Laser therapy is becoming a widely accepted modality in treating various conditions. Leaving alone questions of laser therapy efficacy for a particular condition, or even whether it ever works, in this article we will discuss what lasers should be used for treatments.
There are two types of lasers in the market: Class 3b and Class 4.
Does it matter which one to use?
Let’s start with the definition of the laser classes. Classes of lasers have nothing to do with their clinical utility. They are about safety. There are several laser parameters that affect their classification: wavelength, power, beam divergence, etc. But for laser therapy lasers, which emit light at wavelengths between 630nm and 1100nm, it boils down to the output power – if the power exceeds 500mW it is Class 4, if it’s below 500mW Class 3b. There are also other classes for lasers with an output power below 5mW, but they are not used in laser therapy devices. The main distinction from the safety standpoint is very simple: Class 3b is dangerous to your eyes if you shine in it directly; Class 4 is dangerous even when you are exposed to scattered light (such as the patient’s skin).
Before going any further, let’s take a short excursion in the history of therapy lasers. The first therapy lasers were all Class 3b lasers, or even lower, not because it was scientifically proven that their power works the best, but because laser physics had not developed Class 4 lasers, yet. Low Level Light Therapy (LLLT) term was not to distinguish from High Level Light Therapy (HLLT) – there was no HLLT. Once Class 4 lasers were demonstrated, their proponents started a common spiel “Bigger is better”, while Class 3b equipment manufacturers mounted a defense claiming that Class 4 lasers are dangerous, there are claims that the mechanism behind the clinical efficacy of Class 3b is different from Class 4, the term “cold laser” was coined, and that one can “overexpose” a patient. This fencing match continues to date.
Now, let’s take a deeper look into the issue. First of all, power (measured in Watts) itself is not relevant for clinical applications. One can focus a beam from a Class 3b laser into a tight spot and burn the skin. On the other hand, spreading a beam from a Class 4 laser over a large area will produce no sensation. Therefore, the beam size is an important parameter of the therapy laser, and the power density (irradiance), which is the power per unit of beam diameter on the treatment surface determines what the patient feels. This brings us to a “Cold Laser” fallacy. There is no such thing as a “cold laser”. This is not a scientific or engineering term – this is a marketing term. Companies that market “cold lasers” claim that their lasers produce light that has clinically beneficial effects, while Class 4 lasers produce heat. First of all, nether lasers produce “heat”, they emit light, which in term is absorbed by chromophore in the body, and then quickly, in a matter of milliseconds, cause an increase in the temperature of tissue that absorbed this light. Therefore, both classes of lasers produce light, that has the same (provided the wavelengths are the same) photo-chemical effect in the tissue. Why during treatment with a Class 3b laser patient does not feel heat? Because the amount of light deposited to the tissues is low. There are two ways to make a Class 4 laser therapy device “COLD”. One is to make the beam size on the treatment surface bigger so that the power density will be similar to the Class 3b power density. The second approach is to move the beam across the treatment area fast enough that the patient will not feel the heat. But why bother? Once we dispel the bullshit that the mechanism is different between the two laser classes, there is nothing wrong with a sensation of gentle warmth.
Another common groundless claim of the Class 3 laser manufacturers is that the Class 4 laser system will cause “Overexposure”. Let’s first take a look at what is “exposure”. Exposure in physics terms is the amount of light delivered to the unit of surface area. It is called fluence and is measured in J/cm2. If you do not move a handpiece during treatment, the exposure will be the average power of your laser system divided by the beam spot size and multiplied by the duration of the treatment. If you move the handpiece over a treatment area that is bigger than the beam size, you will have to divide it by the surface size of the treated area. In clinical terms, “Exposure” is the “Dose”. In the same way, as the drug doses are calculated per weight of the patient, in laser therapy the dose is the amount of light per unit of surface to be treated. As you can see from the definition of “Exposure”, it depends on the duration of the procedure. Assuming that there is an “ideal” amount of “exposure” that can be delivered with a Class 3 laser system, the same amount of “exposure” can be delivered with a Class 4 system, just in a shorter amount of time. Therefore, “Overexposure”, if it exists, is independent of the laser Class, but depends on the duration of the procedure.
Moreover, the whole idea of “Overexposure” is nothing but quackery. Of course, one can burn a patient with a Class 4 laser, by delivering a dose that far exceeds any recommendation in laser therapy. But this would have nothing to do with the clinical utility of laser therapy, but rather with misuse of the equipment. When light propagates inside a tissue, it is absorbed by the chromophores in the tissue, and its power reduces exponentially. The reduction of light intensity is described using the propagation depth – the distance at which radiation inside the medium falls to 37% of its original value. As you can see in the following picture, the penetration depth is quite small – just a few millimeters at wavelengths that are commonly referred to as therapeutic window, and where therapeutic lasers emit light. It is important to understand that the penetration depth is not the limit of light penetration. The light will not stop when it reaches the penetration depth, it will continue to go deeper into the tissue, but its intensity will continue to diminish. Why we are talking about the penetration depth? Because it is directly related to the “optimum” dose of laser therapy light. As you can see from our discussion above, there will be different doses delivered at different depths below the skin surface. The question is: if we are to believe the proponents of the Class 3 lasers that their few 100 mW lasers deliver the optimal dose – what is the optimal depth at which the light should be delivered? If we use Class 4 lasers, the same dose of light can be delivered to the deeper tissue, not just to the dermis. I find it hard to believe that in order to treat osteoarthritis one should deliver light to the skin using a Class 3 laser, and not to the joint itself. The same issue is with tendons and ligaments.
Lastly, some Class 3 laser therapy devices output very low power which makes me wonder whether they do anything special beyond just a fancy name “Laser therapy”. For example, the Erchonia VLS Laser System outputs two red beams (640nm) 7.5mW each, covering the treatment area of a couple of square centimeters. For comparison, irradiance from the Sun in the same spectral bandwidth of 50nm is 6.5mW/cm2. Get your pet out for a walk instead of such laser therapy. It will get more light!
There is nothing that Class 3 lasers can do cannot be done with the Class 4 lasers.
The opposite is also may be true – it just takes more time. A lot more. A treatment of musculoskeletal condition of a horse neck will take you under 30 minutes using a Class 4 laser therapy system, such as Eclipse-VSL by Omnilase. If you choose to do the same with Erchonia’s VLS it will take you just over 40 days of non-stop walking around a horse with this laser.