Kim-Kheng Lee

Finite element analysis of cervical spinal instability under physiologic loading.

The definition of cervical spinal instability has been a subject of considerable debate and has not been clearly established. Stability of the motion segment is provided by ligaments, facet joints, and disc, which restrict range of movement. Moreover, permanent damage to one of the stabilizing structures alters the roles of the other two. Although many in vitro studies have been conducted to investigate cervical injuries, to date there are only limited finite element investigations reported in the literature on the biomechanical response of the cervical spine in these respects. A comprehensive, geometric, nonlinear finite element model of the lower cervical spine has been successfully developed and validated under compression, anterior–posterior shear, and sagittal moments. Injury studies were done by varying each spinal component independently from the validated model. Seven analyses were conducted for each injury simulation (model without ligaments, model without facets, model without facets and ligaments, and model without disc nucleus). Results indicate that the role of the ligaments in resisting anterior and posterior shear and flexion and axial rotation moments is important. Under other physiologic loading (anterior–posterior shear, flexion–extension, lateral bending, and axial rotation), the disc nucleus is responsible for the initial stiffness of the cervical spine. The results also highlight the importance of facets in resisting compression at higher loads, anterior shear, extension, lateral bending, and torsion. The results provide new insight through injury simulation into the role of the various spinal components in providing cervical spinal stability. These findings seem to correlate well with experimental results as well as with common clinical experience.

Vibration Characteristics of the Human Spine Under Axial Cyclic Loads : Effect of Frequency and Damping

Study Design. A nonlinear finite element model of lumbar spine segment L3–L5 was developed. The effects of upper body mass, nucleus injury, damping, and different vibration frequency loads were analyzed for the whole body vibration. Objectives. To analyze the influence of whole body vibration on facets of lumbar spine and to analyze the influence of nucleus injury, upper body mass, and damping on the dynamic characteristics of lumbar spine. Summary of Background Data. Many studies have investigated whole body vibration for lumbar spine. However, very few investigations analyzed the influence of whole body vibration on facets and vibration characteristics of the injured spine. Methods. The nonlinear finite element model of the L3–L5 segment was constructed based on the embalmed vertebra geometry and validated. Besides static and modal analyses, transient dynamic analyses were also conducted on the model with an upper body mass under damping and different frequency cyclic loads. Results. In the period of human spine vibration, the vibration effects of different regions of the lumbar spine are not the same. Anterior regions of the L3–L5 segment show small vibration amplitudes, but posterior regions show large amplitudes. The vibration amplitude of facet contact force is more than 2.0-fold as large as that of displacement and stress on vertebrae or discs. To decrease the weight of the upper body will increase the resonant frequency. To remove the nucleus will decrease the resonant frequencies. The vibration displacement, stress, and facet contact force will reduce generally by 50% using damping ratio 0.08. Conclusions. The posterior regions of intervertebral discs of the lumbar spine are easy to injure during long-term whole body vibration compared to anterior regions. The vibration of human spine is more dangerous to facets, especially during whole body vibration approximating a sympathetic vibration, which may lead to abnormal remodeling and disorder of the lumbar spine.

Effects of cervical cages on load distribution of cancellous core: a finite element analysis.

Methods: The study was 1 designed to analyze the load distribution of the cancellous core after implantation of vertical ring cages made of titanium, cortical bone, and tantalum using the finite element (FE) method. The intact FE model of C5–C6 motion segment was validated with experimental results. Results: The percentage of load distribution in cancellous core dropped by about one-third of the level for the intact model after the cage implantation. The difference among cages made of different materials (or different stiffnesses) was not very obvious. Conclusions: These results implied that the influence of the cage on the load transfer in the cancellous core is greatly related to the cage’s dimensions and position within the intervertebral space. The dimension and position of the cage that least disturb the load distribution in cancellous core could be criteria in cage development.

Validation of T10-T11 finite element model and determination of instantaneous axes of rotations in three anatomical planes.

Study Design. A finite element (FE) model of thoracic spine (T10-T11) was constructed and used to determine instantaneous axes of rotation (IARs). Objectives. To characterize the locations and loci of IARs in three anatomic planes. Summary of Background Data. The center of rotation is a part of a precise method of documenting the kinematics of a spinal segment for spinal stability and deformity assessments and for implant devices study. There is little information about loci of IARs in thoracic spine. Methods. A FE model of thoracic spine (T10-T11) was developed and validated against published data. The validated model was then used to determine the locations and loci of IARs in three anatomic planes. Results. Within the validated range, The IARs locations and loci were found to vary with the applied pure moments. Under flexion and extension pure moments, the loci of IARs were tracked anterosuperiorly for flexion and posteroinferiorly for extension with rotation between the superior endplate and the geometrical center of the inferior vertebra T11. Under left and right lateral bending pure moments, the loci were detected to diverge latero-inferiorly from the mid-height of the intervertebral disc, then converge medio-inferiorly toward the geometrical center of the inferior vertebra T11. For axial rotation, the IARs were located between anterior nucleus and anulus and found to diverge in opposite direction latero-posteriorly with increasing left and right axial torque. Conclusions. The results of IARs would provide further understanding to the kinematics and biomechanical responses of the human thoracic spine, which is important for the diagnosis of disc degeneration and implant study.

Determination of Load Transmission and Contact Force at Facet Joints of L2-L3 Motion Segment using FE Method

In the human spine, it is well known that facet joints play a significant role in load transmission and providing stability. It has also been hypothesized to be one of most probable sources of low back pain. Experimental determination of the load-bearing role of lumbar intervertebral joints, such as the facets joints, under axial compression has not been a straightforward task. In this study, the role of the facets in load transmission through a L2–L3 motion segment under axial compression is investigated using a L2–L3 finite element (FE) model, incorporated with an accurate three-dimensional geometry of facet joints with the inclusion of surface-to-surface continuum contact representation. The effects of osteoarthritis on facet force and biomechanical behaviors are also investigated by assuming friction at the facet joints. The study shows that the facet joints resisted 8% more in load for joints with osteoarthritics as compared with the normal joints. High percentage increase in contact facet force was also predicted for joint with osteoarthritics deformity. The use of the analytical FE model provided yet another efficient alternative for predicting the load transmission and contact force for degenerative joints, so as to provide a better understanding of the biomechanics of the spine as well as the pathophysiology of the various spinal disorders and degenerative conditions.

Kinematics of the thoracic T10-T11 motion segment: Locus of instantaneous axes of rotation in flexion and extension

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.

BIOMECHANICAL EFFECT OF THORACIC POSTERIOR VERTEBRAL ELEMENTS ON PATTERNS OF THE LOCI OF INSTANTANEOUS AXES OF ROTATION IN SAGITTAL PLANE

The objective of this study was to investigate the effect of the thoracic posterior vertebral elements on the kinematics of T10–T11 motion segment in sagittal plane by assessing the locations and loci of the instantaneous axes of rotation (IARs) under flexion and extension pure moments using finite element (FE) method. The IAR has proven to be a useful parameter of vertebral motion and it is an indicator of spinal instability. An anatomically accurate FE model of thoracic T10–T11 functional spinal unit (FSU) was used to characterize the loci of centers of rotation for the intact T10–T11 FSU and disc body unit (without posterior vertebral elements) under flexion and extension pure moments. The centers of rotation predicted by the intact model and disc body unit of thoracic T10–T11 for both flexion and extension were directly below the geometrical center of the moving vertebra. However, the loci of the IARs were significantly affected by the posterior vertebral elements. The loci of instantaneous axes of ro...