On the biomechanics of a novel L5-S1 posterior fixation system: A finite element study

Low back pain is estimated to affect more than 70% of the population. Recently, interspinous posterior devices are gaining attention as a less invasive alternative to the traditional pedicle screw systems. However, since most of these devices are not suitable for the L5-S1 segment, the goals for this study are to design a tailored fixation system for the L5-S1 level and to study its effects on the degenerated spine. To that end, a finite element model of the L4-S1 spinal segment was developed, considering three different clinical stages (healthy, mildly degenerated and moderately degenerated). The instrumented spine was then simulated in short-term and long-term post-surgery stages, combined with the degenerated conditions. This system was able to effectively reduce the movement of the implanted segment by up to 96% in flexion and extension, 80% in lateral bending and 83% in axial rotation. In what concerns to the maximum principal stress in the disc region, the implanted model has shown a reduction of 80% in flexion, 76% in extension and 78% in lateral bending. These are promising outputs in terms of reducing the movement and the stress levels of the instrumented spine in all directions of motion, particularly flexion and extension, even if the device would require further experimental, computational and clinical studies. Although the mobility of the L4-L5 segment was not altered in the simulations, minor changes in the stress distribution were found in this segment, suggesting a reduced probability of adjacent disc disease with this system.