Hypothesis paper: Mechanism for Primary Blast Induced Traumatic Brain Injury with Minimal Head Motion

Transit of the human skull by blast waves produces diffuse brain injury. The exact mechanisms are unknown. This paper describes plausible mechanisms in which steep intracranial pressure gradients, demonstrated in prior computational models of blast-skull interaction, produce subsequent deformation and motion of the whole brain within the skull, without obvious movement of the head. Equations of motion are derived to describe the acceleration, velocity, and relative position of both the skull and the brain in response to known extracranial and intracranial pressures both during and several hundred milliseconds after blast wave passage. A finite element model is solved to visualize the resulting dynamics. Whole head displacement is minimal (~ 1 mm) during primary blast wave passage. However, the brain experiences intense acceleration during the first millisecond as the blast wave passes the head and is compressed and stretched for the next 10 to 20 msec, while moving through cerebrospinal fluid toward the inner aspect of the skull, at speeds near 0.5 m/sec. Then cycles of coup and contrecoup collision and rebound occur during the next several hundred milliseconds, producing maximal compressive strains of 20 percent or more. A quantitatively realistic causal sequence, demonstrated in a companion analytical model, includes passage of the shock wave in air past the rounded skull; compression of the skull; generation of intracranial sound waves and pressure gradients; distortion followed by acceleration of the whole brain through cerebrospinal fluid; collision of the brain with the inner aspect of the skull; compressive strain wave propagation through the brain with gross deformation, and subsequent diffuse axonal injury. This physics-based sequence, emphasizing whole brain motion through cerebrospinal fluid within the skull and playing out over much longer durations than are usually modeled, provides a unifying concept relating blast exposure levels to the risk of brain injury that may inform the design of future studies