How does it work?
The effects of laser therapy are photochemical in general and with super-pulsed lasers also photomechanical. Photons enter the tissue and are absorbed in the cell’s mitochondria and at the cell membrane by chromophores. These chromophores are photosensitizers that generate reactive oxygen species following irradiation thereby influencing cellular redox states and the mitochondrial respiratory chain. Within the mitochondria, the photonic energy is converted to electromagnetic energy in the form of molecular bonds in ATP. It is obvious that, in order to interact with the living cell, laser light has to be absorbed by intracellular chromophores.
Cell membrane permeability increases, which promotes physiological changes to occur. These physiological changes affect macrophages, fibroblasts, endothelial cells, mast cells, bradykinin, and nerve conduction rates.
The clinical and physiological effects are obtained by the way in which the tissues absorb laser radiation. This tissue absorption depends on the wavelength of the beam itself and the power to ensure that the laser energy reaches the target tissue at the necessary clinical levels. The use of an improper wavelength laser would not penetrate into the tissue to reach the target area. Furthermore, even if one has a laser with the proper wavelength, if the device does not have enough power to drive the energy into the tissue, the target area may not realize the potential benefits.
Each type of laser emits light at a very specific wavelength which interacts with the irradiated tissue. It also acts in particular with the chromophores present in the tissue, but in a different way. A chromophore, intrinsic or extrinsic, is any substance, colored or clear, which is able to absorb radiation. Among the endogenous chromophores, water and hemoglobin, nucleic acid and proteins can be listed. Among the exogenic chromophores we can instead find porphyrins and hematoporphyrins, which are injected into the organism. These are described as photosensitizers because they fix themselves to the tissue making it photosensitive at specific wavelengths.
How deep into the tissue can laser light penetrate?
The level of tissue penetration by the laser beam depends on its optical characteristics, as well as on the concentration and depth of the chromophores, which according to the wavelength are absorbed at different percentages. For instance, water absorbs almost 100 percent of the laser irradiation at the 10,600 nanometer wavelength, the wavelength of the CO2 gas laser. That is the reason why this type of laser wavelength is used in surgical applications.
Other factors affecting the depth of penetration are the technical design of the laser device and the treatment technique used. There is no exact limit with respect to the penetration of the light. The laser light gets weaker the further from the surface it penetrates with a limit at which the light intensity is so low that no biological effect of the light can be measured. In addition to the factors mentioned above, the depth of penetration is also contingent on tissue type, pigmentation and foreign substances on the skin surface. Bone, muscles and other soft tissues are transparent to certain laser lights, which means that laser light can safely penetrate these tissues.
The radiation in the visible spectrum, that between 400 and 600 nanometers, is absorbed by the melanin, while the whole extension of the visible which goes from 420 to 750 nanometers is absorbed by composite tetrapyrrolics. In the infrared, which covers about 10,000 nanometers of the light spectrum, water is the main chromophore. Fortunately, there exists a narrow band in the light spectrum where water is not a highly efficient chromophore, thereby allowing light energy to penetrate tissue that is rich in water content. This narrow band, which extends approximately from 600 to 1,200 nanometers, is the so-called therapeutic window. That is the reason why the therapeutic lasers in the market today have wavelengths within this therapeutic window. The penetration index is not the same level throughout the therapeutic window. In fact, lasers in the 600 to 730 nanometers have less penetration and are suitable for superficial applications such as in acupuncture.
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