UNC and NC State researchers measure 'brain shearing,' giving new insight into concussions.
New technology shows how this brain movement can lead to concussion
November 29, 2017
A concussion can be like an earthquake...
When an earthquake hits, the sliding disturbance of those millions of tons of rock creates chaos—not only above the ground, but below. All of the instability the earthquake causes can lead to aftershocks, or extra rumblings that come as the initial shock dissipates.
Sometimes, though, that first big blast from an earthquake is just a prelude to an even more powerful quake. That first hit is called a foreshock, and it sets up the fault line for an even more drastic shift.
A severe hit to the head can act a lot like an earthquake. Concussion researchers have known for years that after a hit, the brain has some rebounds that act like aftershocks. Now researchers from UNC and NC State University’s joint biomedical engineering program have discovered that the initial hit may act like a foreshock, producing an even more powerful force inside the brain.
Impacts cause 'brain shearing'
Using an ultrasound technology of their own design, researchers were able to show that a specific type of tissue motion inside the brain can actually grow in force by almost ten times as it moves through the brain. These amplified waves could be a major contributor to concussions, especially on lower-impact hits. The research was published in Physical Review Applied.
Inside the skull, the brain is surrounded by fluid that protects it from direct impacts to the skull. The problem is when the skull comes to a sudden stop, the brain will keep moving and bump into the inside of the skull. When that happens, the brain compresses and then rebounds, like a spring. Many concussion experts study that compression and relaxation as a source of brain tissue damage, but the researchers in this study examined another type of movement in the brain: shear.
Shear occurs when an object is pulled apart along its own plane. It's like how you pull apart an Oreo: you can’t just pull the two sides straight away from each other, you have to sort of slide one cookie off of the cream filling before it comes apart.
Gianmarco Pinton, co-author of the research paper and assistant professor in the joint biomedical engineering program, studies shear stress in the brain. With the other researchers, he developed an ultrasound technology capable of measuring shear force in high-impact scenarios.
They subjected brains from previously deceased pigs to physical shocks and followed the shear forces through the brain. They found that the shear stress would dissipate entirely sometimes and other times, build to 8.5 times the force of the original hit. The unique folded geometry of the brain contributed to how much shear the tissue experienced, says Pinton.
Shear helps inform symptoms of concussion
Shear stress, Pinton says, tends to show up more prominently with lower impact hits, which may help to explain findings from other concussion research. Analysis of football hits shows that a force of 60g is more likely to cause a concussion than a 90-100g hit. The fact that shear waves are more likely to develop at 60g might help explain that disparity.
Showing how a hit affects a dead pig, however, will not be enough to clarify how shear waves work. But as the researchers perform more experiments, they can connect the physics of shear to situations in the human brain, and maybe better predict when that initial earthquake might bring something bigger.
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.