Computational Modeling of the Lumbar Spine: Active Musculature and Intra-abdominal Pressure in Compressive Loading

A current area of interest in lumbar injury is vertical impact loading. This includes effects of underbody blast (UBB), high speed planing boat impacts, and helicopter crashes. Lumbar spine fractures occur in 18% of all wounded in action injuries and 26% of soldiers killed in action exposed to UBB. Further, the US military has moving towards including women in all combat roles, including as the Special Forces beginning in January 2016. Volunteer and cadaveric data exist which suggest that male and female injury risk is not the same for equal stresses or loads. Dynamic injury mechanisms and thresholds have been extensively studied in the cervical spine, but not for the lumbar spine. While many lumbar models are available, no previously developed model is appropriate for high-rate vertical impact loading with intra-abdominal pressure and active musculature. So, the primary objective of this dissertation was to create a biofidelic hybrid multibody/finite element model to compare male and female response and assess the importance of active musculature and intra-abdominal pressure during single accelerative loading impacts (3-15g).

A 50th percentile male model was created using data from literature, experimental data, and medical imaging data. Scaling relationships for the 50th and 85th percentile female were derived, and an 85th percentile female model developed. The 85th percentile female model is mass-matched to the 50th percentile male model. An 11% increase in ischium breadth in the 85th female changes the line of action for muscles inserting on the pelvis. These changes resulted in a female model having increased axial loads over a male model when matched for mass.

The 50th percentile male osteoligamentous model was validated against developed strain energy-force corridors and T12/L1 injury risk functions. A 50th percentile risk of spinal fracture of 5237 N was reported. During failure loading (as seen from experimental tests), the osteoligamentous spine model predicts a 41% risk of failure. While the model slightly underpredicts the risk of injury, the peak compressive load in T12/L1 lies within the 95th percentile confidence intervals for the 50th percentile risk of injury.

In this dissertation, it was hypothesized that men and women do not have the same risk of injury on an effective stress basis. This hypothesis was supported by comparing mass-matched male and female hybrid multibody/finite element models during an underbody blast loading condition. Based on the comparison between the 85th percentile female model to the 50th percentile male model predicts higher axial loads due to changes in musculature. It was also hypothesized that the use of active musculature decreases injury tolerance in compressive loading. This hypothesis was supported by comparing model intervertebral axial loads to both experimental (underbody blast) and epidemiological (electroconvulsive therapy) loading conditions. This research demonstrates higher muscle activations increase risk of lumbar spine injury in vertical impact loading. A tensed activation state contributes 48 percent of the compressive load estimated to fracture the lumbar spine during underbody blast. While this corresponds to a low risk of injury (<10%), this could exacerbate risk of injury during additional compressive loading.

By better understanding the female lumbar spine response, new safety measures can be developed. This work could inform the design of new protective personal equipment, or guide permissible exposure limits and risk of injury.