Objective       To study the characteristics of behind armor blunt trauma extending to the brain and to investigate the biomechanics mechanism of the remote injury.      Methods       Eighteen male Landraces were randomly divided into a sham injury group (n=4)

a bullet velocity 910 m/s group (n=6)

a 740 m/s group (n=4) and a 590 m/s group (n=4). The injury model was anesthetized Landrace in right lateral position and wrapped in a two-layer bullet-proof vest. The outer layer was ceramic hard armor with NIJ III protection grade. The inner layer was police grade II ultra-high molecular polyethylene soft body armor. Caliber 5.8 mm ballistic guns were fired at 25 m range and three different projectile velocities

aimed at the left midclavicular line 4

5 intercostal (heart window). Air bomb shooting was applied to sham injury group. EEG

ECG

invasive arterial blood pressure

heart rate and respiratory changes before and after injury were continuously monitored. Plasma was collected at 1

2 and 3 h before and after injury to detect markers of brain injury. Lumbar puncture was taken at 3 h before and after injury to test specific brain injury protein in cerebrospinal fluid. In 3 h after injury the animals were deep anesthetized and bled to death. Their brain blocks were collected for toluidine blue staining and electron microscopy. The other 4 Landrace pigs were used for biomechanical testing. On the basis of the above model pressure sensors were placed in the subcutaneous layer of precordium at the point of impact

pericardial cavity

left chest

left common carotid artery and cranial cavity. Acceleration sensors and force sensors were placed in the sixth rib at precordial midclavicular line. When the target was hit

each part’s biomechanical parameters and duration were recorded and analyzed.      Results      In the 3 groups of different speed

spectrum intensity of low-frequency EEG Delta and Theta wave within 2 minutes after injury was significantly lower than that before injury (P0.05)

and the decline was about 10%. In 5 min after injury the amplitude was restored to pre-injury level. There was no significant difference among 3 speed groups (P0.05). Three hours after injury

acute injury alterations were found in Nissl stained hippocampal under light microscope. The 910 m/s group had hippocampal neurons atrophy and varying sizes of nuclei; the 740 m/s group had neurons atrophy and nucleus deviation; the 590 m/s group had mild neuron swelling

injury scale 910 m/s group 740 m/s group 590 m/s group sham injury group. When bullets hit precordial vest at three different speed

pressures in pericardial cavity

chest cavity

internal carotid artery and cranial cavity instantly surged

to 23.32±4.41 kPa in 590 m/s group

45.52±27.38 kPa in 740 m/s group

and 88.27±8.81 kPa in 910 m/s group. Each group’s pressure was significantly different from that of the other two groups (P0.01).      Conclusion       Under the protection of NIJ III grade outer layer and police II grade inner layer

as the bullet speed increases

the Landrace has central nervous EEG suppression

central neuron degeneration and demyelination in hippocampus

which may produce early symptoms and late effects. Biomechanical testing suggests that under hard armor protection

high-speed bullet hitting Landrace precordial bulletproof plate causes a small displacement of the animal and kinetic energy of the bullet instantly decreased from extremely high to 0. The energy passes to the organism through body armor deformation. It could reach remote central nervous system through pulmonary hollow organs

heart impact and bone transfer

and this may be the reason for remote injury.