Interesting stuff. A few other ideas:
1. Being hit on the head transmits the force down the whole of the cervical
spine (spine fairly uniformly straight), whereas bending it in a fall you
have it already bent when force is being applied (and nobody's neck bends
regularly) so there are stiff segments acting as stress risers (like the way
asparagus snaps at the junction between the soft and woody bits).
2. Again, neck is straight when head is hit by a bus. There's a lot of
movement possible in joints (absorbing energy) before a fracture. In a fall,
max force is applied to a maximally bent neck, ligaments can't stretch more
and the bone takes the strain. (whereas the head can't flex or extend in
either case)
> Other factors probably also operate; in the "bus" cases the
> kinetic energy of the patient's head is relatively small,
> while in the "fall" cases the kinetic energy of the body is
> much larger, even allowing for squaring of velocity.
3. Energy transmission to the head might actually be larger in the 'bus'
case (as transmitted to head, not whole body). Certainly, rate of
transmission of energy to the head is higher because of the short impact
time). On the other hand, momentum is higher (because of not squaring
velocity) in the fall. Momentum is the same thing as impulse (= Force times
time). Therefore, less energy (which causes brains to mush and skulls to
shatter) but more force (which bends and breaks long thin things like necks)
is transmitted in the fall.
4. Head is hit straight on by bus, so transmits force to skull. Hit a
glancing blow in a fall, so moves the neck a lot; but force is transmitted
over a much larger area of skull.
5. (Unlikely). Different patient populations. People with long necks (thus
more leverage and more likely to break) have their eyes that bit further
away from their feet, can't see where they're going and fall down stairs.
Your more squat/ rotund, 'no neck' build means you keep putting your head
forwards to see over the curvature of your own body, so a bus is more likely
to hit it.
Matt Dunn
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