Acoustic emissions in vertebral cortical shell failure

Expanded Combined Loading Injury Criterion for the Human Lumbar Spine Under Dynamic Compression

Contemporary injury tolerance of the lumbar spine for under-body blast references axial compression and bending moments in a limited range. Since injuries often occur in a wider range of flexion and extension with increased moment contribution, this study expands a previously proposed combined loading injury criterion for the lumbar spine. Fifteen cadaveric lumbar spine failure tests with greater magnitudes of eccentric loading were incorporated into an existing injury criterion to augment its applicability and a combined loading injury risk model was proposed by means of survival analysis. A loglogistic distribution was the most representative of injury risk, resulting in optimized critical values of Fr,crit = 6011 N, and My,crit = 904 Nm for the proposed combined loading metric. The 50% probability of injury resulted in a combined loading metric value of 1, with 0.59 and 1.7 corresponding to 5 and 95% injury risk, respectively. The inclusion of eccentric loaded specimens resulted in an increased contribution of the bending moment relative to the previously investigated flexion/extension range (previous My,crit = 1155 Nm), with the contribution of the resultant sagittal force reduced by nearly 200 N (previous Fr,crit = 5824 N). The new critical values reflect an expanded flexion/extension range of applicability of the previously proposed combined loading injury criterion for the human lumbar spine during dynamic compression.

Primary Creep Characterization in Porcine Lumbar Spine Subject to Repeated Loading

Low back pain (LBP) is a common medical condition worldwide, though the etiology of injuries causing most LBP is unknown. Flexion and repeated compression increase lumbar injury risk, yet the complex viscoelastic behavior of the lumbar spine has not been characterized under this loading scheme. Characterizing the non-injurious primary creep behavior in the lumbar spine is necessary for understanding the biomechanical response preceding injury. Fifteen porcine lumbar spinal units were loaded in repeated flexion-compression with peak compressive stresses ranging from 1.41 to 4.68 MPa. Applied loading simulated real loading exposures experienced by high-speed watercraft occupants. The strain response in the primary creep region was modeled for all tests using a generalized Kelvin-Voigt model. A quasilinear viscoelastic (QLV) approach was used to separate time-dependent (creep) and stress-dependent (elastic) responses. Optimizations between the models and experimental data determined creep time constants, creep coefficients, and elastic constants associated with this tissue under repeated flexion-compression loading. Average R2 for all fifteen models was 0.997. Creep time constants optimized across all fifteen models were 24 s and 580 s and contributed to 20 ± 3% and 30 ± 3% of the overall strain response, respectively. The non-transient behavior contributed to 50 ± 0% of the overall response. Elastic behavior for this porcine population had an average standard deviation of 24.5% strain across the applied stress range. The presented primary creep characterization provides the response precursor to injurious behavior in the lumbar spine. Results from this study can further inform lumbar injury prediction and kinematic models.

A comparison of fracture response in female and male lumbar spine in simulated under body blast component tests

Underbody blasts (UBB) from mines and improvised explosive devices in military combat can cause debilitating spine injuries to vehicle mounted soldiers. Due to the exclusion of females in combat roles in prior US Department of Defense policy, UBB exposure and injury have predominantly affected male soldiers. Recent policy changes have opened many combat roles to women serving in the US Military (Carter, 2015) and have increased the need to understand the injury potential for female Warfighters. The goal of this study was to investigate the fracture response of adult female lumbar spines compared to adult male spines in UBB relevant loading to identify potential differences in either fracture mechanism or force. Results are presented for 15 simulated UBB spine compression tests using three small female (SF), five large female (LF), and seven mid-sized male (MM) post-mortem human subjects (PMHS). These PMHS groups align to 5th- and 75th-percentile female and 50th-percentile males, based on height and weight from the 2012 Anthropometric Survey of U.S. Army Personnel (Gordon et al., 2014). Both small females and large females (similar in size to the males) were included to assess the role of size and/or sex in the response. Tests were conducted at Virginia Tech on a cam-driven linear compression rig, which included a 6-axis load cell and ram accelerometer to evaluate the fracture. Fracture was visualized through high-speed x-ray video. All female and male spines exhibited similar fracture initiation at the end plates and progression through the vertebral body. The resulting severe compression and burst fractures were representative of reported theatre injuries (Freedman et al., 2014). Mean axial fracture forces were -4182 ± 940 N (SF), -6225 ± 1180 N (LF), -5459 ± 1472 N (All Females) and -7993 ± 2445 N (MM). The SF group was found to have statistically significant differences in mean fracture force compared to both LF and MM groups, while no significant difference was found between LF and MM groups, although the mean force at initial fracture was lower for the LF group. The All-Females group Fz mean was significantly different from the MM group. These data suggest that the significant difference in weight between the SF and LF groups, did have an influence on the Fz outcome, when controlling for sex. Conversely, controlling for size in the LF and MM comparison, sex did influence the mean Fz, but was not statistically significant. Groups with combined sex and size differences, however, did show significant differences in mean Fz. Further study is warranted to understand whether sex or size has a larger effect on fracture force. Mean ram displacement (spine compression) values at fracture initiation were -6.0 ± 5.3 mm (SF), -4.4 ± 0.8 mm (LF), -5.0 ± 3.0 mm (All Females), -6.2 ± 4.5 mm (MM). Spine compression did not seem to be largely influenced by either sex or size, and none of the groups was found to have significant differences in mean displacement values.

The Human Lumbar Spine During High-Rate Under Seat Loading: A Combined Metric Injury Criteria

Modern changes in warfare have shown an increased incidence of lumbar spine injuries caused by underbody blast events. The susceptibility of the lumbar spine during these scenarios could be exacerbated by coupled moments that act with the rapid compressive force depending on the occupant's seated posture. In this study, a combined loading lumbar spine vertebral body fracture injury criteria (L_ic) across a range of postures was established from 75 tests performed on instrumented cadaveric lumbar spine specimens. The spines were predominantly exposed to 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. The proposed L_ic utilizes a recommended 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 F_r,crit = 5824 N and M_y,crit = 1155 Nm. The 50% risk of lumbar spine injury 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.