Hong-Wan Ng

Nonlinear Finite-element Analysis of the Lower Cervical Spine (c4–c6) Under Axial Loading

This study was conducted to develop a detailed, nonlinear three-dimensional geometrically and mechanically accurate finite-element model of the human lower cervical spine using a high-definition digitizer. This direct digitizing process also offers an additional method in the development of the finite-element model for the human cervical spine. The biomechanical response of the finite-element model was validated and corresponded closely with the published experimental data and existing finite-element models under axial compressive loading. Furthermore, the results indicated that the cervical spine segment response is nonlinear with increasing stiffness at higher loads. As a logical step, a parametric study was conducted by evaluating the biomechanical response related to the changes in the modeling techniques of the finite-element model and the mechanical properties of the disk annulus. Variations of the predicted horizontal disk bulge were investigated under axial compressive displacements for the normal model, the model without facet articulations, and the model without nucleus. Removal of nucleus fluids causes an inward bulge of the inner annulus layers, with the displacement magnitude dependent on external loads. The result indicates that the nucleus fluid plays an important role in cervical spine mechanics. Simulated facetectomy indicates a decrease in the stiffness of the cervical spine. The study shows that, in reality, the stiffness of the lower cervical spine depends closely on factors such as the spinal geometry and physical properties, thereby resulting in various force and displacement responses.

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.

The biomechanics of lumbar graded facetectomy under anterior-shear load

In this paper, an anatomically accurate three-dimensional finite-element (FE) model of the human lumbar spine (L2-L3) was used to study the biomechanical effects of graded bilateral and unilateral facetectomies of L3 under anterior shear. The intact L2-L3 FE model was validated under compression, tension, and shear loading and the predicted responses matched well with experimental data. The gross external (translational and coupled) responses, flexibilities, and facet load were delineated for these iatrogenic changes. Results indicted that unilateral facetectomy of greater than 75% and bilateral facetectomy of 75% or more resection markedly alter the translational displacement and flexibilities of the motion segment. This study suggests that fixation or fusion to restore strength and stability of the lumbar spine may be required for surgical intervention of greater than 75% facetectomy.

Finite-element analysis for lumbar interbody fusion under axial loading

A parametric study was conducted to evaluate axial stiffness of the interbody fusion, compressive stress, and bulging in the endplate due to changes in the spacer position with/without fusion bone using an anatomically accurate and validated L2-L3 finite-element model exercised under physiological axial compression. The results show that the spacer plays an important role in initial stability for fusion, and high compressive force is predicted at the ventral endplate for the models with the spacer and fusion bone together. By varying the positioning of the spacer anteriorly along anteroposterior axis, no significant change in terms of axial stiffness, compressive stress, and bulging of the endplate are predicted for the implant model. The findings suggest that varying the spacer position in surgical situations does not affect the mechanical behavior of the lumbar spine after interbody fusion.

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.

Influence of Cervical Disc Degeneration after Posterior Surgical Techniques in Combined Flexion-Extension—A Nonlinear Analytical Study

Laminectomy and facetectomy are surgical techniques used for decompression of the cervical spinal stenosis. Recent in vitro and finite element studies have shown significant cervical spinal instability after performing these surgical techniques. However, the influence of degenerated cervical disk on the biomechanical responses of the cervical spine after these surgical techniques remains unknown. Therefore, a three-dimensional nonlinear finite element model of the human cervical spine (C2-C7) was created. Two types of disk degeneration grades were simulated. For each grade of disk degeneration, the intact as well as the two surgically altered models simulating C5 laminectomy with or without C5-C6 total facetectomies were exercised under flexion and extension. Intersegmental rotational motions, internal disk annulus, cancellous and cortical bone stresses were obtained and compared to the normal intact model. Results showed that the cervical rotational motion decreases with progressive disk degeneration. Decreases in the rotational motion due to disk degeneration were accompanied by higher cancellous and cortical bone stress. The surgically altered model showed significant increases in the rotational motions after laminectomies and facetectomies when compared to the intact model. However, the percentage increases in the rotational motions after various surgical techniques were reduced with progressive disk degeneration.

Influence of preload magnitudes and orientation angles on the cervical biomechanics: A finite element study

Objective: Although a number of in vivo, in vitro, and finite element studies have attempted to delineate the natural biomechanics, injury mechanisms, and surgical techniques of the cervical spine, none has explored the influence of various preload magnitudes and orientations on the biomechanical responses. Methods: A nonlinear three-dimensional finite element model of the lower cervical spine (C5–C6) was used for this study. The model was tested under four preload magnitudes and three orientations. For every preload, magnitude, and orientation, pure moments of 1.8 Nm were applied to the superior surface of the moving vertebra (C5) in flexion, extension, lateral bending, and torsion. The resulting rotational motions were obtained and compared against literature data Results: The predicted biomechanical responses under the same loading directions varied, depending on the preload magnitudes and orientations. With flexion and extension, increasing the preload magnitudes and varying the C5–C6 orientation in the sagittal plane changed the rotational motions by 1% and 18%, respectively. Under normal orientation and with increasing preload magnitudes, flexion and extension increased, whereas lateral bending and torsion decreased. These changes were found to be influenced by several spinal components: posterior facets, passive ligaments, and stiffening of the intervertebral disc. The predicted responses under the direction of loading varied significantly, depending on the preload magnitudes and orientations. Under fixed preload magnitudes and varying the three types of orientations, rotational motions were not affected under flexion but changed under extension, lateral bending, and axial rotations. Under normal orientation and increasing preload magnitudes, biomechanical responses under flexion and extension increased, whereas lateral bending and torsion decreased. Changes in the predicted responses were found to be influenced by several spinal components: posterior facets, passive ligaments, and stiffening of the intervertebral disc. Conclusion: The findings of the current study were important for the further understanding of the cervical biomechanics during in vitro testing.

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.