The effect of sagittal alignment on adjacent joint mobility after lumbar instrumentation––a biomechanical study of lumbar vertebrae in a porcine model

Background. The mechanisms and changes in range of motion of neighboring mobile segment (adjacent level) after the instrumented posterior stabilization are not completely understood. This study aims to investigate the effect of sagittal alignment on the adjacent joint mobility after lumbar instrumentation. Methods. Eight fresh porcine lumbar spines were instrumented with pedicle screw implants from L2 to L4. Each specimen was tested in three different sagittal alignments. Group A were instrumented in lordotic alignment (lordosis 20°), Group B in straight alignment (lordosis 0°), and Group C in kyphotic alignment (kyphosis 20°). Hydraulic testing machine was used to generate an increasing moment in flexion and extension respectively for each specimen. The vertebral displacement of the disc between L1–L2 and L4–L5 were measured simultaneously with an extensometer. Findings. There were no significant differences in vertebral displacement between the three different sagittal alignments in both the superior and inferior adjacent segments under extension motion. However, under flexion motion, the vertebral displacement on the superior adjacent segment (L1–L2) with kyphotic alignment was statistically larger than that of the straight and lordotic alignments (P=0.0198 and P=0.000473 respectively), and no differences were found between the three different sagittal alignments on the inferior adjacent segment (L4–L5). Interpretation. The iatrogenically produced kyphotic lumbar spine by posterior instrumentation might cause larger adjacent joint mobility on the superior adjacent joint as compared to the instrumented lordotic lumbar spine. This study implies that an instrumented spine in lordosis is less likely to develop adjacent instability than a kyphotic spine.