An experiment to investigate the effect of preloading on bone impacts by projectiles was undertaken. During this experiment, a total of 59 bones were impacted. Red deer tibias were used, with impacts to the shaft under three different loading conditions. The purpose of the trial was to provide an indication on whether preloading a bone has an effect on chance of perforation, or type and level of fracture sustained. This is important because the United Kingdom (UK) Ministry of Defence (MOD) requires accurate injury models to aid with decision-making and risk analyses. . . . Keywords Injury Bone Loading Ballistic Introduction with a full burden of equipment), an experiment for assessing the effect of prestressed bones on ballistic per- The United Kingdom (UK) Ministry of Defence (MOD) re- formance is required. quires accurate injury models to aid with decision-making and The aim of this experiment was to identify whether there risk analyses. These decisions include what protective equip- is evidence that a preloaded tibia has different damage ment to use and to what specification. Generally, to make a thresholds to an unloaded tibia when impacted by a projec- piece of protective equipment more effective, you add more tile against the parameters: likelihood of perforation, level material (weight). Adding weight could make the soldier less of damage and type of fracture. The assessment was effective in different situations or have an effect on a soldier’s achieved through the analysis of high-speed video (HSV) long-term wellbeing. Understanding what variables are in- of the projectile to collect projectile velocity pre-impact and volved and what effect each has allowed the UK MOD to a record of the projectile impacting the bone; a visual as- make informed decisions. sessment of the bone post-impact was also conducted. For One of these injury models requires an estimation of the this experiment, red deer tibias were used because of their effect of projectile impact on a bone. A literature search likeness to human tibias. conducted by Dstl determined the state of the art in this field. All reviewed experiments that investigated the bal- listic performance of bones were conducted unloaded and Method the bones were held in place by a clamp or a block of gelatin [1–5]. This is not representative of real-world con- Target Details ditions as performing activities will place stresses within the bone. Because of the fact that during some activities a Red deer hind legs were purchased from the food chain leg bone can be loaded up to five times body weight, such sealed within vacuum packs and stored frozen at − 20 °C as when jumping or climbing stairs  (this will increase for a minimum of 2 weeks and a maximum of 6 weeks * J. J. Keirl Red Deer tibia was identified as a result of a review on readily available firstname.lastname@example.org animal bones of appropriate size and shape by surgical subject matter experts. The Red deer legs were purchased from Holme Farmed Venison (Produce) Ltd. Holme Farmed Venison, 9 First Avenue, Aviation Road, Sherburn in Dstl, Platform Systems, Porton Down, Salisbury, Wilts SP4 0JQ, UK Elmet, Leeds, LS25 6PD 4 Page 2 of 6 Hum Factors Mech Eng Def Saf (2018) 2:4 Fig. 1 Loading frame in use Loading Hydraulic Red Deer Loading Arm Tibia Piston Hydraulic Pump behind Polycarbonate sheet prior to the commencement of the experiment. This was highest intended experimental load (i.e. 500 pounds per necessary because the quantity of bones required, and square inch (psi) rather than 400 psi) to ensure that there time needed to plan the experiment meant that the bones was no risk of the bone fracturing as a result of the load- could not be supplied fresh. The age of the deer at the ing strain. The bone compacted under this initial loading. time of slaughter was between 12 and 24 months. The This meant that when a load was applied to a bone, the average time delay between slaughter and freezer was user had to ensure the load stayed constant and the bone 1weekandweretransportedtoDstlinacold boxand was not still yielding under the stress as this would, over placed straight in a freezer. The bones were defrosted by time, reduce the load on the bone for firing. immersion in room temperature Hartmann’s solution for The average muzzle to target distance was 1.06 m, with a minimum of 12 h and a maximum of 36 h. Hartmann’s a range of ± 0.01 m. The impact point was the middle of solution is used because it is electrolytically balanced and the diaphysis and kept the same with all tests. Efforts is designed for rehydration. were made to keep variables the same between test The hind legs were supplied whole following the groups, e.g. all groups contained a similar variety of bone butchering process; prior to immersion, the tibia had to masses and diameters at the impact point. The bones ei- be isolated from the remainder of the leg and cleaned to ther broke or remained intact when impacted by the 6-mm remove any excess tissue. The tibias were removed from diameter steel ball bearing projectile fired from the gas the ankle and knee joint, and then, any tissue was stripped gun; on one occasion, the bone broke under the load be- away on the impact area. Efforts were made to remove yond the recording timespan of the HSV (0.5 s). −1 tissue at the top and bottom of the bone to improve how The maximum velocity was 323 m s and the minimum −1 the bone would fit in the frame, but after the first few velocity was 173 m s . firings, this was deemed unnecessary. Once the bone To achieve the originally intended loads of 1 × body weight was cleaned, it was placed in Hartmann’s solution ready and 5 × body weight, the pressure on the pump would have to for shooting the next day. Each target was inspected for be 69 and 342 psi; this was not marked on the pressure gauge. surface damage prior to firing and only impacted once. Therefore, 100 psi was chosen because 100 psi is closest to The bones were preloaded with the use of a loading frame specifically designed for this experiment. The loading arm on Table 1 Preload exerted on the bone the frame was used to transfer the moment from the hydraulic pump to the bone. Category Actual load (including weight of loading arm)/kg The frame in use is shown in Fig. 1.Beforeanytests No load 7 were conducted, a bone was loaded to + 25% of the 100 psi 124 To make 10 l of Hartmann’s solution requires 10 l deionised water, 1.7 g 400 psi 476 CaCl2.2H20, 3 g KC1, 60 g NaCl and 31.7 ml C3H5Na03. Hum Factors Mech Eng Def Saf (2018) 2:4 Page 3 of 6 4 Fig. 2 Experiment setup Outside Chamber Inside Chamber Loading Frame Gas Gun Velocity Projectile/Impact measurement Camera Camera 1 × body weight and 400 psi was chosen because although HSV is directly affected by light intensity; image clarity 300 psi was slightly closer to 5 × body weight, a larger differ- was therefore constrained by the lighting available within ence in test cases (while still being relevant) was seen as ben- the fragment firing chamber. A 1-m steel rule was used eficial for a comparative study (the 400 psi load case was to take a calibration image at the start of each day, and therefore close to 6 × body weight). The accuracy of the user any time the setup was changed. The accuracy of the was deemed to be ± 25 psi for each firing. From here forward, camera timing unit is either ± 1.25 ns or ± 0.005% of the different test groups will be known as no load, 100 psi and frame rate, whichever is greater. To quantify motion blur, 400 psi (Table 1). the projectile diameter of the ball bearing was measured to within 0.1 mm of 6 mm over several measurements on the video. With these values and others taken into ac- Instrumentation Setup count, the velocity error measurement was calculated as ± 1%. The error bars were calculated by accumulating the The experiment setup is shown in Fig. 2. ± error at each set in the velocity calculation then compar- The experiment used two high-speed video cameras, ing the maximum and minimum final values. one to measure projectile velocity and one to record foot- The camera recording the projectile/bone impact had a age of the impact. Both cameras were triggered with an frame rate of 42,000 FPS, a shutter speed of 1/200,000 s and audio trigger box located near the muzzle of the gas gun. a resolution of 896 × 544 pixels. Due to limited space, this The video measuring the projectile velocity was posi- camera had to be outside the fragment firing chamber (see tioned perpendicular to the shot line, measured to within Fig. 2); however, this had minimal effect on the quality of 0.1° of the gun barrel; this was measured by taking two results. points along the long-axis of the gun and along the width of the camera to measure the angle with reference to the wall of the firing chamber. The camera settings were Firing Schedule 100,000 frames per second (FPS), a shutter speed of 1/800,000 s and a resolution of 896 × 208 pixels. These In total, 59 shots were fired on tibia targets. Of these, seven settings were chosen to maximise the number of frames in shots were deemed not fair and one shot was discounted fol- which the projectile flight would be in view while reduc- lowing trigger failure of the camera used to record velocity. ing image blur as much as possible. Image quality using Over the course of the trial, the temperature range was 23 to 28 °C, the ambient pressure range was 1032 to 1043 mbar and the humidity range was 42 to 60%. These readings were Table 2 Comparison of means taken inside the fragment firing chamber where it was fre- Comparison of means quently warmer than the surrounding room. Variable No load vs No load vs 100 psi vs Lights used were a mixture of standard LED lamps (to avoid 60 Hz flashing 100 psi 400 psi 400 psi from AC lamps) and LED blast lamps that are high intensity but short duration due to overheating risk. Bone diameter p =0.57 p =0.88 p =0.49 Temperature and pressure measured on Fisher Scientific Thermometer, Bone mass p =0.81 p =0.78 p =0.54 Calibration due June 2018. S/N: 160556490. Projectile impact velocity p =0.99 p =0.79 p =0.75 Measured on Traceable Thermo-Hydro. Calibration due February 2018. S/N: 160259896. 4 Page 4 of 6 Hum Factors Mech Eng Def Saf (2018) 2:4 Fig. 3 Target 29, a perforation (entry hole and exit hole) can be seen Fig. 5 Target 56, showing rear face fractures Statistical Analysis of Results No load, 100 psi and 400 psi. The mean mass values for the loading cases are 0.42, 0.40 and 0.4 kg for no load, 100 psi During the experiment, steps were taken to ensure that non- and 400 psi, respectively. The mean bone diameter values for test variables were not influencing the results; therefore, two- the loading cases are 23.6, 22.5 and 23.1 mm for no load, tailed t tests were performed (Table 2)tocheck forstatistically 100 psi and 400 psi, respectively. Unfair shots were excluded significant differences in the variables (bone diameter, bone from the analysis. mass and projectile impact velocity) between the three groups: The results show that there are no significant differences between groups (p < 0.05). This means that none of these var- iables produced confounding effects, which confirms that sta- tistical comparisons between groups can be made for the out- comes: perforation, broken bone and rear damage. Perforation occurred when a projectile had the energy to enter the front surface of the bone, passed through and exit through the rear face of the bone. Impacts that struck the bone but passed along its side (glancing blows) were not counted as perforations. An example of a perforation is shown in Fig. 3. Broken bone occurred when the bone was no longer intact from one end to another. If fragments or large chunks of the bone were removed but the two ends were still connected, the bone was not classified as broken. (Shown in Fig. 4.) Rear damage occurred when there were cracks, fractures, delamination, fragments or swarf on the outer rear face of the bone (not the inside face of the rear compact bone) as well as broken bones. Examples of rear damage are shown in Figs. 5 and 6. Two-sample proportions tests were performed to check for statistically significant differences in the proportions of perfo- rations, broken bones and rear damage between the three groups: No load, 100 psi and 400 psi. Results show that there are no significant differences (where p < 0.05) in the proportions of perforations, broken bones or rear damage between groups (no load, 100 psi, 400 psi) as shown in Tables 3 and 4. Compact bone also known as cortical bone forms the outer shell of most bones. The impact location for the experiment would be comprised of compact Fig. 4 Target 45, showing a broken bone bone with a marrow centre. Hum Factors Mech Eng Def Saf (2018) 2:4 Page 5 of 6 4 400 psi) compared to the baseline (no load), and no significant effects are found to suggest that there is an interactive effect between load and velocity. Results from model 2 show that there is a statistically sig- nificant positive effect of projectile impact velocity on the probability of broken bones (p = 0.04). There is some evi- dence to suggest that the probability of broken bones is lower for load 400 psi compared to no load (p =0.08) for similar impact velocities. There is also some evidence to suggest that the interactive effect between load 400 and velocity is smaller than the interactive effect between no load and velocity (p = 0.07). There was no significant difference between no load and 100 psi. Results from model 3 show some evidence to suggest that there is a statistically significant positive effect between ve- locity and rear damage (p = 0.08). No significant differences are found for either of the loads (100 psi, 400 psi) compared to the baseline (no load), and no significant effects are found for the interactive effects between load and velocity. Owing to the small sample sizes in this study, it is believed that a p < 0.08 is evidence of correlation. Discussion Fig. 6 Target 10, showing rear face fractures, spalling and delamination Despite the small sample size, some statistically significant conclusions can be drawn. Both of the variables perforation One possible explanation for the lack of significant results and broken bone show a statistically significant positive cor- is the small sample sizes, n =20, n = 14 and n = 17 for the relation with projectile impact velocity irrespective of loading. groups no load, 100 psi and 400 psi, respectively, which re- There is also evidence that rear damage has a correlation with sults in low levels of confidence and low statistical power. To velocity irrespective of loading. These conclusions are not check for the possibility of a combined effect between projec- surprising and were expected. tile impact velocity and the three load types, bias-reduced If the bone is subjected to the higher loading case (400 psi, generalised linear models were fitted using the statistical pack- 8 476 kg, ~ 6 × body weight), and the impact velocity is at the age, R. Three models—for perforation, broken bone and rear −1 higher range from this experiment (270 m s or more), there damage—were created. Specifically, the models considered is less chance of the bone breaking compared to the no load the following: condition. This was unexpected but not scientifically un- founded. Ceramics can be subjected to compressive loading & Model 1: Effects of load, velocity and load × velocity on to improve their ballistic performance  and bones are often perforation (baseline = no load) compared to ceramic materials. However, there are fundamen- & Model 2: Effects of load, velocity and load × velocity on tal differences such as visco-elasticity, which would cause a broken bones (baseline = no load) & Model 3: Effects of load, velocity and load × velocity on rear damage (baseline = no load) Table 3 Proportions of targets subjected to certain loading conditions having aspecificoutcome Results from model 1 show that there is a statistically sig- Proportions nificant positive effect of projectile impact velocity on the probability of perforation (p = 0.05). No significant effects Outcome No load 100 psi 400 psi are found for either of the two loaded conditions (100 psi, Perforation 5/20 (≈ 25%) 3/14 (≈ 21%) 3/17 (≈ 18%) 8 Broken bone 10/20 (≈ 48%) 6/14 (≈ 43%) 4/17 (≈ 24%) R version 2.15.2: R Core Team (2013). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Rear damage 16/20 (≈ 80%) 9/14 (≈ 64%) 9/17 (≈ 53%) Austria. ISBN 3-900051-07-0, URL: http://www.R-project.org/. 4 Page 6 of 6 Hum Factors Mech Eng Def Saf (2018) 2:4 Table 4 Comparison of proportions Open Access Content includes material subject to © Crown copyright (2018), Dstl. This material is licensed under the terms of the Open Comparison of proportions Government Licence except where otherwise stated. To view this licence, visit http://www.nationalarchives.gov.uk/doc/open-government-licence/ Outcome No load vs No load vs 100 psi vs version/3 or write to the Information Policy Team, The National 100 psi 400 psi 400 psi Archives, Kew, London TW9 4DU, or email: email@example.com. gov.uk. Perforation p =1.00 p =0.89 p =1.00 Broken bone p =0.95 p =0.19 p =0.45 Rear damage p =0.53 p =0.16 p =0.79 References 1. Kneubuehl BP (Ed), Coupland RM, Rothschild MA, Thali MJ change in stored strain energy over time and a difference in the (2011) Wound ballistics—basics and applications. Translation of failure load relating to the rate of loading. the revised third German edition (2008). Springer-Verlag Berlin Heidelberg,New York.ISBN 978–3–642-20355-8 The data also suggests that the chance of perforation is 2. Kieser DC, Riddel IR, Kieser JA, Theis JC, Swain MV (2014) Bone independent of level of loading. This, combined with de- micro-fracture observations from direct impact of slow velocity pro- creased chance of breaking at higher loads, implies that al- jectiles. J Arch Mil Med 2(1) though the bone’s ballistic performance does not improve 3. Di Maio, V.J.M. Gunshot wounds—practical aspects of firearms, ballistics, and forensic techniques, Second Edition. ISBN 0-8493- when subjected to loading, it is stronger as a complete 8163-0. CRC Press LLC, Boca Raton. 1999 structure. 4. Grundfest H (1945) Penetration of steel spheres into bone. Nat Research Council, Missiles Casualty Report No 10 5. Huelke DF, Buege LJ, Harger JH (1967) Bone fracture produced by Conclusions high velocity impacts. Am J Anat 120:123–131 6. Mundermann A, Dryby CO, D’Lima DD, Colwell CW Jr, Andriacchi TP (2008) In vivo knee loading characteristics during From this experiment, the following conclusions can be activities of daily living as measured by an instrumented total knee drawn: replacement. Wiley InterScience. doi: 10.1002 7. Holmquist TJ, Johnson GR (2005) Modeling prestressed ceramic & There is evidence to suggest that preloading the bone to and its effect on ballistic performance. Int J Impact Engineering 31(2):113–127 ISSN 0734-743X approximately 6 times body mass decreases the probability of breakage but does not affect the chance of rear damage or perforation; & When developing injury models for risk assessments and decision-making, the stresses applied to body parts before a kinetic assault need to be considered.
Human Factors and Mechanical Engineering for Defense and Safety
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Published: May 30, 2018