Posted: 22 Nov 2010 02:26 PM PST
Adding a face shield to the standard-issue helmet worn by U.S. troops could help protect soldiers from traumatic brain injury, the signature wound of the recent wars in Iraq and Afghanistan. A new study that models how shock waves pass through the head finds that adding a face guard deflects a substantial portion of the blast that otherwise would steamroll its way through the brain.
The study, to appear in the Proceedings of the National Academy of Sciences, is part of a spate of new work tackling traumatic brain injury. An estimated 1.5 million Americans sustain mild traumatic brain injury each year, and nearly 200,000 service members have been diagnosed with it since 2000, according to the Armed Forces Health Surveillance Center in Silver Spring, Maryland.
While direct impact, such as banging the head, clearly can injure the brain, the forces endured when explosives send shock waves crashing through the head are much more difficult to characterize.
In the new study, researchers led by Raúl Radovitzky of MIT's Institute for Soldier Nanotechnologies created an elaborate computer model of a human head that included layers of fat and skin, the skull, and different kinds of brain tissue. The team modeled the shock wave from an explosion detonated right in front of the face under three conditions: with the head bare, protected by the currently used combat helmet and covered with the helmet plus a polycarbonate face shield.
The results showed that today's helmet doesn't exacerbate the damage, as some previous research had suggested. But at least in terms of blast protection, the current helmet doesn't help much either. Addition of a face shield would improve matters, the team reports.
"The face shield contributes a lot to deflecting energy from the blast wave and not letting it directly touch the soft tissue," says Radovitzky. "We're not saying this is the best design for a face shield, but we're saying we need to cover the face."
To validate the model, researchers at MIT and elsewhere will have to conduct experiments in the real world. But the work points to an intrinsic flaw in the current helmets.
"These helmets weren't designed to stop a pressure wave; they were designed to stop bullets," says Albert King, director of the Bioengineering Center at Wayne State University in Detroit. "Just like a football helmet wasn't designed to stop a concussion, but to stop skull fracture."
Designing a blast-resistant helmet requires a better knowledge of what happens in the brain when an explosion washes over it. Soldiers experiencing explosions often describe a wind or wave that makes them see stars. "I really got my bell rung," is a common report.
The resulting "mild" traumatic brain injury doesn't lead to long-term loss of consciousness, and brain scans yield normal results. But labeling these injuries as mild is a misnomer, says Douglas Smith, director of the Center for Brain Injury and Repair at the University of Pennsylvania in Philadelphia.
"It is not mild; that term has led people astray," says Smith. "It is something very serious that can lead to severe dysfunction."
Smith and his colleagues have been working on a sensor that could be placed in a helmet or vehicle and that, like the radiation badges worn by nuclear-plant workers, would indicate exposure to blast forces likely to cause brain injury. The sensor is described in a paper to be published in NeuroImage.
While a sensor would indicate exposure to blast forces, it still isn't clear exactly how that energy translates into brain trauma. Under everyday conditions, the brain can easily withstand a little jostling. "Plop down in your chair and your brain blobs around like Jell-O," Smith says. But at tremendously high speeds, instead of gently stretching, brain cells can snap and break (SN: 3/13/10, p. 11) like glass.
The long-term effects of these busted brain cells are largely unknown. In addition to chronic headaches, vertigo and difficulty remembering words, research suggests that when the brain shuts down for even a few minutes, depression is more likely down the road.
Scott Matthews, a psychiatrist at the University of California, San Diego, who studies mild traumatic brain injury in returning veterans, notes that causality can't be established. But among soldiers who were exposed to combat, he sees depression twice as often in people with traumatic brain injury.
"There's more and more evidence that loss of consciousness changes the brain," Matthews says.
Unraveling cause and effect and designing useful experiments to illuminate traumatic brain injury and its aftermath remains extremely challenging. And translating those scientific findings into meaningful policy can be just as difficult. Even implementing something as simple as a helmet with a face shield poses problems, says Smith.
"How do you deploy something like that?" he asks. "There are practical things like temperature issues. And then there's wanting soldiers to be able to meet and greet in villages without looking like spacemen."
Image: Computer model of the head./Michelle Nyein.
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