TY - JOUR
AU - Brozoski, Fredrick
AB - Abstract Military combat helmets protect the wearer from a variety of battlefield threats, including projectiles. Helmet back-face deformation (BFD) is the result of the helmet defeating a projectile and deforming inward. Back-face deformation can result in localized blunt impacts to the head. A method was developed to investigate skull injury due to BFD behind-armor blunt trauma. A representative impactor was designed from the BFD profiles of modern combat helmets subjected to ballistic impacts. Three post-mortem human subject head specimens were each impacted using the representative impactor at three anatomical regions (frontal bone, right/left temporo-parietal regions) using a pneumatic projectile launcher. Thirty-six impacts were conducted at energy levels between 5 J and 25 J. Fractures were detected in two specimens. Two of the specimens experienced temporo-parietal fractures while the third specimen experienced no fractures. Biomechanical metrics, including impactor acceleration, were obtained for all tests. The work presented herein describes initial research utilizing a test method enabling the collection of dynamic exposure and biomechanical response data for the skull at the BFD-head interface. Skull fracture, back-face deformation, behind armor blunt trauma INTRODUCTION Modern military combat helmets are designed to provide a level of protection against penetrating head trauma from ballistic projectiles (shrapnel, bullets, etc.). Combat helmets protect the wearer from ballistic projectiles by absorbing the projectile’s energy through deformation of the helmet shell’s constituent materials and fracturing of the matrix used to bond the constituent materials together. The deformation of the helmet shell results in projectiles being “caught” by the helmet. As the helmet defeats the ballistic projectile, the helmet locally deforms inward toward the wearer’s head. The described phenomenon is known as back-face deformation (BFD). Due to the limited stand-off distance between the interior surface of the helmet and the head, BFD can result in high-rate localized blunt impacts to the head, potentially inducing localized skull fracture. Currently, BFD in combat helmets is assessed using live fire ballistic testing.1 A modified National Institute of Justice (NIJ) ballistic headform is filled with clay, fitted with a helmet, and impacted using a 9 mm projectile.1–3 The depth of the resulting clay deformation is measured and compared to a threshold of 25.4 mm for the front and back impact sites and 16 mm for the side and crown impact sites.1,3 The clay deformation threshold has no medical basis and is not associated with any type or risk of injury.3 A medically based pass-fail criterion for helmet BFD is needed to ensure proper Service Member protection; however, research into the effects of behind-armor blunt-trauma (BABT) to the head has received limited investigation. Early research focused on the effects of armor stand-off distance from the head. Sarron et al4 shot 16.3 mm steel balls into flat aluminum plates, 12.3 mm thick, that were offset from dry human skulls by a distance of 10 mm. Impacts were focused on the frontal bone, parietal bone, and vertex of the skull.4 In a follow-on study, Sarron et al5 investigated the effect of armor stand-off distance on skull fracture incidence. Standard 9 mm projectiles were shot at armor-simulating plates, 9 mm thick, placed between 12 mm and 15 mm stand-off distance from the right parietal bone of dry human skulls.5 Nine fresh-frozen post-mortem human subject (PMHS) head specimens also were tested with varying armor-simulating plates at stand-off distances varying from 0 mm to 8 mm.5 Fracture severity was shown to be indirectly proportional to stand-off distance, and the authors suggested a minimum stand-off distance of 12.7 mm would help limit head injuries.5 The most applicable injury research conducted to date on BFD injuries of combat helmets was conducted using nine fresh-frozen PMHS head specimens fitted with ultra-high molecular weight polyethylene (UHMWPE) helmets.6 Each specimen was fitted to ensure a nominal stand-off distance of 12.7 mm, and the parietal region was impacted with a 9 mm projectile.6 Recently, Rafaels et al7 documented the results from seven of these PMHS specimens in detail. Fracture patterns due to helmet BFD were defined as causing localized depressed fractures at the point of contact with linear fractures appearing distal to the impact site.7 Biomechanical investigations of BABT due to helmet BFD have reported projectile velocity, projectile energy, and impact pressure as the primary metrics associated with skull fracture and injury.4–7 From the collected metrics, Bass et al6 was able to suggest injury criteria based on muzzle velocity and impact pressure. Force and deformation metrics commonly used in assessing fracture tolerance and development of injury criteria have not been determined due to limitations in the measurement of biomechanical metrics at the BFD-head interface. Force and deformation have been investigated in previous studies focusing on skull fracture due to non-lethal munitions;8,9 however, these results might not be applicable due to the differences in the impact shape profile that has been associated with differences in fracture force.10 The present study aims to develop a test method that will be used to generate medically based human skull fracture criteria for localized blunt impacts to the skull for use in assessing injuries from helmet BFD. By replicating the geometry of the BFD at the point of contact with the skull, the need for a helmeted test surrogate is eliminated. Without the obstruction of a helmet, additional engineering metrics (i.e., acceleration, force, and deformation) are able to be investigated. These metrics can be used in the determination of fracture tolerance thresholds and development of medically based standards for helmet developers. METHODS A modified National Operating Committee on Standards for Athletic Equipment (NOCSAE) projectile system (Southern Research Impact Center, LLC, Rockford, TN) was used to conduct all high-rate blunt impacts in the test series. The projectile launcher (Fig. 1A) consists of an accumulator tank connected to a 63.5 mm inner diameter stainless-steel barrel through a fast-opening solenoid valve. Projectiles are loaded into the open end of the barrel and tamped against the breach end to ensure they are seated flush against the valve opening. When triggered, the solenoid valve dumps the high pressure air from the accumulator tank into the barrel, propelling the projectile down the length of the barrel into the test chamber. The test chamber is enclosed in clear polycarbonate panels allowing for video documentation and to provide a washable, non-porous surface. FIGURE 1. Open in new tabDownload slide (A) A NOCSAE projectile system was modified for use with a custom projectile barrel shown here with a Facial and Ocular CountermeasUres for Safety headform installed in the test chamber for projectile and instrumentation verification prior to cadaveric specimen impacts. (B) A representative impact projectile was 3D printed with specified impactor head curvature and internal attachments for on-board instrumentation. FIGURE 1. Open in new tabDownload slide (A) A NOCSAE projectile system was modified for use with a custom projectile barrel shown here with a Facial and Ocular CountermeasUres for Safety headform installed in the test chamber for projectile and instrumentation verification prior to cadaveric specimen impacts. (B) A representative impact projectile was 3D printed with specified impactor head curvature and internal attachments for on-board instrumentation. A representative impactor was designed to be launched from the projectile launcher system for this test series. The impactor was manufactured out of acrylonitrile butadiene styrene plastic through 3D printing and consisted of two parts: a body section, with a tail, and an impactor head (Fig. 1B). The impactor head was designed to replicate the geometry of a helmet BFD profile at a shell deformation of 12.7 mm. The stand-off distance was consistent with previously reported research.6,7 The Army Research Laboratory’s Survivability / Lethality Analysis Directorate mapped the interior surface geometries of helmets impacted by projectiles using a digital image correlation technique described by Hisley et al.11 Back-face deformation geometries were provided to the U.S. Army Aeromedical Research Laboratory’s Injury Biomechanics Division and used to design the impactor head. The resulting impactor head profile had a 38.1 mm radius of curvature. Both the projectile body and head were designed to fit tightly together affixed with screws. The impactor head was instrumented with three 20,000 G single-axis accelerometers (PCB Piezotronics, Depew, NY, USA) mounted beneath a SLICE Nano data acquisition system (Diversified Technical Systems, Inc., Seal Beach, CA, USA) allowing for on-board data collection. The instrumented impactor had a total mass of 0.124 kg. Impactor instrumentation was verified by mounting a Facial and Ocular CountermeasUres for Safety (FOCUS) headform (Humanetics Innovative Solutions, Plymouth, MI, USA) inside the projectile system test chamber and impacting the frontal bone load cells. Calculated impactor force was compared to the measured force recorded in the headform load cells. Three fresh-frozen, male PMHS head and neck specimens (mean age: 63.3 yr; age range: 53–71 yr) were obtained for this study in collaboration with the University of Virginia, Center for Applied Biomechanics (Table I). The study was conducted in accordance with U.S. Army Cadaver Policy12 and was approved by the U.S. Army Medical Research and Materiel Command Office of Research Protections. Specimens were imaged using computed tomography (CT) upon acceptance to ensure no pre-existing bone fractures or abnormalities were present prior to testing. Specimens were stored at −20°C before thawing at room temperature. Pre-instrumentation dissection was conducted to remove the neck and excess tissue and to evacuate the cranial cavity. A 20% ballistics gelatin mixture (Clear Ballistics LLC, Fort Smith, AR, USA) was used to fill the cranial cavity and sealed at the foramen magnum using a polyester resin (Bondo, 3 M, St. Paul, MN, USA). Specimens were fixed to a steel plate using self-taping screws, to allow for mounting to a Hybrid III neck (Humanetics Innovative Solutions, Plymouth, MI, USA) in the test apparatus. An acoustic emission sensor (MISTRAS Group Inc, Princeton Junction, NJ, USA) was secured to the skull apex using a cyanoacrylate adhesive. Uniaxial strain gages (Vishay Precision Group, Inc, Raleigh, NC, USA) were secured superior and posterior to each impact site. Instrumentation placement was documented using CT imaging before testing. Table I. Specimen Age, Weight, and Anthropometric Measurements Specimen . Age (years) . Head Weight (kg) . Head Circumference (cm) . Head Breadth (cm) . Head Length (cm) . IBD001 71 3.93 55.00 14.86 19.25 IBD002 53 3.60 54.20 14.15 18.99 IBD009 66 N/A 57.85 14.88 19.54 Specimen . Age (years) . Head Weight (kg) . Head Circumference (cm) . Head Breadth (cm) . Head Length (cm) . IBD001 71 3.93 55.00 14.86 19.25 IBD002 53 3.60 54.20 14.15 18.99 IBD009 66 N/A 57.85 14.88 19.54 Open in new tab Table I. Specimen Age, Weight, and Anthropometric Measurements Specimen . Age (years) . Head Weight (kg) . Head Circumference (cm) . Head Breadth (cm) . Head Length (cm) . IBD001 71 3.93 55.00 14.86 19.25 IBD002 53 3.60 54.20 14.15 18.99 IBD009 66 N/A 57.85 14.88 19.54 Specimen . Age (years) . Head Weight (kg) . Head Circumference (cm) . Head Breadth (cm) . Head Length (cm) . IBD001 71 3.93 55.00 14.86 19.25 IBD002 53 3.60 54.20 14.15 18.99 IBD009 66 N/A 57.85 14.88 19.54 Open in new tab Specimens were mounted in the projectile launcher test chamber and positioned so that the anatomical region of interest was perpendicular to the barrel. Three anatomical regions were impacted with the representative impactor for each specimen: frontal bone, right temporo-parietal region, and left temporo-parietal region. Frontal bone impacts were centered approximately 50 mm above the bridge of the nose. Temporo-parietal impacts were centered in the vicinity of the squamosal suture joining the temporal and parietal bone. Impacts were conducted using a pre-determined sequence of five energy levels between 5 J and 25 J. A portable X-ray system (Dicom Solutions, Irvine, CA, USA) was used to take images after each test. Fractures were assessed through image analysis, review of acoustic emission and strain gauge data, palpation, local dissection, and india ink staining when necessary. Testing at an impact site was halted once a fracture was identified or all energy levels had been tested. After testing on a specimen was completed, post-test series CT scans were performed. A post-test autopsy was conducted to fully document the extent of injury, if any. During testing, the instrumented impactor was sampled at 100 kHz. Projectile exit velocity was measured using a photoelectric velocity measurement sensor (KME, Troy, MI, USA). Velocity and specimen-mounted sensors were sampled at 1 MHz using a Synergy Data Acquisition System (Hi-Techniques Inc, Madison, WI, USA). Two Phantom v9 cameras (Vision Research Inc, Wayne, NJ, USA) were used to capture high-speed video of each test. One camera was placed laterally, perpendicular to the flight of the projectile, while the other was suspended above the test chamber. High-speed video was collected at a rate of 2,000 frames-per-second. Sensor data were analyzed using custom MATLAB (MathWorks, Natick, MA, USA) scripts. Projectile acceleration was filtered using a fourth order low-pass Butterworth filter with a 5,000 Hz cutoff frequency and used to calculate impact force based on Newton’s second law (force = mass × acceleration). High-speed video was analyzed using TEMA Automotive (Image Systems Motion Analysis, Linkoping, Sweden). RESULTS Thirty-six impacts were conducted on three PMHS specimens (Table II). Target impact energies ranged from 5 J to 25 J with velocities ranging from 8.84 m/s to 20.12 m/s. After test #32, the data acquisition system on-board the projectile stopped communicating with the laboratory computer. The remaining four test shots were conducted with a tethered projectile attached to the Synergy data acquisition system. Fractures were detected in specimen IBD001and IBD002. Impact energy, velocity, and force were recorded for each fracture (Table III). No fractures were observed in specimen IBD009. Table II. Specimen Test Matrix and Target Impact Conditions Specimen . Location . Impact #1 . Impact #2 . Impact #3 . Impact #4 . Impact #5 . IBD001 Temporo-Parietal (L) 5 J 10 J 10 Ja — — 8.84 m/s 12.8 m/s 12.8 m/s Temporo-Parietal (R) 5 J 10 J 10 J 10 Ja — 8.84 m/s 12.8 m/s 12.8 m/s 12.8 m/s Frontal 5 J 10 J 15 J 20 J N/A 8.84 m/s 12.8 m/s 15.54 m/s 17.98 m/s IBD002 Temporo-Parietal (L) 20 Ja — — — — 17.98 m/s Temporo-Parietal (R) 5 J 10 J 20 J 25 Ja — 8.84 m/s 12.8 m/s 17.98 m/s 20.12 m/s Frontal 5 J 10 J 15 J 20 J 25 J 8.84 m/s 12.8 m/s 15.54 m/s 17.98 m/s 20.12 m/s IBD009 Temporo-Parietal (L) 10 J 20 J 25 J 5 J 15 J 12.8 m/s 17.98 m/s 20.12 m/s 8.84 m/s 15.54 m/s Temporo-Parietal (R) 10 J 15 J 25 J 20 J 5 J 12.8 m/s 15.54 m/s 20.12 m/s 17.98 m/s 8.84 m/s Frontal 10 J 15 Jb 25 Jb 5 Jb 20 Jb 12.8 m/s 15.54 m/s 20.12 m/s 8.84 m/s 17.98 m/s Specimen . Location . Impact #1 . Impact #2 . Impact #3 . Impact #4 . Impact #5 . IBD001 Temporo-Parietal (L) 5 J 10 J 10 Ja — — 8.84 m/s 12.8 m/s 12.8 m/s Temporo-Parietal (R) 5 J 10 J 10 J 10 Ja — 8.84 m/s 12.8 m/s 12.8 m/s 12.8 m/s Frontal 5 J 10 J 15 J 20 J N/A 8.84 m/s 12.8 m/s 15.54 m/s 17.98 m/s IBD002 Temporo-Parietal (L) 20 Ja — — — — 17.98 m/s Temporo-Parietal (R) 5 J 10 J 20 J 25 Ja — 8.84 m/s 12.8 m/s 17.98 m/s 20.12 m/s Frontal 5 J 10 J 15 J 20 J 25 J 8.84 m/s 12.8 m/s 15.54 m/s 17.98 m/s 20.12 m/s IBD009 Temporo-Parietal (L) 10 J 20 J 25 J 5 J 15 J 12.8 m/s 17.98 m/s 20.12 m/s 8.84 m/s 15.54 m/s Temporo-Parietal (R) 10 J 15 J 25 J 20 J 5 J 12.8 m/s 15.54 m/s 20.12 m/s 17.98 m/s 8.84 m/s Frontal 10 J 15 Jb 25 Jb 5 Jb 20 Jb 12.8 m/s 15.54 m/s 20.12 m/s 8.84 m/s 17.98 m/s (L) Left; (R) Right. aFracture detected. bTethered projectile. Open in new tab Table II. Specimen Test Matrix and Target Impact Conditions Specimen . Location . Impact #1 . Impact #2 . Impact #3 . Impact #4 . Impact #5 . IBD001 Temporo-Parietal (L) 5 J 10 J 10 Ja — — 8.84 m/s 12.8 m/s 12.8 m/s Temporo-Parietal (R) 5 J 10 J 10 J 10 Ja — 8.84 m/s 12.8 m/s 12.8 m/s 12.8 m/s Frontal 5 J 10 J 15 J 20 J N/A 8.84 m/s 12.8 m/s 15.54 m/s 17.98 m/s IBD002 Temporo-Parietal (L) 20 Ja — — — — 17.98 m/s Temporo-Parietal (R) 5 J 10 J 20 J 25 Ja — 8.84 m/s 12.8 m/s 17.98 m/s 20.12 m/s Frontal 5 J 10 J 15 J 20 J 25 J 8.84 m/s 12.8 m/s 15.54 m/s 17.98 m/s 20.12 m/s IBD009 Temporo-Parietal (L) 10 J 20 J 25 J 5 J 15 J 12.8 m/s 17.98 m/s 20.12 m/s 8.84 m/s 15.54 m/s Temporo-Parietal (R) 10 J 15 J 25 J 20 J 5 J 12.8 m/s 15.54 m/s 20.12 m/s 17.98 m/s 8.84 m/s Frontal 10 J 15 Jb 25 Jb 5 Jb 20 Jb 12.8 m/s 15.54 m/s 20.12 m/s 8.84 m/s 17.98 m/s Specimen . Location . Impact #1 . Impact #2 . Impact #3 . Impact #4 . Impact #5 . IBD001 Temporo-Parietal (L) 5 J 10 J 10 Ja — — 8.84 m/s 12.8 m/s 12.8 m/s Temporo-Parietal (R) 5 J 10 J 10 J 10 Ja — 8.84 m/s 12.8 m/s 12.8 m/s 12.8 m/s Frontal 5 J 10 J 15 J 20 J N/A 8.84 m/s 12.8 m/s 15.54 m/s 17.98 m/s IBD002 Temporo-Parietal (L) 20 Ja — — — — 17.98 m/s Temporo-Parietal (R) 5 J 10 J 20 J 25 Ja — 8.84 m/s 12.8 m/s 17.98 m/s 20.12 m/s Frontal 5 J 10 J 15 J 20 J 25 J 8.84 m/s 12.8 m/s 15.54 m/s 17.98 m/s 20.12 m/s IBD009 Temporo-Parietal (L) 10 J 20 J 25 J 5 J 15 J 12.8 m/s 17.98 m/s 20.12 m/s 8.84 m/s 15.54 m/s Temporo-Parietal (R) 10 J 15 J 25 J 20 J 5 J 12.8 m/s 15.54 m/s 20.12 m/s 17.98 m/s 8.84 m/s Frontal 10 J 15 Jb 25 Jb 5 Jb 20 Jb 12.8 m/s 15.54 m/s 20.12 m/s 8.84 m/s 17.98 m/s (L) Left; (R) Right. aFracture detected. bTethered projectile. Open in new tab Table III. Specimen Fracture and Impact Conditions Specimen . Location . Energy (J) . Velocity (m/s) . Peak Acceleration (G) . Peak Force (N) . Fracture . IBD001 Temporo-Parietal (L) 10.41 12.96 4,216 5,129 Curvilinear Temporo-Parietal (R) 10.53 13.03 4,432 5,392 Depressed, Comminuted IBD002 Temporo-Parietal (L) 20.74 18.29 8,694 10,576 Depressed, Comminuted Temporo-Parietal (R) 24.43 19.85 8,947 10,883 Curvilinear, Comminuted Specimen . Location . Energy (J) . Velocity (m/s) . Peak Acceleration (G) . Peak Force (N) . Fracture . IBD001 Temporo-Parietal (L) 10.41 12.96 4,216 5,129 Curvilinear Temporo-Parietal (R) 10.53 13.03 4,432 5,392 Depressed, Comminuted IBD002 Temporo-Parietal (L) 20.74 18.29 8,694 10,576 Depressed, Comminuted Temporo-Parietal (R) 24.43 19.85 8,947 10,883 Curvilinear, Comminuted (L) Left; (R) Right. Open in new tab Table III. Specimen Fracture and Impact Conditions Specimen . Location . Energy (J) . Velocity (m/s) . Peak Acceleration (G) . Peak Force (N) . Fracture . IBD001 Temporo-Parietal (L) 10.41 12.96 4,216 5,129 Curvilinear Temporo-Parietal (R) 10.53 13.03 4,432 5,392 Depressed, Comminuted IBD002 Temporo-Parietal (L) 20.74 18.29 8,694 10,576 Depressed, Comminuted Temporo-Parietal (R) 24.43 19.85 8,947 10,883 Curvilinear, Comminuted Specimen . Location . Energy (J) . Velocity (m/s) . Peak Acceleration (G) . Peak Force (N) . Fracture . IBD001 Temporo-Parietal (L) 10.41 12.96 4,216 5,129 Curvilinear Temporo-Parietal (R) 10.53 13.03 4,432 5,392 Depressed, Comminuted IBD002 Temporo-Parietal (L) 20.74 18.29 8,694 10,576 Depressed, Comminuted Temporo-Parietal (R) 24.43 19.85 8,947 10,883 Curvilinear, Comminuted (L) Left; (R) Right. Open in new tab Post-test dissection confirmed and revealed the extent of the fractures. Specimen IBD001 was observed to have fractures of the left and right temporo-parietal regions caused by 10 J impacts. The left temporo-parietal bone presented with a linear 61 mm curvilinear fracture superior to and centering on the point of impact beginning in the frontal bone and ending in the squamous portion of the temporal bone (Fig. 2A and B). The right temporo-parietal bone presented a depressed, comminuted fracture approximately 38.1 mm in diameter (Fig. 2C and D). The fracture had stellate features centered on point of impact and involved the parietal, sphenoid, and temporal bones. FIGURE 2. Open in new tabDownload slide Specimen IBD001 experienced fractures in both the left (A,B) and right (C,D) temporo-parietal regions. The curvilinear fracture (A) was visible in autopsy, but not readily visible on the CT reconstruction (B) while the depressed, comminuted fracture was easily visible in both autopsy (C) and CT reconstruction (D). FIGURE 2. Open in new tabDownload slide Specimen IBD001 experienced fractures in both the left (A,B) and right (C,D) temporo-parietal regions. The curvilinear fracture (A) was visible in autopsy, but not readily visible on the CT reconstruction (B) while the depressed, comminuted fracture was easily visible in both autopsy (C) and CT reconstruction (D). Specimen IBD002 was observed to have fractured the left parietal bone at 20 J, while the right parietal fractured at 25 J. The left temporo-parietal region presented with a depressed comminuted fracture approximately 67 mm in diameter (Fig. 3A and B). The fracture had stellate features centered on point of impact and involved the parietal, sphenoid, and temporal bones including the tympanic portion of the temporal bone. The extent of the temporal bone fracture was not further examined within the ear canal due to the amount of tissue present, the nature of the existing fracture, and the possibility of causing further unintended damage. The right temporo-parietal region presented a comminuted fracture approximately 76 mm in diameter with curvilinear elements centered on the point of impact (Fig. 3C and D). The fracture also involved the parietal, sphenoid, and temporal bones and included the tympanic portion of the temporal bone. FIGURE 3. Open in new tabDownload slide Specimen IBD002 experienced fractures in both the left (A,B) and right (C,D) temporo-parietal regions. Like Specimen IBD001, the depressed, comminuted fracture was visible in both autopsy (A) and CT reconstruction (B) while the curvilinear fracture (C) was best identified during autopsy rather than CT reconstruction (D). FIGURE 3. Open in new tabDownload slide Specimen IBD002 experienced fractures in both the left (A,B) and right (C,D) temporo-parietal regions. Like Specimen IBD001, the depressed, comminuted fracture was visible in both autopsy (A) and CT reconstruction (B) while the curvilinear fracture (C) was best identified during autopsy rather than CT reconstruction (D). DISCUSSION A total of 36 impact tests were conducted on three PMHS heads specimens using an impactor with a representative BFD signature. Each specimen was impacted on the frontal bone, and left/right temporo-parietal regions. Fractures were detected in two of the three specimens. Specimen IBD001 and IBD002 both presented with fractures of the left and right temporo-parietal regions. The observed fractures in the current study were characterized as either depressed comminuted fractures with stellate features or curvilinear fractures. In the case of specimen IBD001, the left temporo-parietal region was not depressed; however, it was curvilinear in nature. The fracture in IBD001 is similar to the comminuted fracture in the right parietal bone of specimen IBD002 that presented with a series of curvilinear elements. No temporo-parietal fractures were detected in specimen IBD009. It is interesting to note here the involvement of the sphenoid bone in the fractures seen during the current test series. The sphenoid bone is an unpaired bone with a complex geometry that spans from side to side across the base of the skull. Five of the cranial nerves pass through this bone, and it houses the pituitary gland in the sella turcica. While protected by the temporal muscle, it is unclear what damage (if any) might occur in the soft tissue structures the sphenoid bone protects. No frontal bone fractures were detected during the present study. The presence of fractures in the temporo-parietal region and not in the frontal bone under the same loading conditions might be due to the bone shape at each location. The frontal bone forms a curved dome, while the parietal bone is relatively flat. The shape of the frontal bone may allow for dissipation of the impact energy not capable in the flatter bones that cave inward. The frontal bone is a continuous bone structure whereas the impact location on the temporo-parietal region contains a number of sutures. Sutures are immovable fibrous joints that closely bind the bones of the skull together and may not provide the same strength as the frontal bone itself. Additionally during previous work investigating general frontal bone fracture tolerance, no fractures were recorded for impacts with less than 32 J of energy.13 It is likely that the energy range for this test series was insufficient to cause fracture. The fractures from the current study compared well with fractures reported in the literature. Raymond et al9 reported six depressed comminuted fractures, and a single fracture curvilinear in nature while examining the effect of blunt ballistic impacts to the temporo-parietal region in 14 specimens. Sarron et al4,5 also reported depressed commuted fractures in dry skull BFD experiments in addition to “cupule fractures” similar to the curvilinear fracture detected in the current study. When compared to the work of Rafaels et al7 and Bass et al6 similar depressed fractures were seen at the point of impact; however, linear fractures were different. Rafaels et al7 reported linear fractures distal to the point of impact, which are unlike the curvilinear fracture observed in the current study. There are several potential reasons for the difference in observed linear fractures–the most significant of which is the difference in impact location. Rafaels et al7 impacts centered more on the parietal bone and less on the temporo-parietal region causing the impact location to be further posterior than in the current study. Additionally, differences in helmet materials could lead to different BFD profiles, the effect of which has yet to be quantified. In the current study, temporo-parietal fractures were detected at energies ranging from 10.41 J to 24.43 J, with corresponding peak forces ranging from 5,129 N to 10,883 N (Mean: 7,995 ± 3,161 N). Biological differences between specimens (age, mass, skull thickness, soft tissue thickness) might account for the large energy and force ranges observed; however, the sample size in this study was too small to make that determination. When compared with the previous work, peak forces were in general slightly higher than those described by Raymond et al9 (Range: 3,376–9,529 N; Mean 5633 ± 2095 N) during blunt ballistic impact to the temporo-parietal region. Interestingly peak forces compared well with those described by Yoganandan et al14(Range 5,556–9,918 N; Mean: 7,717 N) during investigations of lateral skull impacts on a flat surface. Currently, further study is required to determine an appropriate frontal bone and temporo-parietal fracture tolerance due to BFD. While there exists a large volume of work that has been conducted on craniofacial trauma, especially in regards to automotive vehicle accidents,13–18 it is unclear how that translates to helmet BABT. The injury mechanics involved in BFD are drastically different than motor vehicle accidents (velocity, geometry, pulse durations, etc). Comparison of future BFD work to prior skull fracture tolerance literature would provide insight to the role that rate and geometry have on skull fracture patterns. There are several limitations to the current study design. The developed impactor represents a constant geometry at the stand-off distance. An actual helmet would continue to deform past the stand-off distance and the BFD profile would likewise continue to change. While it has been reported that the majority of the BFD’s available energy is dissipated before maximum deflection,11 it is unknown how the change in geometry might affect injury patterns. Additionally, the current configuration for mounting the specimen in the test chamber used a mechanical neck and provided a rigid coupling that resulted in unexpected high frequency content in the specimen-mounted sensors. Future work should investigate alternative boundary conditions that would eliminate this artifact. The implementation of multiple tests at each anatomical site made it difficult to assess the extent of soft tissue damage during impact. If the skin at the impact site was observed though palpation to be flattened, it was assumed that the degraded skin could potentially change impact dynamics; therefore, the skin at the impact site was replaced, using skin taken from the specimen’s neck. The replacement skin was assumed to be similar in composition and properties, however this was not verified. Additionally, impacts at similar or lower energy levels may have weakened the bone causing fracture at lower than expected energy levels. Finally, the low video capture speed in this study made it difficult to quantify the exact impact time, impact duration, and impactor deflection. This should be corrected in future testing to allow for the development of force deflection curves. CONCLUSIONS Initial research efforts have developed an instrumented impactor representative of helmet BFD capable of collecting engineering metrics at the BFD-head interface during a blunt impact event. Preliminary test results from the research have shown that a representative impactor can generate intended fracture patterns seen in the literature. Additional instrumented projectile impactors have been developed that vary geometric shapes across a range of impactor curvatures. The developed impactors can be used to generate medically based human skull fracture criteria for localized blunt impacts to the skull for use in assessing injuries from helmet BFD. Presentations Presented as a poster at the 2016 Military Health System Research Symposium (Abstract Number: MHSRS-16–1303). Funding This study was supported by an award from the Defense Health Agency (DHA). Acknowledgements The authors would like to thank SGT Kyle Rybarczyk and Dr. James McGhee for assistance with CT image reconstruction and identification of fracture patterns. References 1 United States Army : CO/PD-05-04, purchase description helmet, advanced combat. October 30th, 2007 . Available at http://www.gentexcorp.com/assets/base/TechnicalPublications/CO-PD-05-041.pdf; accessed April 24, 2017. 2 Department of Defense : Military Standard MIL-STD-662f, v50 ballistic test for armor. 1997 . Available at https://assist.dla.mil/online/start/; accessed December 27, 2016. 3 National Research Council : Review of Department of Defense test protocols for combat helmets . Washington DC , National Academies Press , 2014 . https://www.ncbi.nlm.nih.gov/books/NBK224912/#ref_000081; accessed December 27, 2016. Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC 4 Sarron JC , Caillou JP, Da Cunha J, et al. : Consequences of nonpenetrating projectile impact on a protected head: study of rear effects of protections . J Trauma Acute Care Surg 2000 ; 49 ( 5 ): 923 – 9 . https://www.ncbi.nlm.nih.gov/pubmed/11086786. accessed December 27, 2016. Google Scholar Crossref Search ADS WorldCat 5 Sarron JC , Dannawi M, Faure A, Caillou JP, Da Cunha J, Robert R: Dynamic effects of a 9 mm missile on cadaveric skull protected by aramid, polyethylene or aluminum plate: an experimental study . J Trauma Acute Care Surg 2004 ; 57 ( 2 ): 236 – 42 . https://www.ncbi.nlm.nih.gov/pubmed/15345967; accessed December 27, 2016. Google Scholar Crossref Search ADS WorldCat 6 Bass CR , Bogges B, Bush B, et al. : Helmet behind armor blunt trauma. In the Proceedings of the RTOAVT/HFM Specialists’ “Meeting on Equipment for Personal Protection (AVT-097)” and “Personal Protection: Bio-Mechanical Issues and Associated Physio-Pathological Risks (HFM-102)”, 2003 . Available at https://www.sto.nato.int/publications/STO%20Meeting%20Proceedings/Forms/Meeting%20Proceedings%20Document%20Set/docsethomepage.aspx?ID=34188&FolderCTID=0x0120D5200078F9E87043356C409A0D30823AFA16F602008CF184CAB7588E468F5E9FA364E05BA5&List=7e2cc123-6186-4c30-8082-1ba072228ca7&RootFolder=https://www.sto.nato.int/publications/STO%20Meeting%20Proceedings; accessed December 27, 2016. 7 Rafaels KA , Cutcliffe HC, Salzar RS, et al. : Injuries of the head from backface deformation of ballistic protective helmets under ballistic impact . J Forensic Sci 2015 ; 60 ( 1 ): 219 – 5 . https://www.ncbi.nlm.nih.gov/pubmed/25039407; accessed December 27, 2016. Google Scholar Crossref Search ADS PubMed WorldCat 8 Viano DC , Bir C, Walilko T, et al. : Ballistic impact to the forehead, zygoma, and mandible: comparison of human and frangible dummy face biomechanics . J Trauma Acute Care Surg 2004 ; 56 ( 6 ): 1305 – 11 . https://www.ncbi.nlm.nih.gov/pubmed/15211141; accessed December 27, 2016. Google Scholar Crossref Search ADS WorldCat 9 Raymond D , Van Ee C, Crawford G, et al. : Tolerance of the skull to blunt ballistic temporo-parietal impact . J Biomech 2009 ; 42 ( 15 ): 2479 – 85 . https://www.ncbi.nlm.nih.gov/pubmed/19674749; accessed December 27, 2016. Google Scholar Crossref Search ADS PubMed WorldCat 10 Melvin J , Evans F: A strain energy approach to the mechanics of skull fracture. SAE Technical Paper 1971 ; 710871. http://papers.sae.org/710871/; accessed December 27, 2016. 11 Hisley DM , Gurganus JC, Drysdale AW: Experimental Methodology Using Digital Image Correlation to Assess Ballistic Helmet Blunt Trauma . J Appl Mech 2011 ; 78 ( 5 ): 051022 . http://appliedmechanics.asmedigitalcollection.asme.org/article.aspx?articleid=1476946; accessed December 27, 2016. Google Scholar Crossref Search ADS WorldCat 12 United States Army : Army Policy for use of human cadavers for research, development, test and evaluation (RDT&E), education or training. April 20th, 2012 . Available at http://mrmc.amedd.army.mil/assets/docs/orp/ORP_Cadaver_Submission_Form.docx; accessed April 24, 2017. 13 Cormier J , Manoogian S, Bisplinghoff J, et al. : The tolerance of the frontal bone to blunt impact . J Biomech Eng 2011 ; 133 ( 2 ): 021004 . https://www.ncbi.nlm.nih.gov/pubmed/21280876. Google Scholar Crossref Search ADS PubMed WorldCat 14 Yoganandan N , Zhang J, Pintar FA, et al. : Biomechanics of Lateral Skull Fracture. IRCOBI Conference. Lisbon Portugal, September 2003 . Available at http://www.ircobi.org/; accessed November 30, 2017. 15 Yoganandan N , Pintar FA, Sances A Jr.: Biomechanics of skull fracture . J Neurotrauma 1995 ; 12 ( 4 ): 659 – 68 . https://www.ncbi.nlm.nih.gov/pubmed/8683617; accessed December 27, 2016. Google Scholar Crossref Search ADS PubMed WorldCat 16 Nahum A , Gatts J, Gadd C, et al. : Impact tolerance of the skull and face. SAE Technical Paper 680785, 1968 . https://doi.org/10.4271/680785. Available at http://papers.sae.org/680785/; accessed November 30, 2017. 17 Yoganandan N , Pintar FA: Biomechanics of temporo-parietal skull fracture . Clinical Biomechanics 2004 ; 19 ( 3 ): 225 – 39 . https://www.ncbi.nlm.nih.gov/pubmed/15003337; accessed November 30, 2017. Google Scholar Crossref Search ADS PubMed WorldCat 18 Allsop D , Kennett K: Skull and facial bone trauma. In: Accidental Injury: Biomechanics and Prevention . Edited by Nahum A, Melvin J New York , Springer-Verlag , 2002 . pp. 254 – 76 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Author notes The opinions, interpretations, conclusions, and recommendations are those of the presenter and are not necessarily endorsed by the U.S. Army and/or the U.S. Department of Defense. Citation of trade names in this presentation does not constitute an official Department of the Army endorsement or approval of the use of such commercial items. Research involved “sensitive use of human cadavers” as determined by the MRMC Office of Research Protections. Per the Army Cadaver Policy, dated 20APR2012, the term “sensitive uses” of cadavers means RDT&E, education or training activities that involve exposing cadavers to impacts, blasts, ballistics testing, crash testing, and other destructive forces. In conducting research using human cadavers, the investigator(s) adhered to the Army Policy for Use of Human Cadavers for Research, Development, Test and Evaluation, Education or Training and other statutes relating to the use and transportation of anatomical gifts. Published by Oxford University Press on behalf of the Association of Military Surgeons of the United States 2018. This work is written by (a) US Government employee(s) and is in the public domain in the US. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) Published by Oxford University Press on behalf of the Association of Military Surgeons of the United States 2018. This work is written by (a) US Government employee(s) and is in the public domain in the US.
TI - Preliminary Investigation of Skull Fracture Patterns Using an Impactor Representative of Helmet Back-Face Deformation 
JF - Military Medicine
DO - 10.1093/milmed/usx210
DA - 2018-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/preliminary-investigation-of-skull-fracture-patterns-using-an-impactor-YdifQVjWLl
SP - 287
EP - 293
VL - 183
IS - suppl_1
DP - DeepDyve
ER -