Dynamic stabilization adjacent to single-level fusion: Part I. Biomechanical effects on lumbar spinal motion

Progression of superior adjacent segment degeneration (PASD) could possibly be avoided by dynamic stabilization of an initially degenerated adjacent segment (AS). The current study evaluates ex vivo the biomechanics of a circumferential fixation connected to posterior dynamic stabilization at the AS. 6 human cadaver spines (L2-S1) were stabilized stepwise through the following conditions for comparison: intact spine (ISP), single-level fixation L5-S1 (SLF), SLF + dynamic AS fixation L4-L5 (DFT), and two-level fixation L4-S1 (TLF). For each condition, the moments required to reach the range of motion (ROM) of the intact whole spine segment under +/- 10 Nm (WSP10) were compared for all major planes of motion within L2-S1. The ROM at segments L2/3, L3/4, and L4/5 when WSP10 was applied were also compared for each condition. The moments needed to maintain WSP10 increased with each stage of stabilization, from ISP to SLF to DFT to TLF (p < 0.001), in all planes of motion within L2-S1. The ROM increased in the same order at L3/4 (extension, flexion, and lateral bending) and L2/3 (all except right axial rotation, left lateral bending) during WSP10 application with 300 N axial preload (p < 0.005 in ANOVA). At L4/5, while applying WSP10, all planes of motion were affected by stepwise stabilization (p < 0.001): ROM increased from ISP to SLF and decreased from SLF to DFT to TLF (partially p < 0.05). The moments required to reach WSP10 increase dependent on the number of fixated levels and the fixation stiffness of the implants used. Additional fixation shifts motion to the superior segment, according to fixation stiffness. Therefore, dynamic instrumentation cannot be recommended if prevention of hyper-mobility in the adjacent levels is the main target.