Lumbar Spinal Injury Under Dynamic Compression

Abstract

<p>Spinal trauma is a major issue with considerable societal, economic and physical consequences. Lumbar spine injuries during dynamic compression have been highly prevalent in recent American military conflicts in part due to the increased use of improvised explosive devices. The susceptibility of the lumbar spine during these scenarios can reduce functional capacity of soldiers and result in disability and morbidity in the military population. Characterizing the spine response during dynamic compression scenarios is essential in understanding injury risk in military personnel from underbody blast. Further, translating the experimental response to real world applications is crucial in developing future vehicle designs and mitigation strategies to reduce the incidence of these injuries.To advance the understanding of lumbar spine injury tolerance an axial loading lumbar spine vertebral body fracture injury criterion (Lic) across a range of three postures was established from 75 tests performed on instrumented cadaveric lumbar spine specimens. The spines were predominantly exposed to dynamic axial compressive forces from an upward vertical thrust with 64 of the tests resulting in at least one vertebral body fracture and 11 in no vertebral body injury. A loglogistic parametric survival analysis determined that the tolerance of the lumbar spine was 5569 N, 4618 N, and 4493 N for the tested nominal, pre-flexed, and pre-extended postures respectively. The differences in tolerance across the tested posture suggests that the injury probability is not captured by axial data alone. Therefore, a combined loading injury criteria was developed to provide an improved assessment. The susceptibility of the lumbar spine during underbody blast loading scenarios could also be exacerbated by coupled moments that act with the rapid compressive force depending on the occupant’s seated posture. The influence of the bending moment on the injury tolerance of the lumbar spine was evaluated using a combined metric that considers both the axial force and bending moment of the loading event. This combined loading lumbar spine vertebral body fracture injury criterion (CLic) across a range of postures was established from 75 cadaveric lumbar spine tests. The proposed CLic utilizes a metric (κ), based on prismatic beam failure theory, resulting from the combination of the T12-L1 resultant sagittal force and the decorrelated bending moment with optimized critical values of Fr,crit = 5824N and My,crit = 1155Nm. The 50% risk of lumbar spine vertebral body fracture corresponded to a combined metric of 1, with the risk decreasing with the combined metric value. At 50% injury risk the Normalized Confidence Interval Size improved from 0.24 of a force-based injury reference curve to 0.17 for the combined loading metric. At this point the injury tolerance for the lumbar spine during UBB events to the axial force and a combined loading metric was investigated for a limited range of occupant’s seated postures. However, it is highly desirable to expand the criterion to consider postures and dynamically evolved positions for a wider range of compression and bending moments in the lumbar spine. Therefore, the established lumbar combined injury criterion (CLic) was expanded to a wider range of initial and dynamically evolved postures and account for a higher contribution of the bending moment to the loading scenario. An expanded combined loading injury criterion (ECLic) was developed by testing an additional 15 specimens for initial and dynamically evolving postures that further increased the extension or flexion of the lumbar spine. These were combined with the 75 previously tested specimens in the nominal posture range. The injury criterion established from 90 cadaveric tests, produced a resultant sagittal force and decorrelated bending moment critical values of Fr,crit = 6011N and My,crit = 904 Nm. Out of the 90 specimens tested 77 had at least one vertebral body fracture, with 13 specimens having minor injuries or no injuries. The 50% risk of lumbar spine vertebral body fracture was normalized to a combined metric of 1, with the risk decreasing with the combined metric value. Finally, translating cadaver injury risk to equivalent anthropometric test devices measurements is a vital step for effective injury mitigation efforts. The commonly used matched-pair approach to translate these data consistently overestimates dummy injury risk, which then exaggerates human injury tolerance. A novel translation method based on energy equivalency was proposed to avoid these errors by matching strain energy between cadaver and dummy. To translate a single metric, say force, to an ATD risk assessment (IARC), the force-energy responses for both cadaver and simulated dummy measurements were used to determine the transfer function from cadaver measurement to ATD measurements at iso-energy. Similarly, a generic a combined metric, similar to the combined loading injury criterion (CLic) developed, was used to illustrate the translation of a complex loading scenario by characterizing the energy response of both cadaver and ATD in the corresponding force/moment duplets that define the CLic. </p>