A three-dimensional anatomically accurate two-motion segment model (C4-C5- C6) of the human cervical spine was developed to study the biomechanics under intact and surgically altered conditions. Linear and nonlinear analyses were conducted. The linear model was validated with experimental data under physiologic compression, flexion, extension, lateral bending and axial torsion. This linear model was used to study the responses due to change in geometrical and material characteristics of the spine during graded facetectomy, laminectomy with graded facetectomy, anterior interbody fusion and pediatric cervical spine developmental process. The internal response (stress) of a spinal component was more sensitive to geometrical and material property changes compared to the external response (overall strength). These results provide a basis for clinical observations regarding spinal development and degeneration.

The linear model was refined to develop a complex detailed nonlinear model based on spinal component sensitivity analysis which demonstrated that the soft tissue material properties were the critical parameters affecting the responses. The accurate details of the soft tissues were obtained by conducting an anatomical study to quantify the threedimensional geometrical details of the uncovertebral joint and a pilot finite element study of different modeling approaches to better define the components of the facet joint. The quantified geometrical data of the uncovertebral joint, a better fluid modeling approach to simulate the facet joint, nonlinear material definition o f ligaments, and composite nature of anulus fibrosis and incompressible behavior of nucleus pulposus of the intervertebral disc, were incorporated in the nonlinear model. This model was validated in terms of external response, such as overall force-displacement response, and internal response such as microstrain data on the vertebra with experimental data under compression, flexion, extension, compression-flexion and compression-extension loading. The validated nonlinear model was used to study the effect o f varying loading conditions. The overall external response was not affected due to a change in loading conditions, but the nature and magnitude of the internal responses varied significantly as the loading conditions changed.