TY - JOUR
AU - Martin, John S.
AB - ABSTRACT A shock tube and anthropomorphic headforms were used to investigate eye protection form and fit using eyewear on the Authorized Protective Eyewear List in primary ocular blast trauma experiments. Time pressure recordings were obtained from highly linear pressure sensors mounted at the cornea of instrumented headforms of different sizes. A custom shock tube produced highly reliable shock waves and pressure recordings were collected as a function of shock wave orientation and protective eyewear. Eyewear protection coefficients were calculated as a function of a new metric of eyewear fit. In general, better protection was correlated with smaller gaps between the eyewear and face. For oblique angles, most spectacles actually potentiated the blast wave by creating higher peak pressures at the cornea. Installing foam around the perimeter of the spectacle lens to close the gap between the lens and face resulted in significantly lower pressure at the cornea. In conclusion, current eye protection, which was designed to reduce secondary and tertiary blast injuries, provides insufficient protection against primary blast injury. INTRODUCTION Deployed military personnel are at particular risk of a spectrum of ocular injuries caused by blast, including penetrating eye injury, retinal detachment, eye rupture, intraocular hemorrhage, corneal laceration, etc.1 Blast eye injuries are thought to be caused typically by secondary blast effects where foreign objects come in contact with or penetrate the globe, and most investigations and models of eye injuries have focused on such secondary mechanisms of ocular blast injury. Although some clinical cases of primary blast injury (PBI) have been documented,2,–4 only recently has PBI gained significant visibility within the research community,5,6 and strong evidence revealing clinically significant primary blast ocular trauma was published in 2014.7 To help protect against ocular blast injuries, the Authorized Protective Eyewear List (APEL), which incorporates acquisition guidelines for ballistic protection, was adopted in 2006. With growing laboratory evidence that the blast wave may cause ocular damage as well as higher visual system injuries, we began investigating the effectiveness of APEL eyewear in PBI protection. At the 2014 Military Health System Research Symposium (unpublished data), we reported some limitations for spectacles on the APEL in reducing the blast intensity reaching the cornea. While the spectacles reduced peak corneal pressure for head-on blasts, they did not provide adequate protection from PBI at other angles and even potentiated pressure at the cornea in the vast majority of cases. The APEL goggles, on the other hand, provided an average of about 31% better protection and showed significantly better protection against head-on blasts than the spectacles (as much as 115% in some cases). We hypothesized that this result was due to the foam around the goggles creating a better seal against the face. With the exception of the Wiley X SG1 (which had foam around the inside of the lenses), APEL spectacles are characterized by large gaps between the frame and/or lens and the face, and we posit that the amount of blast pressure allowed to reach the eye is a function of this gap size. Herein, we describe experiments designed to evaluate blast wave protection as a matter of form and fit of the protective eyewear to test our hypotheses. MATERIALS AND METHODS A shock tube was used to produce blast waves for this study. The shock tube barrel and driver have an inner diameter of 6 inches and a thickness of about 1 inch. The length of the barrel is 122 inches, and it feeds into a catenoidal horn that expands from the 6-inch diameter to a 4-foot-square outlet. A Mylar diaphragm at the tube chamber was used to generate the blast wave. A pressure–time curve of the Friedlander wave produced by the shock tube is presented in Figure 1. The free-field overpressure measured at the test section was 1.6 psi with a standard deviation of 1.64% between tests. The positive phase of the shock tube Friedlander wave was approximately 1.5 msec. FIGURE 1 View largeDownload slide Friedlander waveform produced by shock tube. FIGURE 1 View largeDownload slide Friedlander waveform produced by shock tube. Two headforms of different sizes (15th and 95th percentile based on male head circumference8) made from a polyurethane cast were fitted with blast pressure sensors from PCB Piezotronics (Depew, NY) (model no. PCB102B18). The sensors were installed at the center of each eye location, and were flush with a nylon washer representing the corneal surface. The headform was fixed on an adjustable mount placed 13.5 inches from the exit of the shock tube horn. A PCB Piezotronics free-field blast pencil probe (model no. PCB137A23) was affixed 17 inches to the side of the headform at eye level to measure the free-field blast pressure and these pressure readings were used to correct for minor shock-to-shock variability. Data were collected with the headform rotated in 30° intervals. Five tests were performed for each protection case at each headform orientation, resulting in five 1-second measurements for the pencil probe, left, and right eye sensors recorded at a 204.8-kHz sample rate. A National Instruments (Austin, Texas) PXI-4498 data acquisition board with anti-aliasing filters was used to record the measurements and was triggered upon the bursting of the shock tube membrane. Testing was done with four types of eyewear (Fig. 2) affixed to the headform with a combat helmet. No additional mounting schemes were used beyond what was provided by the eyewear and the helmet. Separate experiments were conducted without the eyewear in place and also with foam insulation inserted on each spectacle lens to eliminate the gaps between the face and spectacles (Fig. 3). The added protection provided by the foam inserts was then compared to the protection provided by the Flakjak goggles. FIGURE 2 View largeDownload slide Four sets of eyewear tested. (1) Revision Sawfly, (2) Wiley X Talon, (3) Uvex Genesis, and (4) Arena Flakjak. FIGURE 2 View largeDownload slide Four sets of eyewear tested. (1) Revision Sawfly, (2) Wiley X Talon, (3) Uvex Genesis, and (4) Arena Flakjak. FIGURE 3 View largeDownload slide Large headform with foam inserts on the inside of the Uvex Genesis spectacles. FIGURE 3 View largeDownload slide Large headform with foam inserts on the inside of the Uvex Genesis spectacles. RESULTS AND DISCUSSION To establish a relationship between eyewear fit and PBI protection, three people concurrently took measurements of gap distances between spectacle lens and face at the bottom and side (Fig. 4) as well as the distance from the cornea to the center of the eyewear lens. Table I shows the measured gaps for the small and large headforms with the spectacle lenses. The distance from the cornea to the spectacle lens increased by 3 mm for each set of eyewear on the large headform. Measurements for the frame-to-side metric increased an average of about 2.3 mm for the large headform. The under eye metric showed the least difference between the small and large headform, with a 0.5-mm gap increase for the large headform with the Talon and Sawfly spectacles and a 0.5-mm gap increase for the small headform with the Genesis spectacles. As a means of establishing gap distances as a critical factor in PBI protection, we compared the average of the five peak pressure measurements at each orientation for the small and large headforms taken with the Sawfly and Talon eyewear (Fig. 5). These are polar plots of the average of the five calculated protection coefficients, which are simply a ratio of the peak pressures measured with and without protection at their respective orientations, i.e., a ratio less than one denotes protection, whereas a number greater than one denotes intensification. To account for peak pressure variability at the eye between each test (σ = 0.95%), the peak pressure measured at the eye for the protected and unprotected case was normalized by the peak pressure measured by the pencil probe for each respective run. Note the degree markings for each eye are from the perspective of the blast source. In other words, the 30° azimuth measurement is with the head turned counterclockwise with respect to the shock tube exit. The reduced gap distances noted for the smaller headform led to better protection coefficients for the Talon spectacles (right plot in Fig. 5). Results were mixed for the Sawfly spectacles (left plot in Fig. 5), but the protection coefficients for the small headform were better in most cases with an average difference of about 7%. This suggests that the protection coefficients were worse for the larger headform because of the larger gaps exhibited between the spectacle lens and face, though other anatomical variations between the headforms such as the difference in brow ridge size would have a nonzero but less significant effect. FIGURE 4 View largeDownload slide Under eye (L) and frame-to-side (R) gap sizes for Uvex Genesis spectacles being measured on the large headform. FIGURE 4 View largeDownload slide Under eye (L) and frame-to-side (R) gap sizes for Uvex Genesis spectacles being measured on the large headform. TABLE I Measured Gaps Between Headform and Spectacle Lens     Genesis  Talon  Sawfly  Small Headform  High Frame to Side (mm)  8  10  11.5  Under Eye (mm)  11  7.5  10.5  Cornea to Lens (mm)  24  23  21  Large Headform  Frame to Side (mm)  12  10.5  14  Under Eye (mm)  10.5  8  11  Cornea to Lens (mm)  27  26  24      Genesis  Talon  Sawfly  Small Headform  High Frame to Side (mm)  8  10  11.5  Under Eye (mm)  11  7.5  10.5  Cornea to Lens (mm)  24  23  21  Large Headform  Frame to Side (mm)  12  10.5  14  Under Eye (mm)  10.5  8  11  Cornea to Lens (mm)  27  26  24  View Large TABLE I Measured Gaps Between Headform and Spectacle Lens     Genesis  Talon  Sawfly  Small Headform  High Frame to Side (mm)  8  10  11.5  Under Eye (mm)  11  7.5  10.5  Cornea to Lens (mm)  24  23  21  Large Headform  Frame to Side (mm)  12  10.5  14  Under Eye (mm)  10.5  8  11  Cornea to Lens (mm)  27  26  24      Genesis  Talon  Sawfly  Small Headform  High Frame to Side (mm)  8  10  11.5  Under Eye (mm)  11  7.5  10.5  Cornea to Lens (mm)  24  23  21  Large Headform  Frame to Side (mm)  12  10.5  14  Under Eye (mm)  10.5  8  11  Cornea to Lens (mm)  27  26  24  View Large FIGURE 5 View largeDownload slide Right eye protection coefficients for Sawfly (L) and Talon (R) spectacles for small and large headforms. In this figure and in the other polar plots, the bold circle is at unity. Data points residing inside the unity circle represent net protection and data points residing outside the circle represent an increase in blast energy reaching the cornea when the eyewear is worn. FIGURE 5 View largeDownload slide Right eye protection coefficients for Sawfly (L) and Talon (R) spectacles for small and large headforms. In this figure and in the other polar plots, the bold circle is at unity. Data points residing inside the unity circle represent net protection and data points residing outside the circle represent an increase in blast energy reaching the cornea when the eyewear is worn. Figure 6 shows the Sawfly and Talon protection coefficients for the large headform with and without the foam inserts. The foam provided noticeable improvement in protection, especially for shocks originating from the front and right side. Figure 7 illustrates the similarity in performance between the spectacles with foam inserts and the Flakjak goggles on the large headform, which have the aforementioned foam and rubber seal around the frame. FIGURE 6 View largeDownload slide Right eye protection coefficients for Sawfly (L) and Talon (R) spectacles on the large headform. FIGURE 6 View largeDownload slide Right eye protection coefficients for Sawfly (L) and Talon (R) spectacles on the large headform. FIGURE 7 View largeDownload slide Protection coefficients on the large headform for Sawfly spectacles with foam inserts and Flakjak goggles. FIGURE 7 View largeDownload slide Protection coefficients on the large headform for Sawfly spectacles with foam inserts and Flakjak goggles. Figure 8 shows the average protection coefficients for each of the spectacles for four conditions (no foam, foam, side foam out, bottom foam out). Dramatic improvement was provided by the foam inserts (as high as 54%). There was a small difference in performance between the side and bottom foam inserts, with the bottom foam providing slightly better protection in all cases. This could be due to the bottom foam occupying a greater volume of space than the side foam. FIGURE 8 View largeDownload slide Large headform average protection coefficients for 4 conditions. FIGURE 8 View largeDownload slide Large headform average protection coefficients for 4 conditions. CONCLUSIONS Gap distances between the spectacles and face were greater when using the larger headform, and this provided a test case for assessing gap distance versus PBI protection. As hypothesized, the larger gap size led to increased blast loadings reaching the cornea. When foam inserts were placed on the edges of the spectacles to reduce the gap distances, dramatic improvement in protection was observed. The improvement was comparable to protection seen with the Flakjak goggles. By removing part of the foam inserts, we attempted to evaluate the protection attributable to a particular gap. The performance of the spectacles was slightly worse when the bottom foam was removed compared to the side foam. Although a clear trend between gap size and protection could not be established with our limited metrics, it is apparent that filling the gap with foam dramatically improved the protection of the spectacles. It is clear from these studies that APEL spectacles are poor at reducing reflected blast pressure at the eye, and reducing or eliminating gaps between the spectacle and face would likely improve performance in this area. That said, there are other protection metrics that could be investigated, such as the positive phase duration of the blast wave and the integrated pressure of this duration. The role of these metrics in PBI is not currently well established; nevertheless, we are investigating the differences of these metrics between each protection case and will be presenting our findings as a supplement to the data exhibited here. ACKNOWLEDGMENTS This research was supported in part by an appointment to the Postgraduate Research Participation Program at the U.S. Army Aeromedical Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the U.S. Army Medical Research and Materiel Command. This effort was supported by core funds from the Medical Research and Materiel Command's Military Operational Medicine Research Program. REFERENCES 1. Scott R The injured eye. Phil Trans R Soc B  2010; 366: 251– 60. Google Scholar CrossRef Search ADS   2. Katz E, Ofek B, Adler J, Abramowitz HB, Krausz MM Primary blast injury after a bomb explosion in a civilian bus. Ann Surg  1989; 209( 4): 484– 8a. Google Scholar CrossRef Search ADS PubMed  3. Hamit HF Primary blast injuries. Ind Med Surg  1973; 42( 3): 14– 21. 4. Chalioulias K, Sim KT, Scott R Retinal sequelae of primary ocular blast injuries. J R Army Med Corps  2007; 153( 2): 124– 5. Google Scholar CrossRef Search ADS PubMed  5. Bhardwaj R, Ziegler K, Seo JH, Ramesh KT, Nguyen TD A computational model of blast loading on the human eye. Biomech Model Mechanobiol  2014; 13: 123– 40. Google Scholar CrossRef Search ADS PubMed  6. Alphonse VD, Kemper AR, Strom BTIII, Beeman SM, Duma SM Mechanisms of eye injuries from fireworks. JAMA  2012; 308: 33– 4. Google Scholar PubMed  7. Sherwood D, Sponsel WE, Lund BJ, et al.   Anatomical manifestations of primary blast ocular trauma observed in a postmortem porcine model. Invest Ophthalmol Vis Sci  2014; 55( 2): 1124– 32. Google Scholar CrossRef Search ADS PubMed  8. Gordon CC, Blackwell CL, Bradtmiller B, et al.   2012 Anthropometric Survey of US Army Personnel: Methods and Summary Statistics. Final Report , U.S. Army Natick Research, Development and Engineering Center, Natick, MA, December 2014. Available at www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA611869; accessed January 27, 2016. Footnotes 1 This work was presented at the Military Health System Research Symposium, Fort Lauderdale, FL, August 17–20, 2015. Reprint & Copyright © Association of Military Surgeons of the U.S.
TI - Blast Wave Dynamics at the Cornea as a Function of Eye Protection Form and Fit
JO - Military Medicine
DO - 10.7205/MILMED-D-16-00042
DA - 2017-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/blast-wave-dynamics-at-the-cornea-as-a-function-of-eye-protection-form-v6A0KZ2eP6
SP - 226
EP - 229
VL - 182
IS - suppl_1
DP - DeepDyve
ER -