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)

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/

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

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/

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

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.

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

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

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.

wrote on 12/01/2007 10:29 +0100:

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.


I think that is a false premise. Note it says it in relation to the
presence of a skull fracture not being a good indicator of intracranial
injury as previously though. If what you proposed were true, it would
be a negative indicator i.e intracranial injuries below fracture
energies would be worse than those above. There is no evidence that
that happens. So below the critical energy the skull absorbs the energy
without fracturing and above it absorbs it prior to and during fracture.


--
Tony

"...has many omissions and contains much that is apocryphal, or at least
wildly inaccurate..."
Douglas Adams; The Hitchhiker's Guide to the Galaxy