Characterization of the Mechanical Response of the Lumbar Spine: The Effect of a Compressive Axial Load

Lumbar spine injury is common among those involved in motor vehicle crashes. With the advent of highly automated vehicles, occupant postures are likely to include reclined positions. Recent studies have found reclining a human body model (HBM) results in high lumbar compression combined with flexion loading. Model simulations are used to assist in the design and development of countermeasures to mitigate risk of injury in motor vehicle crashes. Biofidelity evaluations of HBMs are necessary to ensure the restraint designs effectively reduce the risk of injury in vehicle occupants. Currently, there exists no data that describes changes to lumbar stiffness under axial compression loads relevant to motor vehicle crash-loading, thus there is no benchmarking dataset that can be used to evaluate the biofidelity of HBMs.
The goal of this thesis was to characterize the biomechanical response of the lumbar spine under combined loading to assist in biofidelity evaluation of HBMs. To accomplish this goal, three aims were defined: 1) Characterize the kinetic and kinematic response of the lumbar spine in moderate but sub-injurious loading in each physiological direction. 2) Measure the effect of a compressive follower load on the kinetic and kinematic response of the lumbar spine in each physiological direction. 3) Develop a database of tools in the form of average response corridors for the purposes of the biofidelity evaluation of existing and future HBMs and surrogates of the lumbar spine.
Seven post-mortem human surrogate (PMHS) whole lumbar spines were tested using a six degree-of-freedom (DOF) robotic test device. A follower load was used to apply axial compression to the spine during testing. Test directions included flexion, extension, anterior shear, posterior shear, right lateral bending, right lateral shear and clockwise axial torsion. Each direction was tested in three conditions: without a follower load, with a 900 N follower load and an 1800 N follower load. Individual vertebral motion was tracked using rigidly attached motion tracking markers at each level. Shear loads and bending moments were measured by the control load cell located below the specimen.
The average response of the spines tested was stiffest in extension and posterior shear and became stiffest in torsion and posterior shear with the inclusion of a compressive follower load. On average, the kinematic motions of the L1 vertebral body were higher than L3, which were higher than L5, and these motions are further reduced with a follower load. The HBM biofidelity evaluation tools were created in the form of an average response bracketed by a single standard deviation.
The lumbar spine is stiffest in the extension and posterior shear direction with a non-linear deformation response in flexion. A compressive follower load has a statistically significant effect for all directions tested and the effect is dependent on the location on the spine as well as the direction tested. With the corridors created in this study, HBM responses can be evaluated for the biofidelity of their predictions.