>Dr. Mel Siff wrote "but when I hear explanations that the 'pop' caused by
manipulation is due to the explosive release of nitrogen bubbles from the
cerebrospinal fluid and I recall the devastating effects that explosive or
implosive events can have on engineering structures, I am concerned to
know how this catastrophic type of event can be of benefit in the
management of back pain or disability."

This "pop" phenomemon, known as cavitation, likely does not occur in
cerebrospinal fluid. It is a phenomenon of articulations and articular
fluid, hence its presence in locations of the body devoid of CSF (i.e.
phalangeal joints of the hand). Brodeur has written a nice review on this
topic (see references below).

As to the "catastrophic nature" of this event, there are few studies that
have investigated the relationship between cavitation and articular
pain/function. However, in Castellanos et al., 74 habitual knuckle
"crackers" were compared to 226 controls. It was observed that there was
"no increased preponderance of arthritis of the hand in either group;
however, habitual knuckle crackers were more likely to have hand swelling
and lower grip strength." These observations do not support the notion
that cavitation caused by spinal manipulation is "catastrophic" given that
the frequency of a cavitation event in a joint created by a therapeutic
procedure is likely far less than what was observed in the habitual
cavitators in the above study.

While it is conjecture on my part, cavitation in an articulation may be
less damaging that cavitation on an "engineering structure" (i.e. a ship's
propellor) as the synovial structures are more deformable than the
surface material of the propellor. It is also likely, in my opinion, that
the frequency of the cavitation event in these two situations is greatly
different.


References:
Brodeur R; The audible release associated with joint manipulation.
J Manipulative Physiol Ther, 1995 Mar, 18:3, 155-64

Castellanos J, et al; Effect of habitual knuckle cracking on hand function
Ann Rheum Dis, 1990 May, 49:5, 308-9

For Interest
LaFond E, et al; Synovial fluid cavitation during distraction radiography
of the coxofemoral
joint in dogs. J Am Vet Med Assoc, 1997 May, 210:9, 1294-7


__________________________________________________ __________________
Greg Kawchuk D.C., M.Sc.
Clinician, University Health Services
Ph.D. Candidate, McCaig Centre for Joint Injury and Arthritis Research

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