Sensitivity and stability analysis of a nonlinear material model of cervical intervertebral disc under cyclic loads using the finite element method.

It is known that the human spine exhibits non-linear behavior, and its intervertebral discs play a role in the mechanism of internal load transfer. It is important to simulate its nonlinear behavior in computational models for better delineation of intrinsic responses, especially during cyclic loading activities, a mode pertinent to civilian and military populations. For developing a robust “material model” of the disc, this study used experimental tensile-compressive cyclic loading responses from four human cadaver cervical functional spinal units. Disc deformations were measured using an ultrasound system at 42 samples per second. Using experimental data, a three-network non-linear “material model” was developed using an optimization procedure and finite-element analysis. The model used 12 parameters to capture loading and unloading in tension and compression, including hysteresis. A sensitivity analysis performed to test the robustness of the “material model” indicated that seven of the 12 parameters were sensitive to tension, compressive, or both loading modes. Stability analysis was also performed under nine different loading conditions. The developed “material model” is robust and stable to capture intervertebral disc responses in tensile-compressive cyclic loading and can be used in future finite-element models.