Transcutaneous Transmission of Light of Photobiomodulation Therapy Wavelengths at 808 nm, 915 nm, 975 nm, and 1064 nm to the Spinal Canal of Cadaver Dogs

This study compared the transcutaneous target level irradiances from the thoracic to lumbar segments of the interior spinal canal in three cadaver dogs, measured for light at four wavelengths (808 nm, 915 nm, 975 nm, and 1064 nm), common in photobiomodulation therapy (PBMT). Intra-spinal irradiances at nine sites spanning approximately 8 cm in length were measured using a flexible intra-spinal probe under surface application of continuous-wave (CW) light with powers ranging from 0.5 W to 2 W. Surface illumination was applied using an acupuncture treatment head in three modes: non-contact with skin removed, non-contact with skin intact, and contact with skin intact. During surface application, the treatment head was positioned over the spinal canal near the 13th vertebrae (T13, surface site 1), and approximately 4 cm (surface site 5) and 8 cm (surface site 9) caudal to T13. At each position of the treatment head, the light was multiplexed among the four wavelengths at the same power setting. In all three modes of surface application, the target level irradiance at the 1064 nm wavelength was significantly greater than that at the other three wavelengths (p <= 0.0017). At a surface irradiance of similar to 157 mW/cm(2), corresponding to 0.5 W light applied with the treatment head directly in contact with the skin, the intra-spinal irradiance at 1064 nm reached 0.137 +/- 0.095 mW/cm(2). Obtaining a dosage of PBMT-associative wavelengths of this magnitude at the level of the spinal canal may guide focused research into the transcutaneous applicability of PBMT for spinal cord injuries.