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CHAPTER 5 Compared to skin tone, differences in the photobiological properties of muscles and bones might be a bigger contributor to the variation in penetration (Figure 2.3). This difference is likely to be due to the presence of some major chromophores in muscles (Section 1.2.1.2). A more thorough investigation, taking into account the relative thickness of the two tissue types, needs to be carried out in order to establish a model for light penetration in tissues with muscle and bone. As concluded in Chapter 2, the use of cadavers may facilitate investigations of a particular wavelength in sites not readily available in live humans, e.g. the spinal cord. 670 nm light of 100 mW/cm2 is found to penetrate tissues up to 50 mm thick (Figure 2.1). This thickness is more than the distance from the posterior skin to the epidural space at C5-6 in humans (Zhao et al., 2014). This thesis reports ~91% attenuation of 670 nm LED (35 mW/cm2) through a rat’s spinal cord, giving an irradiance of 3.2 mW/cm2 at the targeted site (Figure 3.1). While this data provides some insight into light attenuation through multiple tissue layers (hair, skin, muscle, bone, various connective tissues, and spinal cord) in the rat, translation to humans requires direct measurement of irradiance at the target site in order to attain the desired exposure necessary to achieve therapeutic effects (AlGhamdi et al., 2012). The commonly reported unit for photobiomodulation studies is J/cm2 (1 J = 1 W.s), however this unit, reflecting both the irradiance and the irradiation time, may be rather misleading. It is rather unlikely that a small irradiance, with a long irradiation time, will provide the same effect as a large irradiance, with a short irradiation time; these combinations have largely different outcomes on penetration and therefore tissue exposure at deeper levels. 5.2 Therapeutic effects of 670 nm LED treatment following spinal cord injury Chapter 3 and Chapter 4 focused on the effects of 670 nm treatment, following spinal cord injury in rats. As mentioned in Section 1.1.1, the pathophysiology of spinal cord injury starts with immediate changes in the cellular microenvironment (Section 1.1.1.1), which ultimately lead to long-term functional deficits (Section 1.1.1.2). The results of these two chapters are discussed below and divided into cellular (Section 5.2.1) and functional (Section 5.2.2) changes. 5.2.1 Cellular changes A large variety of cellular events takes place in response to spinal cord injury (Section 1.1.1.1). This thesis examined the impact of red light on inflammation (Section 5.2.1.1), gliosis (Section 5.2.1.2), and cell death (Section 5.2.1.3) in the subacute phase of spinal cord injury, between 1 and 7 days. 5.2.1.1 Inflammation Following spinal cord injury, microglia and macrophages are activated (David and Kroner, 2011). They secrete numerous cytokines and other molecules at the injury site, which cause long-term inflammation, and ultimately function loss. One of the keys to better recovery is a balanced and 114PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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