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How could explosions cause brain injuries without piercing the skull?

PORTLAND, OREGON (May 20, 2009) -- It is easy to understand how explosions involving shrapnel – such as those caused by improvised explosive devices in Iraq – could cause brain damage. But what about such injuries that seem to be caused by blasts themselves, rather than from being thrown or hit by shrapnel?

Researchers have a few ideas, but one scientist has used some of the world’s most powerful computers at the Lawrence Livermore National Laboratory in California to get a better answer. Willy Moss and colleague Michael King used available data on blast waves from explosions and the physical properties of the human skull, brain and cerebrospinal fluid to craft a three-dimensional simulation of a soldier standing less than 15 feet from an explosion of 5 lbs. of C4. (See image to the right.)

“It sweeps over. There’s lots of oscillation. The skull is ringing. It’s not pleasant,” Moss told the audience at the meeting of the Acoustical Society of America here.

Moss says their simulations suggest that the intense pressures of such blasts flex the skull and ripple the brain. Pressures as little as one atmosphere over normal atmospheric pressure can do that kind of damage.

They repeated the simulation to include helmets, first using data from an older style that uses webbing to create space around a soldier’s head. (See video below.) “What you see is the blast sweeps under the helmet. It acts as a wind scoop; it focuses the blast. The blast pressure is bigger between your head and the helmet than if you weren’t wearing the helmet at all.”

But Moss wasn’t saying the helmets were a bad idea: “Now, I’m not saying don’t wear the helmet, because you’ve still got fragments and other stuff coming.”

Newer helmets use pads instead of webbing, but that creates a different problem. While the pads feel soft, they are actually stiff when hit by the sudden shockwave from a blast. “The blast wave comes over. The helmet flexes. The pads are stiff because you are shock-loading them; and that flexure gets transmitted to your skull.”

This research is supported by a Defense Department grant. Moss says he still needs to test his theory in the field, so his team is developing small, cheap sensors that could document when soldiers are hit by blast waves above a certain threshold.

Shockwaves can also do good. Frederic Padilla presented work that he and colleagues, including Robin Cleveland at Boston University, have done to understand how sound blasts help some bone fractures heal. “The biological response of tissue to shockwave has been well-documented, but the physics, for the mechanism applied in this response of the bone tissue, has not been studied yet,” Padilla says.

The researchers used simulations and experiments with animal bones to document how shockwaves and cavitation -- the implosion of tiny bubbles -- stress bone fractures. The work, says Padilla, may help engineers design devices that would optimize the way shocks can stimulate beneficial bone growth.

Perhaps surprisingly, the idea for Moss’s project came one morning while he was having breakfast with his wife, who has a PhD in neuroanatomy. “There was an article in the newspaper on the war in Iraq and closed-head injuries. She looks at me and says, ‘You can simulate this, can’t you?’ I thought about it and said, ‘Yeah, I think we can.’”

See our previous post with news from the Acoustical Society of America meeting.

Diagram and video courtesy of Willy Moss, Lawrence Livermore National Laboratory (click on diagram to enlarge)

 
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