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Biphasic Dose Response in LLLT – An Update the actual light source and spot size; values from 5 to 50 mW/cm2 are common for stimulation and healing, while much higher irradiances (up to W/cm2) can be used for nerve inhibition and pain relief. LLLT is typ- ically used to promote tissue regeneration, reduce swelling and inflam- mation and relieve pain and is often applied to the injury for 30 seconds to a few minutes or so, a few times a week for several weeks. Unlike other medical laser procedures, LLLT is not an ablative or thermal mechanism, but rather a photochemical effect comparable to photosynthesis in plants whereby the light is absorbed and exerts a chemical change. Within a decade of the introduction of LLLT in the 1970s it was real- ized that more does not necessarily mean better. The demonstration of the biphasic dose response curve in LLLT has been hampered by dis- agreement about exactly what constitutes a “dose”. Many practitioners concentrate on fluence as the principle metric of dose, while others pre- fer irradiance or illumination time. The use of very small spot sizes by some practitioners has led to the assertion that they delivered hundreds of mW/cm2 from a 50 mW laser. While this statement is mathematically correct it can give the impression that much higher doses of light were given than actually were delivered. Two years ago we reviewed (Huang et al. 2009) the biphasic dose response in LLLT and found many reports in the literature concerning biphasic dose responses observed in cell cultures, some in animal exper- iments but no clinical reports. We now believe that the time is right to revisit this interesting topic for two reasons. Firstly because we have found more instances in our laboratory both in vitro with cultured cortical neu- rons, and in vivo with LLLT of traumatic brain injuries in mouse models. Secondly because advances have been made in mechanistic understand- ing of how LLLT works at a cellular level that may explain why a little light may be beneficial and at the same time a lot of light might be harmful. MECHANISMS OF LOW LEVEL LIGHT THERAPY. Basic photobiophysics and photochemistry According to the First Law of Photochemistry, the photons of light must be absorbed by some molecular photoacceptors or chromophores for photochemistry to occur (Sutherland 2002).The mechanism of LLLT at the cellular level has been attributed to the absorption of monochro- matic visible and near infrared (NIR) radiation by components of the cel- lular respiratory chain (Karu 1989). Phototherapy is characterized by its ability to induce photobiological processes in cells. The effective tissue penetration of light and the specific wavelength of light absorbed by pho- toacceptors are two of the major parameters to be considered in light therapy. In tissue there is an “optical window” that runs approximately from 650 nm to 1200 nm where the effective tissue penetration of light is 603PDF Image | BIPHASIC DOSE RESPONSE IN LOW LEVEL LIGHT THERAPY AN UPDATE
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