Three-Dimensional Finite Element Analysis In Degenerative Lower Lumbar Spine Of Multiple Factor

Study Design:Foundation and validation of FE model of lumbar motion segment. Objective:To present a new kind of CAD method for constructing a detailed, 3-D, anatomically accurate FE model of lumbar L4-L5 segment from CT data and to thoroughly validate it.Summary and Background Data:Currently there are many spinal FE modeling approaches proposed in the literature, which can be roughly ranged into three major groups based on the source data used. One group extracts geometric information from medical image, one group is based on direct measurement on embalmed vertebra using a digitiser, the third group uses anatomy data from literature. With the manual operation process being tedious and the error bigger, the modelling method basing on digitiser only can survey on the embalmed vertebra of the corpse, cannot completely embody complex anatomical structure such as the physical curve of the spine and is unable to realize the spinal FE modelling individuation. With the modelling technology being rough, the modelling method using anatomy data (for example geometrial model modelling of lumbar spine basing on 3D-MAX software) belongs to the manual modelling method, which is unable to realize the FE modelling digitization and does not contain any advanced algorithm, so the overseas FE studies commonly does not adopt it. Respecting its advantage of automation, standard and prompt modelling, the spinal FEmodeling approach basing CT scans is the predominant research method of reconstruction of lumbar geometry model.At present the lumbar FE modelling methods basing on CT mostly belong to indirect modelling approaches. These kind of indirect modelling approaches first set up the geometry model in other CAD software, which was then imported into FE software through the data connection to carry on the grid division. However, though the modelling efficiency and the quality of the mesh partition of the spinal FE modelling approach basing CT scans have upgraded than that of Goel(1988). When acquiring CT scans data, most methods still took the primitive geometrial data from superposition of monolayer profile information. Meanwhile, the base geometry information was extracted on cross-section planes parallel to orthogonal plane of the CT images, which does not properly account for the preferential orientation feature (e.g. lordosis) of the lumbar spine . Besides, the request of the size and density of FE elements in the different regions of lumbar spine is not uniform, and the foregoing modelling approaches lack the corresponding control mechanism.Methods:A modified "no-seed region segmentation" was done to extract the interest region in the CT scan images and produce a binary image. "Best cross-section planes" accounting for the preferential direction dictated by lumbar spine were placed on the initial iso-surface model, forming a "non-regular piecewise subspace". This subspace and the embedded iso-surface mode were transformed by local affine transforms to a "regular subspace", in which a surface mesh of high quality was generated quickly. Finally a reverse transform procedure was employed to recover the shape feature of the lumbar surface mesh of lumbar L4-L5 in the original 3-D space, which was then importing into ANSYS for the 3-D FE mesh construction. All nodes of the inferior surface of L5 vertebral body and its spinous process were fixed in 6 freedom degree of translation and rotation. Axial compressive force of 3000N and flexion,extension, lateral bending and axial torsion moment of 15Nm were applied in given increments. Under the same boundary conditions, the predicted results of the current FE model are compared with the results from experimental studies in vitro.Results:The developed FE model consisted of 94794 solid elements, 1196 link elements, 1170 shell elements, 768 target elements and 464 contact elements. The model well replicated the actual geometry of all complicated anatomical features of the spine. Three types of non-linearities (i.e. geometrical, material and contact non-linearity) exhibited by the lumbar motion segment were incorporated into the model. The predicted results of FE model correlated well with experimental data under similar loading configurations.Conclusion:Accurately represented surface model of L4-L5 segment implements the total digitization of extraction of binary imaging and reconstruction of lumbar lordosis, taking on the best simulation. The current non-linear FE model of L4-L5 segment acquires adequate validation under different loading condition.Objective:To develop and validate 3-D FE models of the degenerative lumbar L4-L5 segment with different morphological characteristics using CAD technique.Methods:A series of new CAD methods were used to accurately establish FE model of lumbar L4-L5 motion segment. Humane interactive modification means is employed to construct the "interface" which divided the surface model of L4-L5 segment into two basic "structure module" of anterior vertebral body and posterior structure. 9 surface models of the degenerative lumbar spine were constituted by the basic "structure modules" by altering the parameter of disc height, endplate concave angle, sagittal angle of facet joint and lordosis angle of the intervertebral disc. The data of surface model were respectively input into ANSYS to form FE models of "structure module", which then constituted FE models of degenerative lumbar spine with different morphous through gluing and splicing of the interfaces. 18 FE models of degenerative lumbar spine were obtained at last by determining the location of enthesis of anulus fibrosus and lumbar ligaments from their fixed coding. FE models of degenerative lumbar spine were loaded respectively under the axial compression loading of 150N, moment loading of flexion and extension of 7.5Nm and anterior and posterior shear forces of 150N. Under the different loading, the predicted results of FE models of degenerative lumbar spine are compared with the previous findings of experimental biomechanics in theidentical boundary condition. Results:The FE models of lumbar L4-L5 segment represented all complex spinal components. The predicted results of intact and degenerative L4-L5 segment FE models were closely correlated with published experimental data.Conclusions:The individually FE models libraries of degenerative lumbar spine were established basing CT data and CAD method.Objective:To research the association between the sagittal orientation of facet joints and disc degeneration in lumbar spine and to investigate the contribution and significance of which to development of the degenerative spondylolisthesis.Methods:A new effective CAD method was used to accurately establish 9 FE models which were assembled respectively by facet-joint angle 65° ,facet-joint angle 45° , facet-joint angle 25 ° and normal disc , light degenerative disc , severe degenerative disc. The biomechanical parameters of 9 FE models were measured under axial compressive load within physiological range.Results:Compared with FE models with facet-joint angle 45° and 25°, anterior displacement of L4 vertebra in FE models with facet-joint angle 65° was increased, where the maximum von Mises stresses on facet surface and isthmus and the contact force on facet surface in horizontal orientation were obviously increased. Meanwhile, FE models with facet-joint angle 65° showed a decrease in end-plate bulge and an increase in stress of annular matrix. The stiffness in light degenerative disc FE models was reduced and the von Mises stresses on facet surface and isthmus was slightly increased compared with the normal disc FE models. In all FE models, the FE models with facet-joint angle 65° and light degenerative disc displayed a poorest appearance in resisting anterior shear force.Conclusions:Sagittal orientation of facet joints is not only the primary motivation of the degenerative spondylolisthesis, but the secondary pathological change of remodelling of the facet-joints induced by the regional stress change. The inherent instability of lumbar motion segment of sagittal orientation of facet joints is influenced by the lumbar disc degeneration. The lumbar disc degeneration has no manifested contribution to the aggravation of the sagittal orientation of facet joints.Objective:To investigate the effect of variations in vertebral endplate concavity on the mechanical behavior of the lumbar motion segment.Methods:A 3-D nonlinear geometrical and mechanical accurate FE model of lumbar L4-L5 segment was developed. CAD methods were used to establish three FE models with different endplate concave angle, where disc lordosis angle, the gap of facet joint and all other geometrical parameters and FE mesh partition were kept constant. The effect of endplate concavity on the mechanical properties of the lumbar segment was studied for two moment loads (flexion and extension) and for three different direct forces (compression, anterior and posterior shear forces).Results:The decrease in the endplate concavity, simulated by an increasing endplate concave angle would result in decreased strains of the endplate and vertebral body, increased disc stiffness and nucleus pressure, decreased annular fiber stress, radial disc bulge and stress in the annulus ground substance, and simultaneously produce decreased facet contact force and stresses in posterior structure.Conclusions:The decrease of endplate concavity enhances the protective effect of thedisc on the breakage of the vertebral body. Smaller endplate deformations resulted from the decreasing endplate concavity would contribute to reduction of the nutritional diffusion to the disc.