(PHOTOBIOMODULATION)
1. What is photobiomodulation (low-power laser therapy?)
More than 30 year ago the first publications about low-power
laser therapy or photobiomodulation (at that time called laser biostimulation) appeared. Since then approximately 2000 studies have been published on this topic (analysis of these publications can be found in [1]). Medical treatment with coherent light sources (lasers) or noncoherent light (Light Emitting Diodes, LED's) has passed through its childhood and early maturity. Photobiomodulation is being used by physiotherapists (to treat a wide variety of acute and chronic muscosceletal aches and pains), dentists (to treat inflamed oral tissues, and to heal diverse ulcerations), dermatologists (to treat oedema, indolent ulcers, burns, dermatitis), rheumatologists (relief of pain, treatment of chronic inflammations and autoimmune diseases), and by other specialists (e.g., for treatment of middle and inner ear diseases, nerve regeneration). Photobiomodulation is also used in veterinary medicine (especially in racehorse training centers) and in sports medicine and rehabilitation clinics (to reduce swelling and hematoma, relief of pain and improvement of mobility and for treatment of acute soft tissue injuries). Lasers and LED's are applied directly to respective areas (e.g., wounds, sites of injuries) or to various points on the body (acupuncture points, muscle trigger points). For details of clinical applications and techniques used, the books [ 1-3] are recommended.
2. What light sources (lasers, LED's) can be used?
The field of photobiomodulation is characterized by variety of methodologies and use of various light sources (lasers, LED's) with different parameters (wavelength, output power, continuous wave or pulsed operation modes, pulse parameters). These parameters are usually given in manufacturers manuals.
The GaAlAs diodes are used both in diode lasers and LED's, the difference is whether the device contains the resonator (as the laser does) or not (LED). In latter years, longer wavelengths (-800-900 nm) and higher output powers (to 100 mW) are preferred in therapeutic devices.
Should a medical doctor use a laser or a diode? The answer is - it depends on what one irradiates, in other words, how deep tissue layers must be irradiated. By light interaction with a biotissue, coherent properties of laser light are not manifested at the molecular level. The absorption of low-intensity laser light by biological systems is of a purely noncoherent (i.e., photobiological) nature. On the cellular level, the biological responses are determined by absorption of light with photoacceptor molecules (see the section 3 below). Coherent properties of laser light are not important when cellular monolayers, thin layers of cell suspension as well as thin layers of tissue surface are irradiated (Fig. 1). In these cases, the coherent and noncoherent light (i.e., both lasers and LED's) with the same wavelength, intensity and dose provides the same biological response. Some additional (therapeutical) effects from the coherent and polarized radiation (lasers) can occur in deeper layers of bulk tissue only and they are connected with random interference of light waves. An interested reader is guided to the ref. [4] for more details. Here we illustrate this situation by Fig. 1. Large volumes of tissue can be irradiated by laser sources only because the length of longitudinal coherence Lcoh is too small for noncoherent radiation sources [4].
3. Enhancement of cellular metabolism via activation of respiratory chain: a universal photobiological action mechanism
A photobiological reaction involves the absorption of a specific wavelength of light by the functioning photoacceptor molecule. The photobiological nature of photobiomodulation means that some molecule (photoacceptor) must first absorb the light used for the irradiation. After promotion of electronically excited states, primary molecular processes from these states can lead to a measurable biological effect (via secondary biochemical reaction, or photosignal transduction cascade, or cellular signaling) at the cellular level. The question is, which molecule is the photoacceptor.
Fig. 1. Depth (On in which the beam coherency is manifested, and coherence length Lcoh in various irradiated systems: (A) monolayer of cells, (B) optically thin suspension of cells, (C) surface layer of tissue and bulk tissue. Lcoh, - length of temporal (longitudinal) coherence of laser light, hw) marks the radiation.
When considering the cellular effects, this question can be answered by action spectra. Any graph representing a photoresponse as a function of wavelength, wave number, frequency, or photon energy, is called action spectrum. Action spectra have a highest importance for identifying the photoacceptor inasmuch as the action spectrum of a biological response resembles the absorption spectrum of the photoacceptor molecule. Existence of a structured action spectrum is strong evidence that the phenomenon under study is a photobiological one (i.e., primary photoacceptors and cellular signaling pathways exist). Fig. 2 represents some examples of action spectra for eukaryotic cells: two of them (A, B) consider the processes occurring in cell nucleus, and one spectrum (C) is for cell membrane. Fig. 2D shows the absorption spectrum of the monolayer of the same cells.
