A new way of measuring the forces that cause head injuries could change how engineers protect professional and weekend athletes.
Most football fans have seen a player taken from the field after a shot to the head as an announcer says, “He really got his bell rung.” That makes it sound like his brain was clanging back and forth in his skull like the clapper in a bell.
That image is entirely wrong. In 1943, British physicist A. H. S. Holbourn showed that the brain is not loose enough to rattle against the skull. Instead, using math and a gelatin model of the brain, Holbourn showed that shear strains caused by rotation were the likely culprit for most concussions.
But how? While most engineers and physicians eventually came around to Holbourn’s point of view, they had a difficult time measuring rotation of the head accurately. As a result, they could not explain or model exactly how rotation damaged the brain.
Now that has changed, thanks to an innovative methodology developed by researchers at Stanford University. The team, led by bioengineering professor David Camarillo, has not only collected more accurate data, but has used it to develop models that provide critical insights into the mechanisms that cause concussions and the ways we might protect against them.
In the past, most researchers mounted sensors, mostly accelerometers, either on helmets or directly onto the skin. Unfortunately, those sensors did not couple tightly with the cranium, so the data could be off by a factor of two or more. Moreover, they only recorded linear acceleration, ignoring rotation.
Camarillo, who played football at Princeton University, realized he could overcome those limitations by embedding sensors in the athletic mouth guards used by football players, who are prone to head injuries. These protective devices, developed in his CAMLAB laboratory, fit snugly over the teeth of the upper jaw. By embedding both accelerometers and gyroscopes within the mouth guards, the laboratory tracked all six degrees of freedom and slashed data errors to 10 percent or less.
The sensors include a triaxial accelerometer and a triaxial gyroscope to measure rotation, explained Lyndia Wu, a CAMLAB Ph.D. candidate. They enable engineers to record motion in three linear and three rotational directions. They resemble those found in smartphones, but have much higher dynamic ranges (up to 100 G's for the accelerometers).
"Wearable sensors that collect inertial data can register high acceleration events on the field that are not head impacts," Wu said. So, to filter out unwanted data, like a player tossing a mouth guard on the ground or chewing on it, CAMLAB researchers added infrared position sensors, and created data-processing algorithms that discard irrelevant events.
To outfit players on the Stanford football team with the new mouth guards, the investigators first obtain a dental mold from each athlete's upper teeth. Then they use it to form a base layer of molded plastic, fasten the sensors and electronics to that layer, and cover them with a second layer of thermoplastic.
"It's more or less like a regular mouth guard, with only a little bit of added bulkiness," Wu said.
"If you make a device that's custom fit to [each athlete’s] teeth, it snaps in and holds tightly," Camarillo added.
Since the upper teeth are firmly coupled to the bones of the cranium, the mouth guards can provide data accurate enough for the lab to use in finite element models to describe what is happening inside the brain.
Holbourn originally argued that linear acceleration would create only hydrostatic pressure, rather than displacement, and would not damage the brain. This is because the brain is highly hydrated, it behaves much like a liquid and is incompressible. Moreover, it is suspended in cerebrospinal fluid, which fills all voids inside the rigid cranium that surrounds it completely, and the brain has no room to move.
"The best analogy for what is going on is a snow globe," Fidel Hernandez, a recent CAMLAB Ph.D. graduate, said. "If you move the globe in a straight line, even really quickly, the water and flakes inside won't move.”
Neither will the brain. “There's actually no evidence of that,” Hernandez said. “You don't get a lot of brain movement, because it has nowhere to go."
Yet even a slight rotation will agitate a snow globe's flakes due to the shear manifested in the fluid. In engineering terms, Hernandez said, the brain has a very high bulk modulus and a very low shear modulus; it resists compression because of the former, but changes shape readily when rotation causes shear.
Hernandez and the team needed to measure both translation and rotation to understand the mechanics of concussion. Using the instrumented mouth guards, CAMLAB collected a library of head impact events from the Stanford football team. While most produced no concussions, two did. In one case, a player lost consciousness right after impact.
Hernandez found this event differed greatly from other impacts. The player lost consciousness after rapid coronal rotation, where his head moved from near one shoulder to the other.
Hernandez input the incident's kinematics data into a finite element analysis model of the brain developed by KTH Royal Institute of Technology in Sweden. This enabled him to simulate how different structures within the brain responded to the impact, he said.
Hernandez focused on one structure, the corpus callosum, a bundle of nerve fibers connecting the brain's right and left hemispheres.
"It stands to reason that if this structure were damaged, it would produce many of the symptoms that we associate with concussion, like loss of balance and impaired depth perception," Hernandez said.
But the corpus callosum lies deep within the brain, so how would the force of an impact reach it?
According to Camarillo, most of the brain is like Jell-O, gooey and almost soupy. Yet some structures within the brain are much stiffer. This led him to hypothesize that injuries could occur at the interface between tissues having very different material properties, and that these stiffer structures could provide a pathway for energy to penetrate the brain.
Hernandez's computer simulation showed that the falx cerebri appears to be the culprit. It is a rigid vertical sheet that separates the brain's two lobes. It lies right above the corpus callosum and extends upward, attaching to the skull at the very top. It conducts impact energy from the skull deep into the brain, where it oscillates and induces strain in the corpus callosum, Hernandez said.
"We found the strain in the corpus callosum [that the injured athlete] experienced was far larger than for any of the sub-concussive impacts we had recorded," Hernandez said. He believes that this mechanism is one likely cause of chronic traumatic encephalopathy, or CTE, a debilitating illness suffered by many professional athletes.
Camarillo wants to use these findings to prevent head injuries, and his interest is personal. He not only played football at Princeton, but has been injured twice in bicycling accidents despite wearing a helmet.
"You can see some of my motivation," he said.
He argues that today's sports helmets are designed to prevent skull fractures and can save lives. Yet they clearly do not prevent mild traumatic brain injuries like the concussions that can lead to CTE and other maladies.
Although his lab has tested helmets, improved evaluation methods, and helped redefine industry standards, Camarillo's focus is on developing sensing technology.
"Getting good measurements has turned out to be much trickier than you would think," Camarillo said. But his lab has been able to make the first-ever measurements of head rotation during concussions. Now he hopes other researchers will use the lab's methodology to build a database of head injuries.
That would be a giant step toward reducing sports-related concussions. "Until we understand the mechanics of an injury, how do we know the kind of helmet we should be designing? Or if the next one is better than the last?" Camarillo said.
James G. Skakoon is an independent technical writer.
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