A variety of threats lead to head injuries in the battlefield. Since World War II (WWII), the predominant threats have been: fragmentation and ballistic threats from explosions, artillery, and small arms fire; blunt trauma caused by translation from blast, falls, vehicle crashes, and impact with vehicle interiors and from parachute drops; and exposure to primary blasts. Key findings in this chapter indicate the following:
• Wounding from an explosive source (e.g., fragmentation from bombs, mines, and artillery) dominates all wounding, including bullets.
• Non-battle causes, including blunt traumatic injuries, produced nearly 50 percent of the hospitalizations for traumatic brain injury in Iraq/Afghanistan.
• There is no biomechanical link in the current test methodology between the backface deformation assessment and head injuries from behind-helmet deformation.
There is a need to revise test methodologies to focus on the dominant threats. The current protocol addresses primarily rounds from 9-mm pistol fire, which is a relatively small contributor to soldier injuries. It is also important to develop better understanding of the scientific connection between head injuries and the performance metrics used in current test methodology.
The major threats that have caused head injuries in recent conflicts can be classified into three groups: ballistic, blunt, and blast. Table 3-1 identifies their sources and lists potential head injuries. As shown in Figure 3-1, these three categories can also be distinguished by the duration of peak force.1 For example, for blast loading injuries, the time to peak force and pressure occurs over a timescale of less than 100 microseconds. So, blast injuries of a given severity generally have lower associated momentum and strains/displacements than those for blunt impact, which has peak forces occurring at 3 to 50 milliseconds. On the other hand, ergonomics-related injuries, such as those from heat, weight, lack-of-fit, and long-term usage, typically take days and months.
The rest of this chapter describes head injuries and their typical characteristics. The limitations of current injury test methodologies for assessing head injury risk, including the lack of biomechanical links between test methodology and injury, are also discussed.
A number of studies have examined military wounding of U.S. forces in major conflicts since WWII. See, for example, Emergency War Surgery (DoD, 2004); Bellamy et al. (1986); Bellamy (1992); Carey (1996); Carey et al. (1998); and Owens et al. (2008). These studies are based on injuries/treatments reported from hospitalizations, including those who died of wounds in hospital. They show that the extremities are the predominant body region injured followed by head/neck (Table 3-2).
Owens et al. (2008) reported that a total of 1,566 U.S. soldiers sustained 6,609 combat wounds in Afghanistan (Operation Enduring Force [OEF]) and Iraq (Operation Enduring Freedom [OIF]). This implies an average of about 4.2 wounds per soldier, likely due to fragments. The data did not include those killed in action, or returned to duty, but did
1There has been considerable research related to head and neck injuries over the past 40 years (McIntosh and McCrory, 2005; Fuller et al., 2005; Xydakis et al., 2005; and Brolin et al., 2008). However, much of this work is not applicable to high-impact-rate, low-momentum-transfer scenarios that characterize ballistic impact (Bass et al., 2003).
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3 Threats, Head Injuries, and Test Methodologies 3.0 SUMMARY can also be distinguished by the duration of peak force.1 For example, for blast loading injuries, the time to peak force A variety of threats lead to head injuries in the battle- and pressure occurs over a timescale of less than 100 micro- field. Since World War II (WWII), the predominant threats seconds. So, blast injuries of a given severity generally have have been: fragmentation and ballistic threats from explo- lower associated momentum and strains/displacements than sions, artillery, and small arms fire; blunt trauma caused those for blunt impact, which has peak forces occurring at by translation from blast, falls, vehicle crashes, and impact 3 to 50 milliseconds. On the other hand, ergonomics-related with vehicle interiors and from parachute drops; and expo- injuries, such as those from heat, weight, lack-of-fit, and sure to primary blasts. Key findings in this chapter indicate long-term usage, typically take days and months. the following: The rest of this chapter describes head injuries and their typical characteristics. The limitations of current injury test • Wounding from an explosive source (e.g., fragmenta- methodologies for assessing head injury risk, including the tion from bombs, mines, and artillery) dominates all lack of biomechanical links between test methodology and wounding, including bullets. injury, are also discussed. • Non-battle causes, including blunt traumatic injuries, produced nearly 50 percent of the hospitalizations for traumatic brain injury in Iraq/Afghanistan. 3.2 HISTORICAL PATTERNS OF TREATABLE • There is no biomechanical link in the current test INJURIES methodology between the backface deformation A number of studies have examined military wound- assessment and head injuries from behind-helmet ing of U.S. forces in major conflicts since WWII. See, for deformation. example, Emergency War Surgery (DoD, 2004); Bellamy et al. (1986); Bellamy (1992); Carey (1996); Carey et al. There is a need to revise test methodologies to focus on (1998); and Owens et al. (2008). These studies are based on the dominant threats. The current protocol addresses primar- injuries/treatments reported from hospitalizations, including ily rounds from 9-mm pistol fire, which is a relatively small those who died of wounds in hospital. They show that the contributor to soldier injuries. It is also important to develop extremities are the predominant body region injured followed better understanding of the scientific connection between by head/neck (Table 3-2). head injuries and the performance metrics used in current Owens et al. (2008) reported that a total of 1,566 U.S. test methodology. soldiers sustained 6,609 combat wounds in Afghanistan (Operation Enduring Force [OEF]) and Iraq (Operation 3.1 INTRODUCTION Enduring Freedom [OIF]). This implies an average of about 4.2 wounds per soldier, likely due to fragments. The data did The major threats that have caused head injuries in recent not include those killed in action, or returned to duty, but did conflicts can be classified into three groups: ballistic, blunt, and blast. Table 3-1 identifies their sources and lists potential 1There has been considerable research related to head and neck injuries head injuries. As shown in Figure 3-1, these three categories over the past 40 years (McIntosh and McCrory, 2005; Fuller et al., 2005; Xydakis et al., 2005; and Brolin et al., 2008). However, much of this work is not applicable to high-impact-rate, low-momentum-transfer scenarios that characterize ballistic impact (Bass et al., 2003). 15
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16 REVIEW OF DEPARTMENT OF DEFENSE TEST PROTOCOLS FOR COMBAT HELMETS TABLE 3-1 Broad Categories of Threats Threats Sources Potential Head Injuries Ballistic and fragment impacts on the Rifles, handguns, artillery, IEDs Penetrating trauma, behind-armor-blunt-trauma, helmet BFD Blunt: Impacts into ground, vehicles, Falls, vehicle crashes, blast events, and Closed and open head injuries, skull fracture, buildings, etc. other potential sources hematomas, brain contusions Blasts Bombs, artillery, IEDs Brain trauma, meningeal hematomas, contusions, axonal injuries NOTE: BFD, backface deformation; IED, improvised explosive device. Peak Blast Overpressure Peak Force Peak Blunt Accel due to increased thoracic protection (e.g., Belmont et al., (> 3-100 ms) (~200 ms) (> 3,000-50,000 ms) 2010; Wood et al., 2012a). The relative success of thoracic body armor likely contributes to the changes in proportion Blast Impact of GSW wounding from previous conflicts to OEF/OIF Ballistic Impact (Owens et al., 2008). Blunt Impact For Iraq/Afghanistan, Table 3-5 shows that explosions Ergonomics Hours, days, months, years are the primary source of injury across all body regions, ranging from 88 percent for the head to 78 percent for the thorax. Time 200 ms 400 ms 800 ms 1000 ms Wojcik et al. (2010) found results comparable to Owens et al. (2008) for hospitalizations for traumatic brain inju- FIGURE 3-1 Typical timeline of blast, ballistic, blunt injuries ries (TBIs) from battlefield causes in OEF/OIF. About 22 compared to ergonomics-related injuries. percent of personnel had TBIs from all causes (Okie, 2005; Figure 3-1, Warden, 2006; and U.S. Army Medical Surveillance Activ- ity, 2007). For moderate to severe TBI, about 67 percent of include those who died of wounds.2 Table 3-3 shows the loca- the injuries were attributable to explosions; of these, direct tions and distributions of these wounds. The predominant blunt trauma contributed 11 to 13 percent and penetrating location is extremity (54 percent), followed by the abdomen injuries contributed 11 to 16 percent (Figure 3-2a). Note, (11 percent), face (10 percent), and head (8 percent).3 Data in however, that many of the injuries attributable to explosions Owens et al. (2008) also show that the proportion of head and may have been the result of low-rate blunt trauma following neck wounds in OEF/OIF is higher than those from WWII, blast events. Figure 3-2b shows that nearly half of the hos- Korea, and Vietnam wars (16-21 percent). On the other hand, pitalizations for TBIs in OEF/OIF were noncombat injuries. the proportion of thoracic wounds has decreased by about 50 Since helmets are often worn in noncombat scenarios, these percent from those for WWII and Vietnam. figures emphasize the potential role for the combat helmet Table 3-4 shows that explosions (blast and fragmenta- in protecting the head from nonbattle TBI from blunt trauma tion threats) have been the major source of U.S. military and other causes. wounding since WWII, ranging from 65 percent in Viet- nam to more than 80 percent in OEF/OIF (DoD, 2004; The conclusions from these studies can be summarized Owens et al., 2008; Wojcik et al., 2010). In addition, there as follows: is almost a 50 percent reduction in direct gunshot wounds (GSW) from Vietnam to OEF/OIF. This may largely be Finding 3-1. • Historically, head injuries represent 15 to 30 percent 2Owens et al. (2008) noted: “Definitions significantly affect the results of of all wounding by body region. casualty analysis. . . . The inclusion of KIAs, RTDs, and NBIs in any cohort • Wounding from an explosive source (including analyzed will affect the distribution of wounds and mechanism of injury. fragmentation from bombs, mines, and artillery) For example, the inclusion of KIAs in the cohort analyzed may result in an dominates injuries in all major modern conflicts since increase in the number of head and chest wounds seen.” WWII. 3Owens et al. (2008) also reported that there were fluctuations in these • With respect to blast and blunt trauma: figures over time. For example, one of the studies cited there reported a 4-month period of casualties received at Walter Reed Army Medical Cen- —In OEF/OIF, the proportion of blast-associated ter, when they cared for 119 patients with 184 injuries. There were some head injuries (attributed to blast fragments) has differences in the breakdowns: head and neck—16 percent, thorax—14 increased relative to gunshot wounds. percent, abdomen—11 percent, upper extremity—20 percent, and lower —Nonbattle causes, including blunt traumatic inju- extremity—40 percent. The distribution of the sources of these injuries was ries, produced nearly 50 percent of the hospital- also different: 39 percent bullet, 34 percent blunt, and 31 percent explosion. This was during the period of ground warfare and not counterinsurgency. izations for TBI in OEF/OIF.
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THREATS, HEAD INJURIES, AND TEST METHODOLOGIES 17 TABLE 3-2 Relative Body Surface Area and Distribution of Wounds by Body Region (in Percentage) OEF (Afghanistan) and Body Surface Area WWII Korea Vietnam OIF (Iraq) Head and neck 12 21 21 16 30 Thorax 16 14 10 13 6 Abdomen 11 8 9 10 9 Extremities 61 58 60 61 55 NOTE: Based on injuries/treatments from hospitalizations, including personnel who died of wounds. OEF, Operation Enduring Force; OIF, Operation Iraqi Freedom; WWII, World War II. SOURCE: Owens et al. (2008). TABLE 3-3 Distribution of Wounds by Body Region in TABLE 3-5 Distributions of Injury Causes by Body Operation Enduring Force (Afghanistan) and Operation Region (in Percentage) Iraqi Freedom (Iraq) Gunshot Explosion Motor Vehicle Region Wounds Percent Wounds (%) (%) Collision (%) Head 509 8 Head and Neck 8 88 4 Eyes 380 6 Thorax 19 78 3 Face 635 10 Abdomen 17 81 2 Ears 175 3 Extremity 17 81 2 Neck 207 3 SOURCE: Owens et al. (2008). Thorax 376 6 Abdomen 709 11 Extremity 3,575 54 Recommendation 3-2. The Department of Defense should investigate the possibility of increasing blunt impact protec- Total 6,609 100 tion of the combat helmet to reduce head injuries. NOTE: Based on injuries/treatments from hospitalizations, including personnel who died of wounds. SOURCE: Owens et al. (2008). 3.3 THREATS Bullets TABLE 3-4 Percentage of Injuries from Gunshot Wounds The presentation by the Chief Scientist, Soldier Protective and Explosions from Previous U.S. Wars and Individual Equipment,4 listed repeating pistols, such as Conflict Gunshot Wounds (%) Explosion (%) Tokarev (7.62×25-mm caliber) and Makarov (9×18-mm cali- WWII 27 73 ber), as emerging threats. However, for insurgent and guer- Korea 31 69 rilla warfare, published data and anecdotal evidence suggest that AK-47 (7.62×39-mm) and other Kalashnikov-pattern Vietnam 35 65 weapons are the predominant source of ballistic threats in OIF or OEF 19 81 Iraq, Afghanistan, and Somalia (Small Arms Survey, 2012). NOTE: OEF, Operation Enduring Force; OIF, Operation Iraqi Freedom; In a survey of 80,000 small arms and light weapons seizures, WWII, World War II. they found that the “vast majority of illicit small arms in SOURCE: Owens et al. (2008). Afghanistan, Iraq, and Somalia are Kalashnikov-pattern assault rifles. Other types of small arms are comparatively On the other hand, the Department of Defense helmet test- rare” (p. 6). These weapons and their ammunition are inex- ing protocols—the subject of this report—focus mainly on pensive and widely available with continuing production and protective capabilities against gunfire threats. large existing supplies (e.g., Small Arms Survey, 2012; Stohl et al., 2007; Perry, 2004; Jones and Ness, 2012). Recommendation 3-1. The Department of Defense should ensure that appropriate threats, in particular fragmentation threats, from current and emerging threat profiles are used 4James Zheng, Chief Scientist, Soldier Protective and Individual Equip- in testing. ment, PEO Soldier, U.S. Army, presentation to the committee, March 21, 2013.
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18 REVIEW OF DEPARTMENT OF DEFENSE TEST PROTOCOLS FOR COMBAT HELMETS FIGURE 3-2 (a) Traumatic brain injury (TBI) hospitalizations by source for battle injuries categorized by regions in Operation Enduring Force/Operation Iraqi Freedom. (b) TBI hospitalizations by combat/noncombat source. NOTE: BSA, body surface area. SOURCE: Based on data from Wojcik et al. (2010). TABLE 3-6 Representative Standard-Issue Infantry Rifles and Ammunition for Selected Potential Adversaries Country Type Bullet (mm) Use Typical Muzzle Velocity (m/s) China Type 56 7.62 × 39 1956-present 790-930 Type 81 7.62 × 39 1981-present 750 QBZ-95 5.8 × 42 1995-present 735 QBZ-97 5.56 × 45 1995-present Iran M1 Garand 7.62 × 63 1950s-present 850 HK G3A6 7.62 × 51 1980-present 800 S-5.56 5.56 × 45 990 North Korea Type 58 7.62 × 39 1958-present 715 Type 68 7.62 × 39 1968-present 900 Type 88 5.45 × 39 1988-present 900 Russia AKM 7.62 × 39 1959-present 715 AK-74 5.45 × 39 1974-present 900 AK-74M 5.45 × 39 1991-present 900 SOURCE: Jones and Ness (2012). Infantry small arms of potential major adversaries includ- Finding 3-2. Small arms surveys and deployed infantry ing China, Iran, North Korea, and Russia have two pre- weapons from major adversaries suggest that 5.56-mm and dominant calibers (Jones and Ness, 2012). Reserve forces 7.62-mm rounds at muzzle velocities from 735 m/s to more are often issued older types of 7.62×39-mm Kalashnikov- than 800 m/s are the current predominant ballistic threats. pattern weapons. These have more recently transitioned to 5.45×39-mm or 5.56×45-mm (China) types. Muzzle veloci- Fragmentation ties of these types range from 715 m/s to 990 m/s (Jones and Ness, 2012). Realistic threat profiles, however, may involve As discussed earlier, fragmenting weapons, including velocity at typical engagement ranges rather than muzzle artillery, mines, mortars, and other sources of explosions, are velocities. Available bullet types range from copper-jacketed the principal source of wounding on the modern battlefield. lead core bullets through armor-piercing incendiary bullets These weapons, including improvised explosive devices including high explosive fills. Table 3-6 lists the bullets that (IEDs), have a multitude of fills/wounding mechanisms. are potential threats to U.S. forces. They also have a spatial distribution of fragments that them- selves vary by sizes/mass and initial velocities. The relative
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THREATS, HEAD INJURIES, AND TEST METHODOLOGIES 19 risk fragments of each velocity and mass should be included impact to the head. In addition, many blast events likely in the threat profile for testing. involve blunt trauma (Bass et al., 2012). However, there is limited published data for arena tests5 Blunt trauma threats may be rated as a function of the for principal artillery and fragmentation threats. Much of the change in velocity (often reproduced by drop-testing), as extensive work is classified. Nevertheless, several studies shown in Table 3-7. General threats range from approxi- allow order-of-magnitude analyses for this class of weapon, mately 14 ft/sec for half height falls (falls from 3 ft) to more based on mass, and velocity information from typical 105- than 50 ft/sec for typical vehicle crashes at 35 mph. For mm and 155-mm howitzer shells (e.g., ATEC, 1983; Dehn, comparison, the current ACH purchase description specifies 1980; Ramsey et al., 1978; AMC, 1964). A review of these a particular acceleration limit (150 g) for a 10 ft/sec drop, studies leads to the following findings. far smaller than typical threat velocities. A recent study of TBI from conflicts in OEF/OIF by Finding 3-3. Results in the open literature indicate that Wojcik et al. (2010) found that about 15 percent of the the fragment test velocities used in Advanced Combat hospitalizations were associated with direct blunt trauma, a Helmet specification are representative of initial fragment figure that is similar to ballistic penetrating injury. Further, it velocities from 155-mm artillery shells under high explosive is likely that many of the head injuries associated with blast detonation. (about 50 to 60 percent of the cases) were also attributable to low-rate blunt trauma from direct or subsequent contact with Finding 3-4. Results in the open literature show that frag- vehicle interiors, the ground, and so on. For these injuries, ment masses in the ACH specification are generally rep- Wojcik et al. (2010) found that almost 80 percent of person- resentative fragment masses from 155-mm artillery shells nel were wearing a helmet during the incident. It is unclear under high explosive detonation. However, there is a range how much the presence of the helmet mitigates or moderates of fragment masses between 100-grain6 and 200-grain from potential injury, but there is substantial injury exposure even artillery shells that have no counterpart in ACH testing. with current combat helmet use. Data on blunt trauma injuries from more than 120,000 Finding 3-5. IEDs may have dramatically different distribu- parachute jumps during 1941 to 1998 show that blunt trauma tions of fragment size and velocity compared to other frag- injury rates were approximately 8 per 1,000 drops (Bricknell menting weapons such as mortars and artillery. The current and Craig, 1999). Bricknell and Craig (1999) reported that ACH threat profile used in testing was selected before the head injuries were 4 to19 percent of the total injuries across emergence of widespread IED use. a range of studies. A more recent study (Knapik et al., 2011) showed that blunt trauma to the head comprised 30 percent Recommendation 3-3. The Department of Defense should of the total injuries, which is quite large. Overall hospitaliza- reassess helmet requirements for current and potential tion rates for TBI in OIF were estimated to be 0.31 percent future fragmentation threats, especially for fragments ener- (Wojcik et al., 2010). gized by blast and for ballistic threats. The reassessment U.S. drop-qualified personnel are required to make 4 should examine redundancy among design threats, such as jumps/year to retain their jump status (Knapik et al., 2010), the 2-grain versus the 4-grain and the 16-grain versus the and many active personnel make 10-15 or more jumps per 17-grain. Elimination of tests found to be redundant may year (Knapik et al., 2003, 2010). For exposure over a 10-year allow resources to be directed at a wider diversity of realistic career, airborne personnel may have career head injury risk ballistic threats, including larger mass artillery fragments, ranging from 10 percent for 4 jumps per year to 34 percent bullets other than the 9-mm, and improvised explosive device fragments. This effort should also examine the effects of shape, mass, and other parameters of current fragmentation threats and differentiate these from important characteristics TABLE 3-7 Representative Battlefield Threats/Impact of design ballistic threats. Velocities Impact Velocity Blunt Trauma Threat m/s (ft/sec) Fall—half height (3 ft) 4.3 (14) Blunt trauma threats on the battlefield are ubiquitous and include falls, vehicle crashes, impact with vehicle interiors, Fall—full height (6 ft) 6 (20) impact from parachute drops, and other sources of blunt Parachute drop (e.g,. McEntire, 2005) 5.2-6.4 (17-21) Motor vehicle crash—unrestrained occupant 3-15.2 (10-50) 5Arena tests are standard tests of artillery shells in which fragment num- ber, fragment, and velocity spatial distribution are assessed using high speed Motorcycle helmet standards (e.g., FMVSS-218) 5.2-6 (17-20) video and nondestructive capture mechanisms. Current ACH threat 3 (10) 6The grain (gr) is a commonly used unit of measure of the mass of bullets. There are 0.0648 grams per grain. NOTE: ACH, Advanced Combat Helmet.
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20 REVIEW OF DEPARTMENT OF DEFENSE TEST PROTOCOLS FOR COMBAT HELMETS for 15 jumps per year. Thus, there is a great potential for similar diameters. Based on the earlier threat analyses, the blunt injury from this threat. committee focuses mainly on military rifle rounds. Two primary measures are used to assess the performance Finding 3-6. Common blunt trauma threats have impact of helmets: penetration and backface deformation (BFD). velocities of 6.1 m/s (20 ft/s) that are equivalent to drops of (They are formally defined in Chapter 5.) Briefly, a penetra- 190 cm (75 inches). On the other hand, current blunt trauma tion occurs if the ballistic impact causes a projectile to pass threats assessed for the ACH helmet have impact velocities though the helmet shell. BFD is a measure of the deformation of 3.1 m/s (10 ft/s) which are equivalent to drops of 47 cm on the helmet from impact to the head. (18.6 inches). The earliest published standard for assessment of pen- etration with ballistic protective helmets was developed by the National Institute of Standards and Technology’s Law Primary Blast Enforcement Standards Laboratory (National Institute of There is limited information on the effect of primary Justice (NIJ) Standard-0106.01–NIJ-1981). This standard blast on the head (Bass et al., 2012). TBI associated with specifies inertial impact and penetration assessments for blast exposure in OEF/OIF is estimated at up to 20 percent ballistic helmets. Testing of penetration resistance in this of deployed service personnel (e.g., Tanielian and Jaycox, standard uses a fixed headform with witness panels located 2008; Ling et al., 2009). The current helmet is not designed in the mid-coronal plane for a sagittal shot (Figure 3-3) or with considerations for primary blast, but there is substantial mid-sagittal planes for a coronal shot. (See Chapter 4 for experimental evidence that the ACH helmet is protective more details.) against primary blast for most direct exposures (Shridharani The current ACH standard modifies this NIJ headform to et al., 2012). Further, computational models of the human provide deformation resistance using the clay (Roma Plasti- head/helmet system show that helmets with padding do not lina No. 1) used to certify ballistic vests. The empty spaces of exacerbate blast exposure for a range of conditions (Panzer the headform are filled with clay, and the permanent plastic et al., 2010; Panzer and Bass, 2012; Nyein et al., 2010). But backface deformation of the helmet into the clay is recorded it is not clear if primary blasts are an important source of as a BFD measurement. Since the head does not undergo wounding. Data presented to the committee7 indicated that plastic deformation in the same manner as the clay, this pro- more than 1,500 of the 1,922 reported wounded-in-action cedure has no biomechanical basis (NRC, 2012). incidents produced mild or moderate concussions. However, it is not known if the source of these concussions was primary Finding 3-8. The mechanical response of clay is qualitatively blasts or falls/tertiary blasts. different from the response of the human head/skull, which may affect both the penetration and backface deformation Finding 3-7. Epidemiological data, experimental results, and response of the helmet. computational models suggest that the ACH helmet does not exacerbate blast exposure. 3.4 ADVANCED COMBAT HELMET TEST METHODOLOGY AND LINKS TO BIOMECHANICS This section outlines the typical characteristics of each injury type and elucidates the biomechanical basis for pen- etration and behind-armor blunt trauma assessments. Penetrating Trauma Modern ballistic wounding is generally differentiated between rifle and handgun rounds by velocity. For example, high-velocity tumbling rounds such as typical 5.56-mm projectiles (800 m/s or above muzzle velocity) have qualita- tively different wounding behavior than .22 caliber handgun ammunition (~330 m/s muzzle velocity), although they have FIGURE 3-3 Sagittal headform specified in National Institute of Justice Penetration Standard, based on the Department of Trans- portation blunt impact headform. Two similar headforms are used 7Natalie Eberius, Predictive Analysis Team Leader, Army Research for the helmet tests: A modified version of this headform provides Laboratory, “Blast Injury Research” presentation to the committee, April Figure 3-3, fixed advanced combat helmet backface deformation and the basis for the 25, 2013. penetration tests. SOURCE: NIJ (1981).
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THREATS, HEAD INJURIES, AND TEST METHODOLOGIES 21 Clay has been used to assess BFD in military helmets for the past decade.8 However, there is no existing study link- ing clay deformation to head injury. For ballistic vests and body armor, Prather et al. (1977) linked backface response to abdominal injury in goats, and by inference to humans by an indirect process. There is no corresponding study for the head. Even then, the biomechanics are likely inappropri- ate for humans. For example, transient deformation of the abdomen (and by extension the clay) is much larger than the typical deformation to failure from a skin or skull system. Finding 3-9. • Prather et al. (1977) is the basis for use of clay to assess BFD injuries. This study linked abdomi- FIGURE 3-4 Long linear and depressed skull fractures from non- nal response behind deforming soft body armor penetrating helmet BFD in a human cadaveric model. SOURCE: with abdominal injury in goats through an indirect Bass et al. (2003). process. Figure 3-4 • There is no biomechanical link between the BFD assessment in the current test methodology and head Modern protective helmet materials (McManus, 1976; injuries from behind helmet deformation. Carey et al., 2000) may deform sufficiently for the backface of the helmet to make contact with the head, potentially Recommendation 3-4. The Department of Defense should causing head injuries (e.g., Mayorga et al., 2010; Bass et al., vigorously pursue efforts to provide a biomedical basis for 2002, 2003). Possible injuries include both depressed and assessing the risk of helmet backface injuries. long linear skull fractures (Figure 3-4) and other closed-head brain trauma. Owing to the localization from ballistic impact, Head and neck injuries have been the focus of much it is unclear that there is a relationship between low-rate research in the past 40 years (e.g., McIntosh and McCrory, injuries from blunt trauma and potential injuries from BFD. 2005; Fuller et al., 2005; Xydakis et al., 2005; Brolin et al., The injuries may occur either from the deforming of the 2008). This work, however, is not necessarily applicable to undefeated helmet locally onto the head or underlying skull the high-impact-rate, low-momentum-transfer scenarios that or from acceleration loads transmitted through the helmet characterize ballistic impact (e.g., Bass et al., 2003). padding to the head (Bass et al., 2003; Mayorga et al., 2010). For BFD scenarios or scenarios in which the bullet The Advisory Group for Aerospace Research and Devel- remains in the helmet, there is a potential for neck inju- opment (AGARD, 1996) references 29 standards for blunt ries. Such neck injuries are generally associated with large impact assessment, all of which have a similar underlying momentum input or resulting velocity changes from impact basis: the head acts as a rigid body (Bass et al., 2003), and (e.g., see Bass et al., 2006). Increased helmet mass will tend head injury of any type is associated with skull fracture to delay and decrease neck forces and may mitigate the (Versace, 1971; Hodgson and Thomas, 1973; Bass et al., potential for injury. A number of neck injuries are possible 2003). Recent work by Viano demonstrates poor association from head motion following momentum transfer from the between skull fracture and brain injury (Viano, 1988). bullet to the helmet. These include ligamentous injuries (such There are a few studies of head injury that arises from as strains, tears, or distractions), tensile failure in interver- BFD (e.g., Sarron et al., 2000; Bass et al., 2003). Bass et al. tebral endplates or vertebral bodies, or other injuries to the (2003) developed injury criteria for skull fracture and brain osteoligamentous spine (Figure 3-5). injury in human cadaveric heads during ballistic loading of Because neck motion following ballistic impact follows a protective helmet. These tests used ultrahigh-molecular- a timescale comparable to neck motion from vehicle crashes weight polyethylene helmets with 9-mm full metal jacket or falls, automobile criteria are likely appropriate. Current (FMJ) test rounds under various impact velocities to 460 or future helmet ballistic threats have quite low momentum m/s (1,510 ft/s). Measurements taken from cadavers with transfer to the head, resulting in quite low injury risk (NRC, and without skull fracture show no association with existing 2012). For example, direct measurements have been made blunt trauma injury models. Further, there was no obvious of the neck loads following helmet ballistic impact using association of any acceleration-based response with the a 9-mm FMJ round over a range of velocities for human occurrence of BFD fracture. Skull force-based injury criteria are available from Bass et al. (2012), which may be useful in 8James Zheng, Chief Scientist, Soldier Protective and Individual Equip- future test methodologies. ment, PEO Soldier, U.S. Army, presentation to the committee, March 21, 2013.
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22 REVIEW OF DEPARTMENT OF DEFENSE TEST PROTOCOLS FOR COMBAT HELMETS Contusion Rupture 15-20 mm Bridging Vein *d/dt=45 s-1 10-30 mm slip =0.25 Tentorium Rupture Brain Motion Impact 15-25 mm displacement Diffuse Concussion/Coma Axonal Injury *d/dt=45 s-1 Complex strain =0.25 pattern (100-200 g’s) FIGURE 3-5 Typical potential neck injury locations in adults from impact loading. SOURCE: Courtesy of Dale Bass, Duke University. Figure 3-5 fixed FIGURE 3-6 Typical blunt brain trauma diagram. SOURCE: Based cadaver tests. Both the NIJ and beam9 injury assessment on Ommaya et al. (1994). values indicate very low risk of neck injuries (<0.1 percent) for these scenarios, and no neck injuries were seen in test- ing. By extension, injury risk through 7.62×54-mm rounds 80 g has been suggested recently to protect against changes and beyond to muzzle velocities is low. There is, however, in mentation (cf. Duma et al., 2005). Impact energy limits the potential for neck trauma from blunt impact to the head. from these standards are shown in Figure 3-7. Improved helmet blunt impact characteristics may reduce the Other potential assessment techniques include the ACH risk of neck injury from blunt trauma. standard (CO/PD-05-04), which is based on the motorcycle helmet Federal Motor Vehicle Safety Standard–218 (49 CFR Finding 3-10. The risk of neck injuries from momentum Sec 571.218); the National Operating Committee on Stan- transfer from ballistic impact of a nonpenetrating round dards for Athletic Equipment (NOCSAE); and standards that or fragment on the helmet is low for current and near-term incorporate the International Standards Organization (ISO) future threats up to the 7.62×54-mm rounds at muzzle headforms. Recent developments include the star rating velocity. system for football helmets from the Virginia Polytechnic and State University (Rowson and Duma, 2011). The current Blunt Trauma ACH blunt impact test assessment (CO/PD-05-04) restricts peak acceleration to a U.S. Department of Transportation Typical blunt trauma head injuries include skull fractures, (DOT) headform fitted in the ACH to less than 150 g given a hematomas and contusions, and diffuse axonal injuries (e.g., headform impact velocity of 3 m/s (10 fps). At approximately Ommaya et al., 1994). Many tentative mechanical injury tol- 45 J drop energy, the ACH blunt impact assessment is quali- erances have been established for particular injuries (Figure tatively different from many typical blunt threats experienced 3-6), and blunt trauma injury criteria have been promulgated by service personnel. for protective helmets (e.g., AGARD, 1996). Head protection from blunt impact in vehicles and sports has advanced substantially over the past 30 years. Wide- spread use of protective helmets has reduced severity and frequency of head injuries. Many of the improvements in helmet technology have arisen from standardized test meth- odologies based on blunt impact injury criteria. Twenty-nine blunt impact test standards are included in AGARD AR-330 (AGARD, 1996), and the basis for each of these standards is some type of impact acceleration limit. Nineteen have accel- eration or force limits alone, and ten use acceleration/dura- tion levels. Acceleration levels specified in these standards vary from 150 g to 400 g, but a standard of approximately 9Beam is a neck injury criterion that was developed to assess the risk of neck injury from impacts, including the effect of helmets/night vision and FIGURE 3-7 Energy limits for blunt impact injury assessment in other head-supported mass (Bass et al., 2006). AGARD AR-330. SOURCE: Based on data from AGARD (1996). Figure 3-7 fixed
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THREATS, HEAD INJURIES, AND TEST METHODOLOGIES 23 Bass, C.R., M. Bolduc, and S. Waclawik. 2002. Development of a nonpen- Finding 3-11. Numerous established test methodologies are etrating, 9-mm, ballistic trauma test method. Pp. 18-22 in Proceedings of the Personal Armor Systems Symposium (PASS 2002), The Hague, available for assessment of blunt trauma injury with helmets, Netherlands, November 18-22, 2002. Prins Maurits Laboratorium, Rose including supporting injury reference values. International Exhibition Management and Congress Consultancy, The Hague, Netherlands. Recommendation 3-5. Whether or not advanced combat Bass, C.R., B. Boggess, B. Bush, M. Davis, R. Harris, M.R. Rountree, helmet design standards are improved to reflect more realistic S. Campman, J. Ecklund, W. Monacci, G. Ling, G. Holborow, E. Sanderson, and S. Waclawik. 2003. “Helmet Behind Armor Blunt blunt trauma threats, the current testing protocols should be Trauma.” Paper presented at the RTO Applied Vehicle Technology Panel/ revised to more fully reflect common blunt trauma threats Human Factors and Medicine Panel Joint Specialists’ Meeting held in that are prevalent in training and on the battlefield. Koblenz, Germany, May 19-23, 2003. NATO Science and Technology Organization, Neuilly-sur-Seine, France. Bass, C.R., L. Donnellan, R.S. Salzar, S. Lucas, B. Folk, M. Davis, K.A. Primary Blast Rafaels, C. Planchak, K. Meyerhoff, A. Ziemba, and N. Alem. 2006. A New Neck Injury Criterion in Combined Vertical/Frontal Crashes with Models based on animals show that exposure of the iso- Head Supported Mass. Madrid, Spain: International Research Council lated head to primary blast impingement can cause various on the Biomechanics of Impact (IRCOBI). types of injuries including fatality (Säljö et al., 2000, 2008; Bass, C.R., M.B. Panzer, K.A. Rafaels, G. Wood, and B. Capehart. 2012. Rafaels et al., 2011, 2012). The injuries include menin- Brain injuries from blast. Annals of Biomedical Engineering 40(1):185- 202. geal bleeding, skull fractures, axonal injuries, and gliosis. Bell, M.K. 2008. Standardized model is needed to study the neurological However, there are still uncertainties about the relationship effects of primary blast wave exposure. Military Medicine 173(6): v-viii. between primary blast TBI from animal models and mild Belmont P.J., A.J. Schoenfeld, and G. Goodman. 2010. Epidemiology of TBI during military service (e.g., Bell, 2008). For severe TBI combat wounds in operation Iraqi freedom and operation enduring free- from blast exposure, there may be clear neurological changes, dom: Orthopaedic burden of disease. Journal of Surgical Orthopaedic Advances 19(1): 2-7. including reduced levels of mentation, unconsciousness, and Bellamy, R.F., P.A. Maningas, and J.S. Vayer. 1986. Epidemiology of other dysfunctions (Ling et al., 2009). For milder exposures, trauma: Military experience. Annals of Emergency Medicine 15(12): possible consequences include neurological deficits, depres- 1384-1388. sion, anxiety, memory difficulty, and impaired concentration Bellamy, R.F. 1992. The medical effects of conventional weapons. World (Kauvar et al., 2006; Ritenour and Baskin, 2008; Stein and Journal of Surgery 16(5): 888-892. Bricknell, M.C.M., and S.C. Craig. 1999. Military parachuting injuries: A McAllister, 2009). Diagnosis is difficult for milder exposures literature review. Journal of Occupational Medicine 49(1):17-26. because these symptoms strongly overlap with posttraumatic Brolin, K., K. Hedenstiern, P. Halldin, C.R. Bass, and N. Alem. 2008. stress disorder often seen in service members (Capehart and The importance of muscle tension on the outcome of impacts with a Bass, 2011; Bass et al., 2012). major vertical component. International Journal of Crashworthiness Several primary blast injury assessments have been devel- 13(5):487-498. Capehart, B.P., and C.R. Bass. 2011. Mild TBI among veterans returning oped recently using animal models (Rafaels et al., 2011, from Afghanistan and Iraq. Available at http://www.psychiatrictimes. 2012). While scaling of these animal models to human val- com/military-mental-health/traumatic-brain-injury-among-veterans- ues is not fully established (Wood et al., 2012b), these risk returning-afghanistan-and-iraq. assessments suggest that brain injuries may occur at much Carey, M.E. 1996. Analysis of wounds incurred by U.S. Army Seventh lower levels of blast exposure than previously accepted, and Corps personnel treated in Corps hospitals during Operation Desert Storm, February 20 to March 10, 1991. Journal of Trauma 40(3S):165S- potentially much lower levels than pulmonary injury for a 169S. soldier wearing body armor. Carey, M.E., A.S. Joseph, W.J. Morris, D.E. 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