Experimental Characterization of the Anatomical Structures of the Lumbar Spine Under Dynamic Sagittal Bending.

Underbody blast (UBB) events transmit high-rate vertical loads through the seated occupant’s lumbar spine and have a high probability of inducing severe injury. While previous studies have characterized the lumbar spine under quasi-static loading, additional work should focus on the complex kinetic and kinematic response under high loading rates. To discern the biomechanical influence of the lumbar spine’s anatomical structures during dynamic loading, the axial force, flexion-extension moments and range of motion for lumbar motion segments (n=18) were measured during different states of progressive dissection. Pre-compression was applied using a static mass while dynamic bending was applied using an offset drop mass. Dynamic loading resulted in peak axial loads of 4,224±133 N, while maximum peak extension and flexion moments were 19.6±12.5 and -44.8±8.6 Nm in the pre-dissected state, respectively. Upon dissection, transection of the interspinous ligament, ligamentum flavum and facet capsules resulted in significantly larger flexion angles, while the removal of the posterior elements increased the total peak angular displacement in extension from 3.3±1.5 to 5.0±1.7 degrees (p=0.002). This study provides insight on the contribution of individual anatomical components on overall lumbar response under high-rate loading, as well as validation data for numerical models.

Material Parameter Determination of an L4-L5 Motion Segment Finite Element Model Under High Loading Rates.

Underbody blast (UBB) events impart vertical loads through a victim’s lumbar spine, resulting in fracture, paralysis, and disc rupture. Validated biofidelic lumbar models allow characterization of injury mechanisms and development of personal protective equipment. Previous studies have focused on lumbar mechanics under quasi-static loading. However, it is unclear how the role and response of individual spinal components of the lumbar spine change under dynamic loading. The present study leverages high-rate impacts of progressively dissected two-vertebra lumbar motion segments and Split-Hopkinson pressure bar tissue characterization to identify and validate material properties of a high-fidelity lumbar spine finite element model for UBB. The annulus fibrosus was modeled as a fiber-reinforced Mooney-Rivlin material, while ligaments were represented by nonlinear spring elements. Optimization and evaluation of material parameters was achieved by minimizing the root-mean-square (RMS) of compressive displacement and sagittal rotation for selected experimental conditions. Applying dynamic based material models and parameters resulted in a 0.42% difference between predicted and experiment axial compression during impact loading. This dynamically optimized lumbar model is suited for cross validation against whole-lumbar loading scenarios, and prediction of injury during UBB and other dynamic events.