A Finite Element Analysis of the Creep Response of Lumbar Intervertebral Joints in the Rhesus Monkey.

Lumbar intervertebral centra from healthy, adolescent Rhesus Monkeys Macaca Mulatta were subjected to axial compression loads of 15 and 30 pounds for a period of eight hours and displacement-time data was gathered. An axisymmetric finite element model was used to analytically determine material parameters describing the observed creep for each applied load. Analytical stress profiles in two horizontal planes through the vertebral centrum indicated a predominance of stress in cortical bone and the transition of stress from the Trabecular bone to the Cortical bone as creep proceeded. According to the model, the Centrum behaved as though the Cortex was acting like a thin shell constraining the outward flow of a viscoelastic Trabecular bone region. Viscoelastic constants determined for the Trabecular bone region were incorporated into an overall model of the intervertebral joint minus the Articular Facet Joint and associated spinous processes.