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A Software Tool to Assess Injury Risk Associated with Mechanical Exposures From Wearing Head Supported Mass


OBJECTIVE: Develop injury criteria, methodology, and a software tool to assess the risk of neck injury from loads sustained while wearing head supported mass. The software will characterize the hazards endemic to the ground combat environment and will be used to evaluate products and recommend less hazardous designs and usage scenarios. DESCRIPTION: It is imperative that equipment issued to Soldiers is designed to function properly and to minimize the risks of developing disabling chronic musculoskeletal disorders. Experiences from recent military conflicts clearly demonstrate that Soldiers are issued head borne devices that impose loads that exceed functional tolerances and elevate risks of developing disabling neuromuscular disorders. The Personnel Armor System for Ground Troops (PASGT) comes in five sizes that range in weight from 3.1 to 4.2 pounds to which many be attached other devices that add mass or create asymmetrical loads. The PASGT face shield is a common addition. The reactive contraction of posterior cervical muscles to counter the torque imposed by the 3.4 pound PASGT ballistic face shield caused headaches and prompted a request from a field medical officer to locate a substitute product. Although symptoms were noted soon after the face shield was fielded, the medical sequela from long term use and increased risk of severe, acute conditions such as disc herniation and of future degenerative conditions could not be estimated because of the lack of a injury criteria for chronic load effects and an assessment tool that could be applied during design to identify potential problems and alert System Developers to the need for design modifications. Since the Army fielded more than 800,000 PASGTs, tremendous opportunities to avoid costs associated with lost work time, provider salaries, medical treatments were lost by not having the capacity to identify problems early and integrate design changes prior to fielding. Although headgear is aggressively scrutinized, tests traditionally focus on how the physical properties of a helmet will protect the Soldier from insult originating from the environment or how well an attached device will perform its intended function. Aside from fitting, less attention is directed to estimating the adverse medical effects manifested in musculoskeletal tissues subjected to unaccustomed levels of mechanical force over long durations. The reason for this oversight is that the injuries and costs related to impacts from motor vehicle accidents and sports have stimulated medical research to develop models for high velocity collisions, but similar models have not been developed for lower energy, long duration loading. PHASE I: Develop injury criteria that describe the relationship between the physical characteristics of head borne load (mass, symmetry of weight distribution, and location relative to the center of gravity of the head) and the exposure schedule (hours per day, days per week, weeks per year) to determine hazard severities and probabilities for key adverse outcomes: load intolerance from cervical muscle fatigue and increased risk (relative to a comparable normal population) of developing near term acute cervical conditions (such as disc bulges, prolapses and herniations) and long term chronic degenerative tissue diseases of the cervical spine (such as degenerative disc disease and degenerative joint diseases). Build and demonstrate a proof-of-concept exposure assessment model that estimates and displays a biomechanical measure of adverse mechanical stress from a set of data that are either readily measurable or obtainable from the System Developer's specification (such as the item's mass and item center of gravity) or the use scenario. Load conditions shall demonstrate the range of weights of head borne gear (helmets and attached accessories). It shall determine injury risk relative to the full range of male and female anthropometries and display risk using hazard severity and hazard probabilities as defined in AR-40-10. This model shall be limited to static biomechanical calculates that assumes that the Soldier is stationary. PHASE II: Build a software application that integrates all of the features specified in Phase I. Additional exposure assessment criteria shall be developed and integrated into the software that allow simulation of attachments on headgear. The software shall provide a means for simulating a minimum of two attachments at a variety of locations relative to the head piece: a face shield and one other attachment. The software shall be able to resolve the forces from the masses of the main head piece and attachments and generate injury risk assessments using AR-40-10 as described in the previous phase. The exposure assessment model and software shall be expanded to enable injury prediction for the following dynamic activities: marching, running, jumping, controlled parachute fall landing and diving to prone. The software shall accomplish this by allowing the software operator to enter an integer corresponding to time spent either marching or running and integers for the other activities to designate the number of times each activity is performed in a 24 hour period. The model shall estimate the sum of the biomechanical energy to which musculoskeletal tissues are exposed based upon the physical characteristics of the activities performed and the duration of exposure, determine hazard severity and hazard probability in accordance with AR-40-10 definitions and display these data on the graphical user interface. PHASE III DUAL USE APPLICATIONS: The contractor will provide a working software application. The US Army Public Health Command (USAPHC) Ergonomics Program will develop at least five hypothetical scenarios that include equipment and user scenarios that expose Soldiers to head supported mass. The USAPHC Ergonomics Program will use the assessment methodology to conduct analyses, write reports and submit the results through the USAPHC Health Hazard Assessment Program's report review process. USAPHC will collect comments and appraisals from Ergonomics and Health Hazard Assessment Personnel and analyze them. Results will be discussed with the US Army Medical Research and Material Command to identify deficiencies and develop a plan for reconciliation. The injury criteria and assessment software should attract great worldwide commercial interest by all branches of the armed services, law enforcement agencies, and other parties who design headgear for occupational and sports applications. Generally, helmets are designed to protect against insult from rapid change in acceleration (such as used for horseback riding, hang gliding, roller and ice skating, skiing, skateboarding, automotive racing and motocross, bull riding, canoeing, kayaking and bicycle riding) and repeated trauma (such as used for hockey and football). Recent awareness of vulnerability to traumatic brain injury from high force and repeated low force trauma has been favoring a shift to constructing helmets from heavier materials. Exposure to chronic loading from these heavier helmets and other specialized helmets used by selected groups (such as policemen, firemen, welders, miners and autistic children) increase the risk of developing degenerative neck disorders. This head supported mass model would help commercial headgear designers more objectively evaluate the tradeoff between selecting certain design features (i.e., heavy materials or asymmetrical weight distribution) and injury risk. This model would impact a very broad market. Although no statistics were available that summarized the total market, the Bicycle Helmet Safety Institute estimated that between 12 to 15 million bicycle helmets are sold in the United States each year. REFERENCES: 1. Butler BP and Allen NM. Long-Duration Exposure Criteria for Head-Supported Mass. Army Aeromedical Research Lab, Fort Rucker AL. USAARL-97-34, 1997. 2. Ivancevic V and Beagley N. Determining the Acceptable Limits of Head Mounted Loads. Land Operations Division, Systems Sciences Laboratory. DSTO-TR-1577. 2003. 3. LaFiandra M, Harman E, Cornelius N, Frykman P, Gutekunst D, and Nelson G. The Effects of the Personal Armor System for Ground Troops (PASGT) and the Advanced Combat Helmet (ACH) With and Without PVS-14 Night Vision Goggles (NVG) on Neck Biomechanics During Dismounted Soldier Movements. US Army Research Institute of Environmental Medicine, Military Performance Division. T07-09 2007. 4. Manoogian SJ, Kennedy EA and Duma SM. A literature review of musculoskeletal injuries to the human neck and the effects of head-supported mass worn by Soldiers. USAARL Contract Report No. CR-2006-01, 2006. 5. Merkle AC, Kleinberger M, and Uy OM. The Effects of Head-Supported Mass on the Risk of Neck Injury in Army Personnel. Johns Hopkins APL Technical Digest: 26(1):75-83. 2005.
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