Low back pain is experienced by 80% of people sometime in their lives. It accounts for more lost time from jobs than any other ailment. Often surgery is required to relieve the pain. An innovative surgical procedure is vertebral stabilization. It is currently under investigation to separate and stabilize vertebral bodies and to promote a bony fusion through the region of the intervertebral discs. This procedure allows, at some spine levels, fusion through a laparoscope so that it can be performed as an outpatient procedure with very low morbidity. Because the procedure is so new, little has been done to structurally analyze and optimize the interbody cages used in the spinal fusion procedure. The objective of this study was to biomechanically evaluate the human lumbar spine stabilized by intervertebral cages. This study used the finite element method to biomechanically characterize normal spine behavior and to structurally evaluate existing cage designs. It also attempted to determine new, more optimal cage designs by considering stresses in the cages, contact forces between the cages and the bony structure. The design process considered different levels of vertebral bone density and considered physiologic levels of axial, torsional, and bending loads on the lumbar spine.

A new bone remodeling theory based on a volumetric strain criterion was developed in order to investigate the apparent density for cancellous bone. This theory was applied to the cancellous bone of lumbar spine and femur. The theory, which is a new and original interpretation of Wolffs Law, changes the bone apparent density distribution according to volumetric strain changes from mechanical loads.