Page 63 - Veterinary Laser Therapy in Small Animal Practice

Light’s path to all these places in the body 49

limitations in extrapolating these results to conclusions time is virtually always measured in seconds, and so the
on the effects in bulk tissue, as well as some fundamental units of power are watts (1 watt = 1 joule per second).
shortcomings in the breadth of their investigations. This is important to stress: a higher-power laser beam is
The first is the fact that the amount of light incident simply producing more of the same light than a lower-
on a monolayer of cells in a Petri dish is quite different powered laser (when all else is equal). Maybe it is better
than the amount of light that penetrates the skin/fur, to say that a higher power setting on the same laser
through some fat and soft tissue, to an affected bone, produces more of the same light as its lower power
for example. What you CAN take from an in vitro setting. But each photon still packs the same punch;
study on bone cell repair initiated by laser therapy is remember that energy per photon is determined by its
the amount of light it takes to stimulate those cells. wavelength.
From there you need the rest of this discussion to better
understand what proportion of the light you start with So the difference is about how many photons are
at the surface actually gets to the bone, then work your delivered by each laser in a given time.
way backward to understand how much you need to Or from a time perspective, how long it
start with in the first place. will take with each differently powered
The second point of hesitation is the range of doses laser to deliver a given amount of energy.
used and the a priori assumption that there is only one
peak in the biostimulatory spectrum (i.e. one sweet spot
of dose that gives the only hope for healing). Tiina Karu, 6.6.1 Average vs. peak power
among others, has shown this to be an invalid assump-
tion, and that for a given cell line, there may be several A word to the wise: for the purpose of therapy lasers,
peaks with similar biostimulatory effect separated by always always always make any decision (purchasing
several orders of magnitude of dose. [194] There will or protocol selection) based on average power. Again,
clearly be a lower limit, below which no effect is possible. this is the total joules of energy emitted in a second,
But I’ll warn you that if some paper claims an upper limit and given in watts. If you move your focus to surgical
– some maximum dose above which there is no effect, or body contouring or tattoo removal or even equine
or some inhibition – they simply may not have mea- regenerative lasers, then you’ll need to bring peak
sured enough of the dose domain to see the next peak. power into the conversation.
With this there also has to be an admission con-
cerning the opposite end of the dose spectrum. Most
people who preach on the use of higher powers (as Photo-acoustics
we do) quickly dismiss anecdotal reports on the clini- There are some very cool things you can do when
cal efficacy of much lower-powered lasers. And there you have a very bright laser that emits its light in
is certainly a threshold below which no effect will be very short bursts: those short, but powerful pulses
observed. But that threshold, as mentioned here, can be will force absorption events within the tissue that
orders of magnitude different. happen faster than the thermal relaxation time of
the tissue, which causes the creation of a shock
So things treated with a lower dose – or structures wave. These photo-acoustic waves (so named
much deeper into the body treated with a higher dose because they are caused by light – photo – but
– can still be getting the light they need to respond and end up as shock – acoustic – waves) can permeate
repair. However, keep reading for the method to our farther through tissue than the original light and
madness when it comes to using the higher end of the can mechanically shake the tissue in their path.
dose domain. Sometimes this is good for imaging (photo-acoustic
imaging), sometimes for bursting trapped ink
packets (tattoo removal), sometimes for ablating
6.6 Power the water in tissue (surgical), and sometimes even

Dose is the accumulation of energy over a given area. to force the release of growth factors from the
How quickly that energy accumulates is dictated by the extracellular matrix near dormant/hypoxic/necrotic
average output power of the laser. Average power is tissues so as to stimulate healing where the body
otherwise wouldn’t (regenerative laser therapy).
simply the total energy per unit time, where that unit of

REDONDO PRINT (4-COL BLEED).indd 49 08/08/2019 09:47