Analysis of compressive creep behavior of the vertebral unit subjected to a uniform axial loading using exact parametric solution equations of Kelvin-solid models--Part I. Human intervertebral joints.

The creep response phenomena observed on 47 human intervertebral discs subjected to a constant axial compressive stress was analytically studied by two-, three- and four-parameter-solid models employing the Burns- Kaleps 'exact analysis scheme'. The mechanical properties (Young's moduli and viscosity coefficients) associated with each model were calculated for each of the 47 disks, with superior results obtained for the latter two models. Results for the two-parameter-solid model suggest its possible usefulness in simulating creep response that is characteristic of disk degeneration. Results for the three- and four-parameter-solid models were excellent, with an average error for the model predicted strain, epsilon(ti)cal, values from the experimentally measured, epsilon(ti)exp, values of 2.314% for the former model and 4.446% for the latter model on the 47 human spinal segments analyzed. The three-parameter-solid model was most sensitive in its predictability of strain behavior for ti greater than 1 min; whereas the four-parameter-solid model demonstrated greater simulation sensitivity in the 0 less than ti less than or equal to 1 min range.