hello and welcome to another video laser lessons from dr phil my name is dr phil harrington and our topic today is therapy laser depth of penetration through the years that i have been involved with laser therapy a question that i frequently get asked is how deep does laser penetrate that's going to be the topic of my video presentation today but really a better question to ask is to what depth in the body can we deliver a therapeutic dosage now first let's uh lay a little groundwork and do a little bit of review and make sure we're all on the same page here first all when it comes to laser therapy or photobiomodulation there is the gratuis draper law which is the first law of photochemistry and the grout whose draper law states that for light to produce an effect upon matter it must be absorbed in other words during a laser therapy during a photobiomodulation treatment we are shining photons of light onto the tissues and we need to get those photons of light to the cells in the body that need them now those cells could be in muscle or connective tissue or nerves blood vessels etc so in order for us to really undergo photobiomodulation we have to get photons absorbed by the tissues in the body that need them so the goal is you know trying to figure out how do we get those photons of light to those deep structures in the body to review the electromagnetic spectrum comprises everything from gamma rays x-rays through visible infrared and so forth now the photons on the left side of the screen are very high in energy they are ionizing and by definition can damage tissue the beauty of therapy lasers is that we are using low energy photons we are using red and infrared light which is by definition non-ionizing and therefore very very safe one other comment is that when you look at a therapy laser device and this yellow sticker is an example of a sticker that's required to be on all therapedic laser devices you'll notice when it looks at the wavelength when it shows the wavelength it will say a number and then plus or minus 20 or a plus or minus 10 nanometers now that does not mean it's a bad piece of equipment laser therapy devices there's a laser diode a semiconductor diode that produces the laser light and it is simply a physical nature of the semiconductor diode that there's going to be a small margin of error there so this is completely acceptable but just realize that when this laser device says 810 plus or minus 10 nanometers it is perfectly acceptable that it could be delivering 800 or it could be delivering 820 nanometers during the treatment output so let's start to look at a few scientific papers and and the first one i've chosen is a computer simulation uh they're using monte carlo computer simulations on uh tissue uh using computational methods the effective wavelength and beam width on penetration in light tissue interaction using computational methods so this this is a fairly complicated process where they would be inputting the various coefficients of scattering and reflection so on and so forth to determine the depth of penetration of various wavelengths of light into the tissues and so this is a figure from that study and it says light penetration into skin illustrating the depth to which wavelengths penetrate human skin red light is extinguished some four to five millimeters beneath the surface of the skin whereas ultraviolet hardly penetrates at all and blew barely one millimeter into tissue so you know the wavelengths on the left side ultraviolet blue green those would be very very poor choices for a therapy wave therapy laser device because they will only get photons just a couple of millimeters into the tissues red will go a little bit further but then what will go the deepest into the body is the infrared wavelengths and we will look more deeply at those in a moment now this is a a figure that is used frequently by therapy laser companies to discuss absorption as a function of wavelength so on this chart you see on the y-axis they have the absorption coefficient notice that it is a logarithmic scale going from 1 to 10 to 100 to a thousand so it's using powers of 10 on that scale on the left side and then looking at wavelength along the bottom so of course between 400 and 700 is our visible wavelengths and then once we get beyond 700 we're getting into the infrared and they are charting on here the absorption by melanin oxygenated hemoglobin and water now it's a fairly complicated figure here so oftentimes we will see therapy laser companies simplify that into something like this so this represents a particular therapeutic laser device that uses four wavelengths simultaneously one of them red three of them infrared and these wavelengths are chosen because they are at the peak of absorption for various components involved in the mechanism of action for photobiomodulation for the whole process so the 980 nanometer wavelength is at a peak of absorption for water molecules in the tissues 915 is at a peak of absorption for oxygenated hemoglobin so notice that at that 915 wavelength the absorption by water is much less but the absorption by oxygenated hemoglobin is is at a peak similarly with cytochrome oxidase the the enzyme inside of the mitochondria that is responsible for the production of adenosine triphosphate the energy in the mitochondria there is a peak of absorption uh at 810 nanometers by this particular enzyme and then the visible red wavelengths are wonderful for superficial conditions once again those red wavelengths are absorbed in the first few millimeters of tissue so first let's take a look at those red wavelengths now my source for this it's from yantune jan is a friend of mine he's a co-author of the laser therapy handbook and here jan is simply taking a red laser pointer and shining it through his fingertip now we look at that and that seems to be bright and it seems like there is a lot of red light getting through his fingertip but let's take a look at uh you know what happens when we put this onto a laser power meter once again this is from yon's website and he writes however how much is really getting through the finger well not as much as your eye would think look here when the 60 milliwatt laser is measured after passing through the finger 0.27 milliwatts remains 60.33 milliwatts lost so do not trust your eyes when you try to find out about penetration so in other words only 0.5 of the red laser penetrates through your fingertip and uh 99.5 of it is lost that you know it's absorbed in just those few millimeters of the width of your fingertips so once again red laser will only penetrate a few millimeters red laser a very very poor choice for treating deep inside of the body now just for fun let's take a look at the green laser so this is one of my laser pointers it is a green 100 milliwatt laser and we look at that and see how bright it is when i shine it against the wall and now what i will do is simply shine that green laser just through the width of skin in the web between my thumb and forefinger now take a look at your own hand and notice how you know how thin that web of skin is between your thumb and forefinger when i start out that green light looks nice and bright going through just a millimeter or two of tissue but then as i slide it along the web of of my skin there you see that very quickly you cannot see any penetration with the green laser light so green will only penetrate a few millimeters very very poor choice for a therapy laser because because it does not penetrate significantly into the body now this is a chart that i will see sometimes when i see people lecturing on laser therapy and it's interesting and it does indicate that yes there are some receptors at the skin surface that are sensitive to those green photons of light but then they rely on signaling pathways which is relying on the secondary and tertiary effects of laser therapy so i'm a fan of using class 4 therapeutic lasers that are using red and infrared wavelengths that penetrate much much deeper into the body and will get directly those photons will get directly absorbed by the chromophores deep inside of the body and so it's it's very important when we are doing therapy laser treatments we want to get light as deeply into the body as possible here is a figure from another study and this is looking at the absorption spectrum of the human hand now along the left-hand side here they're using a term called optical density so on the y-axis the optical density and here we see the definition it is a logarithmic intensity ratio of the light falling upon the material to the light transmitted through the material in other words the higher the number the less light gets through so looking in at the absorption spectrum of the human hand when we are at a red 600 nanometer wavelength we follow that straight up and we see that we are at an optical density of seven whereas when we get into the infrared wavelengths 700 800 900 nanometers the optical density of the hand drops down significantly and so you can see that with those lower optical density numbers that tells us that infrared light should very easily penetrate the hand so on my previous slide you saw how poorly red light went through the fingertip but now if we look at the human hand and there we see a treatment being done on the hand and what we're doing here is shining a therapy laser on the palm of the hand and then we are using an infrared scope looking at the back side of the hand to see how much light is getting through on the left lower power on the right hand side higher power and so there you see that yes the infrared light will easily penetrate through the palm of the hand we can very easily get infrared light through the palm and then what is also interesting is that yes as we shine brighter light on the palm our side we can see more light coming through the back side of the hand it's simply common sense and that is why i have always been a proponent of class 4 therapy lasers we shine brighter light at the skin surface therefore we can get more light deeper into the tissues so we'll take a look at a few studies here this is an early study looking at penetration of visible red and invisible infrared light into soft tissues and the main point here is that looking at my first red arrow there the depth of penetration of 632 which is red so the depth of penetration of red and infrared light is not related to the average power of the light source so this was at a time in the history of laser therapy that we were not sure we did not know what determined penetration into tissues you know what parameter was it was it power was it pulsing mode was it power density was it wavelength at this point in time we did not know which parameter was most important now this is the first study that i'm aware of that points out that yes it is wavelength wavelength plays a critical role in the depth of penetration of laser light and here it did indicate that the infrared wavelength of 904 penetrates much more than the red wavelength of 632 so two key points here infrared penetrates much deeper than red and also wavelength is the primary determinant of depth of penetration so here's a study that compared 808 and 980 nanometers here they are using bovine or cow tissue samples and the conclusion it was determined that 808 nanometer of light penetrates as much as 54 percent deeper than 980 nanometer light so in other words 800 nanometers penetrates much better than 980 nanometers this study looked at penetration and here they were comparing a continuous 810 and a super pulsed 904 um so this was interesting because they they said that it had been previously suggested that strong pulses may cause a photo bleaching effect in the skin barrier over time so at this point in time once again this study was done in 2012 the thought at this time in the laser community was that when you super pulse when you have a very high power a very high peak power laser pulse and you you shine those pulses on the tissues that it somehow would photo bleach the pigments in the tissues and allow for a deeper penetration it was a somewhat controversial concept and actually a few years later this uh this study was debunked so in 2016 juanita anders who was one of the preeminent scholars the preeminent photobiomodule researchers in the world did the study to once again compare 810 and super pulse 904 and as it says here these data prove that transmission of continuous wave 810 through muscle and skin and skin alone is greater than transmission of super pulse 904 wavelength light conclusions it had been previously reported that super pulsing 904 wavelength light increased depth of penetration over time to photo bleaching based on our data the observed increase in light penetration over time was due to an insufficient warm-up period of the super pulsed laser so in other words throw out throw out that concept of photo bleaching sorry that's wrong but what this does show is that 810 810 is the deepest penetrating wavelength this study looked at uh penetration transcranial and intraparenchymal light penetration in human cadaver brain tissue once again juanita anders involved in this study and it showed yes the the 808 nanometer wavelength light demonstrated superior cns tissue penetration and they were comparing it to red 660 and another infrared 940. so once again in that 800 810 nanometers in that ballpark that is the deepest penetrating this study looked at penetration of visible and near-infrared lasers through the head tissues in animal and human species now i frequently get asked about using infrared laser over the brain and and what i say is that yes infrared laser can penetrate the skull the number that i quote is 10 percent and i use that just because it is a it's a nice round figure it's easy to make calculations you know if you're doing power density or dosage calculations you know you can multiply it by 0.1 to get how much is actually getting through the skull uh so this is is the study that i am using to support that now the range here is 0.2 to 10 and the reason being is that if you recall your anatomy of the human skull you have you know various thicknesses that the human skull is is a very um widely varying topology so then the percent penetration number is going to vary but 10 percent is a nice round ballpark number to use for going through the skull and here's an interesting study looking at penetration for comparing 8 10 and 904 and then looking at before and after ice application and the conclusion of this study was that the penetration of laser light increased significantly through healthy achilles tendons subjected to 20 minutes of cooling so uh the the take on points that i want you to get here and you know let me just pause for a moment and say that i do have other videos other webinars and recordings where i talk about the use of ice on acute injuries i no longer recommend ice on acute injuries now i can you know very highly referenced there and probably at the top of the list is the fact that dr gabe merkin the guy who coined the rice treatment protocol rest ice compression elevation uh he coined that protocol back in the 70s now just a few years ago dr merkin reversed himself and says that using ice on acute injuries actually delays healing so you know it's going to be up to you if you are going to use ice the main point is always use ice before laser always use ice before laser three reasons why it will give you better penetration and realize that laser improves blood flow so if you think about it if you do a laser treatment and improve blood flow if you do ice afterwards you are immediately counteracting the benefits of the laser treatment that you've done so once again if you are going to use ice always use ice before laser this study i want to look at because just because studies get published in journals uh it does not mean that they've escaped the uh the tentacles of bias so let's look at the title of this penetration profiles of a class iv therapeutic laser and a photobiomodulation therapy device in equine skin so right there the the way that they word the title of this it tells me that there's something going on here that they're having some bias why not just call them both a photobiomodulation device so why am i saying this well i have always been a proponent of class 4 therapeutic lasers from 2007 and before i have been spoken out in favor of class 4 therapeutic laser it was so rewarding three years ago 2019 when these four experts there again you see the name juanita anders along with praveen irani david baxter and raymond lanza fame they all said that and let's look at the red arrow both class 3 and class 4 devices are being used successfully to treat patients in human and veterinary medicine alongside other devices an important advance in photobiomodulation therapy was the recognition that optimization of transcutaneous therapeutic parameters should be based on the photonic dose reaching the target tissue and that often requires higher doses of light at the skin surface to reach deeper tissues in other words class 4 therapy lasers are photobiomodulation devices and in other words if you want to get light deep into the body you must use a class 4 therapeutic laser device so i'm going to put the biased label on this study simply by that title alone they are exposing their bias well let's look further at it what are they doing they are comparing and they show the brand names of the two devices here the variable the primary variable in the study is the wavelength and so you know we've already looked at other studies and and hopefully by now you do get the point that these are both infrared wavelengths infrared wavelengths do penetrate deeper into the body and yes it is true that 905 wavelength will penetrate deeper than the 980 wavelength remember that 980 is more absorbed by water in the tissue so with that water content all the way through when we shine 980 laser light on the tissue it is going to get absorbed more you know you saw the study comparing 808 to 980 808 went significantly deeper because it has lower water absorption so i mean this study is okay in the in the sense that yes they're showing that 905 penetrates better than 980 but once again i would be opposed to the title of the study so i want to wrap up here and we'll look at a depth of penetration study that i participated in for this study we were using an 800 nanometer device and there you see the parameters listed three orders three things the first thing we did was with the water phantom you know because our bodies are mostly composed of water very easy to do you have a laser device shine the laser light down onto a power meter and then we are going to put slowly uh increasing amounts of water into that cylinder and as water is added we read the amount of light that is reaching the laser power meter and create a graph so from that we can create a graph and show the amount or the percent intensity of light remaining after it has gone through so much water in the column the next order in the study was to look at a monte carlo computer simulation so similar to the study that i started out with where we are inputting all of the different coefficients involved with the tissue types it's a very complicated process and we're looking at how much dose do we deliver we run the monte carlo computer simulation and we can look at how much of that dose is being delivered to those but really real world thing that we did was we would bury photon sensors in fresh cadaver tissue and shine the laser at the skin surface there you see the laser device shining at the skin surface and measuring how much of that laser light actually reaches that photon sensor it was the first time anything like this had been done and from that we were able to fully develop the absorption curve and we presented this poster at the 2010 meeting of the north american association of laser therapy this poster is available if you if you would like to get it really the main point of of the juxtaposition here is this we're looking at percent intensity on the y-axis and depth it says depth in water here or we could say depth and tissue and really what we do is we look at our desired depth how many centimeters deep do you want to be in the tissue then you follow the line over and then you follow it back on the chart so when we are about 10 centimeters deep in the tissue about 30 percent of the incident light remains now one way to think of that is this if 10 joules per square centimeter are applied at the skin surface at 10 cm 10 centimeter depth the dosage is about 3 joules per square centimeter so you have about 30 percent of that incident light remaining so let's just look at a couple of um you know graphical representations here and and you know pardon my uh my my amateur computer graphics here but i'm just trying to put a representation on it you know if you measure the width of the human thigh you know the human thigh is uh i'm usually using mine as an example my thigh is about 20 centimeters across so if i were to make a cross section at 20 centimeters and just showing shining that laser from a couple of different angles as that laser light penetrates into the thigh the light will get dimmer and dimmer and dimmer and dimmer so then here at this 10 centimeter depth right in the middle of the thigh we have about 30 percent of the light remaining 10 centimeter depth about 30 percent remaining and then here is just taking another uh cross section uh at the leg and here uh just once again using my amateur computer graphics to show that when we are shining that laser here we're shining it from the posterolateral direction on the calf and then here from a medial lateral direction showing that we can get significant penetration through the leg and again once again at that 10 centimeter depth we have about 30 percent of the incident light remaining so i hope that that answered some of your questions about therapeutic laser depth of penetration to summarize red laser penetrates only a few millimeters and is not effective for deep conditions infrared laser wavelengths penetrate deepest into the body therapy laser wavelengths between 790 and 810 will penetrate the deepest and using that 800 nanometer wavelength at a 10 centimeter depth approximately 30 percent of the incident light remains so thank you for your time folks take care and have a good day bye bye