FAQ - The Best Laser
QUESTION: I've been researching laser therapy equipment, and [Laser Manufacturer] says that [Wavelength]/[Power] penetrates the deepest so it's the best. So how deep does laser penetrate anyway, and what influences this?
Please Note!! Although the information below is largely correct, a more up-to-date discussion about wavelength and penetration depth can be found here.
There is no wavelength or power that is any better, per se, than any other, as it all depends upon what you're using it for. For example, if you treat only superficial conditions you don't need deep-tissue penetration. But some applications do require energy to be delivered deep in the tissue, so please read on...
Looking at the physics of tissue optics, the transmission of light depends greatly upon the absorption characteristics of the tissue, and the key components which absorb light most readily are blood, melanin and water.
The 'relative penetration' diagram (from Oshiro's 1988 book, Low Level Laser Therapy) on the [Laser Manufacturer] site has been somewhat superseded by more recent studies which show that, for continuous wave (or modulated/pulsed continuous wave), the maximum penetration is achieved within a roughly 40nm bandwidth from 800nm to 840nm, although this can extend to wavelengths as short as 790nm and as long as 850nm, give or take, depending upon tissue type, body composition, skin color, and so on.
The image at the bottom of this page shows that shorter NIR wavelengths (820nm is highlighted on the diagram) are absorbed more by melanin than 904nm (also highlighted), but less by both water and blood. The same is illustrated in the second graph shown below, from an article on the mechanisms of LLLT by Michael Hamblin. Essentially, there is more blood and water than melanin in most tissues, which accounts for the deeper penetration by wavelengths within the shorter NIR band described above.
However - there's always a 'however!' - there are many other factors that affect penetration depth, such as:
- power/power density of the beam at the tissue surface;
- applicator/probe design;
- application method (non-contact, light contact, or contact with firm pressure);
- operating mode of the laser emitter; and, etc..
Looking at this last point, operating mode, first, there are two main modes of operation of a laser emitter, continuous wave (CW) and super-pulsed. Both CW and super-pulsed lasers can also be further modulated, but this is of no consequence to depth of penetration, nor of great value in terms of therapeutic outcomes.
The 800-840nm waveband will achieve the deepest penetration into most living tissues. When the beam is super-pulsed, however, the penetration will be deeper still. The 904nm wavelength, when delivered by a super-pulsed GaAs laser diode, will penetrate slightly more deeply than the deeper-penetrating continuous wave Near Infra Red wavelengths within the band 800-840nm. Unfortunately one cannot obtain a super-pulsed laser that operates in this waveband, but 904nm GaAs super-pulsed lasers are readily available.
Super-pulsing achieves deeper effective penetration by virtue of a tissue-bleaching effect caused by the very high peak power (50-100 Watts) but very short duration (100-200 nanosecond) pulses. The thing to remember here, though, is that the 904nm laser MUST be super-pulsed for it to achieve the deeper penetration - a CW or modulated CW 904nm laser will not penetrate as far, for the reasons described previously.
Probe design, and its effect upon application methods, also greatly impact the effective penetration, and therefore the efficacy of a device in the treatment of deeper tissues
The key to treatment efficacy is to deliver the correct dosage to the target tissues. Where the target is deep in the tissue, certain design elements, in addition to the wavelength and emission mode, facilitate deeper penetration.
The first is the shape of the lens or window at the laser aperture. This should be convex, and should protrude past any surrounding materials in the applicator head so as to make good contact with the skin surface. Recalling that blood is a major absorber of laser energy, reducing the amount of blood in the tissue between the laser source and the target tissue is key to increasing the amount of laser energy reaching the target.
By using a protruding convex lens, even slight pressure on the applicator will cause any superficial circulation to diminish, essentially 'pushing' the blood away to the sides and allowing more of the laser beam to pass through the centre of the aperture and into the tissue. A flat aperture, or, worse, a concave opening, will not achieve the same outcome.
Increasing the power/power density will increase the effective penetration depth, but only up to a point. Once the source power reaches the point at which it generates significant heat in the tissue, it is no longer Low Level Laser Therapy. Class IV lasers are used for High Intensity Laser Therapy (HILT), which is, to date, a largely un-researched modality that uses very high doses of energy and generates a substantial amount of heat in the treated tissue. This is in contrast to Low Level Laser Therapy, which uses much lower doses of energy and creates little or no heat in the treated tissue. This significant difference in tissue effects means that they likely act therapeutically in very different ways, and so are, effectively, different types of therapeutic devices.
Despite all this, the difference in penetration depths between, say, CW 810nm and CW 904nm is only in the range of 3-5mm over a maximum penetration depth of 35-40mm (this is the greatest depth over which any wavelength delivered in CW mode will achieve effective penetration). Super-pulsed 904nm will achieve slightly deeper penetration, to around 45mm. These depths assume light contact with the tissue, and a sufficient power density to overcome the significant (37-40%) loss of power across the skin barrier.
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