Abstract

Energized fragments from explosive devices have been the most common mechanism of injury to both military personnel and civilians in recent conflicts and terrorist attacks. Fragments that penetrate into the thoracic cavity are strongly associated with death due to the inherent vulnerability of the underlying structures. The aim of this study was to investigate the impact of fragment-simulating projectiles (FSPs) to tissues of the thorax in order to identify the thresholds of impact velocity for perforation through these tissues and the resultant residual velocity of the FSPs. A gas-gun system was used to launch 0.78-g cylindrical and 1.13-g spherical FSPs at intact porcine thoracic tissues from different impact locations. The sternum and rib bones were the most resistant to perforation, followed by the scapula and intercostal muscle. For both FSPs, residual velocity following perforation was linearly proportional to impact velocity. These findings can be used in the development of numerical tools for predicting the medical outcome of explosive events, which in turn can inform the design of public infrastructure, of personal protection, and of medical emergency response.

Introduction

Energized blast fragments from explosive devices have been the most common mechanism of injury to both military personnel and civilians in recent conflicts [1,2] and terrorist attacks [3,4]. Primary fragments originate from the detonation device itself such as munition casings, or from embedded objects within the explosive device such as nuts and bolts [58]. Secondary fragments originate from the surrounding environment and can include soil ejecta, energized debris, and foreign bone fragments from adjacent victims [5,9]. The review by Champion et al. [10] on injuries from explosions, mainly from Iraq and Afghanistan, showed that most explosion-related injuries and subsequent deaths were from fragments which ‘exponentially increase the range and lethality of explosives’.

Penetrating injuries to the thorax are the most common cause of death in explosive events [1113]. Breeze et al. [12] highlighted that penetrating injuries to the torso were the main cause of potentially survivable deaths on the battlefield, whereby injuries to the heart, pulmonary vessels, and the vena cavae were the most common cause of immediate death, and thoracic aortic injuries were three times more likely to be nonsurvivable than abdominal aortic injuries. Eastridge et al. [14] investigating the 4596 U.S. battlefield fatalities in Iraq and Afghanistan between 2001 and 2011 showed that cardiac and thoracic injuries, mainly penetrating in nature, were of the most fatal and accounted for 23.6% instantaneous deaths, 21.8% acute deaths from nonsurvivable injuries, and 24.2% of lethal hemorrhage. Edwards et al. [3] reviewed 58,095 terrorist explosions worldwide between 1970 and 2014; they reported that suicide attacks, which cause injuries mainly to the thoracic and abdominal regions [15], can result in 8.9 times more fatalities than nonsuicide attacks. These studies highlight the inherent vulnerability of the thorax, which includes the heart and great vessels, while having limited protection from surrounding the skeleton, namely, the ribs, vertebrae, scapulae, and clavicles. A few studies detailing injuries to specific locations within the thorax showed that within this region, pleura injuries (pneumothorax and hemothorax) are the most common and cardiac and vascular injuries have the highest mortality rates [1520]. Even though these studies only cover a small fraction of cases sustaining penetrating injuries to the thorax, one thing for certain is that the penetrating threats must have perforated the surrounding chest wall, including the rib cage and scapulae, before inflicting these injuries. In addition, the incidence of injury differs between the military and civilian populations as the former have the benefit of wearing body armor at the time of the incident that reduces injuries by fragments to the torso [21]. Thus, the civilian population is especially vulnerable to penetrating injuries to the thorax.

Tools to predict injury outcomes from explosive events can be used to devise potential mitigation and response approaches. Computational prediction tools have been developed to estimate the injury burden by the blast wave and energized fragments from an explosive event in a crowded environment [22]. However, limited experimental evidence exists to generate the algorithms for the penetration of fragments into a range of tissues used in such computational models [23]. In contrast, there have been substantial efforts on ballistic penetrating injuries, including to the thorax, using various animal models, mostly focused on gunshot wounds [2431]. These studies have established that the kinematics of the bullets (such as velocity, energy, and flight stability) and their designs, together with the biological and mechanical properties of the impacted tissues, are key factors affecting the injury outcome [26,27,32]. Porcine tissue has been the most commonly used physiological and anatomical model for studying ballistic injury, due to “similar tissue architecture and scale to human” [24]. The important kinematic variable commonly used to quantify injury include: (1) impact velocity—the velocity of the projectile when striking the target; (2) residual velocity—the velocity of the projectile after perforating the target; and (3) velocity reduction—the change in velocity of the projectile after the impact event, which can be used to determine the kinetic energy transferred to the tissue [25,32].

Blast-fragment penetrating injuries remain understudied, and extrapolations from the aforementioned ballistic work are challenging, as gunshot wounds and wounds caused by explosive devices do not result in similar injury patterns or medical sequalae [16,33]. To our knowledge, the only published study of fragment penetration to the thorax is by Breeze et al. using three North Atlantic Treaty Organisation-standardized fragment simulating projectiles (FSPs) to impact different regions, including the chest, of cadaveric pigs [34]. The objective of the study, however, was to compare the retardation of the FSPs in the porcine skin and muscle with that in gelatin; as such, it did not specifically report the interactions of the FSPs with individual components of the chest wall or the risk of penetration and perforation through these tissues. More research work is indeed needed to provide experimental data for fragment penetrating injuries to tissues in the thorax, especially those in the chest wall and the vital organs.

The aims of this study were threefold; (i) to investigate the responses of different components of the chest wall to penetration of a range of representative FSPs; (ii) to ascertain velocity thresholds for perforation through these tissues by the chosen FSPs; and (iii) to determine the residual velocity and velocity reduction of the FSP when perforation occurs.

Materials and Methods

A diaphragm-operated stainless steel 32-mm-bore gas-gun system, as described by Nguyen et al. [35], was used to perform the impact tests (Fig. 1(a)). The breech section of the device can be charged to 220 bar-liters with compressed air or helium to rupture Mylar© diaphragms—the firing mechanism—of chosen thicknesses. The release of high-pressure gas can accelerate a projectile unit along the barrel section to an output velocity of up to 600 m/s. The following two carbon steel FSPs were chosen: (1) a cylindrical FSP 4.5 mm in diameter, 6.27 mm in length, and 0.78 g in mass representing the most common metal fragment under 1 g [34]; (2) a spherical FSP 6.5 mm in diameter and 1.13 g in mass representing the most common metallic fragment in the 1–2 g range [36]. A polycarbonate hollow sabot with an aluminum alloy front plate was designed to accommodate both FSPs, and to be separated from the housed FSP by the sabot-stripper unit upon entering the target chamber. The precision manufactured flat contact surfaces between the sabot and the sabot stripper, along with the short distance between the sabot stripper and the sample (less than 10 cm), ensured that the FSPs came out straight (no more than 15-deg tilt from the axes of travel) and did not tumble before impacting the target. A laser diode mounted at the center of the gas gun barrel was utilized to position accurately the sample in the target chamber. High-speed photography (Phantom VEO710 L camera, AMETEK, Wayne, NJ) at 120,000 fps was employed to record the interaction of the FSP with the sample. The camera was setup so that its plane of focus coincided with the travel plane of the FSP. The calibration of pixel to millimeter was carried out for the same plane to later obtain the distance traveled by the FSP. High-speed footage was then used to estimate the impact and residual velocities of the FSP over three frames before and after the respective event occurred. For each frame, edge detection was applied to the image, and the pixel location of the center of the FSP was determined by averaging the pixel coordinates of the FSP edges. Using the predetermined distance calibration and the interframe time, the desired velocity of the FSP could be obtained.

Fig. 1
Diagram of (a) the gas-gun system and experimental setup; (b) the anatomy of the rib cage and scapula and key connections which were implemented in the mounting of (c) the sectioned rib panel, (d) the sternum, (e) the scapula, and (f) the full rib cage; all depicted in top, axial views; and (g) photographs of the full rib cage, rib panel, and scapula samples as mounted in the gas gun setup
Fig. 1
Diagram of (a) the gas-gun system and experimental setup; (b) the anatomy of the rib cage and scapula and key connections which were implemented in the mounting of (c) the sectioned rib panel, (d) the sternum, (e) the scapula, and (f) the full rib cage; all depicted in top, axial views; and (g) photographs of the full rib cage, rib panel, and scapula samples as mounted in the gas gun setup
Close modal

Three complete rib cages, 11 additional sternums and 17 scapulae were harvested from skeletally mature pigs (90–100 kg in overall weight). These were acquired from an abattoir, fresh-frozen and stored at −20 °C and testing was undertaken within one month postmortem. Skin and subcutaneous tissue were removed from the bones to expose the exterior surface of the ribs and the sternum whilst leaving the periosteum and intercostal muscles between the ribs and the parietal pleura intact. Each was then sectioned into four panels of ribs with intercostal muscles (left and right sides from the fourth to the eighth rib and from the ninth to the 13th rib) and the sternum. During setup, the samples were kept moist with sprayed water.

Mounting of samples was carried out so as to ensure biofidelic boundary conditions. The rib panels were clamped vertically at the medial side, which normally connects to the spinal column (Figs. 1(b), 1(c), and 1(g)). The sternum and scapula samples were suspended independently using mounting posts on the sides and at the shoulder joint, respectively, so that the anterior surface of each sternum sample and the posterior surface of each scapula sample were facing the barrel without applying forces in any particular direction (Figs. 1(b), 1(d), 1(e), and 1(g)). Only the fifth to the seventh ribs and the tenth to the 12th ribs were targeted by the FSPs during testing so that the impact location was far enough away from the fixations, thus minimizing any effects due to mounting. The top and bottom ribs were kept intact to preserve the structural response of the specimen [37]. The central region of each rib (the area 30–70% along its length) was targeted by the FSPs during testing as the geometry there is more uniform and flatter in shape, thus, closer to the human anatomy [3840]. The intercostal muscles in these panels were subsequently used for impact if the tissue in the related ribs above and below were still intact.

A total of 148 impact tests were performed: 50 tests on rib bones (28 with cylindrical FSPs and 22 with spherical FSPs), 30 tests on intercostal muscles (12 with cylindrical FSPs and 18 with spherical FSPs), 30 tests on the sternum (13 with cylindrical FSPs and 17 with spherical FSPs), and 38 tests on the scapula (18 with cylindrical FSPs and 20 with spherical FSPs). Each sample underwent multiple impact tests at different locations in order to optimize the use of the samples. For ribs, intercostal muscles and scapulae, the location of impact was set to be at least 3-FSP diameters apart from the damaged area caused by a previous test so as to ensure that the target location of each test was intact prior to testing. Each anatomical section of the sternum was limited to one penetration. The multiple impacts were also performed in the order of increasing FSP velocity to minimize any structural change that may have occurred in a previous impact. Postimpact, the samples underwent radiographic scanning using a mini C-arm (Fluoroscan® InSight™ FD system) to record the resultant fractures and aid in the determination of penetration.

Survival analysis was performed using the ncss statistical software (v11, Utah). As a result of a likelihood-criteria best-fit analysis (Table S1 available in the Supplemental Materials on the ASME Digital Collection) together with visual justification by probability plots, the Weibull regression model was chosen as the probability distribution most appropriate for generating the risk curves with speed at impact being the predictive variable. The data were classified as left-censored if there was perforation (where the FSPs penetrated the samples and exited from the back, Fig. 2) and as right-censored if there was no perforation (where the FSPs penetrated and stopped inside the samples or rebounded without penetration, Fig. 2). The values of impact velocity at 50% risk of perforation (V50) were extracted from the risk curves with 95% confidence interval as uncertainty. For the sets of data where the threshold of perforation was distinctive with no overlapping region in the impact velocity, the Weibull fitting became diverged. For these cases, the values of V50 and its uncertainty were determined from the mean and standard deviation of the three highest impact velocities resulting in no perforation and the three lowest impact velocities resulting in perforation of the corresponding tissue.

Fig. 2
Classification of impact outcome: perforation happens when the FSP penetrates and exits at the back of the sample, either in tumbling or straight motion; no perforation happens when the FSP does not penetrate the sample but rebounds off it or when the FSP penetrates but is brought to rest inside the sample
Fig. 2
Classification of impact outcome: perforation happens when the FSP penetrates and exits at the back of the sample, either in tumbling or straight motion; no perforation happens when the FSP does not penetrate the sample but rebounds off it or when the FSP penetrates but is brought to rest inside the sample
Close modal

To validate the biofidelity of the tests on rib panels and on individual sternum samples, the same experimental procedure was repeated on a full rib cage structure, including the intercostal muscles in between ribs and the sternum, potted with bone cement at the T9 vertebral level of the spinal column to support the rib cage structure (Figs. 1(b), 1(f), and 1(g)). Impact tests with 0.78-g cylindrical FSPs were performed on the fifth to the seventh rib and the sternum. The resultant damages to the ribs and sternum were then compared with those generated in the main setup described earlier.

Results

For both types of FSP, all the tests resulting in perforation through bony tissues of rib bones, sternum, and scapula as shown in Fig. 3 (typically of impact velocity above 80 m/s) produced a drill-hole fracture with a sharp entrance hole similar to the diameter of the FSP and a less sharp exit hole with small fragments of bone around the edges. The higher the impact velocity was, the larger the exit hole was and the more resulting fragments occurred due to the fracture. More cracks were observed to be generated from the punctures in rib bones by the spherical FSP compared to the cylindrical FSP. It was also noticed that at the lower range of impact velocities (under 200 m/s), spherical FSPs tended to slip from the targeted midpoint of the rib bone and thus the fracture occurred near the edge of the bone. For impacts on the intercostal muscle, the FSPs either were brought to rest inside the tissue or perforated through. The penetrated wound (Fig. 3) is larger and more distinctive for high impact velocities (typically above 300 m/s), and harder to detect at low impact velocities (typically below 150 m/s). Tears in the parietal pleura membrane were used to detect the site of perforation in low-velocity impact.

Fig. 3
Radiographs and photographs of postimpacted specimen by (a) cylindrical FSPs and (b) spherical FSPs
Fig. 3
Radiographs and photographs of postimpacted specimen by (a) cylindrical FSPs and (b) spherical FSPs
Close modal

No deflection of the rib panel due to the vertical clamping was detected (Fig. S1, available in the Supplemental Materials on the ASME Digital Collection). Perforations by cylindrical FSPs through the 5th to the 7th rib and the sternum in the full rib-cage setup resulted in visually similar fractures across different impact velocities as those in the sectioned rib panels and individual sternums (Fig. 3). In addition, the linear regressions of FSP velocity before and after perforating the rib bone and sternum in both set-ups did not show statistically significant differences (Fig. S2, available in the Supplemental Materials on the ASME Digital Collection). These observations suggest that the simplified mountings used were appropriate and biofidelic.

The Risk of Perforation Through Different Tissues.

Of all 71 impacts by the cylindrical FSPs to various thoracic tissues, 11/28 resulted in perforation through the rib bone, 4/12 resulted in perforation through the intercostal muscle, 6/13 resulted in perforation through the sternum, and 5/18 resulted in perforation through the scapula. From the survival analysis (Fig. S3, available in the Supplemental Materials), the V50 of perforation by the 0.78-g cylindrical FSPs through the rib bone, intercostal muscle, sternum, and scapula were, respectively, 124 ± 12 m/s, 66 ± 13 m/s, 122 ± 12 m/s, and 89 ± 8 m/s (Fig. 4).

Fig. 4
The impact velocity at 50% risk of perforation through the thoracic tissues by 0.78 g cylindrical and 1.13 g spherical FSPs. The error bars denote 95% confidence intervals of the Weibull fitting or the standard deviation of the V50 values.
Fig. 4
The impact velocity at 50% risk of perforation through the thoracic tissues by 0.78 g cylindrical and 1.13 g spherical FSPs. The error bars denote 95% confidence intervals of the Weibull fitting or the standard deviation of the V50 values.
Close modal

For the 77 impacts by the spherical FSPs, 6/22 resulted in perforation through the rib bone, 3/18 resulted in perforation through the intercostal muscle, 8/17 resulted in perforation through the sternum, and 6/20 resulted in perforation through the scapula. From the survival analysis (Fig. S4, available in the Supplemental Materials), the V50 of perforation by the 1.13-g spherical FSPs through the rib bone, intercostal muscle, sternum, and scapula were, respectively, 93 ± 5 m/s, 53 ± 9 m/s, 131 ± 6 m/s, and 56 ± 5 m/s (Fig. 4).

Across both types of FSP, the sternum had the highest V50 value, followed by the rib bone, the scapula, and then the intercostal muscle.

Postimpact Behavior of Fragment-Simulating Projectiles.

The FSP velocity before and after perforation through various thoracic tissues is shown in Fig. 5 for both types of FSP. Similar to the velocity thresholds of perforation, the reduction in FSP velocity is the greatest in the sternum and rib bone, followed by the scapula and then the intercostal muscle. The correlations between impact and residual velocities of the FSPs are very similar for rib bone and sternum. The scapula behaved more closely to intercostal muscle in terms of velocity reduction, especially at low impact velocities. For impacts on rib bone by cylindrical FSPs, the estimations of the FSP postimpact velocity in the sectioned rib panel setup was more accurate than the full rib-cage setup as the view of the high-speed camera was less obstructed. The velocity relationship derived from tests with sectioned rib panels was essentially the same as the trend presented in Fig. 5(a), which included data from both the sectioned rib panel and the full rib-cage boundary conditions. Furthermore, there was no difference observed for impact at different rib levels for both types of FSP. For impacts on rib bone by cylindrical FSPs, the variation in the data at low velocities (under 200 m/s) is likely due to the possible slipping behavior of the FSP at impact; even though not captured on radiographs, it is highly likely that it happened for the scapula and sternum as well; this variation seems to be greater the thicker the tissue (sternum > rib > scapula).

Fig. 5
Residual velocity (Vo) versus impact velocity (Vi) for rib bone (blue), intercostal muscle (orange), sternum (yellow), and scapula (green) obtained from impact tests with (a) a 0.78 g cylindrical FSP and (b) a 1.13 g spherical FSP
Fig. 5
Residual velocity (Vo) versus impact velocity (Vi) for rib bone (blue), intercostal muscle (orange), sternum (yellow), and scapula (green) obtained from impact tests with (a) a 0.78 g cylindrical FSP and (b) a 1.13 g spherical FSP
Close modal

The behavior of the cylindrical FSPs after impacting the sample is summarized in Fig. 6. Across three bins of impact velocity, namely, 20–100, 100–200, and 200–600 m/s, the FSP motion was generally categorized into “Rebound/Halted” where there was no perforation occurring, “straight” where the FSP did not tumble after perforating the tissue (completely straight or tilted slightly as it traveled on), and “tumbling” where the FSP tumbled as it continued to travel forward after perforation. Tumbling and rebounding only occurred to FSPs impacting the bony tissues. For the few rebounded FSPs observed, the rebound velocity was very small (less than 10 m/s; Fig. S4(a), available in the Supplemental Materials). For perforating impacts, the FSP had less tendency to tumble with higher impact velocity and with softer tissues (intercostal muscle and scapula). The angular velocity of the tumbling FSPs contributed to typically less than 1% of the total kinetic energy of the FSP after perforating the tissues (Figs. S4(b) and S4(c), available in the Supplemental Materials). These behaviors are irrelevant for impacts with the spherical FSPs and so are not reported.

Fig. 6
The motion of the 0.78 g cylindrical FSP after impact with (a) rib bone, (b) intercostal muscle, (c) sternum, and (d) scapula
Fig. 6
The motion of the 0.78 g cylindrical FSP after impact with (a) rib bone, (b) intercostal muscle, (c) sternum, and (d) scapula
Close modal

Discussion

The study was undertaken to enhance our understanding of penetration of energized fragments into the thorax and provide experimental evidence to generate improved algorithms for injury-prediction tools to improve the planning of infrastructure and medical outcome following explosive events. This study investigated the injury to thoracic tissues by energized small metal fragments using an experimental porcine model.

Sectional rib panels and isolated sternums were used instead of the whole rib-cage structure as it was practical to obtain samples in this form, more time-efficient in sample preparation and mounting, and, importantly, it allowed for more accurate measurement of the impact and residual velocities since the rib panels could be orientated vertically, and so ensure no obstruction of view as was the case when a whole rib cage was used. Simplifying the mounting was shown to not affect the response of the samples as (a) similar fracture patterns were observed and (b) the linear regressions of impact versus residual velocity were statistically similar between sectional and full rib-cage boundary conditions. The fact that similar behaviors were observed in different rib levels suggests that the response of the rib bone is not sensitive to rib level and location of the impact insofar as penetrating injury due to small fragments is concerned. This is likely due to the small size of the FSPs making the material properties more relevant than the global structural effects in the response of the tissue. This finding also agrees with existing literature, which suggests that the rib bones behave differently when the overall geometry such as the curvature of the rib or the position of the rib in the rib cage is involved in the interaction, but similarly when testing smaller rib sections as was the case in this study [4146].

The velocity of the FSP was calculated using high-speed photography. As the distance between the exit of the gas gun and the sample was small, less than 10 cm, the parallax error and calibration had a limited effect on the accuracy of the velocity calculation. The main source of uncertainty in the calculation of the velocity of the FSP came from determining the pixel location of the FSP center due to blurring in the captured frames, especially in tests with high impact velocity. The typical error in FSP velocity was between 5 and 20 m/s; the error was larger for higher velocities.

The injury-risk analysis presented focused on the perforation by the FSP instead of penetration (which includes both perforating cases and those where penetration occurs but the FSPs are halted inside the tissue) as it portrays both the injuries to the thoracic tissues as well as potential threats to the vital organs behind them. The obtained perforation threshold of impact velocity (V50) and the FSP velocity reduction (effectively the kinetic energy absorption as the rotational energy was negligible) reflect the resistance to perforation of the tissues. The sternum and rib bone are the most resistant thoracic tissues as they have higher V50 values and more energy absorbed than the others. This is understandable given that they are skeletal tissues. The rib bone, however, was observed to be less resistant to perforation by the spherical FSP compared to the cylindrical FSP while the sternum behaves similarly against both FSPs. It suggests that the thicker bone tissue is not affected by the difference in mass of the two FSPs but rather their geometry. This effect can also be seen in the V50 values of the scapula—the thinnest of the skeletal tissues tested here. The overall resistance of the scapula is between that of the other bones and intercostal muscle, which may be attributed to its thin geometry. The geometry factor is also likely to be responsible for the similar postimpact FSP behaviors (velocity reduction and movement of FSP after impact) of the scapula to soft rather than skeletal tissue. The V50 for the scapula is slightly lower than that for intercostal muscle, suggesting that in this case, the elasticity of the muscle could have been more beneficial to stopping the heavy but rounded projectile than a thin layer of bone tissue. The intercostal muscle shows almost identical resistance against the two types of FSP, suggesting that the soft tissue is unaffected by the change in mass and contact geometry of these two FSPs. Furthermore, the V50 values (66 m/s for the 0.78-g FSP and 53 m/s for the 1.13-g FSP) for the intercostal muscle indicates that it requires approximately 1.5 J of kinetic energy to perforate the 20-mm-thick soft tissue. This is in the same order of magnitude as the 1.2 J of energy required to penetrate a 20-mm thick ballistic gelatin soft-tissue simulant by the same FSP of a similar impact–velocity range [35]; the slightly higher value is likely due to the resilient parietal pleura membrane. This finding suggests that the freezing and thawing processes of the porcine soft tissue did not alter its response to FSP perforation in the range of velocities investigated, which is in agreement with a previous study for velocities under 100 m/s [47]. It also confirms that the behavior of the intercostal muscle was realistic, and not artificially high in strength due to the cadaveric nature of the tissue. Existing studies also confirm that the fresh-frozen cadaveric bone is suitable for biomechanical testing with no changes observed in structural and material properties of the bone [4852].

There was no noticeable difference in terms of reduction in velocity (or kinetic energy) of the two FSPs due to their interactions with the thoracic tissues tested. The difference in contact between the FSPs and the rib bone, due to their different nose shape, contributes to more cracks observed in the rib bone after impact with the spherical FSP. As both FSPs were not pointy (flat and hemispherical nose shape) and the bone target was a semibrittle material, which experienced brittle-to-ductile transition during fracture [53], the mechanism of perforation could have involved initial compression, formation of radial cracks, ductile hole, and plugging [5458]. The sharper edge of the cylindrical FSP may have resulted in shearing of the periosteum, which cushioned the impact during the initial moment [57] and created adiabatic shear bands in the bone earlier [56]; thus, only a few cracks were generated. The higher mass of the spherical compared to the cylindrical FSP also may have contributed to more cracks formed by its impact to the rib bone.

The postimpact behavior of cylindrical FSPs following perforation of the thoracic wall may lead to increased trauma of internal organs: tumbling FSPs deposit greater energy over a larger volume, thereby causing more severe injuries to underlying thoracic organs; although FSPs traveling in a more linear fashion deposit less energy, they can travel a greater distance through the thoracic cavity and, as a result, can damage more vital organs [24,59]. It needs to be noted that the study only focused on the interactions of the FSP through the tissues of the thoracic wall. Further work is recommended to determine the effect of FSP behavior following perforation of the thoracic wall components to organs such as the heart and lungs in order to get a full picture of the severity of overall injury. Further work should also investigate the effect of large temporary cavity that can be created by small projectile with high velocity (greater than this gas-gun's 600 m/s limit) [18], especially to the thoracic organs.

A porcine model use in this study has been one of the most common choices for experimental study of ballistic trauma to the chest and ribs [38,6064]. The central part of skeletally mature porcine ribs has the same pattern and similar shape to those of the human [39,40], and the Haversian tissue microstructure and density of their cortical bone is close to that in the human [65,66]. Similarities between porcine and human tissue allow us to assume comparable behaviors between them, in accordance with other studies using this animal model [24,60,61,64]. This assumption is a limitation of this study and needs to be confirmed or improved by adapting the experimental model to human cadaveric samples. In particular, the study by Waltenberger et al. [67] compared human and porcine rib bones and found no significant differences in their cortical and cancellous bone areas as well as in their trabecular bone volumes, but they found differences in their cortical thickness. The cortical thickness of the porcine rib (0.995 ± 0.361 mm) was reported to be approximately 2.5 times greater than that of human (0.396 ± 0.129 mm) [67]. The overall thicknesses of the porcine rib bone, sternum, and scapula (respectively, 9.04 ± 0.46 mm, 12.30 ± 1.94 mm, and 7.36 ± 1.31 mm, measured in this study) are also slightly greater than those of human (respectively, 7 ± 1 mm [68], 10.8 ± 1.3 [69], and 5.7 ± 1.2 mm [70]). These differences mean that the human thorax may be more susceptible to penetration by a fragment compared to its porcine surrogate and further work on porcine–human transfer functions for these bones are necessary to account for the anatomical differences. Furthermore, soft tissues such as skin or muscle were removed and their effect on the resulting injury was not captured in this study; this was by design so that there was no retardation of the FSP due to contact and perforation through these tissues. The contribution of skin or muscle, as appropriate, can be accounted for separately both in the physical experiments and in computational models of secondary blast injury. For future work, this experimental model can be utilized to test other representative FSPs, including FSPs of different masses, shapes, and materials such as glass shards.

The results of this study can be implemented readily into the numerical algorithms utilized by injury-prediction tools used to assess the risk in individuals and in populations due an explosion [22]. These tools combine information from the explosive threat, the environment, the population distribution, and injury data—such as those produced by this study—to output an overall pattern of injury. Such tools can inform injury-prevention strategies for planning mass events (concerts, games), designing national infrastructure (stadia, train stations, airports), and planning military operations.

Conclusions

This study investigated the perforation through different components of the chest wall by two different FSPs. The impact velocities required to produce a 50% risk of perforation through ribs, intercostal muscle, sternum, and scapula were, respectively, 124 ± 12 m/s, 66 ± 13 m/s, 122 ± 12 m/s, and 89 ± 8 m/s by the cylindrical FSP and 93 ± 5 m/s, 53 ± 9 m/s, 131 ± 6 m/s, and 56 ± 5 m/s by the spherical FSP. The residual velocity of the FSP after perforating a thoracic tissue was directly proportional to its impact velocity. The sternum and rib bones were found to be the most resistant to perforation, followed by the scapula and intercostal muscle. These results can be used to enhance our understanding of penetration of energized fragments into the thorax and generate improved algorithms for injury-prediction tools to improve medical outcome following explosive events.

We would also like to thank Mr Satpal Sangha for his technical assistance and Dr Vincent Huair-Yu Chen for discussions on statistical analysis.

Acknowledgment

This work was supported by the Defence Science and Technology Laboratory (DSTL) under contract DSTLX-1000131908. The authors would like to acknowledge the financial support of the Royal British Legion as the work was conducted in the Royal British Legion Centre for Blast Injury Studies at Imperial College London. Any views expressed in this article are those of the authors and not necessarily representative of the funding organizations.

Funding Data

  • The Defence Science and Technology Laboratory (DSTL) (Grant No. DSTLX-1000131908; Funder ID: 10.13039/100010418).

  • The Royal British Legion (TRBL) (Funder ID: 10.13039/100008631).

Conflict of Interest

The investigators do not have any conflict of interest related to this project.

Nomenclature

     
  • Vi =

    the impact velocity of the fragment simulating projectile just before impacting the tissue

  •  
  • Vo =

    the residual velocity of the fragment simulating projectile after perforating the tissue

  •  
  • V50 =

    the values of impact velocity at 50% risk of perforation

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Supplementary data