Study Design: A finite element analysis of the screw pullout procedure for the osteoporotic cancellous bone using screw-bone unit model without cortical layer. Objective: The objective is to determine the region of effect (RoE) during the screw pullout procedure and predict the proper amount of injection cement (AIC) in screw augmentation. Summary of Background Data: For the osteoporotic spine, the AIC is a critical factor for the augmentation screw performance and leakage risk. There are few studies on the proper AIC in literature. Methods: Three finite element models were established, 2 screw-foam models were used for validation study, and 1 screw-bone model was used for investigation of RoE and AIC. The simulations of screw pullout were conducted. A velocity loading of 0.01 mm/s with a maximum displacement of 2.7 mm was applied on the screw. For the validation, the screw-foam models with 2 different densities were used for comparison of pullout force with those published experimental data. After validation, the screw-bone model was used to investigate the RoE and predict the proper AIC during screw augmentation in spine surgery. Results: In validation, the predicted pullout strengths were 2028.8 N for high-density foam model and 607 N for low-density foam model, respectively. They were in good agreement with those of the published experiment. In the screw-bone model, the simulations demonstrated that the RoE changed with the displacement of screw and reached the maximum when the displacement of screw was 1.8 mm. Similar trend was found for the AIC with the displacement. The proper AIC was 2.6 mL when the displacement of screw was 1.8 mm in this study. Conclusions: The RoE and proper AIC for augmentation were evaluated in the osteoporotic spine. This information could provide practical reference for screw augmentation in spinal decompression and instrumentation in the spine surgery.

In this study, the effects of threshold variation in image segmentation of micro CT images of cancellous bone in the determination of the architectural parameters and stiffness were investigated. A total of 42 samples of 6 × 6 × 6 mm3 cubes with threshold values set between 500–1100 greyscale in increment of 100 of CT images of six human C5 vertebral bodies were analyzed. Threshold value of 800, based on Otsu's method, was set for the control group. From various threshold values, the respective architectural parameters, and the corresponding stiffness in three orthotropic directions (Exx, Eyy, Ezz) of each cube were computed from the voxel-based micro-finite element models under compressive simulation. The results showed that 1% variation of threshold value resulted in a 3.4% variation in BV/TV, 2% in Tb.N, 3.1% in Tb.Th, 2.9% in BS/BV, 1.8% in Tb.Sp, 29.2% in Exx, 28.7% Eyy and 27.7% in Ezz. Statistical analysis showed that 2.9% threshold variation caused significant change in BV/TV, Tb.Th, Exx, Eyy and Ezz values. The study shows that with threshold variation of more than 2.9%, significant differences in the architectural parameters and stiffness compared to those based on Otsu's method.

Background: The mechanical response of the spinal cord during burst fracture was seldom quantitatively addressed and only few studies look into the internal strain of the white and grey matters within the spinal cord during thoracolumbar burst fracture (TLBF).The aim of the study is to investigate the mechanical response of the spinal cord during TLBF and correlate the percent canal compromise (PCC) with the strain in the spinal cord.Methodology/Principal Findings: A three-dimensional (3D) finite element (FE) model of human T12-L1 spinal cord with visco-elastic property was generated based on the transverse sections images of spinal cord, and the model was validated against published literatures under static uniaxial tension and compression.With the validated model, a TLBF simulation was performed to compute the mechanical strain in the spinal cord with the PCC.Linear regressions between PCC and strain in the spinal cord show that at the initial stage, with the PCC at 20%, and 45%, the corresponding mechanical strains in ventral grey, dorsal grey, ventral white, dorsal white matters were 0.06, 0.04, 0.12, 0.06, and increased to 0.14, 0.12, 0.23, and 0.13, respectively.At the recoiled stage, when the PCC was decreased from 45% to 20%, the corresponding strains were reduced to 0.03, 0.02, 0.04 and 0.03.The strain was correlated well with PCC.Conclusions/Significance: The simulation shows that the strain in the spinal cord correlated well with the PCC, and the mechanical strains in the ventral regions are higher than those in the dorsal regions of spinal cord tissue during burst fracture, suggesting that the ventral regions of the spinal cord may susceptible to injury than the dorsal regions.

Truly representative architectural parameters of trabeculea can be extremely difficult to achieve based on scanning images because of variable porosity and distribution of trabeculae within the specific overall scanned volume of bone. Accordingly, in present study different selective volume of interests, measured from centroid of μ-CT scanned human vertebral body, were analyzed to determine the architectural parameters (BV/TV, BS/BV, Tb.Th, Tb.N, Tb.Sp) of trabeculae within these volumes and to suggest an optimal volume for representative architectural parameters of the overall scanned volume. Nonlinear curve fitting method was also applied to obtain the correlation between the parameters and the volume of interests. The results show different volumes of interests give different morphological indices of BV/TV, BS/BV, Tb.N and Tb.Sp within a specific scanned vertebral body. Tb.Th shows relatively small variation (0.8%) even with sample volume of less than (2 mm)3. Statistical analysis shows that with sample volume of less than (6 mm)3, significant different in the measured BV/TV comparing against the control group. Tb.N and Tb.Sp show significant different values against the control group for volume of interest less than (4 mm)3 and (5 mm)3, respectively. However, no significant differences were observed in the indices of BS/BV and Tb.Th. Present study shows that an optimal volume of interests of greater than (6 mm)3 be selected to predict the architectural parameters of trabeculae of human vertebral bodies.

To establish a Finite element model of thoracolumbar spinal cord and investigate the injury mechanism caused by burst fracture using finite element simulation. A three-dimensional finite element model of human spinal cord at T-12 and L-l level was developed. The model was validated with previous published literatures in terms of uniaxial tension, compression. Then, a burst fracture compressive simulation was performed to study the spinal cord injury mechanism. The strain distribution in the eight function regions of white and grey matter in transverse section was quantitatively documented with the bony fragments' enroachment. When the encroachment increased to 2.7 mm, the strain in ventral cord tissue (AHMN (DM): 0.18, AHMN (PM): 0.26, VTF: 0.29 , ASCT: 0.04) is higher than dorsal (SG: 0.02, CN: 0.05, PSCT: 0.01, FG: 0.02). As the encroachement increased to maximum value (5.4 mm), the strains had no difference between ventral and dorsal spinal cord tissue.

Increasingly, musculoskeletal models of the human body are used as powerful tools to study biological structures. The lower limb, and in particular the foot, is of interest because it is the primary physical interaction between the body and the environment during locomotion. The goal of this paper is to adopt the powerful finite element (FE) modelling and analysis approaches to create a state-of-the-art 3D coupled foot-boot model for future studies on biomechanical investigation of stress injury mechanism, foot wear design and parachute landing fall simulation. In the modelling process, the foot-angle model with lower leg was developed first based on 2D medical images (CT images) using softwares of ScanIP (image processing software), Surfacer and ANSYS. Then, the boot was represented by assembling the FE models of upper, insole, midsole and outsole built based on the FE model of the foot-angle, and finally the coupled foot-boot model was generated by putting together the models of the lower limb and boot.

Increasingly, musculoskeletal models of the human body are used as powerful tools to study biological structures. The lower limb, and in particular the foot, is of interest because it is the primary physical interaction between the body and the environment during locomotion. The goal of this paper is to adopt the finite element (FE) modeling and analysis approaches to create a state-of-the-art 3D coupled foot–boot model for future studies on biomechanical investigation of stress injury mechanism, foot wear design and parachute landing fall simulation. In the modeling process, the foot–ankle model with lower leg was developed based on Computed Tomography (CT) images using ScanIP, Surfacer and ANSYS. Then, the boot was represented by assembling the FE models of upper, insole, midsole and outsole built based on the FE model of the foot–ankle, and finally the coupled foot–boot model was generated by putting together the models of the lower limb and boot. In this study, the FE model of foot and ankle was validated during balance standing. There was a good agreement in the overall patterns of predicted and measured plantar pressure distribution published in literature. The coupled foot–boot model will be fully validated in the subsequent works under both static and dynamic loading conditions for further studies on injuries investigation in military and sports, foot wear design and characteristics of parachute landing impact in military.

The main objective of this work is the demonstration of a conceptual approach for efficient interspinous implant evaluation and design using finite element (FE) method. A novel dynamic stabilization system (Interspinous spacer (ISS), registered in France under FR05/003524 and FR05/11161) was developed, which has an important role in the treatment of the degenerative lumbar spine compared to conventional surgical treatment -spinal fusion for chronic low back pain due to degenerative disorders in the lumbar spine. Anatomically realistic FE models of healthy, nucleotomized and implanted models of lumbar L4-L5 motion segment were exercised the physiological loading configurations under flexion, extension, lateral bending and axial rotation to investigate the biomechanical effect of the ISS by comparison of the load-response curves of the three models. Under flexion and extension, the response curves show that the range of motion of the implanted model is greatly reduced in relation to the healthy and nucleotomized models. In lateral bending mode, the response curves show that the implanted segment relative less stiff compared to the nucleotomized segment. Under axial rotation, the response curves suggests that the interspinous relatively effective in the axial rotation mode without affecting the kinematics of the L4-L5 segment. Based on the implanted L4-L5 model, the interspinous spacer has the capability of restoring back the functionality of the dissected posterior ligaments, and the kinematics of the implanted model and nucleotomized model are of similar range. The much greater stiffness of the implanted segment under sagittal bending is an issue needed to be addressed, as the motion is greatly limited.

A finite element model of the T12-L1 motion segment was subjected to dynamic vertical impact to investigate vertebral burst fracture mechanism at the thoracolumbar junction. A rigid ball was directed vertically towards a rigid plate fixed on top of the T12 vertebral body to simulate the axial impact. The results show that upon impact, the T12 vertebra exhibited a vibratory motion. At its maximum compression, the endplates bulged towards their vertebral bodies. The central parts of the endplates adjacent to the nucleus experienced the highest effective stress, and localized stress concentration developed correspondingly within the central parts of the cancellous bone adjacent to the endplates. This appears to confirm the hypothesis that nucleus material is forced to enter the vertebral body, pressurizing it further and squeezing the fat and marrow contents out of the cancellous bone. When the nucleus material enters the vertebral body faster than fat and marrow being expulsed, the vertebral body could burst through the anterior and posterior cortical shell. Upon sudden posterior cortex fracture, the transient fragment encroachment could be further into the spinal canal than the final observed locations, as the fragments are retropulsed to the vertebral body during the bursting process.

Spinal stenosis can be found in any part of the spine, though it is most commonly located on the lumbar and cervical areas. It has been documented in the literature that bilateral facetectomy in a lumbar motion segment to increase the space induces an increase in flexibility at the level at which the surgery was performed. However, the result of bilateral facetectomy on the stability of the thoracolumbar spine has not been studied. A nonlinear three-dimensional finite element (FE) model of thoracolumbar T11–L1 was built to explore the influence of bilateral facetectomy. The FE model of T11–L1 was validated against published experimental results under various physiological loadings. The FE model with bilateral facetectomy was evaluated under flexion, extension, lateral bending and axial rotation to determine alterations in kinematics. Results show that bilateral facetectomy causes increase in motion, considerable increase in axial rotation and least increase in lateral bending. Removal of facets did not result in significant change in the sagittal motion in flexion and extension.

An anatomically realistic finite element (FE) model of thoracolumbar spine T11L1 was developed based on the captured digitized-geometrical properties of the dried cadaveric vertebrae (T11, T12 and L1). The geometrical data were then exported to ANSYS 10 for the three-dimensional (3D) FE meshing. Cortical shell, cancellous core, the intervertebral disc and posterior elements were modeled using eight-noded brick elements. The annulus fibres and ligaments were represented by cable elements. The intact T11-L1 FE model was validated against experimental results reported in literature under various physiological loadings. Spinal stenosis is one of the most frequently experienced spine diseases, and bilateral facetectomy is the surgical treatment to advocate decompression. Therefore, in this study, the validated FE model was used to investigate the effect of this surgical bilateral facetectomy procedure by resecting the two articulating facets of the stenosis T12-L1 motion segment. The objective of this study is to investigate the influence of bilateral facetectomy on the stability of the thoracolumbar spine under different physiological motions (flexion, extension, lateral bending and axial rotation) to evaluate changes in kinematics. The FE model shows that bilateral facetectomy results in increase in motion with significant increase in axial rotation and least increase in lateral bending.

The purpose of this study was to compare the kinematics in terms of the locations and loci of instantaneous axes of rotation (IARs) at levels T11-T12 and T12-L1 of thoracolumbar junction (TLJ). The IAR is one of the kinematics characteristics of a functional spinal unit (FSU) in a plane under load. There is little information about loci of IARs in the TLJ. Validated finite element (FE) models of T11-T12 and T12-L1 FSUs were used to determine the locations and loci of IARs in three anatomical planes. In the sagittal plane, the locations and loci of the IARs were located below the intervertebral disc for T11-T12, and situated in the intervertebral disc for T12-L1. In the frontal plane, they were all located around the mid-sagittal plane for T11-T12 and T12-L1. In the transverse plane, they fell in the medio-anterior region of the movable vertebra T11 for T11-T12, and located near the cortical shell of the upper vertebra T12 for T12-L1. These findings may offer an insight to better understanding the kinematics of the human thoracolumbar spine and provide clinically relevant information for the evaluation of spinal stability and functionality of implant devices.

Results. The simulated kinematics of the head-neck complex showed relatively good agreement with the experiment with most of the predicted peak values fell within one standard deviation of the experimental data [2]. Under rear impact, the whole C0-T1 structure formed a S-shaped curvature with flexion at the upper levels and extension at the lower levels at early stage after impact, during which the lower cervical levels might experience hyperextensions. The results showed that the capsular ligament (CL) should be the major concern under the rear impact condition, followed by anterior longitudinal ligament (ALL). In current study, only the peak strain values of CL were above its failure limit under both accelerations. Under 8G, the ALL also exceeded its failure strains. The strain value of posterior longitudinal ligament kept low under both conditions, while there is definitely no tension in ligamentum flavum and inter spinous ligament. The peak impact acceleration has significant effect on the potential injury of ligaments. Under higher acceleration, more ligaments will reach failure strain at much shorter time immediately after impact. Conclusion. The current model was identified to be available for human neck injury study.

In current study, a detailed three-dimensional C0-C7 FE model of the whole head-neck complex developed previously was modified to include T1 vertebra. Rear impact accelerations of different conditions were applied to Tl inferior surface to validate the simulated variations of the intervertebral segmental rotations of the cervical spine. The simulated kinematics of the head-neck complex showed relatively good agreement with the experimental data. Under rear impact, the whole C0-T1 structure formed a S-shaped curvature with flexion at the upper levels and extension at the lower levels at early stage after impact, during which the lower cervical levels might experience hyperextensions. The current model was identified to be available for human neck injury study.

Degenerative changes to the intervertebral disc structure appear gradually in the aging process, and are likely to affect the fracture patterns of the thoracolumbar vertebral body — a common site of spinal injuries — by changing the mechanical interaction within each motion segment. This study focused on the influence of disc degeneration on the mechanism of load transmission through the thoracolumbar vertebral body. Compressive stresses and ways of load transmission were examined in cases of normal and degenerated discs. The stress analysis was performed using the finite element (FE) method. For normal discs, the highest compressive stresses were found in the central region of the discs, corresponding to the anatomical nucleus. Such high stress made the endplates bulge towards the vertebral bodies. Hence, the fracture would most probably start in the center of the endplate. For degenerated discs, the peak stresses rose to high levels in the annuli. Thus, the annulus was mostly under compression, and so was the periphery of the endplate, with much less deflection at its center. The failure of the vertebra was due to the fracture of the periphery of the endplate. Ways of load transmission through the vertebral body were strongly affected by the disc condition.

The objective of this study was to build and validate the FE models of thoracolumbar junctional T11-T12 and T12-L1 functional spinal units (FSUs) and compare the biomechanical responses of the two FSUs under physiological loading modes: flexion, extension, lateral bending and axial rotation. Anatomically accurate FE models of thoracolumbar T11-T12 and T12-L1 FSUs were developed and validated against published experimental results in terms of load displacement responses and range of motion (ROM) under flexion and extension pure moments of 7.5 Nm, left and right lateral bending pure moments of 7.5 Nm and left and right axial torque of 7.5 Nm. The overall responses predicted by the T11-T12 and T12-L1 FE models showed differences in stiffness under different load configurations. Amongst all loading configurations, the motions at T11-T12 and T12-L1 were the stiffest under axial torque. The lateral bending motions of T11-T12 and T12-L1 were relatively flexible. Under sagittal moments, the motion in extension was greater than in flexion at level T11-T12, while the rotation in flexion was greater than in extension at level T12-L1.

The purpose of this study was to determine the locations and loci of instantaneous axes of rotation (IARs) of the T10-T11 motion segment in flexion and extension. An anatomically accurate three-dimensional model of thoracic T10-T11 functional spinal unit (FSU) was developed and validated against published experimental data under flexion, extension, lateral bending, and axial rotation loading configurations. The validated model was exercised under six load configurations that produced motions only in the sagittal plane to characterize the loci of IARs for flexion and extension. The IARs for both flexion and extension under these six load types were directly below the geometric center of the moving vertebra, and all the loci of IARs were tracked superoanteriorly for flexion and inferoposteriorly for extension with rotation. These findings may offer an insight to better understanding of the kinematics of the human thoracic spine and provide clinically relevant information for the evaluation of spinal stability and implant device functionality.

STUDY DESIGN:A study using an anatomically accurate finite-element model of a L2-L3 motion segment to investigate the biomechanical effects of graded bilateral and unilateral facetectomies of L3 under flexion and extension loadings.OBJECTIVE:To predict the amount of facetectomy on lumbar motion segment that would cause segmental instability, therefore enhancing the understanding concerning the role of the facet under sagittal loadings.SUMMARY OF BACKGROUND DATA:This study provides a quantitative study on the role of facets in preserving segmental lumbar stability. Previous analytical models lack of three-dimensional structural characterization and insufficient element representation for facet joints.METHODS:A validated finite-element L2-L3 model was subjected to sagittal loadings at 7.5 Nm. Effects of ligaments and facets were examined to establish their relative importance on segment response. The effect of iatrogenic changes (graded unilateral and bilateral facetectomy) was then investigated under these loadings to predict the alterations in terms of gross external (angular and coupled) responses, flexibilities, and facet load.RESULTS:This study shows the importance of preserving ligaments to prevent rotational instabilities for motion segment under flexion. The effect of the facetectomy on the motion segment is insignificant under flexion. In extension, unilateral facetectomy and resection on contralateral facet markedly alters the rotational motion and flexibilities as well as coupled motions. Also, unilateral complete facetectomy with resection of less than 100% on contralateral facet generates high facet load.CONCLUSIONS:Clinically, this study suggests that it may be appropriate to incorporate additional stabilization procedure in restoring the spinal strength and stability for surgical intervention of unilateral complete facetectomy and resection on contralateral facet. The exploitation of the finite-element method to simulate clinically related situations permits an improved understanding of lumbar spinal stability to assist in defining clinical expectation for various forms of surgical intervention of the operative procedures.