SAN ANTONIO — In a basement lab at Fort Sam Houston, researchers have been unleashing massive shock waves strong enough to bounce a postmortem pig eye out of the artificial socket and gelatin in which it had been sitting.
Macabre as this experiment may be, exposing pig eyes to shock waves equivalent to the blasts from improvised explosive devices allowed scientists from the University of Texas at San Antonio and other area institutions to discover that the concussive force of an IED's blast alone — without any shrapnel or particles — is powerful enough to cause lasting damage to soldiers' vision.
The discovery could help doctors more quickly catch and treat related injuries and help provide clues to developing better protective gear, researchers said.
“The big thing that we found, that no one had seen before, is that the shockwave itself is sufficient to cause significant damage to the eye,” said Matthew Reilly, a UTSA assistant professor of biomedical engineering. “The current military eyewear that they use to protect soldiers is designed based on particle impact rather than dealing with the blast wave.”
Ocular injuries rank fourth on the list of most common military deployment-related injuries and account for 13 percent of battlefield injuries, according to the study by Reilly and others, which was published this year in the journal Investigative Ophthalmology and Visual Science.
With about $1 million in funding from the U.S. Department of Defense, Reilly and other researchers, including ophthalmologist Dr. William Sponsel and those at the U.S. Army Institute of Surgical Research, used a shock tube — essentially a giant air cannon — to replicate an IED blast “without blowing up the lab” and explore its effects on the eye, Reilly said.
In a soundproofed room, the scientists built up pressure by pumping air behind a flat aluminum plate placed in a large tube. Once the pressure reaches a critical point, it will rupture the aluminum plate and shoot a shock wave of air down the tube. Researchers placed the pig eyes in its path, along with pressure sensors and visual recording equipment, to analyze the impact.
The pig eyes, procured from a slaughterhouse, provide a substitute for the human eye with similar anatomy, Reilly said.
Examining the eyeball before and after allowed researchers to survey the damage, said Sponsel, a UTSA adjunct professor of biomedical engineering. Sponsel said he had developed similar techniques when researching paintball injuries to the eye.
One theory on why there are so many injuries to the eye is that “body armor is really good for the rest of the body, but armor for the eye is not,” said Reilly, describing the current eyewear as similar to sunglasses or goggles. “The eye is also a relatively soft organ. It doesn't take a lot of force to damage it.”
The natural defense against eye damage is to blink, covering the eye with the eyelid. But Reilly said explosions can occur so quickly that a soldier would not have time to blink. The shock wave could cause detachment of the retina or damage to the optic nerve. The retina is “the light-sensitive layer of tissue at the back of the inner eye” and the optic nerve “carries the information of vision from the eye to the brain,” according to the U.S. National Library of Medicine.
Either of those injuries could result in “decreased vision or blindness” depending on the severity, Reilly said.
He said the eyewear now used by soldiers was not “designed to protect against the wave, so they've never really been tested in that regard.” Soldiers also have complained that the protective gear impeded their peripheral vision, which might also pose a safety threat, Reilly said.
The new findings could give military protective gear designers something to consider, he said.
Since World War II, the percentage of eye injuries among those that wounded soldiers have survived has increased, Sponsel said. He said the statistics reflect “the fact that people are surviving bad problems and are more likely to come home with eye injury as well.” But they also reflect the nastiness of IEDs, which “will do horrible things to eyes if the eyes are exposed,” he said.
Sponsel said the research will be “incredibly useful” since these types of injuries are likely to “be part of every blast injury.”