A computational biomechanical investigation of posterior dynamic instrumentation: combination of dynamic rod and hinged (dynamic) screw

Plate fixation optimization for distal femoral fractures with segmental bone loss: Defining the preferred screw distribution using finite element analysis

OBJECTIVES

Distal femur fractures can exhibit extensive comminution, and open fractures may result in bone loss. These injuries are under high mechanical demands when stabilized with a lateral locked plate (LLP), and are at risk of non-union or implant failure. This study investigates the optimal LLP screw configuration for distal femur fractures with a large metadiaphyseal gap of 5 cm.

METHODS

A finite element (FE) model, validated against experimentally measured strains and displacement, evaluated pull-out forces and stress concentration on typical implants under clinical conditions corresponding with the 10 % point during the stance phase of the gait cycle.

RESULTS

Maximum stress was up to 83 % less when the ratio (Cp) between the proximal screw-distribution-length and the distance of the first screw to the fracture was less than 0.2; maximum pull-out force was 99 % less when this ratio was higher than 0.4.

CONCLUSIONS

Screw configuration based on either normal or osteopenic bone quality plays an important role in determining the risk of construct failure for a major (50 mm) distal femoral metadiaphyseal segmental defect. This study provides valuable information when planning definitive fixation for distal femur fractures with extensive comminution or segmental bone defects, to mitigate the risk of implant failure and subsequent nonunion.

Biomechanical Evaluation of Rigid Interspinous Process Fixation Combined With Lumbar Interbody Fusion Using Hybrid Testing Protocol

Rigid interspinous process fixation (RIPF) has been recently discussed as an alternative to pedicle screw fixation (PSF) for reducing trauma in lumbar interbody fusion (LIF) surgery. This study aimed to investigate biomechanics of the lumbar spine with RIPF, and also to compare biomechanical differences between two postoperative stages (before and after bony fusion). Based on an intact finite-element model of lumbosacral spine, the models of single-level LIF with RIPF or conventional PSF were developed and were computed for biomechanical responses to the moments of four physiological motions using hybrid testing protocol. It was found that compared with PSF, range of motion (ROM), intradiscal pressure (IDP), and facet joint forces (FJF) at adjacent segments of the surgical level for RIPF were decreased by up to 8.4%, 2.3%, and 16.8%, respectively, but ROM and endplate stress at the surgical segment were increased by up to 285.3% and 174.3%, respectively. The results of comparison between lumbar spine with RIPF before and after bony fusion showed that ROM and endplate stress at the surgical segment were decreased by up to 62.6% and 40.4%, respectively, when achieved to bony fusion. These findings suggest that lumbar spine with RIPF as compared to PSF has potential to decrease the risk of adjacent segment degeneration but might have lower stability of surgical segment and an increased risk of cage subsidence; When achieved bony fusion, it might be helpful for the lumbar spine with RIPF in increasing stability of surgical segment and reducing failure of bone contact with cage.

The porous cantilever beam as a model for spinal implants: Experimental, analytical and finite element analysis of dynamic properties

Investigation of the dynamic properties of implants is essential to ensure safety and compatibility with the host's natural spinal tissue. This paper presents a simplified model of a cantilever beam to investigate the effects of holes/pores on the structures. Free vibration test is one of the most effective methods to measure the dynamic response of a cantilever beam, such as natural frequency and damping ratio. In this study, the natural frequencies of cantilever beams made of polycarbonate (PC) containing various circular open holes were investigated numerically, analytically, and experimentally. The experimental data confirmed the accuracy of the natural frequencies of the cantilever beam with open holes calculated by finite element and analytical models. In addition, two finite element simulation methods, the dynamic explicit and modal dynamic methods, were applied to determine the damping ratios of cantilever beams with open holes. Finite element analysis accurately simulated the damped vibration behavior of cantilever beams with open holes when known material damping properties were applied. The damping behavior of cantilever beams with random pores was simulated, highlighting a completely different relationship between porosity, natural frequency and damping response. The latter highlights the potential of finite element methods to analyze the dynamic response of arbitrary and complex structures, towards improved implant design.

Computational lumbar spine models: A literature review

BACKGROUND

Computational spine models of various types have been employed to understand spine function, assess the risk that different activities pose to the spine, and evaluate techniques to prevent injury. The areas in which these models are applied has expanded greatly, potentially beyond the appropriate scope of each, given their capabilities. A comprehensive understanding of the components of these models provides insight into their current capabilities and limitations.

METHODS

The objective of this review was to provide a critical assessment of the different characteristics of model elements employed across the spectrum of lumbar spine modeling and in newer combined methodologies to help better evaluate existing studies and delineate areas for future research and refinement.

FINDINGS

A total of 155 studies met selection criteria and were included in this review. Most current studies use either highly detailed Finite Element models or simpler Musculoskeletal models driven with in vivo data. Many models feature significant geometric or loading simplifications that limit their realism and validity. Frequently, studies only create a single model and thus can't account for the impact of subject variability. The lack of model representation for certain subject cohorts leaves significant gaps in spine knowledge. Combining features from both types of modeling could result in more accurate and predictive models.

INTERPRETATION

Development of integrated models combining elements from different model types in a framework that enables the evaluation of larger populations of subjects could address existing voids and enable more realistic representation of the biomechanics of the lumbar spine.

Should Iliac Wing Screws Be Included in Long Segment Dynamic Stabilization?

Background

In this article, clinical satisfaction and radiological results are discussed in a series of patients where the iliac wings participate in dynamic stabilization. Dynamic stabilization is an effective alternative surgical treatment method, especially in clinical pictures that go with pain due to minor instabilities. Practically the unique surgical instrument used in multilevel instabilities is the Dynesys system. The most important drawback of the Dynesys system is that the S1 screws become loose in time. In this article, our aim is to find solution to S1 insufficiency by extension of the system to the iliac wings.

Methods

Nineteen patients (eight females, 11 males) with a mean age of 54.16 were included in the study. Patients had multilevel (level 2 and above) instability, iliac wings were included in the stabilized segments, and Visual Analog Scale (VAS) and Oswestry Disability Index (ODI) were used for patient follow-up.

Results

First year results showed a significant improvement in VAS and ODI. Regarding the complications, infection developed in one patient, loosening in the proximal iliac wing in one patient, and both S1 and iliac proximals in one patient, but no clinical findings were encountered.

Conclusion

When more than two levels of dynamic systems are used in chronic instability, especially in the elderly patients, S1 screws are loosened. In these patients, if the iliac bones are also included in stabilization, this problem is solved successfully. However unfortunately, Dynesys system does not have a screw suitable for the iliac bones.

Development and Validation of a Computationally Efficient Finite Element Model of the Human Lumbar Spine: Application to Disc Degeneration

INTRODUCTION

This study develops and validates an accurate, computationally efficient, 3-dimensional finite element model (FEM) of the human lumbar spine. Advantages of this simplified model are shown by its application to a disc degeneration study that we demonstrate is completed in one-sixth the time required when using more complicated computed tomography (CT) scan-based models.

METHODS

An osseoligamentous FEM of the L1-L5 spine is developed using simple shapes based on average anatomical dimensions of key features of the spine rather than CT scan images. Pure moments of 7.5 Nm and a compressive follower load of 1000 N are individually applied to the L1 vertebra. Validation is achieved by comparing rotations and intradiscal pressures to other widely accepted FEMs and in vitro studies. Then degenerative disc properties are modeled and rotations calculated. Required computation times are compared between the model presented in this paper and other models developed using CT scans.

RESULTS

For the validation study, parameter values for a healthy spine were used with the loading conditions described above. Total L1-L5 rotations for flexion, extension, lateral bending, and axial rotation under pure moment loading were calculated as 20.3°, 10.7°, 19.7°, and 10.3°, respectively, and under a compressive follower load, maximum intradiscal pressures were calculated as 0.68 MPa. These values compare favorably with the data used for validation. When studying the effects of disc degeneration, the affected segment is shown to experience decreases in rotations during flexion, extension, and lateral bending (24%-56%), while rotations are shown to increase during axial rotation (14%-40%). Adjacent levels realize relatively minor changes in rotation (1%-6%). This parametric study required 17.5 hours of computation time compared to more than 4 days required if utilizing typical published CT scan-based models, illustrating one of the primary advantages of the model presented in this article.

CONCLUSIONS

The FEM presented in this article produces a biomechanical response comparable to widely accepted, complex, CT scan-based models and in vitro studies while requiring much shorter computation times. This makes the model ideal for conducting parametric studies of spinal pathologies and spinal correction techniques.

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.

Application of Finite Element Analysis for Investigation of Intervertebral Disc Degeneration: from Laboratory to Clinic

Due to the ethical concern and inability to detect inner stress distributions of intervertebral disc (IVD), traditional methods for investigation of intervertebral disc degeneration (IVDD) have significant limitations. Many researchers have demonstrated that finite element analysis (FEA) is an effective tool for the research of IVDD. However, the specific application of FEA for investigation of IVDD has not been systematically elucidated before. In the present review, we summarize the current finite element models (FEM) used for the investigation of IVDD, including the poroelastic nonlinear FEM, diffusive-reactive theory model and cell-activity coupled mechano-electrochemical theory model. We further elaborate the use of FEA for the research of IVDD pathogenesis especially for nutrition and biomechanics associated etiology, and the biological, biomechanical and clinical influences of IVDD. In addition, the application of FEA for evaluation and exploration of various treatments for IVDD is also elucidated. We conclude that FEA is an excellent technique for research of IVDD, which could be used to explore the etiology, biology and biomechanics of IVDD. In the future, FEA may help us to achieve the goal of individualized precision therapy.

Finite element simulation and clinical follow-up of lumbar spine biomechanics with dynamic fixations

Arthrodesis is a recommended treatment in advanced stages of degenerative disc disease. Despite dynamic fixations were designed to prevent abnormal motions with better physiological load transmission, improving lumbar pain and reducing stress on adjacent segments, contradictory results have been obtained. This study was designed to compare differences in the biomechanical behaviour between the healthy lumbar spine and the spine with DYNESYS and DIAM fixation, respectively, at L4-L5 level. Behaviour under flexion, extension, lateral bending and axial rotation are compared using healthy lumbar spine as reference. Three 3D finite element models of lumbar spine (healthy, DYNESYS and DIAM implemented, respectively) were developed, together a clinical follow-up of 58 patients operated on for degenerative disc disease. DYNESYS produced higher variations of motion with a maximum value for lateral bending, decreasing intradiscal pressure and facet joint forces at instrumented level, whereas screw insertion zones concentrated stress. DIAM increased movement during flexion, decreased it in another three movements, and produced stress concentration at the apophyses at instrumented level. Dynamic systems, used as single systems without vertebral fusion, could be a good alternative to degenerative disc disease for grade II and grade III of Pfirrmann.

Lumbar Single-Level Dynamic Stabilization with Semi-Rigid and Full Dynamic Systems: A Retrospective Clinical and Radiological Analysis of 71 Patients

BACKGROUND

This study compares the clinical and radiological results of three most commonly used dynamic stabilization systems in the field of orthopedic surgery.

METHODS

A total of 71 patients underwent single-level posterior transpedicular dynamic stabilization between 2011 and 2014 due to lumbar degenerative disc disease. Three different dynamic systems used include: (1) the Dynesys system; (2) a dynamic screw with a PEEK rod; and (3) a full dynamic system (a dynamic screw with a dynamic rod; BalanC). The mean patient age was 45.8 years. The mean follow-up was 29.7 months. Clinical and radiological data were obtained for each patient preoperatively and at 6, 12, and 24 months of follow-up.

RESULTS

Clinical outcomes were significantly improved in all patients. There were no significant differences in the radiological outcomes among the groups divided according to the system used. Screw loosening was detected in 2 patients, and 1 patient developed screw breakage. All patients with screw loosening or breakage underwent revision surgery.

CONCLUSIONS

Each procedure offered satisfactory outcome regardless of which system was applied.

Adjacent segmental degeneration following Wallis interspinous stabilization implantation: Biomechanical explanations and the value of magnetic resonance imaging

Adjacent segmental degeneration (ASD) is a major issue after pedicular fixation. This study examined the degeneration of the adjacent levels due to the insertion of the Wallis interspinous stabilization system compared with discectomy, using magnetic resonance imaging (MRI).Thirty-eight patients diagnosed with lumbar degeneration disorders at L4-L5 were reviewed: 19 patients underwent discectomy and Wallis system implantation (group A), and 19 patients underwent discectomy (group B). The Visual Analog Scale (VAS) and Oswestry Disability Index (ODI) were assessed preoperatively and postoperatively. ASD was evaluated by MRI.There was no difference in the preoperative ODI scores between the 2 groups (non-normal distribution, median, 50 (40, 50) vs 50 (50, 50), P = .331), but the postoperative ODI scores were different (non-normal distribution, median, 0 (0, 32) vs 20 (20, 30), P < .005). Similar results were observed for VAS. In group A, ASD occurred in 4 patients (21.1%) in the disc and 8 (42.1%) in the facet joint at L3/4, and in 4 (21.1%) in the disc and 5 (26.3%) in the facet joint at L5/S1. In Group B, ASD occurred in 3 patients (15.8%) in the disc at L3/4, and in 4 (21.1%) in the disc at L5/S1. In general, there was no difference between the 2 groups (P > .05), except at L3/4 (P = .015).ASD of the facet joint in the cranial segment occurred after Wallis system implantation, suggesting that the Wallis system cannot prevent ASD of the facet joint, but could have some other benefits for the discs.

Hybrid Rigid-Deformable Model for Prediction of Neighboring Intervertebral Disk Loads During Flexion Movement After Lumbar Interbody Fusion at L3-4 Level

Knowledge of spinal loads in neighboring disks after interbody fusion plays an important role in the clinical decision of this treatment as well as in the elucidation of its effect. However, controversial findings are still noted in the literature. Moreover, there are no existing models for efficient prediction of intervertebral disk stresses within annulus fibrosus (AF) and nucleus pulposus (NP) regions. In this present study, a new hybrid rigid-deformable modeling workflow was established to quantify the mechanical stress behaviors within AF and NP regions of the L1-2, L2-3, and L4-5 disks after interbody fusion at L3-4 level. The changes in spinal loads were compared with results of the intact model without interbody fusion. The fusion outcomes revealed maximal stress changes (10%) in AF region of L1-2 disk and in NP region of L2-3 disk. The minimal stress change (1%) is noted at the NP region of the L1-2 disk. The validation of simulation outcomes of fused and intact lumbar spine models against those of other computational models and in vivo measurements showed good agreements. Thus, this present study may be used as a novel design guideline for a specific implant and surgical scenario of the lumbar spine disorders.

Comparison of clinical and radiographic results between isobar posterior dynamic stabilization and posterior lumbar inter-body fusion for lumbar degenerative disease: A four-year retrospective study

PURPOSE

A retrospective study was conducted to compare clinical outcome with radiographic data and clinical complications between isobar posterior dynamic stabilization (IPDS, Scient'x, France) and posterior lumbar inter-body fusion (PLIF) for lumbar degenerative disease.

METHODS

113 consecutive patients (IPDS group, N=62; PLIF group, N=51) with lumbar degenerative disease were operated on between March 2009 and November 2011. Patient charts, radiographic films and medical records were reviewed. Clinical outcomes including the visual analog scale (VAS), Oswestry disability index (ODI) scores, and radiographic outcomes, including disk height index (DHI) and range of motion (ROM) were retrospectively analyzed.

RESULTS

The ODI and VAS leg and back pain scores in two groups were significantly improved at 6 and, 24 months and at the final follow-up (all, P<0.05). The degree of improvements in the ODI and VAS back pain scores, the incidence of complications and the rate of adjacent segment degeneration were similar in both groups (P>0.05). However, operation times and blood loss were significantly reduced in the IPDS group (P<0.05).

CONCLUSION

In summary, with similar symptoms improvement and complication rates, the results of this study demonstrate that IPDS is an effective and safe treatment for lumbar degenerative disease. There is currently insufficient evidence to indicate that the IPDS can avoid adjacent segment degeneration therefore, it is essential to conduct prospective, randomized, controlled multicenter studies with larger sample size and longer follow-up.

Biomechanical evaluation of a new pedicle screw-based posterior dynamic stabilization device (Awesome Rod System)--a finite element analysis

Background

Pedicle-screw-based posterior dynamic stabilization devices are designed to alleviate the rate of accelerated degeneration of the vertebral level adjacent to the level of spinal fusion. A new pedicle- screw-based posterior dynamic stabilization device- the Awesome Dynamic Rod System was designed with curve cuts on the rods to provide flexibility. The current study was conducted to evaluate the biomechanical properties of this new device.

Methods

Finite element models were developed for the intact spine (INT), the Awesome Dynamic Rod Implanted at L4-L5 (AWE), a traditional rigid rod system implanted at L4-L5 along with an interbody cage (FUS), and the Awesome Dynamic Rod System implanted at L4-L5 along with an interbody cage as an adjunct to fusion procedures and extension of dynamic fixation to L3-L4 (AWEFUS). The models were subjected to axial loads and pure moments and evaluated by a hybrid method on range of motion (ROM)s, disc stresses, pedicle screws stresses, and facet joint contact forces.

Results

FUS sustained the lowest L4-L5 ROM decrement in flexion and torsion. AWE demonstrated the lowest adjacent level ROM increment in all moments except for extension at L3-L4, and AWEFUS showed the greatest ROM increment at L2-L3. AWE demonstrated lowest adjacent segment disc stress in flexion, lateral bending and torsion at L3-L4. AWEFUS showed the highest disc stress increment in flexion, extension, and lateral bending, and the lowest disc stress decrement in torsion at L2-L3. AWE sustained greater adjacent facet joint contact forces than did FUS in extension and lateral bending at L3-L4, and AWEFUS demonstrated the greatest contact forces concentrating at L2-L3.

Conclusion

The results demonstrate that the Awesome Dynamic Rod System preserved more bridged segment motion than did the traditional rigid rod fixation system except in extension. However, the Awesome Dynamic Rod System bore a greater facet joint contact force in extension. The Awesome Dynamic Rod System did protect the adjacent level of fusion segments, but led to much greater ROM, disc stresses, and facet joint contact forces increasing at the adjacent level of instrumented segments.