Finite element application in implant research for treatment of lumbar degenerative disc disease

Finite element modeling to predict the influence of anatomic variation and implant placement on performance of biological intervertebral disc implants

Background

Tissue-engineered intervertebral disc (TE-IVD) constructs are an attractive therapy for treating degenerative disc disease and have previously been investigated in vivo in both large and small animal models. The mechanical environment of the spine is notably challenging, in part due to its complex anatomy, and implants may require additional mechanical support to avoid failure in the early stages of implantation. As such, the design of suitable support implants requires rigorous validation.

Methods

We created a FE model to simulate the behavior of the IVD cages under compression specific to the anatomy of the porcine cervical spine, validated the FE model using an animal model, and predicted the effects of implant location and vertebral angle of the motion segment on implant behavior. Specifically, we tested anatomical positioning of the superior vertebra and placement of the implant. We analyzed corresponding stress and strain distributions.

Results

Results demonstrated that the anatomical geometry of the porcine cervical spine led to concentrated stress and strain on the posterior side of the cage. This stress concentration was associated with the location of failure of the cages reported in vivo, despite superior mechanical properties of the implant. Furthermore, placement of the cage was found to have profound effects on migration, while the angle of the superior vertebra affected stress concentration of the cage.

Conclusions

This model can be utilized both to inform surgical procedures and provide insight on future cage designs and can be adopted to models without the use of in vivo animal models.

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

Low back pain is associated with degenerative disc diseases of the spine. Surgical treatment includes fusion and non-fusion types. The gold standard is fusion surgery, wherein the affected vertebral segment is fused. The common complication of fusion surgery is adjacent segment degeneration (ASD). The ASD often leads to revision surgery, calling for a further fusion of adjacent segments. The existing designs of nonfusion type implants are associated with clinical problems such as subsidence, difficulty in implantation, and the requirement of revision surgeries. Various surgical approaches have been adopted by the surgeons to insert the spinal implants into the affected segment. Over the years, extensive biomechanical investigations have been reported on various surgical approaches and prostheses to predict the outcomes of lumbar spine implantations. Computer models have been proven to be very effective in identifying the best prosthesis and surgical procedure. The objective of the study was to review the literature on biomechanical studies for the treatment of lumbar spinal degenerative diseases. A critical review of the clinical and biomechanical studies on fusion spine surgeries was undertaken. The important modeling parameters, challenges, and limitations of the current studies were identified, showing the future research directions.

Numerical analysis of the mechanical behaviour of intact and implanted lumbar functional spinal units: Effects of loading and boundary conditions

The objective of this study was to develop an improved finite element (FE) model of a lumbar functional spinal unit (FSU) and to subsequently analyse the deviations in load transfer owing to implantation. The effects of loading and boundary conditions on load transfer in intact and implanted FSUs and its relationship with the potential risk of vertebral fracture were investigated. The FE models of L1-L5 and L3-L4 FSUs, intact and implanted, were developed using patient-specific CT-scan dataset and segmentation of cortical and cancellous bone regions. The effect of submodelling technique, as compared to artificial boundary conditions, on the elastic behaviour of lumbar spine was examined. Applied forces and moments, corresponding to physiologic movements, were used as loading conditions. Results indicated that the loading and boundary conditions considerably affect stress-strain distributions within a FSU. This study, based on an improved FE model of a vertebra, highlights the importance of using the submodelling technique to adequately evaluate the mechanical behaviour of a FSU. In the intact FSU, strains of 200-400 µε were observed in the cancellous bone of vertebral body and pedicles. High equivalent stresses of 10-25 MPa and 1-5 MPa were generated around the pars interarticularis for cortical and cancellous regions, respectively. Implantation caused reductions of 85%-92% in the range of motion for all movements. Insertion of the intervertebral cage resulted in major deviations in load transfer across a FSU for all movements. The cancellous bone around cage experienced pronounced increase in stresses of 10-15 MPa, which indicated potential risk of failure initiation in the vertebra.

A graphical guide for constructing a finite element model of the cervical spine with digital orthopedic software

Three-dimensional (3D) reconstruction and finite element analysis (FEA) have been extensively used to simulate cervical biomechanics. However, instructive articles providing full descriptions for operating Mimics software, Geomagic software, and FEA are rare in the literature. This omission has hindered research and development related to cervical spine biomechanics. Herein, we expound a detailed and easily understandable protocol for performing a digital biomechanics study which may facilitate a better understanding of the internal anatomy mechanics and the investigation of novel screw fixation techniques. We describe step-by-step instructions for use of Mimics and Geomagic software in FEA, along with a concise literature review. The key procedures of digital FEA stepwise instruction are presented, accompanied by a brief but complete report on the computed tomography (CT) imaging data for establishing the final finite element model. Previous publications regarding the commonly used software are also reviewed and discussed. Each piece of software performs a specific function for digital FEA establishment and each has its inherent shortcomings, making it is necessary to combine the software to leverage the advantages of each in order to best serve finite element research. For reasons of brevity, this study only provides an illustrative report on a small key part of finite element research in the cervical spine. These stepwise instructions can guide orthopedic researchers in conducting FEA studies in digital cervical biomechanics.

Application of Simulation Methods in Cervical Spine Dynamics

Neck injury is one of the most frequent spine injuries due to the complex structure of the cervical spine. The high incidence of neck injuries in collision accidents can bring a heavy economic burden to the society. Therefore, knowing the potential mechanisms of cervical spine injury and dysfunction is significant for improving its prevention and treatment. The research on cervical spine dynamics mainly concerns the fields of automobile safety, aeronautics, and astronautics. Numerical simulation methods are beneficial to better understand the stresses and strains developed in soft tissues with investigators and have been roundly used in cervical biomechanics. In this article, the simulation methods for the development and application of cervical spine dynamic problems in the recent years have been reviewed. The study focused mainly on multibody and finite element models. The structure, material properties, and application fields, especially the whiplash injury, were analyzed in detail. It has been shown that simulation methods have made remarkable progress in the research of cervical dynamic injury mechanisms, and some suggestions on the research of cervical dynamics in the future have been proposed.

Biomechanics of spinal implants-a review

Spinal instrumentations have been classified as rigid fixation, total disc replacement and dynamic stabilization system for treatment of various spinal disorders. The efficacy and biomechanical suitability of any spinal implant can be measured through in vitro, in vivo experiments and numerical techniques. With the advancement in technology finite element models are making an important contribution to understand the complex structure of spinal components along with allied functionality, designing and application of spinal instrumentations at preliminary design stage. This paper aimed to review the past and recent studies to describe the biomechanical aspects of various spinal implants. The literatures were grouped and reviewed in accordance to instrumentation category and their functionality in the spinal column at respective locations.

Design of Patient Specific Spinal Implant (Pedicle Screw Fixation) using FE Analysis and Soft Computing Techniques

BACKGROUND

This work uses genetic algorithm (GA) for optimum design of patient specific spinal implants (pedicle screw) with varying implant diameter and bone condition. The optimum pedicle screw fixation in terms of implant diameter is on the basis of minimum strain difference from intact (natural) to implantation at peri-prosthetic bone for the considered six different peri-implant positions.

METHODS

This design problem is expressed as an optimization problem using the desirability function, where the data generated by finite element analysis is converted into an artificial neural network (ANN) model. The finite element model is generated from CT scan data. Thereafter all the ANN predictions of the microstrain in six positions are converted to unitless desirability value varying between 0 and 1, which is then combined to form the composite desirability. Maximization of the composite desirability is done using GA where composite desirability should be made to go up as close as possible to 1. If the composite desirability is 1, then all 'strain difference values in 6 positions' are 0.

RESULTS

The optimum solutions obtained can easily be used for making patient-specific spinal implants.

Prediction of biomechanical behavior of lumbar vertebrae using a novel semi-rigid stabilization device

The work investigates the effect of proposed novel semi-rigid stabilization device on lumbar segment L2-L3 so as to preserve motion at vertebral level. Here, the biomechanical behavior of intact spine with three instrumented spinal models (semi-rigid stabilization device, rigid implant and dynamic stabilization system NFlex) have been compared under the motion conditions of flexion, extension, bending and twist. Three-dimensional non-linear finite models of intact spine, semi-rigid stabilization device, rigid implant and dynamic stabilization system NFlex were developed in the present study. All the four models were subjected to a combined load of 400 N in axial compression along with 2, 4, 6, 8 and 10 N m as bending moment individually. Dynamic stabilization system NFlex shows the maximum variation in motion and reflects range of motion as 89.7% during lateral bending, 53.4% in flexion, 34.6% in twist and 28.0% in extension with respect to intact spine. However, semi-rigid stabilization device and rigid implant shows the range of motion of 60%, 48.7%, 32% and 21.8% and 60%, 32.3%, 22.3% and 21.7% of intact, respectively, during bending, flexion, twist and extension. Finite element simulation results reveal that semi-rigid stabilization device shows comparatively lower values than dynamic stabilization system NFlex and higher as compared to rigid implant for measured intradiscal pressure and von Mises strain at intervertebral disc-23.

Simplifying the human lumbar spine (L3/L4) material in order to create an elemental structure for the future modeling

The human lumbar spine incorporates the best joints in nature due to its optimal static and dynamic behavior against the internal and external loads. Developing an elemental structure based on this joint requires simplification in terms of the materials employed by keeping the mechanical and anatomical behaviors of the human lumbar spine. In the present study, the finite element (FE) of two motion segments of the human lumbar spine (L3/L4) was developed based on the CT scan data as the base for vertebrae geometry, verified geometry properties for another part of two motion segments, and combination of materials and loads obtained from the validated resources. Then, simplification occurred in four continuous steps such as omitting the annual fibers of annual matrix, representing the material of the annual matrix to the nucleus, demonstrating the material of annual matrix to the endplates too, and omitting the trabecular part of vertebrae. The present study aimed to propose the method for developing the basic structure of the human lumbar spine by simplifying its materials in the above-mentioned steps, analyzing the biomechanical effects of these four steps in terms of their internal and external responses, and validating the data obtained from the FE method. The validated simplified way introduced in this study can be used for future research by making implants, prosthesis, and modeling based on the human lumbar spine in other fields such as industrial design, building structures, or joints, which results in making the model easier, cheaper, and more effective.

Biomechanical Evaluation of a Novel Apatite-Wollastonite Ceramic Cage Design for Lumbar Interbody Fusion: A Finite Element Model Study

Objectives

Cage design and material properties play a crucial role in the long-term results, since interbody fusions using intervertebral cages have become one of the basic procedures in spinal surgery. Our aim is to design a novel Apatite-Wollastonite interbody fusion cage and evaluate its biomechanical behavior in silico in a segmental spinal model.

Materials and Methods

Mechanical properties for the Apatite-Wollastonite bioceramic cages were obtained by fitting finite element results to the experimental compression behavior of a cage prototype. The prototype was made from hydroxyapatite, pseudowollastonite, and frit by sintering. The elastic modulus of the material was found to be 32 GPa. Three intact lumbar vertebral segments were modelled with the ANSYS 12.0.1 software and this model was modified to simulate a Posterior Lumbar Interbody Fusion. Four cage designs in different geometries were analyzed in silico under axial loading, flexion, extension, and lateral bending.

Results

The K2 design had the best overall biomechanical performance for the loads considered. Maximum cage stress recorded was 36.7 MPa in compression after a flexion load, which was within the biomechanical limits of the cage.

Conclusion

Biomechanical analyses suggest that K2 bioceramic cage is an optimal design and reveals essential material properties for a stable interbody fusion.

The effect of screw tunnels on the biomechanical stability of vertebral body after pedicle screws removal: a finite element analysis

PURPOSE

Posterior reduction and pedicle screw fixation is a widely used procedure for thoracic and lumbar vertebrae fractures. Usually, the pedicle screws would be removed after the fracture healing and screw tunnels would be left. The aim of this study is to evaluate the effect of screw tunnels on the biomechanical stability of the lumbar vertebral body after pedicle screws removal by finite element analysis (FEA).

METHODS

First, the CT values of the screw tunnels wall in the fractured vertebral bodies were measured in patients whose pedicle screws were removed, and they were then compared with the values of vertebral cortical bone. Second, an adult patient was included and the CT images of the lumbar spine were harvested. Three dimensional finite element models of the L1 vertebra with unilateral or bilateral screw tunnels were created based on the CT images. Different compressive loads were vertically acted on the models. The maximum loads which the models sustained and the distribution of the force in the different parts of the models were recorded and compared with each other.

RESULTS

The CT values of the tunnels wall and vertebral cortical bone were 387.126±62.342 and 399.204±53.612, which were not statistically different (P=0.149). The models of three dimensional tetrahedral mesh finite element of normal lumbar 1 vertebra were established with good geometric similarity and realistic appearance. After given the compressive loads, the cortical bone was the first one to reach its ultimate stress. The maximum loads which the bilateral screw tunnels model, unilateral screw tunnel model, and normal vertebral model can sustain were 3.97 Mpa, 3.83 Mpa, and 3.78 Mpa, respectively. For the diameter of the screw tunnels, the model with a diameter of 6.5 mm could sustain the largest load. In addition, the stress distributing on the outside of the cortical bone gradually decreased as the thickness of the tunnel wall increased.

CONCLUSIONS

Based on the FEA, pedicle screw tunnels would not decrease the biomechanical stability and strength of the vertebral body. A large diameter of screw tunnel and thick tunnel wall were helpful for the biomechanical stability of the vertebral body.

A database of lumbar spinal mechanical behavior for validation of spinal analytical models

Data from two experimental studies with eight specimens each of spinal motion segments and/or intervertebral discs are presented in a form that can be used for comparison with finite element model predictions. The data include the effect of compressive preload (0, 250 and 500N) with quasistatic cyclic loading (0.0115Hz) and the effect of loading frequency (1, 0.1, 0.01 and 0.001Hz) with a physiological compressive preload (mean 642N). Specimens were tested with displacements in each of six degrees of freedom (three translations and three rotations) about defined anatomical axes. The three forces and three moments in the corresponding axis system were recorded during each test. Linearized stiffness matrices were calculated that could be used in multi-segmental biomechanical models of the spine and these matrices were analyzed to determine whether off-diagonal terms and symmetry assumptions should be included. These databases of lumbar spinal mechanical behavior under physiological conditions quantify behaviors that should be present in finite element model simulations. The addition of more specimens to identify sources of variability associated with physical dimensions, degeneration, and other variables would be beneficial. Supplementary data provide the recorded data and Matlab® codes for reading files. Linearized stiffness matrices derived from the tests at different preloads revealed few significant unexpected off-diagonal terms and little evidence of significant matrix asymmetry.

Finite element simulation of an artificial intervertebral disk using fiber reinforced laminated composite model

Degeneration of intervertebral disk (IVD) has been increased in recent years. The lumbar herniation can be cured using conservative and surgical procedures. Surgery is considered after failure of conservative treatment. Partial discectomy, fusion, and total disk replacement (TDR) are also common surgical treatments for degenerative disk disease. However, due to limitations and disadvantages of the current treatments, many studies have been carried out to approach the best design of mimicking natural disk. Recently, a new method of TDRs has been introduced using nature deformation of IVD by reinforced fibers of annulus fibrosis. Nonetheless, owing to limitations of experimental works on the human body, numerical studies of IVD may help to understand load transfer and biomechanical properties within the disks with reinforced fibers. In this study, a three-dimensional (3D) finite element model of the L2-L3 disk vertebrae unit with 12 vertical fibers embedded into annulus fibrosis was constructed. The IVD was subjected to compressive force, bending moment, and axial torsion. The most important parameters of disk failures were compared to that of experimental data. The results showed that the addition of reinforced fibers into the disk invokes a significant decrease of stress in the nucleus and annulus. The findings of this study may have implications not only for developing IVDs with reinforced fibers but also for the application of fiber reinforced IVD in orthopedics surgeries as a suitable implant.

Comparison of eight published static finite element models of the intact lumbar spine: predictive power of models improves when combined together

Finite element (FE) model studies have made important contributions to our understanding of functional biomechanics of the lumbar spine. However, if a model is used to answer clinical and biomechanical questions over a certain population, their inherently large inter-subject variability has to be considered. Current FE model studies, however, generally account only for a single distinct spinal geometry with one set of material properties. This raises questions concerning their predictive power, their range of results and on their agreement with in vitro and in vivo values. Eight well-established FE models of the lumbar spine (L1-5) of different research centers around the globe were subjected to pure and combined loading modes and compared to in vitro and in vivo measurements for intervertebral rotations, disc pressures and facet joint forces. Under pure moment loading, the predicted L1-5 rotations of almost all models fell within the reported in vitro ranges, and their median values differed on average by only 2° for flexion-extension, 1° for lateral bending and 5° for axial rotation. Predicted median facet joint forces and disc pressures were also in good agreement with published median in vitro values. However, the ranges of predictions were larger and exceeded those reported in vitro, especially for the facet joint forces. For all combined loading modes, except for flexion, predicted median segmental intervertebral rotations and disc pressures were in good agreement with measured in vivo values. In light of high inter-subject variability, the generalization of results of a single model to a population remains a concern. This study demonstrated that the pooled median of individual model results, similar to a probabilistic approach, can be used as an improved predictive tool in order to estimate the response of the lumbar spine.

EXPERIMENTAL METHODS FOR THE BIOMECHANICAL INVESTIGATION OF THE HUMAN SPINE: A REVIEW

In vitro mechanical testing of spinal specimens is extremely important to better understand the biomechanics of the healthy and diseased spine, fracture, and to test/optimize surgical treatment. While spinal testing has extensively been carried out in the past four decades, testing methods are quite diverse. This paper aims to provide a critical overview of the in vitro methods for mechanical testing the human spine at different scales. Specimens of different type are used, according to the aim of the study: spine segments (two or more adjacent vertebrae) are used both to investigate the spine kinematics, and the mechanical properties of the spine components (vertebrae, ligaments, discs); single vertebrae (whole vertebra, isolated vertebral body, or vertebral body without endplates) are used to investigate the structural properties of the vertebra itself; core specimens are extracted to test the mechanical properties of the trabecular bone at the tissue-level; mechanical properties of spine soft tissue (discs, ligaments, spinal cord) are measured on isolated elements, or on tissue specimens. Identification of consistent reference frames is still a debated issue. Testing conditions feature different pre-conditioning and loading rates, depending on the simulated action. Tissue specimen preservation is a very critical issue, affecting test results. Animal models are often used as a surrogate. However, because of different structure and anatomy, extreme caution is required when extrapolating to the human spine. In vitro loading conditions should be based on reliable in vivo data. Because of the high complexity of the spine, such information (either through instrumented implants or through numerical modeling) is currently unsatisfactory. Because of the increasing ability of computational models in predicting biomechanical properties of musculoskeletal structures, a synergy is possible (and desirable) between in vitro experiments and numerical modeling. Future perspectives in spine testing include integration of mechanical and structural properties at different dimensional scales (from the whole-body-level down to the tissue-level) so that organ-level models (which are used to predict the most relevant phenomena such as fracture) include information from all dimensional scales.

The use of X-shaped cross-link in posterior spinal constructs improves stability in thoracolumbar burst fracture: a finite element analysis

Posterior instrumentation is a common fixation method used to treat thoracolumbar burst fractures. However, the role of different cross-link configurations in improving fixation stability in these fractures has not been established. A 3D finite element model of T11-L3 was used to investigate the biomechanical behavior of short (2 level) and long (4 level) segmental spine pedicle screw fixation with various cross-links to treat a hypothetical L1 vertebra burst fracture. Three types of cross-link configurations with an applied moment of 7.5 Nm and 200 N axial force were evaluated. The long construct was stiffer than the short construct irrespective of whether the cross-links were used (p < 0.05). The short constructs showed no significant differences between the cross-link configurations. The XL cross-link provided the highest stiffness and was 14.9% stiffer than the one without a cross-link. The long construct resulted in reduced stress to the adjacent vertebral bodies and screw necks, with 66.7% reduction in bending stress on L2 when the XL cross-link was used. Thus, the stability for L1 burst fracture fixation was best achieved by using long segmental posterior instrumentation constructs and an XL cross-link configuration. Cross-links did not improved stability when a short structure was used.

Biomechanics of posterior dynamic stabilization systems

Spinal rigid instrumentations have been used to fuse and stabilize spinal segments as a surgical treatment for various spinal disorders to date. This technology provides immediate stability after surgery until the natural fusion mass develops. At present, rigid fixation is the current gold standard in surgical treatment of chronic back pain spinal disorders. However, such systems have several drawbacks such as higher mechanical stress on the adjacent segment, leading to long-term degenerative changes and hypermobility that often necessitate additional fusion surgery. Dynamic stabilization systems have been suggested to address adjacent segment degeneration, which is considered to be a fusion-associated phenomenon. Dynamic stabilization systems are designed to preserve segmental stability, to keep the treated segment mobile, and to reduce or eliminate degenerative effects on adjacent segments. This paper aimed to describe the biomechanical aspect of dynamic stabilization systems as an alternative treatment to fusion for certain patients.

A short history of posterior dynamic stabilization

Interspinous spacers were developed to treat local deformities such as degenerative spondylolisthesis. To treat patients with chronic instability, posterior pedicle fixation and rod-based dynamic stabilization systems were developed as alternatives to fusion surgeries. Dynamic stabilization is the future of spinal surgery, and in the near future, we will be able to see the development of new devices and surgical techniques to stabilize the spine. It is important to follow the development of these technologies and to gain experience using them. In this paper, we review the literature and discuss the dynamic systems, both past and present, used in the market to treat lumbar degeneration.

Effect of side holes in cervical fusion cages: a finite element analysis study

The objective of this study was to investigate the effect of side holes on the predicted von Mises stress levels in cervical spinal fusion cages subjected to compressive loading. Models with between zero and ten side holes were developed. Finite element analysis (FEA) was used to simulate compression of the cage, made from the polymer PEEK (polyetheretherketone), between two adjacent vertebrae. The analyses were validated by experimental tests. In all of the models, the von Mises stress was highest at the cage–vertebrae interface with peak stresses of between 14 and 18 MPa. Increasing the Young’s modulus of the vertebrae from 12 to 30 GPa increased the peak stress on average by 29 per cent. The stresses in the models were lower than the compressive strength of PEEK (118 MPa), and are well within the PEEK fatigue strength reported (60 MPa at 10 million cycles). This study suggests that the number of side holes had a negligible effect on the stress distribution within the cage; the stress magnitudes were fairly constant across all of the models and did not change substantially with the number of holes. Hence, a cervical cage with side holes is unlikely to fail in compression.

NUMERICAL VALIDATION OF A FIBER-REINFORCED HYPERELASTIC CONSTITUTIVE MODEL FOR HUMAN INTERVERTEBRAL DISC ANNULUS FIBROSUS

This paper validates a constitutive model for human intervertebral disc annulus fibrosus via numerical simulations on a lumber spine motion segment. This anisotropic hyperelastic fiber-reinforced constitutive model was previously developed by the authors. Based on three-dimensional (3D) lumbar spine segments that are constructed from CT scanning images, a detailed and anatomically accurate human lumbar spine finite element (FE) model for L3–L4 motion segment is developed. The FE model includes vertebral bodies, intervertebral disc, and various ligaments. Numerical simulations are carried out by using commercial CAE software package ABAQUS/Standard. The loading cases considered in the numerical analysis are set to be consistent with sets-up of cadaveric specimen testing available in the literature. Numerical results such as load–displacement curves and nucleus pressure are compared with experimental data. Simulation results show good consistency with cadaveric experimental data, and have good biomechanical fidelity. The constitutive model can be used for human intervertebral disc modeling and biomechanical analysis of human spine column.