Identifying Spinal Injury Patterns in Underbody Blast to Develop Mechanistic Hypotheses

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.

Hybrid III Manikin Lumbar Spine Loading Under Vertical Impact

INTRODUCTION

Clinical investigations have attributed lumbar spine injuries in combat to the vertical vector. Injury prevention strategies include the determination of spine biomechanics under this vector and developing/evaluating physical devices for use in live fire and evaluation-type tests to enhance Warfighter safety. While biological models have replicated theater injuries in the laboratory, matched-pair tests with physical devices are needed for standardized tests. The objective of this investigation is to determine the responses of the widely used Hybrid III lumbar spine under the vertical impact-loading vector.

MATERIALS AND METHODS

Our custom vertical accelerator device was used in the study. The manikin spinal column was mounted between the inferior and superior six-axis load cells, and the impact was delivered to the inferior end. The first group of tests consisted of matched-pair repeatability tests, second group consisted of adding matched-pair tests to this first group to determine the response characteristics, and the third group consisted of repeating the earlier two groups by changing the effective torso mass from 12 to 16 kg. Peak axial, shear, and resultant forces at the two ends of the spine were obtained.

RESULTS

The first group of 12 repeatability tests showed that the mean difference in the axial force between two tests at the same velocity across the entire range of inputs was <3% at both ends. In the second group, at the inferior end, the axial and shear forces ranged from 4.9-25.2 kN to 0.7-3.0 kN. Shear forces accounted for a mean of 11 ± 6% and 12 ± 4% of axial forces at the two ends. In the third group of tests with increased torso mass, repeatability tests showed that the mean difference in the axial force between the two tests at the same velocity across the entire range of inputs was <2% at both ends. At the inferior end, the axial and shear forces ranged from 5.7-28.7 kN to 0.6-3.4 kN. Shear forces accounted for a mean of 11 ± 8% and 9 ± 3% of axial forces across all tests at the inferior and superior ends. Other data including plots of axial and shear forces at the superior and inferior ends across tested velocities of the spine are given in the paper.

CONCLUSIONS

The Hybrid III lumbar spine when subjected to vertical impact simulating underbody blast levels showed that the impact is transmitted via the axial loading mechanism. This finding paralleled the results of axial force predominance over shear forces and axial loading injuries to human spines. Axial forces increased with increasing velocity suggesting the possibility of developing injury assessment risk curves, i.e., the manikin spine does not saturate, and its response is not a step function. It is possible to associate probability values for different force magnitudes. A similar conclusion was found to be true for both magnitudes of added effective torso mass at the superior end of the manikin spinal column. Additional matched-pair tests are needed to develop injury criteria for the Hybrid III male and female lumbar spines.

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.

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.

Spinal Fractures during Touristic Motorboat Sea Cruises: An Underestimated and Avoidable Phenomenon

PURPOSE

Each summer, many vacationers enjoy the Mediterranean Sea shores. Among the recreational nautical activities, motorboat cruise is a popular choice that leads to a significant number of thoracolumbar spine fractures at our clinic. This phenomenon seems to be underreported, and its injury mechanism remains unclear. Here, we aim to describe the fracture pattern and propose a possible mechanism of injury.

METHODS

We retrospectively reviewed the clinical, radiological, and contextual parameters of all motorboat-related spinal fracture cases during a 14-year period (2006-2020) in three French neurosurgical level I centers bordering the Mediterranean Sea. Fractures were classified according to the AOSpine thoracolumbar classification system.

RESULTS

A total of 79 patients presented 90 fractures altogether. Women presented more commonly than men (61/18). Most of the lesions occurred at the thoracolumbar transition region between T10 and L2 (88.9% of the levels fractured). Compression A type fractures were seen in all cases (100%). Only one case of posterior spinal element injury was observed. The occurrence of neurological deficit was rare (7.6%). The most commonly encountered context was a patient sitting at the boat's bow, without anticipating the trauma, when the ship's bow suddenly elevated while crossing another wave, resulting in a "deck-slap" mechanism hitting and propelling the patient in the air.

CONCLUSIONS

Thoracolumbar compression fractures are a frequent finding in nautical tourism. Passengers seated at the boat's bow are the typical victims. Some specific biomechanical patterns are involved with the boat's deck suddenly elevating across the waves. More data with biomechanical studies are necessary to understand the phenomenon. Prevention and safety recommendations should be given before motorboat use to fight against these avoidable fractures.

Injuries in Underbody Blast Fatalities: Identification of Five Distinct Mechanisms of Head Injury

Previous research has shown that injuries to the head and neck were prevalent in 73% of all mounted fatalities of underbody blast. The mechanisms that cause such injuries to the central nervous system (CNS) are not yet known. The aim of this study was to identify the head and spinal injuries in fatalities due to underbody blast (UBB) and then develop hypotheses on the causative mechanisms. All U.K. military fatalities from UBB with an associated head injury that occurred during 2007-2013 in the Iraq and Afghanistan conflicts were identified retrospectively. Computed tomography post-mortems (CTPMs) were interrogated for injuries to the head, neck, and spine. All injuries were documented and classified using a radiology classification. Pearson's chi-square and Fisher's exact tests were used to show a relationship between variables and form a hypothesis for injury mechanisms. There were 50 fatalities from UBB with an associated head injury. Of these, 46 had complete CTPMs available for analysis. Chi-square and Fisher's exact tests showed a relationship between lateral ventricle blood and injuries to the abdomen and thorax. Five partially overlapping injury constellations were identified: 1.multiple-level spinal injury with skull fracture and brainstem injury, 2.peri-mesencephalic hemorrhage, 3.spinal and brainstem injury, 4.parenchymal contusions with injury to C0-C1, and 5.an "eggshell" pattern of fractures from direct impact. These injury constellations can now be used to propose injury mechanisms to develop mitigation strategies or clinical treatments.

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.

An improved spinal injury parameter model for underbody impulsive loading scenarios

Underbody blast events such as aircraft ejection, mine blast, and helicopter crashes pose a serious threat to occupants. These impulsive excitations exert substantial axial loads on the thoracolumbar spine causing severe injuries. The Dynamic Response Index (DRI), which is commonly used as the injury parameter for underbody loading scenarios, suffers from inherent disadvantages and has been reported to underpredict the chances of injury. The main reasons are the inability of the DRI model to account for bending loads and posture of the spine. Thus, a novel lumped full spine model capable of modelling the spine in different posture along the sagittal plane is formulated. The unavailable data for the model were obtained using inverse parameter identification approach by eigenfrequency matching. Each vertebra has three degrees of freedom: axial, shear, and rotary motion to model the flexion of the spine. A new injury parameter is proposed based on the sum of compressions caused due to axial and rotary springs at each vertebral level, to account for wedge compression and burst fractures. The results indicate that the model was able to predict the motions of vertebrae under different postures of the spine according to trends in literature.

Fatal head and neck injuries in military underbody blast casualties

Introduction

Death as a consequence of underbody blast (UBB) can most commonly be attributed to central nervous system injury. UBB may be considered a form of tertiary blast injury but is at a higher rate and somewhat more predictable than injury caused by more classical forms of tertiary injury. Recent studies have focused on the transmission of axial load through the cervical spine with clinically relevant injury caused by resultant compression and flexion. This paper seeks to clarify the pattern of head and neck injuries in fatal UBB incidents using a pragmatic anatomical classification.

Methods

This retrospective study investigated fatal UBB incidents in UK triservice members during recent operations in Afghanistan and Iraq. Head and neck injuries were classified by anatomical site into: skull vault fractures, parenchymal brain injuries, base of skull fractures, brain stem injuries and cervical spine fractures. Incidence of all injuries and of each injury type in isolation was compared.

Results

129 fatalities as a consequence of UBB were identified of whom 94 sustained head or neck injuries. 87 casualties had injuries amenable to analysis. Parenchymal brain injuries (75%) occurred most commonly followed by skull vault (55%) and base of skull fractures (32%). Cervical spine fractures occurred in only 18% of casualties. 62% of casualties had multiple sites of injury with only one casualty sustaining an isolated cervical spine fracture.

Conclusion

Improvement of UBB survivability requires the understanding of fatal injury mechanisms. Although previous biomechanical studies have concentrated on the effect of axial load transmission and resultant injury to the cervical spine, our work demonstrates that cervical spine injuries are of limited clinical relevance for UBB survivability and that research should focus on severe brain injury secondary to direct head impact.

The association between type of spine fracture and the mechanism of trauma: A useful tool for identifying mechanism of trauma on legal medicine field

BACKGROUND

Determining the association between mechanism of trauma, and the type of spine column fracture is a useful approach for exactly describing spine injury on forensic medicine field. We aimed to determine mechanism of trauma based on distribution of the transition of spinal column fractures.

METHODS

This cross-sectional survey was performed on 117 consecutive patients with the history of spinal trauma who were admitted to emergency ward of Rasoul-e-Akram Hospital in Tehran, Iran from April 2015 to March 2016. The baseline characteristics were collected by reviewing the hospital recorded files.

RESULTS

With respect to mechanism of fracture, 63.2% of fractures were caused by falling, 30.8% by collisions with motor vehicles, and others caused by the violence. Regarding site of fracture, lumbosacral was affected in 47.9%, thoracic in 29.9%, and cervical in 13.7%. Regarding type of fracture, burst fracture was the most common type (71.8%) followed by compressive fracture (14.5%). The site of fracture was specifically associated with the mechanism of injury; the most common injuries induced by falling from height were found in lumbosacral and cervical sites, and the most frequent injuries by traffic accidents were found in thoracic site; also the injuries following violence were observed more in lumbar vertebrae. The burst fractures were more revealed in the patients affected by falling from height and by traffic accidents, and both burst and compressive fractures were more observed with the same result in the patients injured with violence (p = 0.003).

CONCLUSION

The type of spine fracture due to trauma is closely associated with the mechanism of trauma that can be helpful in legal medicine to identify the mechanism of trauma in affected patients.

Mediastinal injury is the strongest predictor of mortality in mounted blast amongst UK deployed forces

BACKGROUND

Blast injury has been the most common cause of morbidity and mortality encountered by UK forces during recent conflicts. Injuries sustained by blast are categorised by the injuring component of the explosion and depend upon physical surroundings. Previous work has established that head injuries and intra cavity haemorrhage are the major causes of death following exposure to under body (mounted) blast but has yet to explore the precise nature of these torso injuries nor the effect of particular injuries upon survival. This study examines the patterns of torso injury within the mounted blast environment in order to understand the effect of these injuries upon survivability.

METHODS

This retrospective study examined the UK Joint Theatre Trauma Registry to determine precise injury patterns of mounted blast casualties within a 13year period of UK military deployments. Survival rates of individual injuries were compared and a multivariable logistic regression model was developed in order to assess the effect that each injury had upon likelihood of death.

RESULTS

426 mounted casualties were reviewed of whom 129 did not survive. Median NISS and ISS for non-survivors was found to be 75. Torso injuries were significantly more common amongst non-survivors than survivors and high case fatality rates were associated with all haemorrhagic torso injuries. Multivariable analysis shows that mediastinal injuries have the largest odds ratio for mortality (20.4) followed by lung laceration and head injury.

CONCLUSIONS

Non-compressible torso haemorrhage is associated with mortality amongst mounted blast. Of this group, mediastinal injury is the strongest predictor of death and could be considered as a surrogate marker of lethality. Future work to link blast loading characteristics with specific injury patterns will inform the design of mitigating strategies in order to improve survivability of underbody blast.