Biomechanical evaluation of a new anterior spinal implant.

OBJECTIVE

To biomechanically evaluate the construct of a new anterior spinal implant for osteoporotic patients.

DESIGN

Mechanical tests and finite element analysis were designed to evaluate the spinal implant.

BACKGROUND

Many osteoporotic thoracolumbar spine fractures often need surgical intervention to relieve pains, stabilize progressive kyphosis or improve neurologic impairment. Progressive kyphosis, instrumentation failure and pseudoarthrosis are often seen post-operatively. These results may be due to insufficient bonding between the screw and the vertebral body. As the end-plate is the most rigid part of the vertebral body, the spinal implant was designed so that the end-block would fix onto the end-plates of the vertebral bodies.

METHODS

Two biomechanical evaluations of the new anterior spinal implant were conducted to evaluate the construct stiffness and the bonding strength between the spinal implant and the vertebral body. In the evaluation of the construct stiffness, the biomechanical tests between the new spinal implant and the Kaneda device were performed on six fresh intact porcine spines at the thoracolumbar region. In the prediction of the loading transfer between the spinal implant and the vertebral body, a three-dimensional finite element model (FEM) was built to simulate the osteoporotic vertebral body and the new anterior spinal implant.

RESULTS

The results of the mechanical tests showed that the compressive stiffnesses of the Kaneda device and the new implant were 357 +/- 37 N mm(-1) and 297 +/- 98 N mm(-1). Their flexional stiffnesses were 0.339 +/- 0.126 N-m mm(-1) and 0.364 +/- 0.107 N-m mm(-1), respectively. Their torsional stiffnesses were 6.37 +/- 0.28 N-m deg(-1) and 5.30 +/- 0.71 N-m deg(-1), respectively. There were no significant differences (p > 0.01). The results of FEM showed that the new implant had high stress concentration on the junction of the screw and plate, screw and end-blocks, and between the end-plate and the end-blocks.

CONCLUSIONS

The stabilization effect of the new implant was similar to that of the Kaneda device. With less rigid mechanical properties, the new implant can be more beneficial to the remodeling process of the spinal structure after instrumentation. The new anterior spinal implant showed a high potential for application to osteoporotic patients.