A type of brain injury that could be made worse by wearing a bike helmet
A few years ago I flew over my handlebars while cruising at about 25 mph. I landed head-first on the road and proceeded to do three cartwheels (per a witness). I was wearing a helmet at the time. It got scuffed up and cracked, but it stayed on, held together, and did its job. Unfortunately, I was knocked out cold and was looking pretty lifeless - other than a few grand mal seizures. I spent three days in a coma and ultimately I was diagnosed with Diffuse Axonal Injury or DAI. Most people with DAI are vegetables until they die. I was lucky.
I had what is called a closed-head traumatic brain injury. My skull was perfectly intact. (I'm still pretty - to quote Ali.) The damage to my brain was the result of rapid angular acceleration. In essence, my head suddenly rotated when my helmet hit the pavement. If you ever watch a plane's wheel make contact with the ground when it's landing, you get an idea how your head would experienced a rapid acceleration in a case like this.
The rotation/acceleration generated shearing forces that tore up bits of my brain. There was bleeding here there and everywhere inside my head.
I'm an engineer, so I've given this a little analysis...
The helmet effectively increased the twisting force that was spun into my brain in two ways:
1) The helmet "grabbed" the pavement better than my scalp would have. (Without the helmet, a piece of my scalp might have been ripped off. Not pretty, but it's better than a large torque causing a TBI and being a vegetable for life.)
2) The helmet increased the length of the moment arm that the friction force acted through. This resulted in a greater torque and an increased angular acceleration of my head.
My helmet resulted in a more severe twisting force being cranked into my head by the impact. I'm very confident that in my case, my helmet exacerbated my injury.
For many or most accidents, it's likely that a helmet reduces the severity of an injury. But there are exceptions.
I would love to know if any of the helmet manufacturers have considered this scenario and if they have determined how frequently this type of injury happens to crashing bike riders wearing helmets.