Light delivery for potential bacterial disinfection (UV/blue) and photobiomodulation (near-IR) requires specific, concentrated and controllable local irradiance and dose. Dental targets for light irradiation involve dentine, which scatters, absorbs and reflects light, reducing local irradiance. This study compared the effectiveness of LEDs (400-900nm) and lasers (660nm and 810nm) to penetrate dentine. Caries-free wisdom teeth were sectioned through the Pulpchamber by either cutting perpendicular to the crown, the buccal aspect or obliquely. Specimens were wet-polished to 1, 2 or 3mm thicknesses to expose the dentine on opposing surfaces. The beam profile of the LEDs/lasers were measured through dentine specimens (n=5) to obtain beam width following optical calibration, and spatial irradiance distribution following photodiode power calibration. There were no significant differences in the percentage power and irradiance transmitted through different dentine specimens between LEDs and lasers (P>0.05). However, light penetration through tissue was wavelength dependent and highest for red and near-IR wavelengths (P<0.05) for specimens cut perpendicular to the crown compared with buccal and oblique specimens. The beam diameters increased and irradiance decreased significantly (P<0.05) with increasing specimen thickness/distance for both LEDs and lasers. There was a noticeable shift in beam position for all light sources in buccal and oblique specimens. Data indicated that dentine tubule orientation may alter the direction of light through the tissue. Optimal light penetration and distribution through dentine at specific distance is best achieved with a flat-top beam distribution vertically through the crown of the tooth.
Human dental pulp cells (DPCs) were isolated and cultured in phenol-red-free α-MEM/10%-FCS at 37ºC in 5% CO<sub>2</sub>. DPCs at passages 2-4 were seeded (150μL; 25,000 cell/ml) in black 96-microwell plates with transparent bases. 24h post-seeding, cultures were irradiated using a bespoke LED array consisting of 60 LEDs (3.5mW/cm<sup>2</sup>) of wavelengths from 400-900nm (10 wavelengths, n=6) for time intervals of up to 120s. Metabolic and mitochondrial activity was assessed via a modified MTT assay. Statistical differences were identified using multi-factorial analysis of variance and post-hoc Tukey tests (P=0.05). The biological responses were significantly dependent upon post-irradiation incubation period, wavelength and exposure time (P<0.05). At shorter wavelength irradiances (400nm), a reduction in mitochondrial activity was detected although not significant, whereas longer wavelength irradiances (at 633, 656, 781 and 799nm) significantly increased mitochondrial activity (P<0.05) in DPCs. At these wavelengths, mitochondrial activity was generally increased for exposures less than 90s with 30s exposures being most effective with 24h incubation. Increasing the post-irradiation incubation period increased the measured response and identified further significance (P<0.05). The biological responses of human DPCs were wavelength, exposure-time and incubation period dependent. The optimisation of irradiation parameters will be key to the successful application of LLLT in dentistry.
Low level laser/light therapy (LLLT) or photobiomodulation is a biophysical approach that can be used to reduce pain, inflammation and modulate tissue healing and repair. However, its application has yet to be fully realized for dental disease treatment. The aim of this study was to assess the modulation of dental pulp cell (DPC) responses using two LLLT lasers with wavelengths of 660nm and 810nm. Human DPCs were isolated and cultured in phenol-red-free α- MEM/10%-FCS at 37°C in 5% CO<sub>2</sub>. Central wells of transparent-based black walled 96-microplates were seeded with DPCs (passages 2-4; 150μL; 25,000 cell/ml). At 24h post-seeding, cultures were irradiated using a Thor Photomedicine LLLT device (THOR Photomedicine, UK) at 660nm (3, 6 or 13s to give 2, 5 and 10J/cm<sup>2</sup>) or 810nm (for 1, 2 or 5s to deliver 5, 10 and 20J/cm<sup>2</sup>). Metabolic activity was assessed via a modified MTT assay 24h post-irradiation. Statistical differences were identified using analysis of variance and post-hoc Tukey tests (P=0.05) and compared with nonirradiated controls. Significantly higher MTT activity was obtained for both lasers (P<0.05) using the high and intermediate radiant exposure (5-20J/cm<sup>2</sup>). The MTT response significantly decreased (P<0.05) at lower radiant exposures with no statistical significance from control (P>0.05). Consequently, enhanced irradiation parameters was apparent for both lasers. These parameters should be further optimised to identify the most effective for therapeutic application.