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CHAPTER 2 LMER), there was no significant effect of intensity increase on penetration between 100 and 300 mW/cm2 (p = 0.256) and 300 and 500 mW/cm2 (p = 0.466) in cadaver structures. There were an additional 55 structures with thickness between 95 and 150 mm that were tested in the 9 live (but not cadaver) subjects. These structures included the shoulder, humerus, coronal elbow, hamstring, femur, sagittal and coronal knee, calf, and sagittal ankle. No detectable readings were obtained at 100 and 300 mW/cm2 intensities, however, 5.5% of structures demonstrated red light penetration at an intensity of 500 mW/cm2. These data indicate: 1) 100 mW/cm2 is sufficient to reliably penetrate through tissue structures of 50 mm or less in live subjects, 2) increasing the intensity to 500 mW/cm2 increases the penetration, and 3) complete penetration of red light is negligible in tissues with an optical path greater than 95 mm, even at 500 mW/cm2. 2.4.2 670 nm LED penetration is not affected by skin tone As melanin is a key chromophore in the skin (Anderson and Parrish, 1981), the next objective was to examine the effect of skin tone on red light penetration. First, the minimum intensity necessary for red light to penetrate structures as a function of tissue thickness for each skin tone (light, n = 21; medium, n = 14; dark n = 18) and cadaveric tissues (n = 22; Figure 2.1d) was plotted. Skin tone did not have a significant effect on red light penetration (p = 0.52, AOV), nor did the subjects (live vs cadaveric tissue; p = 0.83, AOV). A piecewise envelope was then applied that captures the minimum intensity required to penetrate tissues at a given thickness. This envelope comprises of two boundary lines (dotted, Figure 2.1d), one relatively flat and other relatively steep, that captures all data points with the exception of three (13.6%) cadaver outliers. The two boundary lines intersect at an intensity of 100 mW/cm2 and a tissue thickness of 50 mm, indicating that 100 mW/cm2 is sufficient to penetrate all tissues (i.e. skin tones and cadaver material) to a depth of 50 mm, with the exception of one cadaver outlier which came from the humerus site of a cadaver. For the majority of tissues greater than 50 mm of thickness, a disproportionately greater intensity is required for successful penetration (Figure 2.1d). At an intensity of 100 mW/cm2 and at tissue thicknesses of ≤ 50 mm, there was no difference in red light penetration among the different skin tones of live subjects (p = 0.62, LMER), or between live vs cadaveric tissues (p = 0.20, LMER). To minimise the effect of tissue thickness when evaluating the effect of skin tone on red light penetration, wrist, thumb and middle finger sites were selected for comparisons. These structures were selected because: 1) there was no significant difference among each of the respective structures across all live and cadaver subjects (Figure 2.2a; p = 0.17, AOV), 2) all these structures from live and cadaver subjects were ≤ 50 mm and therefore penetrated by red light, and 3) each respective structure contains the same layers of tissue elements, thereby minimising any random factors between groups. The mean light penetration at all intensities investigated for each skin tone and cadaveric tissues are shown for the wrist, thumb and middle figure (Figure 2.2b-d 42PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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