Biomechanical analysis of the durability of a modified S1 vertebrae transpedicular screws insertion technique

One of the most important elements of the transpedicular screw implantation technique, which enables a strong screw-bone interface, is the precise choice of the site of screw insertion and the screw's trajectory. Due to the complex biomechanics of the lumbosacral interface and different shape of the sacrum, fixation of this segment remains a challenge for surgeons. Because of this, Kubaszewski et al. proposed a modified technique in which the entry point for screw insertion in the S1 vertebra is changed.

Six human cadaver specimens of the S1 vertebrae were examined. Two transpedicular screws were inserted into the body of each examined vertebra using two implantation methods with different screw entry points and trajectories. The screws were subjected to cyclic preloading, followed by the pull-out test. The ultimate pull-out force, displacement, stiffness, and failure energy were measured.

The average pull-out force obtained for the standard method of implantation was 498 N (SD 201), whereas for the modified technique, it was 1308 N (SD 581). Displacement of the inserted screws in the new method was 36% higher than in the case of the standard method. This method is also characterized by the greater stiffness of the obtained interface and greater failure energy than the normally used technique.

The obtained results demonstrate that the use of the new technique of implantation significantly increases the strength of the obtained screw-bone interface. It should also increase the success rate of the performed fixations and increase the safety of such fixations in clinical practice.