This paper describes the results of the fundamental study of a dynamic analysis of the lumbar vertebrae. The authors constructed a finite element model with linear brick elements and nonlinear truss elements for the study of dynamic responses of the fifth and fourth lumbar vertebrae.

First, to reduce the calculation time and maintain accuracy, the effects of changing the brick element formulas for rigidity and numerical integration are examined. The results of the analyses showed the following : For estimating the global behavior of vertebrae, it is an efficient method to treat the stiff parts of the bone as rigid elements and apply the 8-point integration to the other soft brick elements.

Second, to verify the function as a rotator of the disk, the impact response analyses that dropped block comes into contact with the upper side of the lumbar vertebrae is performed. Because the nucleus pulposus has a nearly incompressible property, the disk is expected to function as a rotator in extension or flexion. We obtained the results that an incompressible model rotates larger than the compressible one.

Finally, the function of the nucleus pulposus under compressive loading is studied. For the stiffening of the annulus fibrosus, it is more difficult for the nucleus pulposus to change its volume and the pressure becomes higher.