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Y.-Y. Huang and others FIGURE 8. Idealized biphasic dose response curve (often termed Arndt-Schulz curve) typically reported in LLLT studies. In the context of LLLT the increasing “stimulus” may be irradiation time or increased irradiance. This non-linear effect contradicts the Bunsen-Roscoe rule of reciprocity (which was originally formulated for visual detection of light by photoreceptors (Brindley 1952)), which pre- dicts that if the products of exposure time in seconds and irradiance in mW/cm2 are equal, i.e. the energy density is the same, then the changes in biological endpoint will be equal. This inverse linear relationship between irradiance and time has frequently failed in LLLT research (Karu and Kolyakov 2005; Lubart et al. 2006). A “biphasic” curve can be used to illustrate the expected dose response to light at a subcellular, cellular, tissue or clinical level. Simply put, it suggests that if insufficient energy is applied there will be no response (because the minimum threshold has not been met), if more energy is applied the then a threshold is crossed and biostimulation is achieved but when too much energy is applied then the stimulation dis- appears and is replaced by bioinhibition instead. An idealized illustration (Figure 8) similar to that suggested by Sommer (Sommer et al. 2001) helps understand the concept. 3.2. Biphasic Response—irradiance As early as 1978 Endre Mester observed a “threshold phenomenon” after laser irradiation of lymphocytes in vitro (Mester et al. 1978). Peter Bolton in 1991 irradiating macrophages with two different irradiances (W/cm2) but the same energy density (J/cm2) recorded different results (Bolton et al. 1991). Karu (Karu and Kolyakov 2005) found a dependence of stimulation of DNA synthesis rate on light intensity at a constant ener- gy density 0.1 J/cm2 with a clear maximum at 0.8 mW/cm2. In another study (Karu et al. 1997) the same group found no less than seven maxima in the dose vs. biological effect curves using a pulsed 810-nm diode laser.PDF Image | BIPHASIC DOSE RESPONSE IN LOW LEVEL LIGHT THERAPY
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