The Mechanical Response and Tolerance of the Anteriorly-Tilted Human Pelvis Under Vertical Loading

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.

Comparison of Adult Female and Male PMHS Pelvis and Lumbar Response to Underbody Blast

The goal of this study was to gather and compare kinematic response and injury data on both female and male whole-body Post-mortem Human Surrogates (PMHS) responses to Underbody Blast (UBB) loading. Midsized males (50th percentile, MM) have historically been most used in biomechanical testing and were the focus of the Warrior Injury Assessment Manikin (WIAMan) program, thus this population subgroup was selected to be the baseline for female comparison. Both small female (5th percentile, SF) and large female (75th percentile, LF) PMHS were included in the test series to attempt to discern whether differences between male and female responses were predominantly driven by sex or size. Eleven tests, using 20 whole-body PMHS, were conducted by the research team. Preparation of the rig and execution of the tests took place at the Aberdeen Proving Grounds (APG) in Aberdeen, MD. Two PMHS were used in each test. The Accelerative Loading Fixture (ALF) version 2, located at APG's Bear Point range was used for all male and female whole-body tests in this series. The ALF was an outdoor test rig that was driven by a buried explosive charge, to accelerate a platform holding two symmetrically mounted seats. The platform was designed as a large, rigid frame with a deformable center section that could be tuned to simulate the floor deformation of a vehicle during a UBB event. PMHS were restrained with a 5-point harness, common in military vehicle seats. Six-degree-of-freedom motion blocks were fixed to L3, the sacrum, and the left and right iliac wings. A three-degree-of freedom block was fixed to T12. Strain gages were placed on L4 and multiple locations on the pelvis. Accelerometers on the floor and seat of the ALF provided input data for each PMHS' feet and pelvis. Time histories and mean peak responses in z-axis acceleration were similar among the three PMHS groups in this body region. Injury outcomes were different and seemed to be influenced by both sex and size contributions. Small females incurred pelvis injuries in absence of lumbar injures. Midsized males had lumbar vertebral body fractures without pelvis injuries. And large females with injuries had both pelvis and lumbar VB fractures. This study provides evidence supporting the need for female biomechanical testing to generate female response and injury thresholds. Without the inclusion of female PMHS, the differences in the injury patterns between the small female and midsized male groups would not have been recognized. Standard scaling methods assume equivalent injury patterns between the experimental and scaled data. In this study, small female damage occurred in a different anatomical structure than for the midsized males. This is an important discovery for the development of anthropomorphic test devices, injury criteria, and injury mitigating technologies. The clear separation of small female damage results, in combination with seat speeds, suggest that the small female pelvis injury threshold in UBB events lies between 4 - 5 m/s seat speed. No inference can be made about the small female lumbar threshold, other than it is likely at higher speeds and/or over longer duration. Male lumbar spine damage occurred in both the higher- and lower lower-rate tests, indicating the injury threshold would be below the seat pulses tested in these experiments. Large females exhibited injury patterns that reflected both the small female and midsized male groups - with damaged PMHS having fractures in both pelvis and lumbar, and in both higher- and lower- rate tests. The difference in damage patterns between the sex and size groups should be considered in the development of injury mitigation strategies to protect across the full population.

Biomechanical characteristics of arteries during pelvic fracture reduction and dynamic simulation analysis

During robot-assisted reduction of pelvic fracture, blood vessels are susceptible to tensile and shear forces, making them prone to injury. Considering the impact of pelvic reduction on the risk of arterial injury, the biomechanical characteristics of arteries during the pelvic fracture reduction process are studied, and a refined coupled composite model of the damaged pelvic structure is established. Dynamic simulations of pelvic fracture reduction are conducted based on the planned reduction path. The simulation results show that during the reduction process, when the affected side is rotated, the stress and strain of the artery are maximum, particularly at the locations of the iliac common artery, internal iliac artery, and the superior gluteal artery arch endure significant stress and strain. After reduction, the maximum stress is observed in the right superior gluteal artery, and the maximum strain occurs at the intersection of the right iliac common artery. The stretch ratio of both the left and right iliac common arteries is considerable. Therefore, it can be concluded that the superior gluteal artery and the internal iliac artery are prone to injury, particularly the segment from the origin of the superior gluteal artery to its passage around the greater sciatic notch. After reduction, substantial traction on the iliac common artery, which makes it more susceptible to deformation, carries a risk of arterial rupture and aneurysm formation. This study provides a reference for planning the safe reduction path of pelvic fracture surgery and improving safety.

Pelvis-Sacrum-Lumbar Spine Injury Characteristics From Underbody Blast Loading

INTRODUCTION

Combat-related injuries from improvised explosive devices occur commonly to the lower extremity and spine. As the underbody blast impact loading traverses from the seat to pelvis to spine, energy transfer occurs through deformations of the combined pelvis-sacrum-lumbar spine complex, and the time factor plays a role in injury to any of these components. Previous studies have largely ignored the role of the time variable in injuries, injury mechanisms, and warfighter tolerance. The objective of this study is to relate the time or temporal factor using a multi-component, pelvis-sacrum-lumbar spinal column complex model.

MATERIALS AND METHODS

Intact pelvis-sacrum-spine specimens from pre-screened unembalmed human cadavers were prepared by fixing at the superior end of the lumbar spine, pelvis and abdominal contents were simulated, and a weight was added to the cranial end of the fixation to account for torso effective mass. Prepared specimens were placed on the platform of a custom vertical accelerator device and aligned in a seated soldier posture. An accelerometer was attached to the seat platen of the device to record the time duration to peak velocity. Radiographs and computed tomography images were used to document and associate injuries with time duration.

RESULTS

The mean age, stature, weight, body mass index, and bone density of 12 male specimens were as follows: 65 ± 11 years, 1.8 ± 0.01 m, 83 ± 13 kg, 27 ± 5.0 kg/m2, and 114 ± 21 mg/cc. They were equally divided into short, medium, and long time durations: 4.8 ± 0.5, 16.3 ± 7.3, and 34.5 ± 7.5 ms. Most severe injuries associated with the short time duration were to pelvis, although they were to spine for the long time duration.

CONCLUSIONS

With adequate time for the underbody blast loading to traverse the pelvis-sacrum-spine complex, distal structures are spared while proximal/spine structures sustain severe/unstable injuries. The time factor may have implications in seat and/or seat structure design in future military vehicles to advance warfighter safety.

A Novel Paradigm to Develop Regional Thoracoabdominal Criteria for Behind Armor Blunt Trauma Based on Original Data

INTRODUCTION

For behind armor blunt trauma (BABT), recent prominent BABT standards for chest plate define a maximum deformation distance of 44 mm in clay. It was developed for soft body armor applications with limited animal, gelatin, and clay tests. The legacy criterion does not account for differing regional thoracoabdominal tolerances to behind armor-induced injury. This study examines the rationale and approaches used in the legacy BABT clay criterion and presents a novel paradigm to develop thoracoabdominal regional injury risk curves.

MATERIALS AND METHODS

A review of the original military and law enforcement studies using animals, surrogates, and body armor materials was conducted, and a reanalysis of data was performed. A multiparameter model analysis describes survival-lethality responses using impactor/projectile (mass, diameter, and impact velocity) and specimen (weight and tissue thickness) variables. Binary regression risk curves with ±95% confidence intervals (CIs) and peak deformations from simulant tests are presented.

RESULTS

Injury risk curves from 74 goat thorax tests showed that peak deflections of 44.7 mm (±95% CI: 17.6 to 55.4 mm) and 49.9 mm (±95% CI: 24.7 to 60.4 mm) were associated with the 10% and 15% probability of lethal outcomes. 20% gelatin and Roma Plastilina #1 clay were stiffer than goat. The clay was stiffer than 20% gelatin. Penetration diameters showed greater variations (on a test-by-test basis, difference 36-53%) than penetration depths (0-12%) across a range of projectiles and velocities.

CONCLUSIONS

While the original authors stressed limitations and the importance of additional tests for refining the 44 mm recommendation, they were not pursued. As live swine tests are effective in developing injury criteria and the responses of different areas of the thoracoabdominal regions are different because of anatomy, structure, and function, a new set of swine and human cadaver tests are necessary to develop scaling relationships. Live swine tests are needed to develop incapacitation/lethal injury risk functions; using scaling relationships, human injury criteria can be developed.

Human pelvis injury risk curves from underbody blast impact

INTRODUCTION

Underbody blast loading can result in injuries to the pelvis and the lumbosacral spine. The purpose of this study was to determine human tolerance in this region based on survival analysis.

METHODS

Twenty-six unembalmed postmortem human surrogate lumbopelvic complexes were procured and pretest medical images were obtained. They were fixed in polymethylmethacrylate at the cranial end and a six-axis load cell was attached. The specimens were aligned in a seated soldier posture. Impacts were applied to the pelvis using a custom vertical accelerator. The experimental design consisted of non-injury and injury tests. Pretest and post-test X-rays and palpation were done following non-injury test, and after injury test medical imaging and gross dissections were done. Injuries were scored using the Abbreviated Injury Scale (AIS). Axial and resultant forces were used to develop human injury probability curves (HIPCs) at AIS 3+ and AIS 4 severities using survival analysis. Then ±95% CI was computed using the delta method, normalised CI size was obtained, and the quality of the injury risk curves was assigned adjectival ratings.

RESULTS

At the 50% probability level, the resultant and axial forces at the AIS 3+ level were 6.6 kN and 5.9 kN, and at the AIS 4 level these were 8.4 kN and 7.5 kN, respectively. Individual injury risk curves along with ±95% CIs are presented in the paper. Increased injury severity increased the HIPC metrics. Curve qualities were in the good and fair ranges for axial and shear forces at all probability levels and for both injury severities.

CONCLUSIONS

This is the first study to develop axial and resultant force-based HIPCs defining human tolerance to injuries to the pelvis from vertical impacts using parametric survival analysis. Data can be used to advance military safety under vertical loading to the seated pelvis.

Loading rate effect on tradeoff of fractures from pelvis to lumbar spine under axial impact loading

Objectives: The transmission of impact loading from the seat-to-pelvis-to-lumbar spine in a seated occupant in automotive and military events is a mechanism for fractures to these body regions. While postmortem human subject (PMHS) studies have replicated fractures to the pelvis or lumbar spine using isolated/component models, the role of the time factor that manifests as a loading rate issue on injuries has not been fully investigated in literature. The objective of this study was to explore the hypothesis that short duration pulses fracture the pelvis while longer pulses fracture the spine, and intermediate pulses involve both components.Methods: Unembalmed PMHS thoracolumbar spine-pelvis specimens were fixed at the superior end, and a six-axis load cell was attached. The specimens were mounted on a vertical accelerator, and noninjury and injury tests were conducted by applying short, medium, or long pulses with 5, 15, or 35 ms durations, respectively. Peak axial, shear and resultant forces were obtained. Injuries were documented using posttest x-ray and computed tomography images and scaled using the AIS (2015).Results: The mean age, stature, weight, body mass index, and BMD of twelve specimens were 64.8 ± 11.4 years, 1.8 ± 0.01 m, 83 ± 13 kg, 26.7 ± 5.0 kg/m², and 114.5 ± 21.3 mg/cc, respectively. For the short, long, and medium duration pulses, the mean resultant forces were 5.6 ± 0.9 kN, 5.9 ± 0.94 kN, and 5.4 ± 1.8 kN, and time durations were 4.8 ± 0.5 ms, 16.3 ± 7.3 ms, and 34.5 ± 7.5 ms, respectively. For the short pulse, pelvis injuries were more severe in 3 out 4 specimens, for the medium pulse, they were distributed between the pelvis and spine, and for the long pulse, spine injuries were more severe in 3 out of 4 specimens.Conclusions: While acknowledging the limitations of the sample size, the results of this study support the hypothesis of the time variable in the tradeoff between pelvis and spine injuries with pulse duration. The tradeoff pattern is attributed to mass recruitment: short pulse biases injuries to pelvis while limiting spinal injuries, and the opposite is true for the longer pulse, thus supporting the hypothesis. It is important to account for the time variable in injury analysis.

Whole Body PMHS Response in Injurious Experimental Accelerative Loading Events

Previous studies involving whole-body post-mortem human surrogates (PHMS) have generated biomechanical response specifications for physically simulated accelerative loading intended to reproduce seat and floor velocity histories occurring in under-body blast (UBB) events (e.g.,. References 10, 11, 21 These previous studies employed loading conditions that only rarely produced injuries to the foot/ankle and pelvis, which are body regions of interest for injury assessment in staged UBB testing using anthropomorphic test devices. To investigate more injurious whole-body conditions, three series of tests were conducted with PMHS that were equipped with military personal protective equipment and seated in an upright posture. These tests used higher velocity and shorter duration floor and seat inputs than were previously used with the goal of producing pelvis and foot/ankle fractures. A total of nine PMHS that were approximately midsize in stature and mass were equally allocated across three loading conditions, including a 15.5 m/s, 2.5 ms time-to-peak (TTP) floor velocity pulse with a 10 m/s, 7.5 ms TTP seat pulse; a 13 m/s, 2.5 ms TTP floor pulse with a 9.0 m/s, 5 ms TTP seat pulse; and a 10 m/s, 2.5 ms TTP floor pulse with a 6.5 m/s, 7.5 ms TTP seat pulse. In the first two conditions, the seat was padded with a ~ 120-mm-thick foam cushion to elongate the pulse experienced by the PMHS. Of the nine PMHS tests, five resulted in pelvic ring fractures, five resulted in a total of eight foot/ankle fractures (i.e., two unilateral and three bilateral fractures), and one produced a femur fracture. Test results were used to develop corridors describing the variability in kinematics and in forces applied to the feet, forces applied to the pelvis and buttocks in rigid seat tests, and in forces applied to the seat foam in padded seat tests. These corridors and the body-region specific injury/no-injury response data can be used to assess the performance and predictive capability of anthropomorphic test devices and computational models used as human surrogates in simulated UBB testing.

Quantifying the Effect of Pelvis Fracture on Lumbar Spine Compression during High-rate Vertical Loading

Fracture to the lumbo-pelvis region is prevalent in warfighters seated in military vehicles exposed to under-body blast (UBB). Previous high-rate vertical loading experimentation using whole body post-mortem human surrogates (PMHS) indicated that pelvis fracture tends to occur earlier in events and under higher magnitude seat input conditions compared to lumbar spine fracture. The current study hypothesizes that fracture of the pelvis under high-rate vertical loading reduces load transfer to the lumbar spine, thus reducing the potential for spine fracture. PMHS lumbo-pelvis components (L4-pelvis) were tested under high-rate vertical loading and force and acceleration metrics were measured both inferior-to and superior-to the specimen. The ratio of inferior-tosuperior responses was significantly reduced by unstable pelvis fracture for all metrics and a trend of reduced ratio was observed with increased pelvis AIS severity. This study has established that pelvis fracture reduces compression forces at the lumbar spine during high-rate vertical loading, thus reducing the potential for fracture to the lumbar spine. Therefore, pelvis injury potential should be considered when implementing lumbar injury criteria specific to UBB.