The term "laser" originated as an acronym that describes its process, ie, light amplification of stimulated emission of radiation. Like acupuncture and massage, laser therapy lessens pain, relaxes muscles, and improves circulation. It accomplishes this by altering the physiology of cells and tissue by means of light (photons) instead of an acupuncture needle or manual pressure. Therefore, treatment effectiveness and the types of responses seen depend heavily on if and how light enters living tissue.
Mechanisms of Action:
For tissue to absorb light and alter its physiology, a photochemical or photobiologic event must occur. Ideally, this event would take place within the target tissue(s), whether it be skin, muscle, fascia, nerves, vessels, bones, and/or joints. A "photoacceptor" molecule, also known as a "chromophore," responds to light by initiating a series of physiologic responses that engender healing and improved tissue homeostasis. When a chromophore (such as cytochrome c oxidase in the mitochrondria respiratory chain) absorbs a photon from laser-treated tissue, an electron within the chromophore becomes excited and jumps from a low- to a higher-energy orbit. This increased electron energy provides the impetus for the system to perform cellular activities geared toward growth and repair.
The effects of laser on mitochondria, cells, and tissue is called "photobiomodulation." This collective process encompasses not only the effects of lasers but also those of light-emitting diodes (LEDs) and other light sources. Light therapy also causes vasodilation by relaxing endothelial smooth muscle, likely through nitric oxide mechanisms. Vasodilation improves tissue oxygenation and supports the migration of immune cells into tissue, further aiding recovery.
Many factors impact how light influences tissue, including its power, wavelength, strength, pulse characteristics, tissue contact, and the nature of its beam. As indicated above, photobiomodulation entails changes at the subcellular, cellular, and tissular levels. Within the mitochondria, activated photons engender increases in production of ATP, modulation of reactive oxygen species, and induction of transcription factors. These factors encourage cell proliferation and migration, normalized cytokine levels, enhanced production of growth factors, modulated levels of inflammatory mediators, and improved oxygenation of tissue.
Power and Dose:
The term "low level laser therapy" (LLLT) refers to the use of light at much lower levels than those used for tissue ablation or photocoagulation. Newer, high-powered therapy devices that deliver power similar to surgical lasers (but with a less concentrated beam) no longer constitute LLLT. Some even heat tissue, meaning that the term "cold laser" is also inaccurate.
The specific dose(s) of laser required to heal tissue and treat pain remains unclear. Calculating actual joules of energy delivered requires calculations of considerable complexity. Fortunately, a wide range of doses shows benefit for people and experimental animals, despite the wide variety in size, color, and hair coat.
Most therapy units use red or near infrared light, from 600 nm to 1070 nm. This range constitutes the "optical window" wherein effective absorption into tissue is maximal. That said, units with green, blue, and violet light (~400 nm range) are becoming more popular. Visible light ranges from 390 to 760 nm, progressing from violet to blue, green, yellow, orange, and red at 600 nm.
The types and depth of tissue that respond to light therapy depend on the wavelength delivered. Certain molecules, such as melanin and hemoglobin, preferentially absorb light in the 600 nm range. To reach deeper tissues, wavelengths (810 nm, 980 nm) that absorb less in superficial tissue can be used, leaving more light to reach for deeper sites such as bone, the brain, and internal organs.
Certain laser therapy units emit two or more beams to target a variety of tissues.
Laser beams differ from other types of light therapy, including LEDs, by being monochromatic (existing within a narrow band of wavelengths), coherent (tightly aligned), and collimated (photons travel in parallel). The more light scatters within tissue, the less intense the biologic impact, which may or may not be the desired outcome. However, debate continues about the relative value and differences between laser light and LEDs.
It remains unclear whether pulsed wave or continous wave treatment is preferable for certain conditions. Pulsing reduces tissue warming, which is especially important to deliver light to deeper tissue; this requires more power to provide adequate energy supply to the deep tissue target(s). Furthermore, pulsing limits damage to nerves and surrounding tissue.
Pulsing may also improve tissue responses by resonating with a fundamental frequency to which cells innately respond, commonly exhibited by neural structures.
For more than half a century, scientists have recognized the potential for photomedicine approaches to reduce inflammation, pain, and swelling, as well as to speed wound healing. More recently, medical researchers have uncovered additional benefits pertaining to serious conditions, such as myocardial infarction, spinal cord injury, traumatic brain injury, and stroke. Additional applications include alleviation of pain, trigger point pathology, and joint dysfunction.
Contraindications to direct laser treatment include carcinoma, thyroid gland, active hemorrhage, and autonomic nerve centers. Laser therapy should be avoided in patients in which immune stimulation is not desired, including those with lymphoma or on immunosuppressant medications. In immature patients, higher powered laser therapy devices may stimulate premature closure of epiphyses. Thus, caution is warranted over long bones in animals <1 yr old.
Properly used, laser therapy appears to be very safe. However, higher powered lasers run the risk of inducing thermal burns when improperly used. Tattoos, when lasered, can cause intense pain due to the high amount of light absorption by deposited pigment. Questions remain about the ability of laser therapy to stimulate neoplastic growth and, if so, at what wavelength(s) and power(s).
Laser light can damage the retina, whether reflected off shiny surfaces or shown directly into the eye. Laser goggles protect against indirect exposure but not against direct. One should never look into the applicator of a laser therapy device.
Laser light longer than 760 nm is invisible to the human eye. As such, unless a treatment applicator produces a visible finder beam or audible signal, the practitioner will not know when the laser is emitting light. This could cause inadvertent eye exposure and retinal damage. It also could cause confusion about where the beam is pointing. For this reason, infrared laser devices that lack a clear indicator of when the beam is on pose potential safety hazards.
The immunostimulatory effects of laser therapy may counteract immunosuppressive actions of certain medications. In addition, photosensitizing agents such as hypericin in St. John's wort may augment the dermatologic impact of laser light.
The main controversies surrounding laser therapy involve questions and unproven claims related to pulsed therapy, the advantage of low- or high-powered units, and the longterm safety of high-powered treatment.