJECTIVE

Non-fusion dynamic stabilization surgery is increasingly popular for treating degenerative lumbar disc disease. However, changes in spine biomechanics after application of posterior dynamic fixation devices during whole-body vibration (WBV) remain unclear. The study aimed to examine the effects of non-fusion dynamic stabilization on biomechanical responses of the implanted lumbar spine to vertical WBV.

METHODS

By modifying L4-L5 segment of the healthy human L1-sacrum finite element model, single-level disc degeneration, dynamic fixation using the BioFlex system and anterior lumbar interbody fusion (ALIF) with rigid fixation were simulated, respectively. Dynamic responses of stress and strain in the spinal levels for the healthy, degenerated, BioFlex and ALIF models under an axial cyclic loading were investigated and compared.

RESULTS

The results showed that endplate stress at implant level was lower in the BioFlex model than in the degenerated and ALIF models, but stress of the connecting rod in the BioFlex system was greater than that in the rigid fixation system used in the ALIF. Compared with the healthy model, stress and strain responses in terms of disc bulge, annulus stress and nucleus pressure at adjacent levels were decreased in the degenerated, BioFlex and ALIF models, but no obvious difference was observed in these responses among the three models.

CONCLUSIONS

This study may be helpful to understand variations in vibration characteristics of the lumbar spine after application of non-fusion dynamic stabilization system.

Biomechanical Effect of L4 -L5 Intervertebral Disc Degeneration on the Lower Lumbar Spine: A Finite Element Study

OBJECTIVE

To ascertain the biomechanical effects of a degenerated L₄ -L₅ segment on the lower lumbar spine through a comprehensive simulation of disc degeneration.

METHODS

A three-dimensional nonlinear finite element model of a normal L₃ -S₁ lumbar spine was constructed and validated. This normal model was then modified such that three degenerated models with different degrees of degeneration (mild, moderate, or severe) at the L₄ -L₅ level were constructed. While experiencing a follower compressive load (500 N), hybrid moment loads were applied to all models to determine range of motion (ROM), intradiscal pressure (IDP), maximum von Mises stress in the annulus, maximum shear stress in the annulus, and facet joint force.

RESULTS

As the degree of disc degeneration increased, the ROM of the L₄ -L₅ degenerated segment declined dramatically in all postures (flexion: 5.79°-1.91°; extension: 5.53°-2.62°; right lateral bending: 4.47°-1.46°; left lateral bending: 4.86°-1.61°; right axial rotation: 2.69°-0.74°; left axial rotation: 2.69°-0.74°), while the ROM in adjacent segments increased (1.88°-8.19°). The largest percent decrease in motion of the L₄ -L₅ segment due to disc degeneration was in right axial rotation (75%), left axial rotation (69%), flexion (67%), right lateral bending (67%), left lateral bending right (67%), and extension (53%). The change in the trend of the IDP was the same as that of the ROM. Specifically, the IDP decreased (flexion: 0.592-0.09 MPa; extension: 0.678-0.334 MPa; right lateral bending: 0.498-0.205 MPa; left lateral bending: 0.523-0.272 MPa; right axial rotation: 0.535-0.246 MPa; left axial rotation: 0.53-0.266 MPa) in the L₄ -L₅ segment, while the IDP in adjacent segments increased (0.511-0.789 MPa). The maximum von Mises stress and maximum shear stress of the annulus in whole lumbar spine segments increased (L₄ -L₅ segment: 0.413-2.626 MPa and 0.412-2.783 MPa, respectively; adjacent segment of L₄ -L₅ : 0.356-1.493 MPa and 0.359-1.718 MPa, respectively) as degeneration of the disc progressively increased. There was no apparent regularity in facet joint force in the degenerated segment as the degree of disc degeneration increased. Nevertheless, facet joint forces in adjacent healthy segments increased as the degree of disc degeneration increased (extension: 49.7-295.3 N; lateral bending: 3.5-171.2 N; axial rotation: 140.2-258.8 N).

CONCLUSION

Degenerated discs caused changes in the motion and loading pattern of the degenerated segments and adjacent normal segments. The abnormal load and motion in the degenerated models risked accelerating degeneration in the adjacent normal segments. In addition, accurate simulation of degenerated facet joints is essential for predicting changes in facet joint loads following disc degeneration.

Alterations in the Geometry, Fiber Orientation, and Mechanical Behavior of the Lumbar Intervertebral Disc by Nucleus Swelling

Soft tissues observed in clinical medical images are often prestrained in tension by internal pressure or tissue hydration. For a native disc, nucleus swelling occurs in equilibrium with osmotic pressure induced by the high concentration of proteoglycan in the nucleus. The objective of this computational study was to investigate the effects of nucleus swelling on disc geometry, fiber orientation, and mechanical behavior by comparing those of prestrained and zero-pressure (unswelled) discs. Thermoelastic analysis techniques were repurposed in order to determine the zero-pressure disc geometry which, when pressurized, matches the prestrained disc geometry observed in clinical images. The zero-pressure geometry was then used in simulations to approximately represent a degenerated disc, which loses the ability of nucleus swelling but has not undergone distinct soft tissue remodeling/disruption. Our simulation results demonstrated that the loss of nucleus swelling caused a slight change in the disc geometry and fiber orientation, but a distinct deterioration in the resistance to intervertebral rotations including sagittal bending, lateral bending, and axial torsion. Different from rotational loading, in compression (with a displacement of 0.45 mm applied), a much larger stiffness (3.02 KN/mm) and a greater intradiscal pressure (IDP) (0.61 MPa) were measured in the zero-pressure disc, compared to the prestrained disc (1.41 KN/mm and 0.52 MPa). This computational study could be useful to understand mechanisms of disc degeneration, and guide the future design of disc tissue engineering material and biomimic disc implants.

Intervertebral Disc Diseases PART 2: A Review of the Current Diagnostic and Treatment Strategies for Intervertebral Disc Disease

With an aging population, there is a proportional increase in the prevalence of intervertebral disc diseases. Intervertebral disc diseases are the leading cause of lower back pain and disability. With a high prevalence of asymptomatic intervertebral disc diseases, there is a need for accurate diagnosis, which is key to management. A thorough understanding of the pathophysiology and clinical manifestation aids in understanding the natural history of these conditions. Recent developments in radiological and biomarker investigations have potential to provide noninvasive alternatives to the gold standard, invasive discogram. There is a large volume of literature on the management of intervertebral disc diseases, which we categorized into five headings: (a) Relief of pain by conservative management, (b) restorative treatment by molecular therapy, (c) reconstructive treatment by percutaneous intervertebral disc techniques, (d) relieving compression and replacement surgery, and (e) rigid fusion surgery. This review article aims to provide an overview on various current diagnostic and treatment options and discuss the interplay between each arms of these scientific and treatment advancements, hence providing an outlook of their potential future developments and collaborations in the management of intervertebral disc diseases.

Mechanisms of Failure Following Simulated Repetitive Lifting: A Clinically Relevant Biomechanical Cadaveric Study

STUDY DESIGN

A biomechanical analysis correlating internal disc strains and tissue damage during simulated repetitive lifting.

OBJECTIVE

To understand the failure modes during simulated safe and unsafe repetitive lifting.

SUMMARY OF BACKGROUND DATA

Repetitive lifting has been shown to lead to lumbar disc herniation (LDH). In vitro studies have developed a qualitative understanding of the effect of repetitive loading on LDH. However, no studies have measured internal disc strains and subsequently correlated these with disc damage.

METHODS

Thirty human cadaver lumbar functional spinal units were subjected to an equivalent of 1 year of simulated repetitive lifting under safe and unsafe levels of compression, in combination with flexion (13-15°), and right axial rotation (2°) for 20,000 cycles or until failure. Safe or unsafe lifting were applied as a compressive load to mimic holding a 20 kg weight either close to, or at arm's length, from the body, respectively. Maximum shear strains (MSS) were measured, and disc damage scores were determined in nine regions from axial post-test magnetic resonance imaging (MRI) and macroscopic images.

RESULTS

Twenty percent of specimens in the safe lifting group failed before 20,000 cycles due to endplate failure, compared with 67% in the unsafe group. Over half of the specimens in the safe lifting group failed via either disc protrusion or LDH, compared with only 20% via protrusion in the unsafe group. Significant positive correlations were found between MRI and macroscopic damage scores in all regions (rs > 0.385, P < 0.049). A significant positive correlation was observed in the left lateral region for MSS versus macroscopic damage score (rs = 0.486, P < 0.037) and MSS versus failure mode (rs = 0.724, P = 0.018, only specimens with disc failure). Pfirrmann Grade 3 discs were strongly associated with subsequent LDH (P = 0.003).

CONCLUSION

Increased shear strains were observed in the contralateral side to the applied rotation as disc injury progressed from protrusion to LDH. Larger compressive loads applied to simulate unsafe lifting led to frequent early failure of the endplate, however, smaller compressive loads at similar flexion angles applied under safe lifting led to more loading cycles before failure, where the site of failure was more likely to be the disc. Our study demonstrated that unsafe lifting leads to greater risk of injury compared with safe lifting, and LDH and disc protrusion were more common in the posterior/posterolateral regions.

LEVEL OF EVIDENCE

N/A.

Intervertebral disc swelling maintains strain homeostasis throughout the annulus fibrosus: A finite element analysis of healthy and degenerated discs

Tissues in the intervertebral disc have a large capacity to absorb water, partially due to the high glycosaminoglycan (GAG) content, which decreases linearly from the nucleus pulposus (NP) in the center to the outer annulus. Our recent work showed that fiber network and GAG distribution contributes to development of residual stresses and strains that were compressive in the inner annulus to tensile in the outer annulus. GAG loss in the inner annulus, as observed with early to moderate degeneration, reduced swelling capacity and circumferential-direction stress by over 50%. However, our previous model was not capable of evaluating interactions between the NP and annulus fibrosus (AF) during swelling. In this study, we evaluated the effect of degeneration (GAG content or swelling capacity) on residual stress development throughout the disc. Simulations of moderate to severe degeneration showed a 40% decrease in NP swelling capacity, with a 25% decrease in AF and cartilaginous endplate swelling. Together, these changes in tissue swelling resulted in a decrease in NP pressure (healthy = 0.21 MPa; severe degeneration = 0.03 MPa) that was comparable to observations in human discs. There was a 60% decrease in circumferential-direction residual deformations with early degeneration. Radial-direction stretch switched from compressive to tensile with degeneration, which may increase the risk for tears or delamination. Degeneration had a significant impact on residual stress/stretch and fiber stretch in the posterior AF, which is important for understanding herniation risk. In conclusion, degenerative changes in disc geometry and intradiscal deformations was recreated by only altering NP and AF GAG composition. Since most computational models simulate degeneration by altering material stiffness, this work highlights the importance of directly simulating biochemical composition and distribution to study disc biomechanics with degeneration. STATEMENT OF SIGNIFICANCE: Tissues in the intervertebral disc have a large swelling capacity, due to its high glycosaminoglycan content. Our recent work demonstrated the importance of fiber network and glycosaminoglycan distribution residual stresses and strains development. In this study, we evaluated the effect of swelling on intradiscal deformations between the nucleus pulposus and annulus fibrosus. We also investigated the effect of degenerative glycosaminoglycan loss on swelling-based intradiscal deformations of the intact disc and its subcomponents. Decreases in nucleus glycosaminoglycan content resulted in morphological changes observed with degenerated discs and may help to explain mechanisms behind the increases in annular tears and mechanical dysfunction with degeneration.

Modular hemipelvic endoprosthesis with a sacral hook: a finite element study

Background

A novel hemipelvic endoprosthesis with a sacral hook was introduced previously, and its clinical outcome with midterm follow-up showed decreased prosthesis-related complications, especially decreased rate of aseptic loosening. The aim of present study was to evaluate the role of a sacral hook in prosthesis stability and the biomechanical properties of this hemipelvic endoprosthesis.

Methods

A three-dimensional model of the postoperative pelvis was developed using computed tomography (CT) images. A force of 500 N was applied, and the distribution of stress and displacement was evaluated. Comparisons were performed to explore the role of the sacral hook in prosthesis stability. Prosthesis improvement was simulated to reduce unexpected breakage of the pubic connection plate.

Results

In the reconstructed hemipelvis, stress distributions were concentrated on the superior area of the acetabulum, sacral connection component, and sacral hook. A maximum stress of 250 MPa was observed at the root of the sacral connection component. The sacral hook reduced the maximum stress and displacement by 14.1% and 32.5%, respectively, when the prosthesis was well fixed and by 10.0% and 42.1%, respectively, when aseptic loosening occurred. Increasing the thickness of the pubic connection plate from 2 to 3.5 mm reduced the maximum stress by 32.0% and 15.8%, respectively.

Conclusion

A hemipelvic endoprosthesis with a sacral hook fulfills the biomechanical demands of the hemipelvis and is safe under static conditions. The sacral hook is important for prosthesis stability. Increasing the thickness of the pubic connection plate can reduce the maximum stress and risk of fatigue breakage.

The influence of bilateral pedicle screw fixation on vibration response of the disc degenerated human lumbar spine: A finite element stress analysis

BACKGROUND

Very few studies have evaluated biomechanical characteristics of the disc degenerated human lumbar spine after bilateral pedicle screw fixation (BPSF) under whole body vibration (WBV) that is typically present in vehicles.

OBJECTIVE

To examine the influence of BPSF on stress responses of the disc degenerated human lumbar spine to WBV using finite element (FE) method.

METHODS

Two previously validated L1-S1 FE models with different grades of disc degeneration (mild and moderate) at L4-L5 were employed, and the two degenerated models were instrumented with bilateral pedicle screws and rods across the L4-L5 level, respectively. Transit dynamic analyses were performed on all these models under a 400 N compressive follower preload and a 40 N sinusoidal vertical vibration load. Intradiscal pressure (IDP) and von Mises stress (VMS) of the annulus ground substance in all disc levels of the degenerated models and the corresponding implanted models were recorded and compared.

RESULTS

BPSF decreased maximum response values and vibration amplitudes of the IDP and annulus VMS in both the degenerated and adjacent levels of the lumbar spine.

CONCLUSIONS

Application of the BPSF system is helpful in prevention of further injury of the disc degenerated lumbar spine during WBV.

Development and validation of a geometrically personalized finite element model of the lower ligamentous cervical spine for clinical applications

Epidemiological and clinical studies show that the magnitude and scope of cervical disease are on the rise, along with the world's rising aging population. From a biomechanical perspective, the cervical spine presents a wide inter-individual variability, where its motion patterns and load sharing strongly depend on the anatomy. This study aimed to first develop and validate a geometrically patient-specific model of the lower cervical spine for clinical applications, and secondly to use the model to investigate the spinal biomechanics associated with typical cervical disorders. Based on measurements of 30 parameters from X-ray radiographs, the 3D geometry of the vertebrae and intervertebral discs (IVDs) were developed, and detailed finite element models (FEMs) of the lower ligamentous cervical spine for 6 subjects were constructed and simulated. The models were then used for the investigation of different grades of IVD alteration. The multi directional range of motion (ROM) results were in alignment with the in-vitro and in-Silico studies confirming the validity of the model. Severe disc alteration (Grade 3) presented a significant decrease in the ROM and intradiscal pressure (flexion, extension, and axial rotation) on the C5-C6 and slightly increase on the adjacent levels. Maximum stress in Annulus Fibrosus (AF) and facet joint forces increased for Grade 3 for both altered and adjacent levels. The novel validated geometrically-personalized FEM presented in this study potentially offers the clinical community a valuable quantitative tool for the noninvasive analyses of the biomechanical alterations associated with cervical spine disease towards improved surgical planning and enhanced clinical outcomes.

Relative Nucleus Pulposus Area and Position Alters Disc Joint Mechanics

Aging and degeneration of the intervertebral disc are noted by changes in tissue composition and geometry, including a decrease in nucleus pulposus (NP) area. The NP centroid is positioned slightly posterior of the disc's centroid, but the effect of NP size and location on disc joint mechanics is not well understood. We evaluated the effect of NP size and centroid location on disc joint mechanics under dual-loading modalities (i.e., compression in combination with axial rotation or bending). A finite element model was developed to vary the relative NP area (NP:Disc area ratio range = 0.21 - 0.60). We also evaluated the effect of NP position by shifting the NP centroid anteriorly and posteriorly. Our results showed that compressive stiffness and average first principal strains increased with NP size. Under axial compression, stresses are distributed from the NP to the annulus, and stresses were redistributed towards the NP with axial rotation. Moreover, peak stresses were greater for discs with a smaller NP area. NP centroid location had a greater impact on intradiscal pressure during flexion and extension, where peak pressures in the posterior annulus under extension was greater for discs with a more posteriorly situated NP. In conclusion, the findings from this study highlight the importance of closely mimicking NP size and location in computational models that aim to understand stress/strain distribution during complex loading and for developing repair strategies that aim to recapitulate the mechanical behavior of healthy discs.

The Mechanical Role of the Radial Fiber Network Within the Annulus Fibrosus of the Lumbar Intervertebral Disc: A Finite Elements Study

The annulus fibrosus (AF) of the intervertebral disc (IVD) consists of a set of concentric layers composed of a primary circumferential collagen fibers arranged in an alternating oblique orientation. Moreover, there exists an additional secondary set of radial translamellar collagen fibers which connects the concentric layers, creating an interconnected fiber network. The aim of this study was to investigate the mechanical role of the radial fiber network. Toward that goal, a three-dimensional (3D) finite element model of the L3–L4 spinal segment was generated and calibrated to axial compression and pure moment loading. The AF model explicitly recognizes the two heterogeneous networks of fibers. The presence of radial fibers demonstrated a pronounced effect on the local disc responses under lateral bending, flexion, and extension modes. In these modes, the radial fibers were in a tensile state in the disc region that subjected to compression. In addition, the circumferential fibers, on the opposite side of the IVD, were also under tension. The local stress in the matrix was decreased in up to 9% in the radial fibers presence. This implies an active fiber network acting collectively to reduce the stresses and strains in the AF lamellae. Moreover, a reduction of 26.6% in the matrix sideways expansion was seen in the presence of the radial fibers near the neutral bending axis of the disc. The proposed biomechanical model provided a new insight into the mechanical role of the radial collagen fibers in the AF structure. This model can assist in the design of future IVD substitutes.

The biomechanical influence of anterior vertebral body osteophytes on the lumbar spine: A finite element study

BACKGROUND CONTEXT

Anterior vertebral body osteophytes are common with degeneration but their biomechanical influence on the whole lumbar spine remains unclear.

PURPOSE

To investigate the biomechanical influence of anterior vertebral body osteophytes on the whole lumbar spine.

STUDY DESIGN/SETTING

This is a study using finite element analysis.

OUTCOME MEASURES

Intersegmental rotation, maximum Mises stress, and intradiscal pressure on the intervertebral discs of different lumbar levels were calculated.

METHODS

A finite element model of an intact lumbar spine was constructed and validated against in vitro studies. The modified models, which had different degrees of anterior vertebral body osteophyte formation (OF) in combination with disc space narrowing, were applied with physiological loadings.

RESULTS

The lumbar levels with various degrees of OF altered the kinematics of these levels, which also affected the whole lumbar spine. In flexion and lateral bending, the segment that was one level inferior to the vertebra with OF showed a trend towards increased range of motion. On the intervertebral discs that were one level inferior to the OF level, a trend towards increase in the maximum von Mises stress was found on the annulus.

CONCLUSIONS

Segments adjacent to levels with anterior vertebral body osteophytes showed increased intersegmental rotation and maximum stress. Further clinical observation should be performed to verify the results in vivo.

Case-specific non-linear finite element models to predict failure behavior in two functional spinal units

Current finite element (FE) models predicting failure behavior comprise single vertebrae, thereby neglecting the role of the posterior elements and intervertebral discs. Therefore, this study aimed to develop a more clinically relevant, case-specific non-linear FE model of two functional spinal units able to predict failure behavior in terms of (i) the vertebra predicted to fail; (ii) deformation of the specimens; (iii) stiffness; and (iv) load to failure. For this purpose, we also studied the effect of different bone density-mechanical properties relationships (material models) on the prediction of failure behavior. Twelve two functional spinal units (T6-T8, T9-T11, T12-L2, and L3-L5) with and without artificial metastases were destructively tested in axial compression. These experiments were simulated using CT-based case-specific non-linear FE models. Bone mechanical properties were assigned using four commonly used material models. In 10 of the 11 specimens our FE model was able to correctly indicate which vertebrae failed during the experiments. However, predictions of the three-dimensional deformations of the specimens were less promising. Whereas stiffness of the whole construct could be strongly predicted (R²  = 0.637-0.688, p < 0.01), we obtained weak correlations between FE predicted and experimentally determined load to failure, as defined by the total reaction force exhibiting a drop in force (R²  = 0.219-0.247, p > 0.05). Additionally, we found that the correlation between predicted and experimental fracture loads did not strongly depend on the material model implemented, but the stiffness predictions did. In conclusion, this work showed that, in its current state, our FE models may be used to identify the weakest vertebra, but that substantial improvements are required in order to quantify in vivo failure loads. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodical, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 36:3208-3218, 2018.

Research on Spinal Lumbar Sacral Degeneration Finite Element Model

Recent research has shown that lumbar disease has become common in China. Since the structure of the lumbar spine is extremely complex, a finite element analysis method was used to perform biomechanical simulation and analysis of stress and strain on the L3–L4 lumbar segment to provide both a scientific and theoretical basis for clinical diagnosis and medical research. The MC volume-rendering 3D reconstruction method was the first step to accurately constructing the finite element model of the L3–L4 lumbar sacral segment, which was simulated prior to the addition of the ligaments, fibrous ring, and other major spinal tissue. The finite element model network was classified and the material properties of the corresponding parts were described. According to the normal model, careful simulation and deformation were performed, in addition to intervertebral disc degeneration in various cases. We have provided a detailed and professional analysis of the biomechanical properties, providing a powerful biomechanical basis for the diagnosis of intervertebral disc bulge and degeneration.

A Finite Element Analysis of Stress Distribution and Disk Displacement in Response to Lumbar Rotation Manipulation in the Sitting and Side-Lying Positions

OBJECTIVE

This study aimed to investigate stress distribution and disk displacement in healthy and degenerated intervertebral disks during simulated lumbar rotation manipulation (LRM) in the sitting and side-lying positions.

METHODS

Three-dimensional (3D) finite element models of healthy, mildly degenerated and moderately degenerated L4/5 spinal units were reconstructed. Lumbar rotation manipulation in the sitting and side-lying positions were simulated, and alterations in stress distribution and disk displacement in the lumbar disks were observed.

RESULTS

The application of LRM in the sitting or side-lying position resulted in a similar stress distribution in healthy, mildly degenerated, and moderately degenerated disks. Stress was concentrated at the anterior right side of the annulus. In all disks, intradiskal pressure (IDP) and maximum von Mises stress were higher during LRM in the sitting position than during LRM in the side-lying position. During these manipulations, Intradiskal pressure and stress in the annulus of moderately degenerated disks were higher than in mildly degenerated disks. Displacement was most obvious in healthy disks.

CONCLUSIONS

Mildly and moderately degenerated lumbar disks were subject to higher stress during LRM in the sitting position than during LRM in the side-lying position. Intradiskal pressure and the maximum von Mises stress in the annulus of moderately degenerated disks increased, suggesting the need for caution when treating patients with moderately compromised disks. Although our results are in accordance with previously published data, they are simulated and preliminary and do not necessarily replicate the clinical condition.