|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
Whether or not you wear a helmet you may suffer concussion in an
accident. There is an interesting discussion of the mechanism in the current issue of the New England Journal of Medicine: http://content.nejm.org/cgi/content/.../166?query=TOC (free full text available). "The brief loss of consciousness that characterizes concussion appears to be the result of rotational forces exerted at the junction of the upper midbrain and thalamus that cause transient disruption of the functioning of the reticular neurons that maintain alertness." There a possibility that wearing a helmet might increase rotational forces, in which case concussion might be more likely. The article includes a discussion of concussion during athletics; from this it seems unlikely that a single episode would have prolonged effects as a rule but repeated episodes, as in boxing, are more problematic. -- Anthony Campbell - Microsoft-free zone - Using Linux Gnu-Debian http://www.acampbell.org.uk (blog, book reviews, on-line books and sceptical articles) |
Ads |
#2
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
On 11 Jan 2007 14:19:21 GMT,
Anthony Campbell wrote: Whether or not you wear a helmet you may suffer concussion in an accident. There is an interesting discussion of the mechanism in the current issue of the New England Journal of Medicine: http://content.nejm.org/cgi/content/.../166?query=TOC (free full text available). "The brief loss of consciousness that characterizes concussion appears to be the result of rotational forces exerted at the junction of the upper midbrain and thalamus that cause transient disruption of the functioning of the reticular neurons that maintain alertness." There a possibility that wearing a helmet might increase rotational forces, in which case concussion might be more likely. The article includes a discussion of concussion during athletics; from this it seems unlikely that a single episode would have prolonged effects as a rule but repeated episodes, as in boxing, are more problematic. This is rather interesting: In the past, the presence of a skull fracture was considered^ a marker of intracranial injury,^10 but it has since been recognized^ that a simple fracture actually dissipates much of the energy^ of an impact and is not a strong indicator of intracranial bleeding. I know there are some people here with knowledge of materials science; would a helmet - that presumably changes a sudden impact into something where the forces increases (slightly) more slowly - reduce the chance of a skull fracture and hence increase the energy transferred to the brain? I'd assume that evolution would give us a skull no stronger than needed - so the brain ought to reasonably be expected to survive straightforward impacts up to the point the skull fractures. Given that the skull itself absorbs a lot of energy when it fractures it would make most sense if evolution had actually made the skull less strong than fracturing at the point you are dead anyway - so say if the brain can survive 1000J and the skull can dissipate 500J then a sensible fracture point would be around 800J giving a survivable range of 0-1500J. (These are made up figures - I've got absolutely no idea what real figures would be) Tim. -- God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t," and there was light. http://tjw.hn.org/ http://www.locofungus.btinternet.co.uk/ |
#3
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
Tim Woodall wrote on 11/01/2007 21:38 +0100:
I know there are some people here with knowledge of materials science; would a helmet - that presumably changes a sudden impact into something where the forces increases (slightly) more slowly - reduce the chance of a skull fracture and hence increase the energy transferred to the brain? Cycle helmets are designed for impacts of 100J or less. Skulls fracture at 700J or more. Thus a helmet makes a minimal difference in a skull fracture accident. -- Tony "...has many omissions and contains much that is apocryphal, or at least wildly inaccurate..." Douglas Adams; The Hitchhiker's Guide to the Galaxy |
#4
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
On Thu, 11 Jan 2007 22:12:16 +0000,
Tony Raven wrote: Tim Woodall wrote on 11/01/2007 21:38 +0100: I know there are some people here with knowledge of materials science; would a helmet - that presumably changes a sudden impact into something where the forces increases (slightly) more slowly - reduce the chance of a skull fracture and hence increase the energy transferred to the brain? Cycle helmets are designed for impacts of 100J or less. Skulls fracture at 700J or more. Thus a helmet makes a minimal difference in a skull fracture accident. That's completely missed my point which was that a skull fracture actually dissipates a lot of energy (the article I quoted said "a simple fracture actually dissipates much of the energy of an impact") My question was whether changing the impact from one where the forces go to maximum almost instantaneously to one where the forces increase over a few milliseconds and be spread out over a larger area of the head might actually reduce the risk of a skull fracture and so increase the energy transferred to the brain. If a fracturing skull can dissipate more energy than a crushing helmet then this could well be a net loss for helmet wearing for all cases except the trivial "prevented a bruise" Tim. -- God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t," and there was light. http://tjw.hn.org/ http://www.locofungus.btinternet.co.uk/ |
#5
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
Tim Woodall wrote on 11/01/2007 23:42 +0100:
On Thu, 11 Jan 2007 22:12:16 +0000, Tony Raven wrote: Tim Woodall wrote on 11/01/2007 21:38 +0100: I know there are some people here with knowledge of materials science; would a helmet - that presumably changes a sudden impact into something where the forces increases (slightly) more slowly - reduce the chance of a skull fracture and hence increase the energy transferred to the brain? Cycle helmets are designed for impacts of 100J or less. Skulls fracture at 700J or more. Thus a helmet makes a minimal difference in a skull fracture accident. That's completely missed my point which was that a skull fracture actually dissipates a lot of energy (the article I quoted said "a simple fracture actually dissipates much of the energy of an impact") My question was whether changing the impact from one where the forces go to maximum almost instantaneously to one where the forces increase over a few milliseconds and be spread out over a larger area of the head might actually reduce the risk of a skull fracture and so increase the energy transferred to the brain. If a fracturing skull can dissipate more energy than a crushing helmet then this could well be a net loss for helmet wearing for all cases except the trivial "prevented a bruise" It does answer your question. The effect of the polystyrene is minimal in relation to the energy needed to fracture the skull. Its like asking whether being shot is less dangerous if you are wearing a t-shirt to slow down the bullet. -- Tony "...has many omissions and contains much that is apocryphal, or at least wildly inaccurate..." Douglas Adams; The Hitchhiker's Guide to the Galaxy |
#6
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
Tony Raven wrote: It does answer your question. The effect of the polystyrene is minimal in relation to the energy needed to fracture the skull. Its like asking whether being shot is less dangerous if you are wearing a t-shirt to slow down the bullet. It doesn't answer my question. (Maybe it's completely obvious to a materials scientist but it's not to me) I would expect that a brittle material (and I'm assuming that bone counts as brittle) is more likely to fracture with a sudden shock than with a gradually increasing force. So I would expect to be able to fracture a (small) cast iron bar held between two supports with a hammer but put a small piece of polystyrene on the face of the hammer and, despite the polystyrene only absorbing a tiny amount of the energy in the hammer the bar not to fracture for the same energy input. (I'll accept that I could be completely wrong here - but this is my gut feeling) Taking this to the skull; an impact of 1000J is _meant_ to fracture the skull so as to absorb a lot of the energy and leave the brain only absorbing a small fraction of that. Putting a helmet on means that, instead of say 500J being absorbed by the skull and 500J by the brain, 100J is absorbed by the helmet and 900J by the brain. Tim. |
#7
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
Tony Raven wrote:
Tim Woodall wrote on 11/01/2007 23:42 +0100: On Thu, 11 Jan 2007 22:12:16 +0000, Tony Raven wrote: Tim Woodall wrote on 11/01/2007 21:38 +0100: I know there are some people here with knowledge of materials science; would a helmet - that presumably changes a sudden impact into something where the forces increases (slightly) more slowly - reduce the chance of a skull fracture and hence increase the energy transferred to the brain? Cycle helmets are designed for impacts of 100J or less. Skulls fracture at 700J or more. Thus a helmet makes a minimal difference in a skull fracture accident. That's completely missed my point which was that a skull fracture actually dissipates a lot of energy (the article I quoted said "a simple fracture actually dissipates much of the energy of an impact") My question was whether changing the impact from one where the forces go to maximum almost instantaneously to one where the forces increase over a few milliseconds and be spread out over a larger area of the head might actually reduce the risk of a skull fracture and so increase the energy transferred to the brain. If a fracturing skull can dissipate more energy than a crushing helmet then this could well be a net loss for helmet wearing for all cases except the trivial "prevented a bruise" It does answer your question. The effect of the polystyrene is minimal in relation to the energy needed to fracture the skull. Its like asking whether being shot is less dangerous if you are wearing a t-shirt to slow down the bullet. No it doesn't answer Tim's question. Think of a soft foam h***** that absorbs no energy at all, but slows down the impact. Instead of 700J being imparted to the skull in 1 millisecond, it now takes 50 milliseconds. The overall energy is the same, but the rate of application is different. In the first case, you might get a skull fracture that dissipates a fair fraction of the energy. In the second, there is no fracture, and all the energy goes into the brain instead. Personally, I'm not convinced by the argument. I think it unlikely that enough energy will be dissipated by fracturing the skull to make up for the huge differential in g forces on the brain in the two cases above. TL |
#8
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
The Luggage wrote on 12/01/2007 09:23 +0100:
No it doesn't answer Tim's question. Think of a soft foam h***** that absorbs no energy at all, but slows down the impact. Instead of 700J being imparted to the skull in 1 millisecond, it now takes 50 milliseconds. The overall energy is the same, but the rate of application is different. Work it out. 700J is approx 30mph is approx 500 inches/s. So to compress 1" of foam will take approx 2ms -- Tony "...has many omissions and contains much that is apocryphal, or at least wildly inaccurate..." Douglas Adams; The Hitchhiker's Guide to the Galaxy |
#9
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
Tony Raven wrote: The Luggage wrote on 12/01/2007 09:23 +0100: No it doesn't answer Tim's question. Think of a soft foam h***** that absorbs no energy at all, but slows down the impact. Instead of 700J being imparted to the skull in 1 millisecond, it now takes 50 milliseconds. The overall energy is the same, but the rate of application is different. Work it out. 700J is approx 30mph is approx 500 inches/s. So to compress 1" of foam will take approx 2ms But I would also expect the impact to be spread out over a larger area of the skull. Note that my initial premise is that the skull is _meant_ to fracture at the high end of survivable impacts. My hypothesis is that a helmet might prevent these fractures without absorbing sufficient energy to save a live - i.e. a helmet means death+no fracture instead of life+fracture. Tim. |
#10
|
|||
|
|||
Concussion mechanism, possibly relevant to helmets
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Certainly, not possibly; was Concussion mechanism, possibly relevant to helmets | [email protected] | UK | 0 | January 11th 07 02:51 PM |
relevant importance of variables when choosing a new bike | Mark W | UK | 29 | September 10th 06 04:56 PM |
Is this relevant to some of our odder posters? | Bill C | Racing | 8 | June 25th 06 07:58 PM |
van Moorsel Concussion? | B. Lafferty | Racing | 3 | August 19th 04 07:32 AM |