The diagram below shows the range of emissions called the electromagnetic spectrum.
Note that the more damaging emissions, such as x-rays and ultraviolet light, are the shorter wavelengths. The extreme power of these short wavelengths can break the bonds of atoms and produce ions, and for that reason, these shorter wavelengths are sometimes called ionizing radiation. Typically, at the other end of the spectrum, radiation in the infrared or visible red spectrum does not cause ionization, but it does generate heat when absorbed in tissue.
Water has very high absorption for light at wavelengths longer than 1500 nm, and since water is the major constituent of muscle tissue, this radiation does not penetrate significantly below the skin. We thus are left with an optimum treatment window which is the optical window, between approximately 600 nm to 900 nm where the radiation does not cause ionization and can penetrate beneath the skin to affect the underlying tissue.
This is well visualized in the chart below. In the field of LED therapy, we use the term optical window to describe the range of 600–900 nm, because LEDs that have a wavelength greater than 900 nm produce a lot of heat and are more uncomfortable than the LEDs of shorter wavelengths with the same output power.
Note in the diagram below that light in the 600–900 nm wavelength is in the “optical window.” This range of wavelengths is most likely to penetrate deeply into the tissues because the photons are not strongly absorbed by hemoglobin or water. When light is absorbed by hemoglobin and water it will be absorbed into the circulatory system and prevented from deeper penetration.
Depth of Light Penetration
The accepted depth of penetration of an LED depends on its
wavelength, but, on average, penetrates about 5–7 cm (2–3+
in.). The majority of photons are absorbed in the first few millimeters.
As the beam passes into the body, more superficial tissues absorb most of the photons, thus reducing the number of photons that reach deeper layers. However, as these photons enter the body, they create a powerful physiological effect by inducing local metabolic changes and the creation of second messengers.
Second messengers are molecules that relay signals received at
receptors on the cell surface that target molecules in the nucleus of the cell to modify physiological and genetic informatio
Second messengers also serve to greatly amplify the strength of the signal, causing massive changes in the biochemical activities within the cell. Thus, as the effect of the photons diminishes with increasing depth, the physiological effects rapidly multiply, creating a profound and systemic effect. This is how photons quickly create systemic effects. It was Mester’s experiments in 1966–67 that first documented the widespread, systemic effect of light therapy.
In general, it is well accepted that longer wavelengths, such as 800–900 nm, penetrate slightly deeper than shorter wavelengths, such as 600–700 nm. However, these differences are small and can be overcome by power. Thus, it is most important, when choosing an LED device, to balance wavelength and power.
Generally, lasers are more deeply penetrating, but LEDs are less expensive, slightly more superficial, and are very effective when used in unattended LED pads or large laser clusters. Assuming a similar wavelength, a more powerful LED will provide faster results.
Important Terms and Parameters
Treatment time is expressed in seconds (sec). This is the most important parameter because it determines the total dose.
Power is expressed in milliwatts (mW) or Watts (W). 1,000 mW is one Watt. The terms 1,000 milliwatts and 1 Watt are interchangeable.
Total dose or energy is expressed in joules (J). This is the power multiplied by the time, Watts x seconds.
Wavelength is expressed in nanometers (nm).
The projection or intensity of an LED is measured in millicandelas (mcd).
Light Energy Expressed in Joules
Light energy is expressed in joules (J). This energy is the result of multiplying the number of watts by the treatment time expressed in seconds.
Energy (joules) = Power (Watts) x Time (Seconds)
Some scientists describe a joule as a “Watt-Second,” signifying that the number of watts multiplied by the number of seconds is the output in joules. Therefore:
A 2,000 mW LED (2 Watt) applied for 60 seconds, deliver 120 joules of energy.
A2 W x 60 secs = 120 joules
A 1,000 mW LED (1 Watt) applied for 60 seconds, delivers 60 joules of energy.
W x 60 secs = 60 joules
A 500 mW (.5W) LED would deliver 30 joules of energy in 60 seconds.
0.5W x 60 secs = 30 joules
A 250 mW (.25W) LED would produce 15 joules of energy in 60 seconds
0.25W x 60 secs = 15 joules
How Much Light is Absorbed?
It is important that light be applied to clean skin, free from extra oils or creams. Oil will reduce the penetration of the photons by reflecting them off of the skin surface. We have an acronym for how the penetration of photons can occur: “R.A.T.S.,” which stands for Reflection, Absorption, Transmission, and Scattering.
As you can see in the diagram below, light will react in varied ways with different types of surfaces and materials. For example, light will reflect off of any oily surface, will be absorbed and transmitted through many cells and will scatter off of metal and plastic implants.
Ideally, the LED should be applied directly on the skin. If the structure you are treating is buried deep beneath adipose tissue, pressure should be applied to the device to increase the depth of contact and move the beam closer to the area being treated.
This also displaces fluids, such as blood, preventing too many of the photons from being absorbed on their way to the desired tissue. However, when treating wounds to prevent contamination, keep the probe about 1/4 to 1/2 inch above the skin.
If you are a professional, good note taking is important in any medical treatment. Always note the Subjective, Objective, Assessment, and Planning information in your daily notes.
Subjective stands for the patient’s symptoms, Objective describes any objective observations and/or orthopedic testing, Assessment is the patient’s diagnosis or response to treatment, and Plan refers to your treatment plan or what therapy was provided. In the treatment Plan you can note the following factors:
- Treatment time in seconds. This is the most important parameter to note as it determines the total dose, since typically the probe power is fixed.
- The anatomical location treated.
- Total output power of your device in mW or W. If you only use one probe, this would be optional since you only have one laser instrument.
- Wavelength of your device. If you only use one device, this would again be optional.
There are still a number of safety precautions to consider:
Contraindications of Light Therapy
Outside of the minor precautions regarding eye safety, there are a number of other precautions or contraindications. In spite of the fact that we have no documented evidence that the following problems will occur, it is better to be safe than sorry.
Do not treat over a known or suspected tumor or skin cancer. Although there has been no proof that this is dangerous, we do know that light therapy can stimulate the growth of many types of cells. However, in the field of oncology, light therapy is being used with many types of cancer patients to help manage pain and oral mucositis.
If you are not sure if laser is appropriate, ask the patient’s oncologist or physician.
Be cautious or avoid treatment during pregnancy. There has been no proof that light poses danger to pregnancy or pregnant women. Yet, due to the delicate nature of pregnancy and the natural possibilities for spontaneous abortion, a practitioner would be wise to avoid the use of light during pregnancy.
Photosensitivity reactions can occur because of many drugs, including antibiotics such as tetracycline, Retin-A, St. John’s Wort, some thyroid medications, and possibly any medication with a warning to avoid bright sunlight.
Some practitioners believe that light should not be applied over the thyroid gland because iodine in the thyroid is a strong photo-absorber and could be stimulated by the laser. However, research indicates that low doses are quite
Be cautious when treating over tattoos because dark-colored tattoos will absorb light and could become hot and painful.
Never treat directly over a bacterial infection. If you are treating an infected wound make sure the patient is on antibiotics and being monitored by a physician.
Do not treat patients on immune suppressant drugs since light therapy can stimulate the immune system and possibly interfere with the medical treatment.
Be careful treating children or sensitive adults. Always start with a very small total dose such as 25–50 joules for the first treatment.
Suspend therapy if the patient feels pain, weakness, or presents any other unusual reaction.
General Treatment Parameters
The treatment frequency or dose of treatment for Neuromusculoskeletal (NMS) syndromes and wounds depends on the severity of the syndrome. More acute problems require more frequent treatments. With chronic problems, the initial treatment needs a slightly lower dose than for acute injuries, and then also requires less frequent therapy:
Treat 2 to 3 times per week until symptoms subside.
Subacute or Chronic Syndromes:
Start with 2 times per week, followed by 1 to 2 times per week until symptoms have resolved.
Minimizing Adverse Side Effects:
The 100 Joule Rule
Excessive light dosage can occasionally cause an adverse response.
healthy, adult patient
If you are treating a very healthy, adult patient, the total treatment dose for the first session should not exceed 100 joules. With a very high powered LED, this may increase to 200 joules.
If the patient is suffering from any type of serious disorder, such as a non-healing wound or chronic pain, it is recommended that the first session utilize no more than 50 joules.
Treating Children: 50 Joule Rule
Use lower power and smaller doses when treating children. It is recommended that when treating children below the age of 12 years, to use an output of 50 joules or less during the first treatment, with a maximum of one-third to one-half the adult dosage,
based on body weight.
Maximum Dose Depends on Power
- Lower power LEDs (5 to 100 mW) require less total joules per treatment because they are more efficient and a higher percentage of the photons are absorbed by the tissues.
- Higher power LEDs (1,000–10,000 mW) require more total joules because the large number of photons can produce heat and many of the photons are not utilized
Symptoms of Excessive Treatment
If the dose is too high, the patient may exhibit any of the following symptoms:
Mild fatigue or nausea
Pain or heat at the treatment site
An increase in symptom severity
These symptoms can occur during or after treatment and typically last from 1 to 48 hours.
Initial Treatment: 50% Rule
Assuming you are using a 1,000 mW device, it is safest to start an adult with no more than 100 joules on the first session and for a child, 50 joules. This dose can be increased as long as each successive dose is increased by no more than 50% each treatment.
Thus, for adults, the first session would be 100 joules, the next session 150, etc. Obviously, this can be modified depending on the type of patient and/or condition. See the section on Dark and Light Skin, for more information on how different variables can affect the first and subsequent doses. It is best to warn the patient of the possibility of some increase in discomfort, even though that is rare.
Optimal Dose for Each Patient
The optimal dose for each patient can vary. This can be judged in the following way:
Ask the patient to report any sensation during treatment.
A warming or tingling sensation or a lessening of the symptoms can be good signs.
An immediate increase in pain is a bad sign. Stop treatment immediately. Next time, lower the dose.
Look for an observable reduction in any swelling or inflammation.
Test the patient for improvement in functional capacity. Ask the patient to perform a movement that triggered pain before the treatment.
Palpate the treated area for reduced tenderness and increased tone.
Ask the patient to note any negative reactions or improvements in the 24-hour period following treatment.