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On Fri, 6 May 1994 deepreef@bi*.bi*.ha*.or* wrote:

>On Fri, 6 May 1994 lanier@ex*.cs*.ti*.co* wrote:
>
>> Hold on a second here - I don't believe there is any significant amount of 
>> gas in this "bubble" in the syringe in the above example.  That is a 
>> "vacuum" bubble.  Given time the bubble will fill with gas from the liquid 
>> beneath, but there is hardly time for this to occur in this example or in 
>> joint cracking.  If you see bubbles forming within the liquid volume (as in 
>> boiling), then you have gas.  A bubble temporaily formed at the top surface 
>> is essentially vacuum.
>
>But, if the "vacuum bubbles" were truely vacuums, they would instantaneously
>vanish when exposed to any ambient pressure.  Aren't the bubbles in the above
>example formed by "pulling" dissolved gas molecules out of solution?

Indeed correct.  Cracking knuckles results in the production of bubbles
which are mostly nitrogen by analysis.   Cineradiographs of the process
show the sudden appearance of a bubble which actually causes sudden
expansion of the joint space (since gasses can expand and contract
unlike liquids or solids).  It is this sudden motion of the two bone
surfaces away from one another that seems to produce the crack.  The 
bubble remains present for a long time (many minutes to an hour or so)
which keeps the joint from "cracking" again right away.  The gas has
been sampled by someone and, I am told, is mostly N2.  None of this
answers the question of whether cracking a joint would predispose to DCS
since the gas bubble should be confined to a joint space and not be
likely to have systemic effects.  There is very little blood flow in
cartilage (like almost none) so the bubble would not have easy access to
the circulation.  Cracking your knuckle or back at 250fsw and creating a
bubble then ascending might well have pretty adverse effects!

Tom
which prevents the joint from c

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