Evaluation of Skin Penetration from Less Lethal Impact Munitions and Their Associated Risk Predictors

Introduction: The use of less lethal impact munitions (LLIMs) by law enforcement has increased in frequency, especially following nationwide protests regarding police brutality and racial injustice in the summer of 2020. There are several reports of the projectiles causing severe injuries when they penetrate the skin including pulmonary contusions, bone fractures, liver lacerations, and, in some cases, death. The penetration threshold of skin in different body regions is due to differences in the underlying structure (varying degree of muscle, adipose tissue, and presence or absence of bone). Objective: The objective of this study was to further investigate what factors affected the likelihood of skin penetration in various body regions and to develop corresponding penetration risk curves. Methods: A total of eight, fresh/never frozen, unembalmed, postmortem human specimens (PMHS) were impacted by two projectile sizes: a 1″ and 5/8″ neoprene rubber ball in various body regions. Impacted body regions included the thigh, abdomen, anterior torso between ribs, anterior torso on a rib, sternum, scapula, posterior torso on a rib, and lower back for a total of a minimum of 24 shots per PMHS. To achieve both a penetrating and non-penetrating shot for each set of impacts, the impact location was assessed post impact to determine if penetration occurred, and the velocity of the next shot was adjusted to target the alternate outcome on the contralateral side within the same body region. Post-test, each PMHS underwent X-rays to determine if any other additional injuries occurred. Results: A binary logistic regression analysis was performed to determine which factors (e.g., velocity and energy density) were statistically significant at predicting the risk of penetration. Energy density was utilized as the primary predictor to evaluate the two projectiles’ data together and additional parameters (e.g., skin thickness and BMI) were also tested as co-factors. Statistical significance was obtained with energy density alone for the thigh (p = 0.004), anterior torso between ribs (p = 0.043), lower back (p = 0.04), scapula (p = 0.03), and posterior torso on a rib (p = 0.005). The abdomen region was not significant with energy density alone (p = 0.085) but when BMI was added as a co-factor significance was found to be (p = 0.021). The sternum and anterior torso on a rib were not found to have statistical significance with any of the predictors analyzed. The 50% risk of penetration was found for each region that had statistical significance. The thigh had a 50% risk at 12.62 J/cm2, 22.3 J/cm2 for the anterior torso between ribs, 28.6 J/cm2 for the lower back, 33.3 J/cm2 for the scapula, and 34.3 J/cm2 for the posterior torso on ribs. Conclusion: The results support that energy density is a good predictor for estimating the likelihood of the skin to penetrate and that the risk of penetration varies by body region.