A Comparison of Nonhuman Primate Injuries in Horizontal Versus Vertical Sled +Gz (Head-to-Foot) Impact Accelerations.

INTRODUCTION Accelerative events commonly expose military pilots to potentially injurious + Gz (axial, caudal to cranial) accelerations. The Naval Biodynamics Laboratory exposed nonhuman primates (NHPs) to + Gz loading in two subject orientations (supine or upright) to assess the effect of orientation and accelerations associated with injury at accelerations unsafe for human participation. MATERIALS AND METHODS Archived care records, run records, and necropsy and pathology reports were used to identify acceleration-related injuries. Injuries were categorized as central nervous system (CNS), musculoskeletal (MSK) system, or thoracic (THR). The occurrence of injuries relative to corresponding peak sled acceleration (PSA) and subject orientation were compared. A t-test was applied within each injury category to test for a significant difference in mean PSA between orientations. RESULTS For all 63 + Gz runs conducted, PSA ranged between 6 and 86 G. Of these runs, 17 (6 supine, 11 upright) resulted in acceleration-related injury. The lowest PSAs associated with injury for supine and upright were 69.8 G and 39.6 G, respectively. Individual injury occurrences for supine runs (CNS [n = 5], MSK [n = 6], and THR [n = 6]) occurred at/above 75.7 G, 69.8 G, and 69.8 G, respectively. For upright runs, injury occurrences (CNS [n = 3], MSK injuries [n = 9], and THR injuries [n = 6]) occurred at/above 60.1 G, 39.6 G, and 50.5 G, respectively. The applied t-test showed significant difference between the mean orientation accelerations within each category. Injuries to supine NHPs included compression fracture, organ damage, brain hemorrhage, spinal cord hemorrhage, cervical hemorrhage, paresis/paraplegia, and THR bruising, whereas injuries to upright NHPs included compression fracture, organ damage, spinal cord hemorrhage, paresis/paraplegia, THR bruising, and difficulty breathing. CONCLUSIONS Axial loading to supine occupants produced more CNS injuries, whereas upright produced more MSK injuries. Both orientations produced equal THR injuries. NHP injuries reported reflected those reported following human + Gz acceleration events, highlighting the importance of orientation during acceleration to mitigate injury for next generation equipment design and testing.