Biomechanical Stability of Isthmic Spondylolisthesis Under Compressive Preload With and Without Transpedicular Instrumentation

Summary: This study investigated the stabilizing effect of physiologic compressive follower preload with and without transpedicular instrumentation in an in-vitro model of unstable adult onset isthmic spondylolisthesis. Both follower preload and transpedicular instrumentation stabilized the range of motion of the spine, but transpedicular instrumentation was unable to impede slip, thereby possibly limiting its effectiveness in treatment of an unstable isthmic spondylolisthesis.

Introduction: Adult progressive isthmic spondylolisthesis, most common at L5-S1, may require surgical intervention. This study investigated the stabilizing effect of physiologic compressive follower preload with and without transpedicular instrumentation in an in-vitro model of unstable isthmic spondylolisthesis.

Methods: Seven human cadaveric spines (L1-S1, mean age 54yrs) were used. Each intact spine was loaded to 400N and 800N using follower load technique. The L5-S1 load path was then varied from follower (pure compression) to vertical (equal shear and compression) orientation. Moments of 8Nm flexion and 6Nm extension were applied to L1 without preload and with preloads of 400N and 800N under both follower and vertical load paths. Testing was repeated following bilateral L5 pars fracture, partial L5-S1 discectomy and resulting spondylolisthesis, and L5-S1 transpedicular instrumentation. Flexion-extension motion (FE-ROM) and slip at L5-S1 were measured.

Results: Strong positive correlation between shear load and slip was seen (R2=0.78, p<0 .05). Slip under follower load was significantly smaller than vertical load, both in upright (11%vs.19%, p="0.029)," and flexed posture (9.3% vs.18%, L5-S1 FE-ROM without preload larger with spondylolisthesis intact (24° vs.10°, restored to 400N (p="0.006)." less 800N Transpedicular instrumentation reduced the increased associated isthmic (p<0.033), motion below level of segment However, it unable prevent progression slip load.

Discussion: The decrease in slip and FE-ROM under a follower preload demonstrates the stabilizing effect of follower load, and suggests a role for muscles in stabilizing an unstable lumbar spine. Transpedicular instrumentation was able to limit FE-ROM to less than intact, but not impede slip, possibly limiting its effectiveness when used to treat unstable isthmic spondylolisthesis.