Biomechanical investigation on the influence of the regional material degeneration of an intervertebral disc in a lower lumbar spinal unit: A finite element study

Validation and Estimation of Obesity-Induced Intervertebral Disc Degeneration through Subject-Specific Finite Element Modelling of Functional Spinal Units

(1) Background: Intervertebral disc degeneration has been linked to obesity; its potential mechanical effects on the intervertebral disc remain unknown. This study aimed to develop and validate a patient-specific model of L3-L4 vertebrae and then use the model to estimate the impact of increasing body weight on disc degeneration. (2) Methods: A three-dimensional model of the functional spinal unit of L3-L4 vertebrae and its components were developed and validated. Validation was achieved by comparing the range of motions (RoM) and intradiscal pressures with the previous literature. Subsequently, the validated model was loaded according to the body mass index and estimated stress, deformation, and RoM to assess disc degeneration. (3) Results: During validation, L3-L4 RoM and intradiscal pressures: flexion 5.17° and 1.04 MPa, extension 1.54° and 0.22 MPa, lateral bending 3.36° and 0.54 MPa, axial rotation 1.14° and 0.52 MPa, respectively. When investigating the impact of weight on disc degeneration, escalating from normal weight to obesity reveals an increased RoM, by 3.44% during flexion, 22.7% during extension, 29.71% during lateral bending, and 33.2% during axial rotation, respectively. Also, stress and disc deformation elevated with increasing weight across all RoM. (4) Conclusions: The predicted mechanical responses of the developed model closely matched the validation dataset. The validated model predicts disc degeneration under increased weight and could lay the foundation for future recommendations aimed at identifying predictors of lower back pain due to disc degeneration.

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

STUDY DESIGN

Mechanical simulations.

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Second-generation bone cement-injectable cannulated pedicle screws for osteoporosis: biomechanical and finite element analyses

BACKGROUND

Biomechanical and finite element analyses were performed to investigate the efficacy of second-generation bone cement-injectable cannulated pedicle screws (CICPS) in osteoporosis.

METHODS

This study used the biomechanical test module of polyurethane to simulate osteoporotic cancellous bone. Polymethylmethacrylate (PMMA) bone cement was used to anchor the pedicle screws in the module. The specimens were divided into two groups for the mechanical tests: the experimental group (second-generation CICPS) and control group (first-generation CICPS). Safety was evaluated using maximum shear force, static bending, and dynamic bending tests. Biomechanical stability evaluations included the maximum axial pullout force and rotary torque tests. X-ray imaging and computed tomography were used to evaluate the distribution of bone cement 24 h after PMMA injection, and stress distribution at the screw fracture and screw-cement-bone interface was assessed using finite element analysis.

RESULTS

Mechanical testing revealed that the experimental group (349.8 ± 28.6 N) had a higher maximum axial pullout force than the control group (277.3 ± 8.6 N; P < 0.05). The bending moments of the experimental group (128.5 ± 9.08 N) were comparable to those of the control group (113.4 ± 20.9 N; P > 0.05). The screw-in and spin-out torques of the experimental group were higher than those of the control group (spin-in, 0.793 ± 0.015 vs. 0.577 ± 0.062 N, P < 0.01; spin-out, 0.764 ± 0.027 vs. 0.612 ± 0.049 N, P < 0.01). Bone cement was mainly distributed at the front three-fifths of the screw in both groups, but the distribution was more uniform in the experimental group than in the control group. After pullout, the bone cement was closely connected to the screw, without loosening or fragmentation. In the finite element analysis, stress on the second-generation CICPS was concentrated at the proximal screw outlet, whereas stress on the first-generation CICPS was concentrated at the screw neck, and the screw-bone cement-bone interface stress of the experimental group was smaller than that of the control group.

CONCLUSION

These findings suggest that second-generation CICPS have higher safety and stability than first-generation CICPS and may be a superior choice for the treatment of osteoporosis.

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.

Impact of extracellular matrix and collagen network properties on the cervical intervertebral disc response to physiological loads: A parametric study

Current intervertebral disc finite element models are hard to validate since they describe multi-physical phenomena and contain a huge number of material properties. This work aims to simplify numerical validation/identification studies by prioritizing the sensitivity of intervertebral disc behavior to mechanical properties. A 3D fiber-reinforced hyperelastic model of a C6-C7 intervertebral disc is used to carry out the parametric study. 10 parameters describing the extracellular matrix and the collagen network behaviors are included in the parametric study. The influence of varying these parameters on the disc response is estimated during physiological movements of the head, including compression, lateral bending, flexion, and axial rotation. The obtained results highlight the high sensitivity of the disc behavior to the stiffness of the annulus fibrosus extracellular matrix for all the studied loads with a relative increase in the disc apparent stiffness by 67% for compression and by 57% for axial rotation when the annulus stiffness increases from 0.4 to 2 MPa. It is also shown that varying collagen network orientation, stiffness, and stiffening in the studied configuration range have a noticeable effect on rotational motions with a relative apparent stiffness difference reaching 6.8%, 10%, and 22%, respectively, in lateral bending. However, the collagen orientation does not affect disc response to axial load.

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

Background

To explore the biomechanical differences in oblique lumbar interbody fusion (OLIF) augmented by different types of instrumentation.

Methods

A three-dimensional nonlinear finite element (FE) model of an intact L3-S1 lumbar spine was built and validated. The intact model was modified to develop five OLIF surgery models (Stand-alone OLIF; OLIF with lateral plate fixation [OLIF + LPF]; OLIF with unilateral pedicle screws fixation [OLIF + UPSF]; OLIF with bilateral pedicle screws fixation [OLIF + BPSF]; OLIF with translaminar facet joint fixation + unilateral pedicle screws fixation [OLIF + TFJF + UPSF]) in which the surgical segment was L4–L5. Under a follower load of 500 N, a 7.5-Nm moment was applied to all lumbar spine models to calculate the range of motion (ROM), equivalent stress peak of fixation instruments (ESPFI), equivalent stress peak of cage (ESPC), equivalent stress peak of cortical endplate (ESPCE), and equivalent stress average value of cancellous bone (ESAVCB).

Results

Compared with the intact model, the ROM of the L4–L5 segment in each OLIF surgery model decreased by > 80%. The ROM values of adjacent segments were not significantly different. The ESPFI, ESPC, and ESPCE values of the OLIF + BPSF model were smaller than those of the other OLIF surgery models. The ESAVCB value of the normal lumbar model was less than the ESAVCB values of all OLIF surgical models. In most postures, the ESPFI, ESPCE, and ESAVCB values of the OLIF + LPF model were the largest. The ESPC was higher in the Stand-alone OLIF model than in the other OLIF models. The stresses of several important components of the OLIF + UPSF and OLIF + TFJF + UPSF models were between those of the OLIF + LPF and OLIF + BPSF models.

Conclusions

Our biomechanical FE analysis indicated the greater ability of OLIF + BPSF to retain lumbar stability, resist cage subsidence, and maintain disc height. Therefore, in the augmentation of OLIF, bilateral pedicle screws fixation may be the best approach.

Investigation of geometric deformations of the lumbar disc during axial body rotations

Background

Quantitative data on in vivo vertebral disc deformations are critical for enhancing our understanding of spinal pathology and improving the design of surgical materials. This study investigated in vivo lumbar intervertebral disc deformations during axial rotations under different load-bearing conditions.

Methods

Twelve healthy subjects (7 males and 5 females) between the ages of 25 and 39 were recruited. Using a combination of a dual fluoroscopic imaging system (DFIS) and CT, the images of L3–5 segments scanned by CT were transformed into three-dimensional models, which matched the instantaneous images of the lumbar spine taken by a double fluorescent X-ray system during axial rotations to reproduce motions. Then, the kinematic data of the compression and shear deformations of the lumbar disc and the coupled bending of the vertebral body were obtained.

Results

Relative to the supine position, the average compression deformation caused by rotation is between + 10% and − 40%, and the shear deformation is between 17 and 50%. Under physiological weightbearing loads, different levels of lumbar discs exhibit similar deformation patterns, and the deformation patterns of left and right rotations are approximately symmetrical. The deformation patterns change significantly under a 10 kg load, with the exception of the L3–4 disc during the right rotation.

Conclusion

The deformation of the lumbar disc was direction-specific and level-specific during axial rotations and was affected by extra weight. These data can provide new insights into the biomechanics of the lumbar spine and optimize the parameters of artificial lumbar spine devices.

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

Backgrounds

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

Methods

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

Results

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

Conclusions

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

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.

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.

The response surface method-genetic algorithm for identification of the lumbar intervertebral disc material parameters

Long-term compressive load on the lumbar intervertebral disc (IVD) might lead to lumbar IVD herniation. Exploring the material parameters of normal and degenerative enucleated IVDs is the basis for studying their mechanical behavior. According to the inverse analysis principle of the parameter estimation, an optimization method was proposed to identify the parameters of the porous material of the lumbar IVD based on finite element inverse analysis. The poroelastic finite element models were established in line with the compression creep experiment. The material parameters were combined by Box-Behnken design (BBD), and the response surface (RS) models were constructed using a quadratic polynomial with cross terms and optimized by genetic algorithm (GA). The results showed that the simulation result of the best material parameter combination had a good agreement with the experiment. Compared with the normal lumbar IVD, the elastic modulus and permeability decreased, and Poisson's ratio increased for the enucleated disk, resulting in a significant difference in mechanical properties. The algorithm used in this study can reduce the parameter identification error compared with only the RS method, and decrease the number of finite element simulations compared with only the GA.

A comparison of rigid, semi-rigid and flexible spinal stabilization devices: A finite element study

Pedicle-screw-based spinal fixation system has shown its success for treating degenerative disc disease-related back pain and spinal instability. In the last few decades, several non-fusion implants ( 'flexible' or 'dynamic' fixation) are developed for treating slight degenerate disc disease. The aim of this study is to characterize and compare the biomechanical responses of pedicle-screw-based fusion with various rod materials and a flexible spinal stabilization device on the lumbar spine (L3L5). Computed tomography scan-based finite element model and pedicle screw fixation with rigid rod material stainless steel rod, semi-rigid rod material poly-ether-ether-ketone and flexible rod device made of stainless steel are used in this study. Intact model of the lumbar spine and treated with all the different implants are simulated under typical physiological loading conditions. Compared with the intact model, pedicle screw with the stainless steel rod fixation system is found to offer very less range of motion. Poly-ether-ether-ketone rod system increased range of motion 3.8, 7 and 1.8 times for axial rotation, lateral bending and flexion-extension, respectively, compared to the stainless steel rod system. The flexible rod device rod system is found to reduce stress on vertebral body, carrying out more loads as compared to poly-ether-ether-ketone rods. In the case of stainless steel rods, range of motion is almost restricted on the fusion zone, which is overcome by the poly-ether-ether-ketone rod system to some extent and farther improved by the flexible rod device rod system. So, the poly-ether-ether-ketone rod and flexible rod device rod systems may be implemented for better clinical results after succeeding experimental validation and clinical trial.

Reducing the extent of facetectomy may decrease morbidity in failed back surgery syndrome

Background

Percutaneous transforaminal endoscopic discectomy (PTED) is widely used for the treatment of lumbar disc herniation. Facetectomy in PTED is necessary for accessing the intraspinal region and for decompressing the exiting nerve roots in patients who suffer from hypertrophy of the facet joints. However, this may increase morbidity in failed back surgery syndrome (FBSS) and has not been clearly elucidated.

Methods

A three-dimensional lumbosacral model was reconstructed and validated. And corresponding models after PTED with one-quarter and one-half excisions of the superior articular process were reconstructed. The maximum shear stress on the annulus in L5, von Mises stress of the facet cartilage, maximum principle capsular strain and deformation of the lumbosacral model were calculated using finite element methods.

Results

Calculated results show no significant differences in the complete model and the model with one-quarter excision of the superior articular process, but all biomechanical indexes have been deteriorated under most of the loading conditions tested in the model with one-half excision of the superior articular process.

Conclusions

Less facetectomy is better because it may reduce the risk of biomechanical deterioration and consequently, that of FBSS.