Regarding this subject, I would direct you all to two meta-analysis articles
published in the July (?)issue of Physical Therapy. The authors demonstrate
through their analysis of the literature that not only is there no proven
physiologic effect of ultrasound in living human tissue, but that there is
also no evidence to show efficacy in any patient population.
As to the notion that energy is applied so tissue changes must occur, there
is no evidence to support that US has an effect in muscle tissue.
A friend of mine performed an experiment at Sandia National Laboratories
with a research doctor there. They developed a muscle and bone model out of
substances chosen to acoustically mimic actual muscle and bone tissue. They
then applied ultrasound at the usual paramenters for several minutes and
then thermographed the models.
They found that the attenuation of US energy in muscle at 1 and 3 MHz was
nearly nill. The greatest attenuation occured at the surface of the "bone,"
thus explaining the "thermal effects" felt by many patients (i.e. the
sensation most patients feel from ultrasound, if any, is the attenuation of
sound energy at the periosteum... which is much more sensate than muscle
tissue). However, the lack of attenuation of energy in the muscle model
means that since very little energy is transferred here, very little effect
would be seen.
This may also include "non-thermal" effects such as cavitation. To date, no
study I'm aware of demonstrates cavitation effects of ultrasound in vivo.
The one area where effects can be seen is at the granular layer of the
epidermis. Here, ultrasound produces profound changes in the permeability
of the tissue. Thus, medication is more easily transmitted through the skin
(i.e. phonophoresis). The problem is, if you are using hydrocortisone cream
(the medication of choice), you are blocking all US energy from reaching the
skin as creams do not transmit sound energy well. You also need to allow
the medication time to absorb so standard application times need to be
lengthened past the US treatment... but I digress.
Check out the articles! They are good reads.
Warm regards,
Geoff Mosley, PT, NCS
MRC
Mt. Vernon, MO
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