Rigid fixation of the lumbar spine alters the motion and mechanical stability at the adjacent segment level

This study aims to examine the motion and stability of the adjacent segment following rigid fixation of the lumbar spine.

The study included 17 fresh-frozen lamb lumbar spines (including the sacrum to T12). Biomechanical testing was performed using an axial compression testing machine. Axial compression was applied to all the specimens using a loading speed of 5 mm min-1. A specifically designed fixture was used to generate torque ≤8400 Nmm. The fixture was used with each specimen to achieve flexion and extension, axial neutral compression, and right and left bending. All specimens were tested intact, and again after implantation using posterior pedicle screws and rod fixation. During testing intervertebral displacement at the adjacent level (L5-S1) was recorded continuously via extensometry.

Axial compression and superior-inferior displacement were lower in the adjacent segment (L5-S1) than anterior-posterior displacement following rigid fixation. Statistical analysis showed that there was a numerical difference and a significant change between the intact spine and the adjacent segment in the axial compression and extension positions (p<0.027). The intact spines demonstrated the maximum displacement and the difference in extension positions were significant (p<0.015).

Rigid fixation of the lumbar spine altered the range of motion at the adjacent segment level. As such, abnormal stress on the adjacent segment causes spinal instability, which may subsequently cause facet joint degeneration and low back pain.