Biomechanical mechanism of blunt spinal cord trauma after rifle bullets hitting back of body armor

Objective To investigate the transmission pattern of the pressure wave generated by rifle bullets impacting the back of the body armor and the relationship of spinal injury with pressure and acceleration generated by the bullets with different velocities. Methods Twenty male pigs were randomly divided into high velocity group, middle velocity group, low velocity group and control group, with 5 pigs per group. Behind armor blunt trauma animal models were applied. Degree of spinal cord injury was evaluated by the variation of the muscle strength. Another 5 male pigs were used for the examination of pressure of common carotid artery, intracranial pressure and acceleration at T10 vertebrae. The results were analyzed through regression. Results Bilateral hindlimb score was 5 points in 3 pigs and 7 points in 7 pigs in high velocity group. Except for 1 pig of 8 points in bilateral hindlimb score, the remained presented normal motor function in middle velocity group. There was no motor dysfunction in low velocity and control groups. High, middle and low velocity groups showed intracranial pressure of (98.7±36.8)kPa, (41.1±7.8)kPa, and (38.9±8.7)kPa respectively (P<0.05) and acceleration at T10 of (3 853.4±806.0)g, (2 501.3±226.9)g, and (1 949.0±118.4)g respectively(P<0.01). Logistic analysis revealed positive correlation of pressure with acceleration (R2=0.85, P<0.01), positive correlation of acceleration with pressure at spinal cord (R2=0.56, P<0.01), positive correlation of velocity with acceleration at spinal cord (R2=0.70, P<0.01), and linear correlation of injury severity with acceleration (F=137.05, R2=0.94, P<0.01). Conclusion Rifle bullet impacting the back of body armor generates pressure wave and acceleration which leads to blunt trauma of the spinal cord. Key words: Spinal cord injuries; Firearms; Protective clothing