The effects of design and configuration on the biomechanical response of an internal spinal fixator

This study examines the biomechanical performance of an internal spinal fixator and the effects of specific design features under a range of loading modes. The commercial device was mounted on plastic vertebrae in a corpectomy injury model and attached by a series of experimental jigs to an appropriate material testing machine and tested under axial compression, torsion and flexion and extension moments. Results from the torsional tests indicated that increasing the clamp tightening torque from 5 to 15 N m significantly increased the rigidity of the fixation system. The inclusion of the transverse elements resulted in a significant increase in the torsional stiffness, with the increase largely overriding the effect of clamp tightening torque. By contrast, under compressive and both flexion and extension loads, neither of the design features of the fixator had a marked effect on the overall measured stiffness of the system. However, under extension loads, there were specific interactions between the two design parameters. The present study clearly indicates the need for the optimization of the design of the clamps and for alternative configurations of the transverse elements to enhance their performance under sagittal loads.