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Pointing light at musculoskeletal and neurological conditions: clinical applications 109

• How warm the surface gets: since we will be using
more power and power density than in skin prob- 9.1.3 Frequency (Hz)
lems, this is something you need to keep in mind. Continuous wave (CW) is always the fastest way to
deliver the calculated energy, but is also more warming
Remember, it is not just power that influences how (there are no dark periods between pulses to allow for
warm the surface gets, but also the wavelength used, thermal dissipation in the tissue), and when working
the color of the patient, how fast you move the hand- with higher power densities, as happens with muscu-
piece, and power density, meaning how that power is loskeletal conditions, warming can become significant,
concentrated (W/cm ). Usually musculoskeletal condi- especially for darker coats. If there is significant ery-
2
tions are treated with higher power than wounds, and thema in the treatment area, try not to use a lot of CW,
also in contact with the patient, to improve penetration. since it has a stronger vasodilating effect than pulsed
So keep that in mind: do not exceed 2–3 W/cm unless radiation. Pulsing the light will allow you to do the
2
you are moving the hand-piece relatively fast. If your following.
hand-piece has an adjustable spot size, use the maxi-
mum available size if you are working in contact mode. • Use a higher power. Average power (Pa) will be the
Power density is quite similar to the strength of a result of peak power (Pp) times the duty cycle (DC),
massage: a higher power density feels like a stronger which represents the percentage of each second that
manipulation. When you have a sore muscle or back, the light is “on.” DC can be calculated by multiplying
sometimes you may need a stronger massage to allevi- the frequency of the pulsing (Hz) times the duration
ate it. But if the main problem of a tissue or area is acute of those pulses (ton). The range of effective pulses
inflammation, you definitely don’t want any strong described for pain is very broad, though, from 2 to
manipulation there. In the same way, some painful con- 8000 Hz. [48, 201]
ditions need a more intense approach to reduce pain,
but usually your patient with severe acute inflamma- P = P × DC
a
p
tion will prefer a more subtle approach. DC = Hz × pulse duration (t )
on
“Can I burn my patient? Can I burn them without if you do not move the hand-piece and treat
even knowing it?” These are questions many in contact. That would mean treating with
practitioners have in mind when they start using a high power, using a small hand-piece or
LT. The answer is of course a bit more complex concentrated spot size, while not moving.
than yes or no. Let me start with the second • “What if I make a mistake?” A Class IV laser
question. can produce heat on the surface of the patient,
especially those with dark and/or thick fur. It
• “Can I burn my patient without even knowing is unlikely and easy to avoid if you are treating
I am doing so?” NO, unless the patient or the properly, but if on some of the larger-body-part
area you are treating is anesthetized or has no protocols (which call for higher power) you stay
pain perception. If the patient is awake and in one spot for too long, heat can accumulate,
the area has pain perception, there is no way even to the point of singeing the hair. This will
you could be burning him and not realizing. cause an unpleasant smell and will probably
The feeling for the patient would first be scare your staff (and the pet owner), but the good
discomfort, and if you persisted, the patient thing is that it likely won’t hurt the patient. To
would feel pain and try to run away/bite/ prove this to yourself, take some hair clippings
scratch you. There is no way you would not from a dark dog, put them on your palm, and
notice. start to treat that hair until it singes. You’ll
• “Can I burn my patient?” If you try really hard, notice that your palm didn’t burn. Another
maybe. Burning comes from an accumulation exercise may be to take those clippings and start
of energy, or heat, in a tissue that goes beyond treating them, but move the hand-piece around
its capacity to disperse that energy or cool off. as you do so. You’ll notice that you won’t singe
The way to “try hard” to burn your patient is the hair, because you are spreading the power
by using too high a power density, especially around.

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