Biomechanical investigation of pedicle screw-vertebrae complex: a finite element approach using bonded and contact interface conditions.

This study used finite element simulation to investigate the load transfer mechanisms within the screw/vertebra complex under different interface conditions, and under varying screw lengths. Both bonded and contact conditions were employed to demonstrate the interface between the screw and vertebra. Loadings were applied at the superior surface of the vertebra and screw unthreaded end, respectively, to represent two modes of flexion loads. The results indicated that the screw within the vertebra underwent a series of discontinuities of loading, identified by the localized high contact pressures, thus creating localized bending moments. The peak stress of screw was located at the junction of the screw's hub and thread, which is consistent with the location of screw failure observed in a clinical setting and the values of peak stress in the screw were proportional to the amount of moments generated by the two loading modes. The interface condition plays an important role in transferring the force within the screw/vertebra complex. A contact interface condition induces significantly higher stress in the screw than the bonded condition. Therefore providing a binding surface (with HA, or porosity coating on the screw surface) between the screw and the vertebra might be the most effective way to prevent screw failure. The influences of screw length on the peak stress in the screw become negligible when the screw is of sufficient length to extend fully into the vertebral body.