By David Bradley, DVM,FASLMS
There is nothing inherently good or bad or better or worse about a high-powered vs a low powered laser. It depends on what you want the product to do. Laser light is just a part of the electromagnetic spectrum. It’s interesting to note that nowhere else along this spectrum will you have a debate over power. No one will tell you a dental x-ray unit will be able to take an abdominal x-ray; that a 4-Watt light bulb will light up a room as well as a 40 Watt light bulb; that a 2 Watt surgical Laser will cut as fast as a 20 Watt surgical Laser; or that an 800 Watt microwave will cook as fast as a 1200 Watt one. But in the narrow spectrum of laser therapy between 600nm and 1100nm wavelengths, they try and tell you the low powered lasers will do everything the higher powered can do… or more!? This is some of the confusing (Misleading?!) information out here that jades people on Laser therapy. The question of Power and Dosage continues to come up frequently. Unfortunately, there is still too much misleading or skewed information being disseminated. Here are a few definitions.
‘Power’ is the rate of delivery of energy and is measured in Watts which is 1 Joule/sec.
‘Energy’ is the total number of Joules delivered and is simply calculated by multiplying your power times time in seconds. Power (W) X Time (Sec) = Dosage (Joules)
‘Irradiance’ is the amount of power (Watts) delivered to a specific area. The rate of delivery per area.
‘Fluence’ is the total amount of energy (Joules) delivered to a specific area. In Laser therapy this would be your dose to a given area.
Dosage is calculated the same for all lasers. It does not matter if it is a Class 1M, Class III, Class IV, CW, Pulsed, or Superpulsed. The math and physics related to light penetration and the optics of human skin is beyond the explanation needed here but relates to the wavelength of light which dictates the decay of the incident beam of light as it travels thru tissue. This is calculated by things such as the Boltzmann Transport Equation, Diffusion Equation, scattering coefficient, etc. Obviously, this is an important physical property to consider when determining proper dosage and delivery of laser therapy to a small superficial wound vs a deep musculoskeletal condition.
Irradiance and fluence are what we really care about even though, we often talk just about power and energy. Power only dictates the rate of delivery but is an important component when determining efficiency, efficacy, and safety. (Being able to adjust the power should be one of the features you look for when deciding which laser is best for your practice.) If the laser energy is delivered too rapidly especially to a very small area (high irradiance), then superficial tissue heating certainly could occur. If delivered more slowly and/or over a broader area then all the same positive effects will be experienced but in a larger amount of direct tissue stimulation in a more reasonable time. So, the total energy (fluence) delivered to a tissue or body part will be a direct measure of the irradiance (Power delivered per unit area) at that depth times the time of exposure.
Let’s translate this into something that may be more intuitive; IV fluid therapy. When we IV deliver fluids, we first calculate total dosage. This could be equated to the energy we calculate for laser therapy. This dosage is based on whether we are hydrating a 2-pound kitten or a 100 pound Labrador or a horse. This would be the fluence. If we delivered 60 mls to a kitten, that would be an appropriate total dosage (fluence). If we delivered the same ‘dosage’ to a Lab, that would be much too low (Same Energy or Joules but too low a fluence in laser terms).
Once we calculate the dosage, we then decide at what rate (drops per second or mls per minute) we want to deliver it. This can be considered the ‘power’- the rate of doing work or delivering the fluids. If we deliver the fluids in a normal drip set at one drop per second to the kitten, that would be much too rapid a delivery rate or a very high ‘irradiance’ in laser terms. (At one drop per second a standard IV drip line would deliver 6 mls/minute so the kitten would receive his total daily requirement in 10 minutes!) That same rate of delivery (irradiance) could be just about right for the 100# lab if we wanted to deliver the total daily requirement over an 8-hour day (about 3000 mls). Another very important concept though is that an inadequate irradiance (too low a power) cannot be made up in time. If we could only deliver the fluids at one drop per minute (a very slow rate or very low power), we would never be able to hydrate that 100# Labrador. No matter how long we let the fluids run, at the end of the day we only delivered about 144 mls. Those fluids are being absorbed, dispersed, metabolized by the dog and the rate of delivery for this sized patient (Irradiance) is not adequate to keep up with metabolic demands.
Light works in a similar fashion. Some will use the argument that you can just use the laser longer and it will deliver the same dosage. This is true but they are ignoring the decay of the incident beam as it travels into tissue which is related to the irradiance. Power(W) X Time (Sec) does equal your Joules (Dosage). But just like those fluids being absorbed and dispersed, light is constantly being scattered, reflected and absorbed. It does not just sit there. You cannot turn on a 4 Watt light bulb in a large room and just wait for the room to get brighter. Those photons being delivered at 4 Watts are too low a power (rate of delivery) to fill the room with enough photons fast enough (irradiance) to make it bright. So, you will see a lot of laser protocols that give you the power and the total number of joules to deliver. But this has to be based on the irradiance and fluence or it will not translate into an adequate dosage and therefore a poor clinical response.
Keep in mind that the body also can only absorb so much so fast. So, too rapid a delivery of photons (Or fluids!?) may not allow for full absorption and full benefit. We still need to balance efficiency with practicality and clinical benefits. We make similar decisions regarding the amount and rate of fluid delivery to our patients every day. There is no question that IV fluids delivered over hours (8, 12, 24?) is ideal. Sometimes practicality, patient compliance, or expense dictates that we bolus with SQ. The same dosage may be going in and the patient will benefit but what is the BEST way to get the BEST result.
Dosages listed in the literature, which continue to be studied and actually continue to increase, range from 1-10 Joules/cm2 depending on size and depth of lesion, severity, and chronicity. This mean for a simple wound that is one inch squared it would roughly have an area of 10 cm2. To deliver 1 J/cm2 that would require 10 Joules. With a 500 mWatt Laser that would take 20 seconds. For a 7 mWatt Laser it would take 1428 seconds or 23 minutes! If you want to saturate a deep musculoskeletal area of 500 cm2 with 8 Joules/cm2, that would be 4000 Joules. You can do the math…
Emphasizing once again-even though power times time is your dosage, you can not completely make up for a lack of power with time. Light is constantly scattered, reflected, and absorbed. You have to have a certain threshold of power for adequate results. Just as you could never deliver fluids fast enough to a horse with a 25-gauge catheter or turn on a 4 Watt light bulb in a room and just wait long enough for the room to get brighter; you cannot take a very low powered laser and just leave it in place long enough and hope the energy accumulates, saturates, or reaches the area you want.
This is in no way meant to distinguish or categorize lasers as good or bad. There is no good or bad laser (Although there are some inferior products). This is not a question of high-powered vs low-powered it is a question of dosage and the right tool for the job you want to do. A superficial wound or acupuncture point needs very little laser energy and a low irradiance is often adequate. If you want to get a direct photochemical effect on deep musculoskeletal conditions on larger patients more efficiently and consistently, then a higher-powered laser will be the right tool. And remember, a class IV lasers can be turned down for those very small superficial and/or delicate conditions as well.
Understanding the math and physics related to laser therapy is just as important as the physiology and biochemistry. As we understand this aspect even more, we can better quantify the desired dosage needed for a wider variety of conditions and lasers can be preset to deliver these safely and accurately to simplify treatments. One great consolation is that there is a very wide margin of safety for laser therapy. The improved tissue saturation at higher powers and improved versatility with the capability of delivering very low powers, is what allows a Class IV laser to treat more patients and conditions more effectively in a clinical setting.