Even if doctors could reliably diagnose concussions, identifying the injury does little to protect against it; for that, scientists need an accurate picture of what’s happening inside the head. For generations, doctors believed that concussions were a sort of bruising of the brain’s gray matter at the site of impact and on the opposite side, where the brain presumably bounced off the skull. The reality is not nearly that simple: Concussions happen deep in the brain’s white matter when forces transmitted from a big blow strain nerve cells and their connections, the axons.
To understand how that happens, it’s important to recognize that different types of forces—linear and rotational acceleration—act on the brain in any physical trauma. Linear acceleration is exactly what it sounds like, a straight-line force that begins at the point of impact. It causes skull fracture, which makes perfect sense: You hit the bone hard enough, it breaks.
Rotational acceleration is less intuitive. It occurs most acutely during angular impacts, or those in which force is not directed at the brain’s center of gravity. You don’t have to know much about football or hockey to realize that rotation is a factor in a whole lot of hits. “Think about it,” says Robert Cantu, a neurosurgeon at Boston University School of Medicine and the author of 29 books on neurology and sports medicine. “Because most hits are off-center and because our heads are not square, most of the accelerations in the head are going to be rotational.”
Further complicating matters, the human brain is basically an irregularly shaped blob of Jell-O sitting inside a hard shell lined with ridges and cliffs. After a football tackle or a hockey check, that blob moves, and does so in irregular ways. “Rotational forces strain nerve cells and axons more than linear forces do,” Cantu says. “They’re not only stretching, but they’re twisting at the same time. So they have a potential for causing greater nerve injury.”
So what’s the problem? If scientists know that a concussion is nerve strain caused largely by rotation of the brain, why can’t they figure out a way to stop the rotation?
Just as the actual injury isn’t visible to medical imaging technology, the rotation that causes the injury isn’t measurable in impact conditions; scientists cannot be inside an athlete’s brain measuring its movement. But in a grisly 2007 study, researchers at Wayne State University in Detroit used a high-speed x-ray to observe the brains of human cadaver heads fitted with football helmets and struck from various angles. The research, corroborated by computer models, showed that the brains moved very little—just millimeters. Yet those small movements are enough to cause nerve strain and affect neurological function.
Making things even more difficult is that every brain is different. Young brains respond differently than older brains, female brains differently than male. Researchers have also found that weaker, subconcussive hits can have a cumulative effect over time and lead to CTE, which is likely the cause of many former-player suicides. But how many hits it takes, and what kind, is unclear—and the condition can’t be diagnosed while the player is alive. Only when his brain is cut open can researchers spot the dead zones in the tissue.
The scientific ambiguity surrounding concussions clearly impedes the development of better helmets. But there’s another reason helmet technology hasn’t improved, one more troubling than gaps in our knowledge: a self-regulated industry governed by badly outdated safety standards.
Picture the head of a typical crash-test dummy, the kind you see in car commercials. It’s attached to a rigid metal arm that hangs above a cylindrical anvil topped with a hard plastic disc. A lab technician straps a football helmet to the headform, cranks the arm up to precisely five feet above the anvil, and lets it drop—crack. Inside the dummy head, an accelerometer positioned at the center of gravity records the linear acceleration transmitted during impact. This brutish trial is called a vertical drop test, and it’s the basis for how all football helmets are certified safe by the National Operating Committee on Standards for Athletic Equipment (NOCSAE), an association funded by equipment manufacturers, which in turn funds much of the research on sports-related head trauma. The standard has remained largely unchanged since its creation in 1973.
Now think back to Austin Collie’s concussion in August 2012—the jerking of the head after the initial hit, the collisions with Larry Foote’s elbow and the ground. Those impacts don’t look much like the straight-line force of the NOCSAE drop test. And that brings up a very important question, perhaps the central question scientists and helmet makers are trying to solve today: Is the linear acceleration measured by a drop test correlated to rotational acceleration, and if so, by how much?single page
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