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Photo-neuro-modulation (PNM), or photobiomodulation therapy (PBMT) of neuronal tissue, has demonstrated stimulating axonal growth of the spinal cord after acute structural damage in rodent models. The effectiveness of PNM for rodent models of spinal cord injury has been established at terminal irradiances of approximately 3.2mW/cm2. For clinical translation however, the existing surface irradiation protocol results in significant treatment dose attenuation by the vertebral bones as the light travels to the spinal cord. We reported that with a surface CW power and irradiance as intense as 10W and 3.14W/cm2 respectively less than 100µW/cm2 irradiance reached the spinal cord level of dogs. In exploring clinical transmission of photobiomodulatory irradiance to the spinal cord level, we piloted in a cadaveric rabbit model local light delivery to the cerebral-spinal-fluid (CSF) space using a 200µmx1.75mm (diameterxlength) diffuser fiber at L6/7 via a spinal needle. The irradiances at multiple sites of up to 9 centimeters distance along the spinal canal were measured sequentially by a second diffuser fiber via the corresponding vertebrae disks. At an intra-spinal irradiation of 8mW CW 830nm light which was 1/1,250 of the 10W surface power used for the transcutaneous dosimetry in dogs, irradiances at 1.5~2cm from the local delivery site were 1~2 mW/cm2 (2 rabbits, 4 measurements). The distance-dependent irradiances along the spinal canal corresponded to an effective attenuation coefficient of 0.1021±0.0213 mm-1 (n=4). In comparison (p=0.0274), the distance-dependent irradiances along the epaxial muscle upon interstitial delivery corresponded to an effective attenuation coefficient of 0.0677±0.0105 mm-1 (n=4). “Epidural” PNM is feasible.
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