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CHAPTER 2 tones, as well as human cadavers of unknown skin tone (grey). (a) Thickness of tissues subjected to light penetration measurements are shown for live subjects of each skin tone as well as cadaver subjects. (b) Penetration of 660 nm through the wrist at different red light intensities. (c) Penetration of 660 nm through the thumb at different red light intensities. (d) Penetration of 660 nm through the middle finger at different red light intensities. Data expressed as mean ± SEM; sample sizes (n) indicated; ns p > 0.05 (a: AOV; b-d: LMER). 2.4.3 670 nm LED penetrates better in tissues with bone component Since bone and muscle tissue have different optical properties (Jacques, 2013), it is worthwhile examining the effect of bone/muscle content on red light penetration. The penetration between the thumb and the middle finger was first compared. These two structures were not significantly different in thickness (Figure 2.2a; live subjects, p = 0.12; cadaver subjects, p = 0.65, paired t- test), but contain vastly different proportions of bone and muscle content. It was found that the middle finger joint, a structure with a greater proportion of bone to muscle compared to the thumb, had significantly more penetration across live and cadaver subjects (p < 2e-16; LMER). The penetration of the upper arm through two sites was then compared; one that passed exclusively through muscle (biceps site) and the other that includes muscle and the long bone (humerus site). There was no significant difference in tissue thickness among the different skin tone groups (p = 0.60, AOV) of the biceps (Figure 2.3a) or humerus (Figure 2.3b) in live subjects, however, the thickness of cadaver specimens was greatly reduced compared to the live tissues for both structures (p = 3.76e-08, AOV). Furthermore, the humerus site in live subjects was overall significantly thicker compared to the biceps site (p = 0.003, AOV), but this was not apparent for the cadaver subjects alone (p = 0.48, Tukey HSD post-hoc). Next, all subjects that were penetrable at ≤ 500 mW/cm2 were selected and plotted penetration against light intensity for the biceps (Figure 2.3c) and humerus (Figure 2.3d). To obtain an overview of the effect of skin tone, and live vs. cadaveric tissues, data from each individual was colour coded to their respective groups. The left end of each line represents the minimum intensity by which red light penetrated the tissue; the right end of each line indicates the penetration at maximum red light intensity. In live tissues, there was no obvious relationship between skin tone and the minimum intensity required to penetrate, or penetration at maximum red light intensity over the ranges investigated. The two cadaver specimens demonstrated a greater penetration in the biceps (Figure 2.3c), however, statistical analysis on all available data indicated that the only difference between live and cadaver subjects occurred at intensities of 60-100 mW/cm2 (p ≤ 0.039, Kruskal-Wallis rank sum test). Colour coding was applied for tissue thickness below and above 50 mm. Colour coding was applied for tissue thickness below and above 50 mm (Figure 2.3e-f for biceps and humerus respectively) to facilitate the interpretation of Figure 2.3c-d. Despite the 47PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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