Medical Blunt Force Trauma

  1. I'm doing a little research on road safety for motorcycles and wasn't really sure if this was a medical question or a physics question. What speed would be fatal if a motorcyclist had a head-on collision with say a truck(the rider has neglected to wear a helmet) and assuming that he goes head first into the truck ( does it matter which side of the head takes the greatest impact) and what's the amount of force the human skull can withstand?

    Edit : Would landing on the back of your head after the impact assure certain death as the cerebellum and medulla oblongata are located there and would impact on the front increase the chances of paralysis rather than immediate death
    Last edited: Jan 31, 2012
  2. jcsd
  3. Ryan_m_b

    Staff: Mentor

    I'm not sure of the exact force needed to crack a human skull (googling brings up a variety of results) however people can have fatal head injuries just from falling over onto a hard surface. A head-on collision at even low speeds could probably be fatal.
  4. rhody

    rhody 710
    Gold Member


    I know of a situation involving a bicycle rider, not wearing a helmet, was almost stopped and the front wheel hooked on his bike and he fell headfirst into a pile of rocks off the side of the road (couldn't get his feet out of his toe clips fast enough, this has happened to me, but I wear a helmet), the total distance from to the point of impact couldn't have been more than 6 feet tops and speed less than 5 mph, he suffered brain damage and is partially paralyzed from the impact. I am sure the shape of the rock and the impact point or points to his skull had a lot to do with the severity of his injury.

  5. @Ryan : Thanks for the reply...i had the exact same problem googling for a exact number and i got answers varying from 15 psi - 200 psi which is weird

    @rhody : Thanks a lot...this was helpful!
  6. Moonbear

    Moonbear 11,955
    Staff Emeritus
    Science Advisor
    Gold Member

    With head injuries due to impact, you do not need to fracture the skull to injure the brain, and, yes, direction of impact matters. Think about your brain as floating in a thin layer of fluid contained in a sac (the meninges) with spots attached to the skull. There are also little veins running from the surface of the meninges through tiny holes in the skull (emissary veins). If you are moving forward, and the skull is abruptly stopped (impact), the brain sloshes forward. There is a tent of meninges in front of the brainstem that it smacks into in that type of collision, and can be very abruptly fatal since that part of the brain controls vital functions. Less severe injuries can rip those little veins and cause bleeding and compression of the brain. But, just the sloshing about (think about how a big bowl of jello wiggles) can cause more diffuse brain injury too (much of the severity of a gunshot wound to the head is due to the ripple-effect of the wave generated as the bullet passes through rather than the hole it makes, per se). If you get injury, you get inflammation and swelling. The brain swelling and squishing through holes it's not supposed to go through results in a lot of the slower reactions to brain injury. This too can be fatal.

    As for skull fractures, the skull is not uniform thickness. A lateral impact to the temple is more likely to result in a fracture than a frontal impact. There's a major artery running along the temporal bone, and much the way glass breaks most easily where you score it, the skull breaks most easily along the groove where this artery runs, and then cuts or shears the artery. In other places, a fracture alone is not so serious as long as it isn't compressed pushing the bone into the brain.

    Hitting the very back of your head doesn't hit the brainstem...that's tucked further under and usually well protected...except from that tent in front of it that is supposed to protect the brain except in sudden collisions. The part of the brain that perceives vision is all the way in the back, so hitting the back of your head might blind you, or at least make you see stars (a visual hallucination), but won't kill you unless a blood vessel ruptures.

    So, probably the most important part of a helmet isn't the hard shell that prevents fractures and general scraping along asphalt, but more likely, the padding inside that cushions the impact. On the other hand, in talking with a first responder on the issue, her impression (anecdotal, but from many incidents) is that if you hit head-first into pavement or another vehicle, it really won't matter. They just call the helmet a brain bucket and usually the victim is DOA. Where it's likely more useful is if another part of the body strikes first and absorbs a lot of the impact, such as a shoulder, and the head hits pavement secondarily, scraping along, or you tumble and roll before your head strikes anything.

    According to neurologists I've discussed this with, the most important aspect of a helmet is reducing the severity of the injury you will survive. Keep in mind, they only see patients who survived the accident and made it to them. So taken together, I'd interpret it that a helmet might not help you survive a motorcycle accident, but if you do survive it, you're less likely to be a vegetable and to make a more complete recovery.
  7. bobze

    bobze 651
    Science Advisor
    Gold Member

    In deed Moonbear, I don't think that is anecdotal at all. Like you pointed your brain is "floating"--A helmet does nothing much to help reduce the severity of head impact and the momentum of your brain moving forward in your cranial vault. I suppose in theory someone could design a large helmet designed to crumple in a way that the front of the car does, extending your stopping distance--But I don't think logistically that would work, it would be bulky and probably be more dangerous with it on.
  8. berkeman

    Staff: Mentor

    The crumple zone concept is indeed used in helmet design. The foam liner is designed to deform permanently on impact. Once a helmet has gone through an accident, it is meant to be discarded for a new one.
  9. bobze

    bobze 651
    Science Advisor
    Gold Member

    Nice. Do you know how much give helmets are supposed to have?
  10. berkeman

    Staff: Mentor

    That's a good question. It's not a lot, but I'm guessing the deformable foam padding thickness is somewhere between 1cm and 1inch. It looks like it's according to a standard "Federal Motor Vehicle Safety Standard No. 218" linked to about half-way down this page:

    I'll try to follow the link tomorrow if I have time.
  11. bobze

    bobze 651
    Science Advisor
    Gold Member

    Sounds like something interesting to look into. That doesn't sound like a lot of "crumple" space. I suspect (like I originally said, and Moonbears anecdote says) at 60 mph that doesn't provide nearly enough slower deceleration to save your noggin. Especially considering that you get people in the ER from car wrecks where they have those couple of feet the car can crumple, yet they still tear their bridging veins (those funny little veins Moonbear was talking about)--Even though they didn't actually bang their noggin on anything!
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