Characteristics And Mechanism Of Blunt Trauma After The Armored Spine Injured By Rifle Bullets

BackgroundBody armor had a long history, which was begun to use in the ancient times. Thedevelopment of modern armors originated from the World WarⅠ. Modern Armors aremainly used by army, police and guarantors. There are three kinds of body armorscategorized by materials: soft, hard and combined armors. Body armor can avoid projectiles(such as bullets or blast fragments) penetrating the chest and abdomen of human being, andlower the mortality of the wounded. It was estimated that the mortality of British soldiersdecreased by58%when the armors were applied in the World War I. But the body armorcan not completely eliminate the damage of the projectiles in the body. Behind armor blunttrauma (BABT) is one kind of typical damage. BABT can occur in the body shielded bysoft body armor or hard body armor. When the projectiles, especially high-speed projectilesimpacted with the armors, the energy generated in the course of collision could conduct tothe body through body armor, and lead to different degrees of damage in various organsunder the armor, even to death.In recent years, with the widespread application of body armors and high-energyweapons, people began to pay more attention to BABT and have been doing a lot ofresearches about the BABT on the chest. The researches ranged from minor skin lesions tosevere visceral injury, such as pulmonary contusion, myocardial contusion and rupture ofliver.However, characteristics of BABT in bullet injuries of back have not been reported.Whether spinal cord would be injured concurrent with back injuries is also not clear.Investigating of the characteristics of body armor blunt trauma in the back will contributenot only to the deep comprehension of BABT but also to the operational capabilityassessment of injured soldiers.High-speed bullets on the chest could cause BABT of the central nervous system (brain) featured by inhibition of electroencephalogram. Vertebral column is concatenatedwith skull, and spinal cord is concatenated with brain. Whether the BABT on the backcould cause damages in the brain has not been reported. It is not clear whether functionaldamage of brain and change in level of specific biomarkers in the serum or cerebrospinalfluid would occur after BABT. So, investigating the characteristics of BABT in the backwill also contribute to diagnosis, treatment and prognosis of brain injury caused by BABTon the back.What is the biomechanical mechanism of BABT? It has been reported that thepressure wave and the shear generated after the collision were the main reasons. Previousstudies are mostly confined to the injury on the chest; and there is no report on theconduction laws of pressure wave generated after BABT on the back in vivo. Therelationship between the amplitude of pressure wave and bullets kinetic energy is also notclear. Clarifying these issues can provide the basis for body armor improvements andprevention of BABT.ObjectiveTherefore, the aim of present study was to uncover the pathological characteristics ofdifferent vital organs and the biomechanical characteristics in the spine and central nervoussystem when armored animals were wounded by three different velocities bullets andinvestigate the neurological, functional characteristics of the armored animals when injuredby high-velocity bullet.ContentsThe backs of animals were protected by combined armors consisted of4.2mm Al2O3ceramic plate and ultra-high molecular polyethylene. Then the eighth thoracic vertebra (T8)was injured by bullets of three velocities (590m/s,720m/s, and910m/s) at the distance of25m. The following researches were on the basis of this animal model.1. Pathological changes of central nervous system and other vital organs in chest andabdomen were investigated after injury. Physiological changes including electrocardiogram(ECG), electroencephalogram (EEG) and somatosensory evoked potential (SEP) were alsoinvestigated after the thoracic spine injured by bullets of three velocities (590m/s,720m/s,910m/s). The level of myelin basic protein (MBP), neuron-specific enolase (NSE), and theglial cytoplasmic protein S-100β were measured in the cerebrospinal fluid (CSF) and serum.2. Motor-function changes of armored animals were evaluated when injured on thethoracic spine. Cognitive-function was evaluated when injured by high-velocity (910m/s)bullet.3. To investigate the biomechanical characteristics of pressure wave generated by theimpact, the pressure in the spine, brain and common carotid artery was measured. Theacceleration in vicinity of target point was also measured.Results1. Damages of armors: All bullets did not penetrate the ceramic armors and causedhemispheric projecture on armors. The mean height and area of projecture were significantdifferences between the group of910m/s velocity bullets and the other groups (p <0.01).2. Skin injuries: There formed a pale area surrounded by a flush area in the skin afterinjured. The flush area in the group of910m/s was statistically different compared with theother groups (p=0.008,740m/s; p=0.000,590m/s).3. Organs injuries: There were no vertebras broken in all injured animals.The injurieswere severer in a higher velocity group.When velocity was910m/s, spine cord contusion and subarachnoid hemorrhage (22%),discrete hemorrhaged areas in the lung lobes (100%), hemorrhage in the valve in theheart(33%), and hemorrhage under the envelope of liver(33%) and kidney(50%) wereobserved. The average MAIS was2.22±0.67.(The numbers in the brackets were the ratioof injured)When the velocity was740m/s, discrete hemorrhaged areas in the lung lobes (100%),hemorrhage in the kidney (33%) were observed. The average MAIS was2.00±0.00.When the velocity was590m/s, discrete hemorrhaged areas in the lung lobes (40%)were observed. The average MAIS was1.17±0.41.Pathological investigation showed neuronal degeneration and demyelination in thespinal cord and degeneration of non-medullated nerve fiber in the hippocamp when thebullets velocity was910m/s.4. Biomarkers of central nervous system: MBP, NSE, and S-100β were highlyexpressed in serum and CSF, there was significant difference between control and injuredgroup (p <0.01) when the velocity was910m/s. However there were no significant differences compared with control group when the velocity was740m/s or590m/s5. ECG investigations:. The ECG had no significant changes in each group.6. EEG investigations: The EEG showed that low frequency band decreasedsignificantly in the right frontal lobe and parietal lobe within1min after trauma andrecovered within3-6minutes when the velocity was910m/s. The EEG had no significantchanges when the velocity was740m/s or590m/s.7. Somatosensory evoked potential (right femoral nerve): The incubation period wasnot statistically different between pre-and post-injury in each group. The wave amplitudeof post-injury decreased in each group. When bullet speed was higher, the decrease ofamplitude was more significant. There were statistical differences between the injuredgroups and control group (p=0.001,910m/s; p=0.002,740m/s; p=0.023,740m/s).There was significant difference between the group of910m/s and590m/s (p=0.004).8. Motor and cognitive function: It can cause motor and cognitive dysfunction ofanimals when the bullets velocity was910m/s. The severest motor dysfunction manifestedthat the hind limbs could not support the body and only dragged (33%). The motordysfunction would last about48h. The cognitive dysfunction manifested memorydysfunction (100%), which would last less than24h.9. Biomechanical investigations: When the bullets shot on the armor, pressure wasdetected in the spine, brain and the common carotid artery. Acceleration was also detectedon the spinous process (T10). The pressure and acceleration increased with the bulletsbecome faster. The pressure detected in the brain was higher than that detected in thecarotid artery (p <0.05). The acceleration has linear relationship with the motordysfunction.ConclusionAlthough the eighth thoracic vertebra (T8) was protected by combined armors whichconsisted of4.2mm Al2O3ceramic plate and ultra-high molecular polyethylene, the highvelocity (910m/s) bullets impact from25m distance could induce degeneration of neuronand demyelination in the spinal cord, as could lead to motor dysfunction which would lastabout48h. The injury also could cause degeneration of non-medullated nerve fiber in thehippocamp, as could lead to cognitive dysfunction which would last less than24h. If thesame BABT occurred in the soldiers, it could make them incapacitation. The acceleration was perhaps the key reason responsible for the spinal cord injuryafter BABT.