2014 Volume 9 Issue 2 Pages 13-00163
In spinal fusion with instrumentation for the treatment of osteoporotic vertebral fracture, it is common for pedicle screws, which are inserted into vertebrae and strongly immobilized by a rigid rod, to loosen and dissociate. One of their expected causes is failure and reduction of fixity around a screw. An improved rod with a damper structure has thus been proposed to increase the freedom of movement of the rod and allow more flexible fixation of the spine. To evaluate the availability of the proposed structure and establish appropriate improved design of screws and rods with fewer occurrences of loosening and dissociation, effects of the structural change to the improved rod on the distribution of failure risks around the screw need to be investigated. In the present study, we performed nonlinear fracture analysis for spinal instrumentation surgery using an osteoporosis model and evaluated the failure distribution in the vertebrae while changing the apparent stiffness of the damper joint. Our finite element analysis showed that there were few expected failures in the flexible fixation model, indicating the effectiveness of the improved structure of the rod in reducing the loosening and dissociation of screws. It also suggested that the reduction was derived from the allowance of horizontal deformation in the damper joint in the improved rod. The potential of the structural improvement and the mechanism responsible for reducing the risks of loosening and dissociation are discussed.