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CHAPTER 2 In summary, the results indicate that in live subjects, irrespective of skin tone, red light readily penetrates tissues up to 50 mm of thickness and requires 100 mW/cm2 or less. The presence of bone and/or reduction of muscle increase the level of penetration in live subjects. Cadaveric tissues generally appear to require similar levels of light intensity to that of live tissues to achieve penetration but demonstrate greater variability. 2.5 Discussion PBM is increasingly becoming popular as an alternative therapy for sports related injuries (Chang et al., 2014; Vanin et al., 2016), however there are no standard treatment regimens at present. Irradiation intensity is a key parameter that influences treatments but may be affected by numerous factors. The main aim was to determine clinically relevant parameters for red (670 nm) light irradiation, by improving the understanding of the relationship between light intensity and tissue penetration under the influence of factors such as tissue thickness, skin tone and bone/muscle composition. The secondary objective was to compare penetration in live and cadaveric tissues so that future studies using cadaveric tissues can extrapolate findings to live tissues. The results indicated that penetration of 670 nm irradiation: i) is complete up to 50 mm below the skin surface in tissues comprising skin, tendon, bone and muscle at an intensity of 100 mW/cm2, ii) is largely unaffected by skin tone, and iii) is improved in tissues that comprise a greater proportion of bone and/or reduced proportion of muscle for a given tissue thickness. Furthermore, cadaveric tissues can be used to model live tissue, however with a few caveats that will be discussed below. The main finding is that red light can successfully penetrate up to 50 mm below the skin surface in the tissues and intensities that were examined in live and cadaver subjects. Future studies can use the equations to estimate the penetration of 670 nm light for any given tissue thickness (between 20 and 49 mm) at 100, 300, and 500 mWcm2. It was also demonstrated that increasing the intensity beyond 100 mW/cm2 did not greatly improve penetration, and therefore it is proposed to use 100 mW/cm2 as an intensity starting point for red light treatment for tissues of less than 50 mm of thickness. However, while penetration is evident up to 50 mm of tissue thickness, the intensity is in the μW/cm2 range. The biological effect (if any) at this level of red light irradiation warrants further investigations. For structures approaching 50 mm of thickness, one strategy to increase photon delivery to tissues would be to apply the light source from multiple sides of the site of interest. Light intensity and tissue thickness are two important factors governing the level of red light penetration through human tissues. Increasing the intensity results in an increase in the delivery of photons to the site, and thereby improves penetration, while tissue layers act as a barrier that absorb and scatter the incoming photons and thereby reduces penetration (Anderson and Parrish, 1981). Tissue thickness has a nonlinear effect on light penetration because changing this 49PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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