Low level laser therapy (LLLT) is also known as “soft laser therapy” and bio-stimulation. The use of LLLT in health care has been documented in the literature for more than three decades. Numerous research studies have demonstrated that LLLT is effective for some specific applications in dentistry .
The LLLT literature is large, with more than 1000 papers published on this topic. A problem in dissecting this literature is the variation in methodology and dosimetry between different studies. Not only have a range of different wavelengths been examined, but exposure times and the frequency of treatments also vary. The inclusion of sham-irradiated controls in clinical studies is an important element, since placebo effects can be important, particularly in terms of the level of pain experienced and reported following treatment .
While broad band light can exert effects on cells [2-3], interest has been concentrated on using lasers as a light source because of their greater therapeutic effect. While much of the initial work with LLLT used the helium-neon gas laser (632.8 nm), nowadays most LLLT clinical procedures are undertaken using semi-conductor diode lasers, for example, gallium arsenide-based diode lasers operating at 830 nm or 635 nm wavelengths . Since wavelength is the most important factor in any type of photo-therapy, the clinician must consider which wavelengths are capable of producing the desired effects within living tissues.
The typical power output for a low level laser device used for this therapy is in the order of 10-50 milliWatts, and total irradiances at any point are in the order of several Joules. Thermal effects of LLLT on dental tissues are not significant , and do not contribute to the therapeutic effects seen. The wavelengths used for LLLT have poor absorption in water, and thus penetrate soft and hard tissues from 3 mm to up to 15 mm. The extensive penetration of red and near-infrared light into tissues has been documented by several investigators . As the energy penetrates tissues, there is multiple scattering by both erythrocytes and microvessels. Because of this, both blood rheology and the distribution of microvessels in the tissue influence the final distribution pattern of laser energy .
2. Mechanism of action
The mechanisms of low level laser therapy are complex, but essentially rely upon the absorption of particular visible red and near infrared wave lengths in photoreceptors within sub-cellular components, particularly the electron transport (respiratory) chain within the membranes of mitochondria [2,7]. The absorption of light by the respiratory chain components causes a short-term activation of the respiratory chain, and oxidation of the NADH pool. This stimulation of oxidative phosphorylation leads to changes in the redox status of both the mitochondria and the cytoplasm of the cell. The electron transport chain is able to provide increased levels of promotive force to the cell, through increased supply of ATP, as well as an increase in the electrical potential of the mitochondria membrane, alkalization of the cytoplasm, and activation of nucleic acid synthesis . Because ATP is the "energy currency" for a cell, LLLT has a potent action that results in stimulation of the normal functions of the cell. The specific actions of LLLT are summarized in Table 1.
Karu, who has studied the bio-stimulative effects of light on cell cultures in great detail, has demonstrated that cell cultures which are initially irradiated with laser light show a range of biological effects [7,9,10]. Of importance, if these cultures are then irradiated with nonmonochromatic and incoherent light, the previous laser-produced biological effects are almost nullified. This suggests that there are more complex mechanisms at work than the simple excitation of polarization-sensitive chromophores in the cell.
It is crucial to recognize the optical distinction between irradiating human tissues, in which light will scatter very widely, and a thin transparent monolayer of cells in a laboratory. In this context, a key issue is polarization of the light, since polarized and non-polarized light can bring about different biological responses. In a thin layer of cells in culture, the polarization of laser light is maintained through the entire thickness of the cell layer. The work of Mester , which used leucocytes in the laboratory setting, indicates that both polarized laser light and polarized incoherent light can evoke bio-stimulation, whilst no such stimulation occurs with non-polarized incoherent light.
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