Effects of L4-L5 Posterior Fusion on Stress Distribution: Improved Predictions Using a Uniquely Validated Finite Element Model

Finite element models (FEMs) of spine segments validated in their intact states are often used to make predictions following structural modifications simulating surgical procedures, including posterior fusion with pedicle screws and rods (PSR) and laminectomy (removal of posterior column bone to decompress the spinal cord). The gold standard for spine FEM validation compares predicted vs. experimental intervertebral ranges of motion (ROM). Given that muscle co-contraction compresses the spine, validation that considers compression may produce a more robust FEM. One research goal was to evaluate an experimental method of compressing a lumbar spine segment through its sagittal plane balance (pivot) point (BP) using a 6DOF robotic test system. Experimental data supported the hypothesis that structural modifications, such as PSR and laminectomy alter the segment’s BP location and its compressive stiffness. However, evaluation showed that the experimental BP method is sensitive to specimen posture in the robotic test frame; slight flexion or extension produced shear loads during compression that affect BP location and should be included in specimen-specific FEMs to ensure similar load conditions. Another goal was to develop a uniquely calibrated specimen-specific FEM of an intact L4-5 motion segment using the experimental BP data. A specimen-specific FEM was created and calibrated using experimental BP compressive stiffness data, however matching experimental BP location data was unsuccessful. The BP-compression calibrated FEM was evaluated by comparing predicted responses to loads following simulated PSR and laminectomy to specimen-specific experimental data. Predictions using the BP-calibrated and ROM-calibrated FEMs were compared. The BP-calibration process helped identify an unrealistic FEM disc geometry (nucleus pulposus size and location). Both BP-compression and ROM-calibrated FEMs predicted effects of PSR on stiffness (compressive and flexural) that were greater than experimental, which helped identify a problem with simplified representations of bone in the posterior column and at the anterior column interface. The BP-compression calibrated FEMs predicted relative shifts in BP locations and bone surface strains during compression that were closer to experimental data than similarly modified ROM-calibrated FEMs. Collectively, these results support the use of BP measures in experimental and model-based investigations of surgical modifications of the spine.