Response of the lumbar spine in combined flexion and axial compression up to failure: Postmortem human subject experiments and human body model assessment.

OBJECTIVE: Previous whole lumbar spine PMHS testing evaluated flexion loading with and without axial compression, but an insufficient flexion limit was used to capture the full response. Additional PMHS testing was performed to address that limitation by sequentially increasing the flexion angle in consecutive tests until failure, but that loading scheme resulted in progressive accumulation of damage with a non-repeatable flexion moment versus angle response. The goals of this study were to investigate the PMHS lumbar spine kinetic and kinematic response to combined compression and flexion at loads that surpassed the failure threshold and to simulate the PMHS tests with HBMs to evaluate their performance. METHODS: Ten PMHS whole lumbar spines were tested using a 6-degree-of-freedom robotic test device, with a follower load mechanism. A single test to 90 degrees flexion was performed with 1800 N of axial compression for all specimens. Loads were applied to the superior end of the spine (T12) while reaction forces and moments were recorded with a load cell fixed inferior to the rigidly held sacrum. Joint motions were calculated using 3D translations and rotations from motion capture markers attached to each bone. The PMHS tests were then replicated using finite element simulation with the GHBMC, THUMS, and VIVA+ HBMs to evaluate model performance. RESULTS: Flexion moment versus angle and joint versus whole spine flexion angle were collected for each PMHS test. PMHS data corridors were generated from the population using the ARCGen method and validated against data from the previous study. Characteristic behavior of an initial increase, subsequent flattening, and secondary stiffening at 25-35 degrees of flexion was found in the flexion moment versus angle response. Joint-specific contributions to total spine flexion revealed an uneven contribution by joint to flexion. The HBM evaluation found linear, lower magnitude moment-angle response rather than the bilinear response observed in the PMHS corridor. HBM flexion angle contributions aligned well with quasistatic PMHS corridors highlighting regional soft tissue behavior accuracy. CONCLUSIONS: PMHS testing identified previously uncollected properties at high angles and corridors were developed for comparison to future data or HBMs. Simulation evaluations found disagreement between the PMHS data and HBM response across all models and a lack of uniformity in flexion kinematic response between models. Disagreement is likely due to limitations of the data sources used to define the HBM soft tissues.