Finite element analysis of vertebral end-plate failure under high dynamic axial load and its relation to age

Thesis (Ph.D.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering."July 2005."This research was aimed to develop the analytical model of functional spinal unit with the intent of understanding the contribution of Intervertebral disc condition on the failure mechanism of End-plates under high rate dynamic loading, The condition of the disc for young people is healthy as compared to old person. The simple single degree of freedom DRI (Dynamic response Index) model was utilized to measure the spinal deflection under the influence of dynamic loading is incapable of estimating the potential injury due to disc swelling. The failure considered is always the vertebral body failure due to crushing, but in reality, condition of disc plays important role in deciding the failure of the vertebrae. The detailed axis-symmetry model was utilized to understand the effect of various factors on the end-plate failure mechanism and it is observed that deflection of the end plate is critical failure criteria. The age and the peak load were found to be the significant factors dictating end-plate failure. In young people the deflection of end plate was higher as compared to the old people due to hydrated disc capable of swelling under dynamic loads. The simplified three dimensional model of vertebral motion segment L2-L3 was developed in order to understand the effect of nucleus pulpous pressure in end-plate deflection and to incorporate this model into the DRI model in order to estimate the effect of dynamic loading on the resulting spinal injury. The three dimensional model utilizes the MADYMO material models. The relaxation modulus for the nucleus and the annulus are obtained from the published results and the material parameters were mapped from these published data. Results of a sensitivity analysis indicated the major contribution of age on the end-plate failure. The result clearly indicated the need of modifying the present way of modeling the nucleus as incompressible fluid to viscoelastic incompressible fluid. This study indicated that failure of the end-plate was an important link in the failure mechanism of the spinal unit; this failure lead to the transmission of the disc tissue into the vertebrae and caused the burst fracture