Concussion mechanism, possibly relevant to helmets

On 11 Jan 2007 14:19:21 GMT, Anthony Campbell <[email hidden]>

Quoted message said:

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/full/356/2/166?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.

Very worrying - think of the childen; in some places they must wear
these things by law!

On 11 Jan 2007 14:19:21 GMT,

Anthony Campbell said:

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/full/356/2/166?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:

Quoted message said:

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 said:

Tim Woodall wrote on 11/01/2007 21:38 +0100:

Quoted message said:

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:

Quoted message said:

On Thu, 11 Jan 2007 22:12:16 +0000,

Tony Raven said:

Tim Woodall wrote on 11/01/2007 21:38 +0100:

Quoted message said:

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 said:

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 said:

Tim Woodall wrote on 11/01/2007 23:42 +0100:

Quoted message said:

On Thu, 11 Jan 2007 22:12:16 +0000,

Tony Raven said:

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:

Quoted message said:

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 said:

The Luggage wrote on 12/01/2007 09:23 +0100:

Quoted message said:

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.

[email hidden] wrote on 12/01/2007 10:29 +0100:

Quoted message said:

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

Tony Raven said:

[email hidden] wrote on 12/01/2007 10:29 +0100:

Quoted message said:

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.

Why would I expect intracranial injuries below fracture to be worse
than above?

Evolution will dictate that the skull has been optimized for strength,
energy absorption, weight.

Assuming that intercranial bleeding is always bad I would expect that
the skull will fracture before intercranial bleeding becomes a
significant concern - hence why there is poor correlation between a
skull fracture and intercranial bleeding.

There's no benefit in the skull being too weak but there's also little
benefit in the skull being too strong. Although there might be some
impacts that are survivable with a stronger skull, there will be more
impacts that are now not survivable because the skull doesn't fracture
and there is now more damage to the brain. I would also expect that the
frequency of impact/energy of impact exhibits something like
exponential decay. I must have bumped my head hundreds of times in my
life (maybe thousands). I've had a few that have "really hurt". I've
never been knocked out but I've seen one person knock themselves out
momentarily

What a helmet is doing is shifting this evolutionarily selected balance
point. And for the majority of the time, cyclists are only at speeds
that humans have been encountering throughout time.

I still think risk compensation is the most likely mechanism for the
increased head injury rate for helmet wearers. But this is another
possible mechanism I've never seen mentioned before.

Tim.

Flying a kite here..

If the skull can be considered to either fracture inelastically or not
fracture elastically (I've probably misused the terms but I mean that
if it fractures it will absorb energy, if it doesn't, it won't) then if
a helemt reduces the energy of impact from (taking figures almost from
the air) 750J to 650J, one would be looking at 650J being transferred
to the brain, instead of 750-700=50J plus a cracked skull.

Obviously this effect would only occur in a small number of cases
(those where the impact is between skull cracking and skull cracking
plus helmet squashing).

...d.

"[email hidden]" <[email hidden]>typed

Quoted message said:

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.

Wrong assumption IMO.

There is much fibrous tissue in bone, which bends A LOT before it
breaks. I don't think sudden nature of an impact would hugely increase
fracture probability.

--
Helen D. Vecht: [email hidden]
Edgware.

"[email hidden]" <[email hidden]>typed

Quoted message said:

Assuming that intercranial bleeding is always bad I would expect that
the skull will fracture before intercranial bleeding becomes a
significant concern - hence why there is poor correlation between a
skull fracture and intercranial bleeding.

Wrong again.

Subdural bleeds frequently occur in an unfractured skull.

Bridging veins between the skull and brain can tear/puncture with
minimal deceleration. This is more likely when the brain has shrunk due
to age or alcohol.

--
Helen D. Vecht: [email hidden]
Edgware.

Tony Raven said:

The Luggage wrote on 12/01/2007 09:23 +0100:

Quoted message said:

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

OK so I guessed at the wrong numbers for the time scale. It was just an
example. It doesn't even need the time scale numbers in it:
Consider either an unprotected head with 2mm of skin, or one with a 1"
foam hat. Both coverings act as perfect springs, ie absorbs no energy,
and brings the head to zero velocity just at full compression. The full
amount of kinetic energy is still transferred to the head. The
decelerations could be up to a factor of 13 different. I think that
Tim's point was that the unprotected skull might fracture and absorb
some of the 700J, reducing injury to the brain. The 'protected' skull
has a lower deceleration, possibly spread over a larger area,
preventing fracture. The full 700J is then transferred to the brain,
possibly resulting in more injury than if the skull had fractured.

I still don't think it's a viable mechanism for reducing the energy
imparted to the brain. Although we've had statements that it takes 700J
to produce a skull fracture, I'm not sure that this means that a skull
fracture will reduce a sufficiently large impact by 700J.

TL

If this

Helen Deborah Vecht said:

"[email hidden]" <[email hidden]>typed

Quoted message said:

Assuming that intercranial bleeding is always bad I would expect that
the skull will fracture before intercranial bleeding becomes a
significant concern - hence why there is poor correlation between a
skull fracture and intercranial bleeding.

Wrong again.

Subdural bleeds frequently occur in an unfractured skull.

Bridging veins between the skull and brain can tear/puncture with
minimal deceleration. This is more likely when the brain has shrunk due
to age or alcohol.

I'd assume that that was more a feature of rotational forces which
neither a skull nor a helmet can help protect the brain against.

The sorts of impacts that will lead to a simple skull fracture are also
likely to be the sorts of impacts that don't cause significant damage
to the brain. (of course there may also be a rotational force)

At very high accelerations the brain can crush itself against the
inside of the skull - but we're talking detached retinas, eyes full of
blood sorts of accelerations.

Dr Stapp personally demonstrated that a human can survive 45g with a
rate of onset of 500g/s in the forward position when appropriately
restrained (the highest known g force voluntarily endured)

"But could he see? George Nichols wasn't sure, and what he vividly
remembers from that day, fifty years later, were John Stapp's eyes. He
had suffered a complete red out. "When I got up to the sled I saw his
eyes... Just horrible," recalls Nichols, his voice cracking with
emotion. "His eyes ...were completely filled with blood." When the
Sonic Wind had hit the water brake, it had produced 46.2 Gs of force.
And for an astonishing 1.1 seconds, Stapp'd endured 25 Gs. It was the
equivalent of a Mach 1.6 ejection at 40,000 feet, a jolt in excess of
that experienced by a driver who crashes into a red brick wall at over
120 miles per hour. Only it had lasted perhaps nine times longer. And
it had burst nearly every capillary in Stapp's eyeballs.

As George Nichols and some flight surgeons helped Stapp into a waiting
stretcher, Stapp worried aloud that he'd pushed his luck too far. "This
time," he remarked, "I get the white cane and the seeing eye dog." But
when surgeons at the hospital examined him, they discovered that
Stapp's retinas had not detached. And within minutes, he could make out
some "blue specks" and a short time later he could discern one of the
surgeons' fingers. By the next day, his vision had returned more or
less to normal."

According to some googling I've done, the skull will fracture at
between about 30g at the nose to 100-200g for a one square inch impact
on the frontal bone.

Tim.

Helen Deborah Vecht said:

"[email hidden]" <[email hidden]>typed

Quoted message said:

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.

Wrong assumption IMO.

There is much fibrous tissue in bone, which bends A LOT before it
breaks. I don't think sudden nature of an impact would hugely increase
fracture probability.

Fair enough. And thinking about it the skull has to undergo deformation
before fracturing or it won't absorb much energy.

Tim.

[email hidden] wrote on 12/01/2007 13:41 +0100:

Quoted message said:

Helen Deborah Vecht said:

I'd assume that that was more a feature of rotational forces which
neither a skull nor a helmet can help protect the brain against.

Helen's the Doctor with experience here - I would give a lot of credence
to what she says.

--
Tony

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

[email hidden] wrote on 12/01/2007 11:33 +0100:

Quoted message said:

Why would I expect intracranial injuries below fracture to be worse
than above?

Let's take an extreme example of you presumption. Intracranial injuries
occur with and without fracture - it's how to diagnose the latter that
the paper is about. So at 699J impact, the skull is intact and
according to your model 699J goes through to the brain. At 701J, the
skull fractures and only 50J say goes through to the brain. One would
expect therefore to see an inverse correlation of intracranial injury
with fracture (no helmets involved at all).

--
Tony

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