The assessment of bullet wound trauma dynamics and the potential role of anatomical models

Abstract

Background It is hypothesised that an anatomical simulant model, that replicates the heterogeneous nature of human organs and tissues, will provide a more reliable and accurate method of evaluating the pathological features and incapacitation potential of ammunition in a weapons system than homogeneous bare ordnance gelatine alone. The use of frozen and thawed cadavers for simulant development was also examined. To develop a model, the most critical organs and tissues that sustain bullet wound trauma within the thorax and abdomen must be determined. Next a suitable method for establishing and matching the relevant biomechanical properties with candidate simulant materials must be developed, and an appropriate scoring system adopted. Method De-identified wound trauma data from 197 homicidal gunshot post mortem examinations in Israel were obtained between 2000-2001 and 2004-2008. The corresponding forensic ballistics data was only available for the cases between 2004 and 2008. The major organs involved, type of wounds, cause of death (COD), most common bullet paths, distances involved, firearm calibres and bullet types were established. Tensile strength tests were undertaken on selected tissue samples from an un- embalmed cadaver that had been frozen and thawed five times, which maximised the effects of repeated cycles. The universal test equipment Hounsfield H50KM machine was used to apply uniaxial tension until tissue failure occurred. The maximum tensile strength results in g/mm² were compared against corresponding data from the literature. Energy loss tests were conducted on fresh porcine organs/tissues using steel 4.5mm BB pellets fired from a Daisy® brand air rifle. Each organ/tissue was tested at room temperature and 37°C (body temperature). They were compared to Federal Bureau of Investigation (FBI) and North Atlantic Treaty Organisation (NATO) specification ordnance gelatine, as well as a candidate simulant material. A limited number of tests were also conducted at 4°C for further comparison purposes. Two chronographs measured BB pellet velocity before and after each test material was perforated and the difference was established in m/s. The resulting energy loss was established using the formula KE = ½ mv². FBI and NATO specified ordnance gelatine of 250 and 285 Bloom strengths were manufactured using tap water, reverse osmosis (RO) water and de-ionized water. They were allowed to cure for 21 hours, 100 hours and 3 weeks. The FBI calibration standard was used for all formulations as there is no separate standard for the NATO formulation in the literature. An Australian Defence Force (ADF) AUSTEYR model F88 ICW (individual combat weapon) in calibre 5.56x45mmNATO was used with standard issue ASF1 ball ammunition. Large FBI specification ordnance gelatine blocks were manufactured and thin gelatine/composite plates were used to simulate subcutaneous tissue and fat, as well as to provide a platform for the attachment of a skin simulant and to embed bone/rib composite within. A 250mm air gap and bubble wrap was used to simulate an expanded lung. The gelatine/composite plates were secured to a wooden cradle and the gelatine blocks were positioned behind it. The F88 ICW was fixed in a remote firing device 50m from the target and a chronograph 3m in front of the rifle measured bullet velocity. Test results were recorded using two high speed ‘Photron Fastcam’ digital cameras. Maximum three dimensional permanent cavity dimensions were obtained using a vernier gauge, and temporary cavity measurements were taken from high speed video images. Results The homicide study established that males represent 91% of gunshot victims. Of the 999 bullet wounds recorded, males were struck in the body an average of 5.2 occasions, with 2.2 of these bullets striking the thorax and/or abdomen. A contributing factor to the frequency of bullet strikes was the type of firearms involved, namely semi automatic pistols in the predominant calibre 9mm Luger, and assault rifles in calibre 5.56x45mm and calibre 7.62x39mmSoviet. Full metal jacket bullets were used in most instances and the majority of shootings (N=124) occurred at ranges estimated at 1m or greater. The most common bullet path was front to back in 66% of cases, followed by back to front in 27% of cases. Entry wounds occurred more often on the left side of the thorax, abdomen and back (N=253) compared to the right (N=172). The most common critical organs/tissues to sustain bullet trauma in descending order were; heart, lungs, liver, aorta, spleen, kidneys and vena cava. Ribs were struck by most bullets that entered the thorax. Multiple organ injury was listed in 146 of the 192 cases where a specific COD was determined by the pathologist. The following tensile strength results were achieved from the cadaver study: heart 3.56g/mm², kidney 10.27g/mm², oesophagus 22.08g/mm², skeletal muscle 29.46g/mm², ascending aorta 59.98g/mm², trachea 155.40g/mm², spleen 4.65g/mm², liver 10.83g/mm², pancreas 15.18g/mm², lung 29.94g/mm², pericardium 136.84g/mm², skin (abdomen) 355.26 g/mm² and skin (thorax) 407.88g/mm². These data were compared to published results obtained from non-frozen tissues from elderly persons, recognising that tensile strength values were only available for the following organs and tissues at the 95% degree of confidence: heart 9.2±0.95g/mm²; kidney 4±0.20g/mm², oesophagus 51±1.1g/mm², skeletal muscle 9±0.30g/mm², ascending aorta 68±2.4g/mm², trachea 150±6.5g/mm². It can be seen that some results from the test cadaver were higher and some lower than the published results, with trachea recording the only similar result. This indicates that the freezing and thawing process may change the tensile strength of tissues in unpredictable ways. Therefore, bio- mechanical research should avoid the use of frozen/thawed tissues and organs. The major agreement between the porcine energy loss tests were: FBI specification gelatine was similar (p>0.05) to heart and lung at room temperature and 37°C; spleen was similar to NATO specification gelatine at room temperature and 37°C; candidate Simulant ‘A’ was similar to hindquarter muscle at room temperature and 37°C and hindquarter muscle, kidney and spleen were similar to each other at room temperature and 37°C. Liver and kidney, and liver and fat were similar to each other at 4°C. The use of different water types had no effect upon ordnance gelatine calibration results. However, different temperatures, concentrations and curing times did have a significant effect. Neither of the two NATO 20% formulations met the same calibration standard as the FBI 10% formulation. The penetration depths achieved for the FBI formulations at both 3°C and 4°C were closest to the recommended calibration standard after 3 weeks curing time. A 20% concentration of 285 Bloom at 20°C met the same FBI calibration standard after 100 hours of curing and can be considered comparable. The anatomical model pilot tests demonstrated the benefit of using simulants that are more representative of the heterogeneous nature of human organs/tissues. It was found that by combining skin, bone and other simulant materials with ordnance gelatine, the behaviour of a military full metal jacket (FMJ) rifle bullet changes with regard to the earlier onset of temporary cavitation, reduced penetration depth and a higher degree of bullet yaw compared to simulations using only bare FBI specification ordnance gelatine. This occurs because more energy is consumed negotiating the various anatomical simulants, which means wounding is likely to occur much earlier, and organs that are deeper within the body may not be affected to the same degree. These factors will impact significantly upon injury severity in real tactical scenarios. Conclusion The experimental studies provide the framework for the development of a heterogeneous model for bullet trauma simulations of the thorax and abdomen. This model would be more representative of actual wound trauma than bare ordnance gelatine alone. This conclusion was arrived at by identifying the most critical organs/tissues for modelling purposes. Their energy loss values (J/m) were established and the method adopted allows for comparable simulants to be developed. Porcine energy loss tests showed that FBI specification gelatine is similar to heart and lung, but different to hind quarter muscle and most of the other ‘critical’ organs and tissues within the thorax and abdomen. NATO specification gelatine is a suitable simulant for spleen, and test Simulant ‘A’ is a suitable simulant for both hindquarter muscle and kidney. A separate simulant would be required for liver, fat and aorta. Frozen and thawed cadaveric tissue was shown to produce unpredictable tensile strength data and is therefore unsuitable for simulant development. The limitations of using FBI and NATO specification ordnance gelatine was highlighted when changes to bloom number, temperature and curing times altered calibration results. Therefore, temperature stable synthetic simulants such as Simulant ‘A’ are preferable. The anatomical model pilot tests clearly demonstrated that the addition of simulant materials directly affects wound severity simulations compared with bare ordnance gelatine alone. This in turn affects interpretation of real life situations. The AIS 2005/2008 and MAXISS scoring systems are deemed appropriate to grade the lethality potential of model simulations. Therefore, the original hypothesis has been validated.