Preliminary Head-Supported Mass Performance Guidance for Dismounted Soldier Environments.

INTRODUCTION: The helmet is an ideal platform to mount technology that gives U.S. Soldiers an advantage over the enemy; the total system is recognized quantitatively as head-supported mass (HSM). The stress placed on the head and neck is magnified by adding mass and increasing the center of mass offset away from the atlanto-occipital complex, the head's pivot point on the spine. Previous research has focused on HSM-related spinal degeneration and performance decrement in mounted environments. The increased capabilities and protection provided by helmet systems for dismounted Soldiers have made it necessary to determine the boundaries of HSM and center of mass offset unique to dismounted operations. MATERIALS AND METHODS: A human subject volunteer study was conducted to characterize the head and neck exposures and assess the impact of HSM on performance in a simulated field-dismounted operating environment. Data were analyzed from 21 subjects who completed the Load Effects Assessment Program-Army obstacle course at Fort Benning, GA, while wearing three different experimental HSM configurations. Four variable groups (physiologic/biomechanical, performance, kinematic, and subjective) were evaluated as performance assessments. Weight moments (WMs) corresponding to specific performance decrement levels were calculated using the quantitative relationships developed between each metric and the study HSM configurations. Data collected were used to develop the performance decrement HSM threshold criteria based on an average of 10% total performance decrement of dismounted Soldier performance responses. RESULTS: A WM of 134 N-cm about the atlanto-occipital complex was determined as the preliminary threshold criteria for an average of 10% total performance decrement. A WM of 164 N-cm was calculated for a corresponding 25% average total performance decrement. CONCLUSIONS: The presented work is the first of its kind specifically for dismounted Soldiers. Research is underway to validate these limits and develop dismounted injury risk guidance.

Fracture Injury Risk of the Restrained Mandible to Anterior-Posterior Blunt Impacts.

This study describes the results of anterior-posterior impacts conducted on the mandibles of 22 male postmortem human subjects (PMHSs). The objective of this study was to develop an injury criterion for the mandible based on blunt impact while the jaw was restrained. Previous studies have attempted to characterize the injury risk of blunt impact to the mandible; however, due to the translation of the mandible during impact and a limited number of fractured specimens, previous studies were not able to produce an injury criterion. Blunt impact to a restrained mandible is relevant to a wide array of helmeted individuals, including the military population and sports that require helmets with chinstraps. Therefore, in this study, specimens were positioned with restrained jaws and impacted using a monorail drop tower with a gravity-driven cylindrical impactor. Nineteen of 22 specimens sustained at least one fracture during testing. Injury cases had an average impact energy of 15.0 ± 5.7 J (11.1 ± 4.2 ft-lb) and a fracture force of 2684 ± 726 N (603 ± 163 lbf). Results were used to generate an impactor force based injury criterion through survival analysis. Risk of injury was modeled using a Weibull distribution and a 50% risk of injury was found to occur at approximately 2834 N (637 lbf). The developed injury risk curve can be used to characterize injury to the restrained mandible for future testing and research studies, especially in the development of maxillofacial protective equipment.

A Novel Application of Head Tracking Data in the Analysis and Assessment of Operational Cervical Spine Range of Motion for Army Aviators.

INTRODUCTION: Neck pain among rotary-wing aviators has been established as an important issue in the military community, yet no U.S. Army regulation defines exactly what cervical spine range of motion (CROM) is adequate for flight. This lack of regulation leaves flight surgeons to subjectively determine whether an aviator affected by limited CROM is fit to maintain flight status. The U.S. Army Aeromedical Research Laboratory is conducting a study among AH-64 and UH-60 pilots to define CROM requirements in simulated and actual flight using optical head tracking equipment. Presented here is a preliminary analysis of head position data from a pilot and co-pilot in two AH-64 missions. METHODS: Maintenance data recorder (MDR) files from two AH-64 missions were provided by the Apache Attack Helicopter Project Management Office. Data were filtered down to three-dimensional pilot and co-pilot head position data and each data point was analyzed to determine neck posture. These neck postures were then categorized as neutral, mild, and severe for flexion/extension, lateral bending, and twist rotation postural categories. RESULTS: Twist rotation postures reached 90 degrees, particularly early in the flight; additionally, a few instances of 90-degree lateral bends were observed. Co-pilots spent more time than pilots in mild and severe twist rotation posture for both flights. Co-pilots also spend a high percentage of time in mild flexion and twist rotation. CONCLUSION: This investigation provides a proof of concept for analysis of head tracking data from MDR files as a surrogate measure of neck posture in order to estimate CROM requirements in rotary-wing military flight missions. Future studies will analyze differences in day and night flights, pilot versus co-pilot CROM, and neck movement frequency.

Evaluation of Head and Body Kinematics Experienced During Parachute Opening Shock.

INTRODUCTION: The U.S. Army conducts airborne operations in order to insert soldiers into combat. Military airborne operations are physically demanding activities with a unique loading environment compared with normal duties. A significant amount of research surrounding airborne operations has focused on assessing the incidence and type of associated injuries as well as the potential risk factors for injuries. During parachute opening shock and other high-acceleration events (e.g., fixed wing flight or vehicle crashes), the neck may be vulnerable to injury if inertial loads overcome the voluntary muscular control of the cervical spine and soft tissue structures. A recent epidemiological survey of sport skydivers showed that the neck, shoulders, and back were the most frequently reported sites of musculoskeletal pain. In addition, the survey indicated that wing loading (a measure of the jumper's weight divided by the size of the parachute canopy) was a potential contributing factor for developing musculoskeletal pain. Recently, there have been efforts to measure the severity of parachute opening shock as an additional potential risk factor for injury; however, no studies have measured both head and body accelerations and no studies have measured head or body angular rate during parachute opening shock. The purpose of this study was to measure and characterize the accelerations and angular rates of both the head and body during parachute opening shock as well as investigate potential factors contributing to higher severity opening shock, which may link to the development of musculoskeletal pain or injury. MATERIALS AND METHODS: Data were collected from the U.S. Army Parachute Team, The Golden Knights, under an approved Medical Research and Material Command Institutional Review Board protocol. Subjects were instrumented with a helmet- and body-mounted sensor package, which included three angular rate sensors and three single-axis accelerometers each. Data were collected at 2,500 samples per second. Kruskal-Wallis tests were used to determine if helmet-mounted equipment (e.g., cameras), neck length, neck circumference, or wing loading (the ratio of jump weight to the size of the main parachute canopy) affected the accelerations or angular rates of the head or body. RESULTS: A total of 54 jumps conducted by 19 experienced free-fall jumpers were analyzed. For the head, the mean (± SD) resultant accelerations and angular rates were 5.8 (± 1.6) g and 255.9 (± 74.2) degrees per second (deg/s), respectively. For the body, the resultant accelerations and angular rates were 4.3 (± 1.5) g and 181.3 (± 61.2) deg/s, respectively. A wing loading above 1.4 pounds per square foot (lb/ft2) was found to have a significant effect on head (P = .001) and body (P = .001) resultant acceleration as well as body angular rate about the Y-axis (P = .001). CONCLUSIONS: There is evidence to suggest that wing loading has an influence on individual head and body resultant accelerations. However, no significant effects were found for the other variables (e.g., neck length and circumference, helmet-mounted equipment, etc.). Future research should focus on identifying additional factors that result in changes in accelerations and angular rates of the head and body during parachute opening shock events.

Retrospective Analysis of Injuries in Underbody Blast Events: 2007-2010.

Underbody Blast (UBB) exposure emerged as a substantial cause of morbidity and mortality of Service Members in Iraq and Afghanistan, which was unique to OIF/OEF due to the frequent use of improvised explosive devices. Improvised explosive devices under the vehicle delivered high-rate vertical loading to the vehicle translating energy to the occupant(s) resulting in injuries. Injury mitigating technologies needed to be developed; however, technologies rely on biomechanical human response data for research and development. Widely accepted human response corridors have been developed and established for slower frontal and side impact exposures. Currently, there are no accepted human response data for high-rate vertical exposures, like those experienced during UBB events. To understand the mechanisms and replicate the exposures, analyses of injuries caused by UBB events were required. Medical injury data from UBB events during OIF/OEF were examined. Data were categorized by disposition, body region, injury type, and severity. Data analyses were performed on 555 Service Members receiving a total of 3,844 injuries. The Torso and the Head/face regions were the most injured and sustained predominately fractures/dislocations and internal organ injuries. This work will allow others to prioritize injuries to develop the methodology required to create response metrics to improve energy mitigating technology.

Development of an injury risk curve for pelvic fracture in vertical loading environments.

OBJECTIVE: Pelvis injury mechanisms are dependent upon loading direction (frontal, lateral, and vertical). Studies exist on the frontal and lateral modes; however, similar studies in the vertical mode are relatively sparse. Injury risk curves and response corridors are needed to delineate the biomechanical responses. The objective of the study was to derive risk curves for pelvis injuries using postmortem human subjects (PMHSs). METHODS: Published data from whole-body PMHSs loaded axially through the pelvis were analyzed. Accelerometers were placed on the pelvis/sacrum and seat. Specimens were loaded along the inferior to superior direction using a horizontal sled or a vertical accelerator device. Specimens were positioned supine in the horizontal sled and seated upright on the vertical accelerator. Pre- and posttest images were obtained and autopsies were completed to document the pathology. Variables used in the development of risk curves included velocity, acceleration, time to peak acceleration, pulse duration of acceleration, and jerk for the seat and sacrum. Survival analysis was used for risk curves. To determine the best predictor of pelvis injury, the Brier Score metric (BSM) was used. The best parametric distribution was determined using the corrected Akaike information criterion (AICc). Injury data points were treated as either uncensored or left/interval censored. Noninjury data points were treated as right censored. RESULTS: Twenty-four PMHS specimens were identified from 3 published data sets. Fifteen PMHS specimens sustained injuries and 9 remained intact. The BSM ranged from 1.24 to 24.75 and, in general, the BSMs for the seat metric-related scores were greater than the sacrum data. The sacrum acceleration was the optimal metric for predicting pelvis tolerance (lowest BSM). The Weibull distribution had the lowest AICc, with right and left/interval-censored data. This was also true when injury data were treated as exact (uncensored) observations. The 50% probability of injury was associated with 229 G for the uncensored analysis and 139 G for the censored analysis, and the quality indices in both cases were in the "good" range. CONCLUSIONS: Statistical determination of the best injury metric will help improve the accuracy of injury prediction, prioritize instrumentation choice in dummy development, and improve design criteria for crash mitigation. The present study showed that injury risk curves using response data are better biomechanical descriptors of human responses than exposure data. These data are important in automotive safety because complex loading of the pelvis, including submarining, occurs in frontal car crashes.

Development and validation of a synthetic eye and orbit for estimating the potential for globe rupture due to specific impact conditions.

The Facial and Ocular CountermeasUre Safety (FOCUS) headform is intended to aid safety equipment design in order to reduce the risk of eye and facial injuries. The purpose of this paper is to present a three part study that details the development and validation of the FOCUS synthetic eye and orbit and the corresponding eye injury criteria. The synthetic eye and orbit were designed to simulate the force-deflection response to in-situ dynamic impacts. In part I, the force-deflection response of the eye was determined based on dynamic blunt impact tests with human eyes. These data were used to validate the appropriate material for a biofidelic synthetic eye. In part II, force-deflection corridors developed from ten dynamic in-situ eye impacts were used to validate the design and material selections for the synthetic orbit assembly. In part III, 82 experimental tests on the FOCUS headform were conducted using steel BB projectiles to develop a conservative injury risk criteria for the FOCUS headform based on the response of the eye load cell. Injury criterion for globe rupture is strongly correlated to the data from the FOCUS eye load cell (R(2) = 0.995). Based on the response of the FOCUS eye load cell, a 50% risk of globe rupture from a 4.5 mm BB impact is shown to be 107 N. With a biofidelic synthetic eye and this projectile-specific injury criteria, the FOCUS headform can be used to conservatively evaluate the risk of globe rupture from > or = 4.5 mm diameter projectile impacts to the eye.

Upper extremity interaction with a helicopter side airbag: injury criteria for dynamic hyperextension of the female elbow joint.

This paper describes a three part analysis to characterize the interaction between the female upper extremity and a helicopter cockpit side airbag system and to develop dynamic hyperextension injury criteria for the female elbow joint. Part I involved a series of 10 experiments with an original Army Black Hawk helicopter side airbag. A 5(th) percentile female Hybrid III instrumented upper extremity was used to demonstrate side airbag upper extremity loading. Two out of the 10 tests resulted in high elbow bending moments of 128 Nm and 144 Nm. Part II included dynamic hyperextension tests on 24 female cadaver elbow joints. The energy source was a drop tower utilizing a three-point bending configuration to apply elbow bending moments matching the previously conducted side airbag tests. Post-test necropsy showed that 16 of the 24 elbow joint tests resulted in injuries. Injury severity ranged from minor cartilage damage to more moderate joint dislocations and severe transverse fractures of the distal humerus. Peak elbow bending moments ranged from 42.4 Nm to 146.3 Nm. Peak bending moment proved to be a significant indicator of any elbow injury (p = 0.02) as well as elbow joint dislocation (p = 0.01). Logistic regression analyses were used to develop single and multiple variate injury risk functions. Using peak moment data for the entire test population, a 50% risk of obtaining any elbow injury was found at 56 Nm while a 50% risk of sustaining an elbow joint dislocation was found at 93 Nm for the female population. These results indicate that the peak elbow bending moments achieved in Part I are associated with a greater than 90% risk for elbow injury. Subsequently, the airbag was re-designed in an effort to mitigate this as well as the other upper extremity injury risks. Part III assessed the redesigned side airbag module to ensure injury risks had been reduced prior to implementing the new system. To facilitate this, 12 redesigned side airbag deployments were conducted using the same procedures as Part I. Results indicate that the re-designed side airbag has effectively mitigated elbow injury risks induced by the original side airbag design. It is anticipated that this study will provide researchers with additional injury criteria for assessing upper extremity injury risk caused by both military and automotive side airbag deployments.