Abstract
Low-Level Laser Therapy (LLLT), also known as photobiomodulation (PBM), has emerged as a promising non-invasive therapeutic approach with diverse applications in healthcare. This review synthesizes current research findings on the effects of different laser wavelengths, including 635 nm, 660 nm, 808 nm, 810 nm, 904 nm, and 905 nm, focusing on their interactions with skin, therapeutic potential in spinal cord injury (SCI) recovery, thermal effects on human skin, and modulation of oxidative stress in diabetic wound healing. Key findings highlight the influence of skin color and tissue thickness on laser transmittance and reflectance, the efficacy of combined wavelengths in promoting functional recovery after SCI, wavelength-specific thermal responses in different skin types, and the ability of LLLT to reduce oxidative stress in diabetic wounds. Understanding these multifaceted effects is crucial for optimizing LLLT protocols and advancing its clinical utility.
1. Introduction
Low-Level Laser Therapy (LLLT) utilizes light in the visible to near-infrared spectrum to stimulate cellular functions and elicit therapeutic effects on living tissues (Svobodova et al., 2019). Over the past decade, significant progress has been made in elucidating the mechanisms and applications of LLLT across various medical conditions. From its interactions with skin tissue to its potential in neurorehabilitation and wound healing, LLLT has demonstrated versatility and promise. This review integrates four key studies to provide a comprehensive overview of the current state of knowledge regarding LLLT, with a focus on wavelength-specific effects, skin-related factors, therapeutic outcomes in SCI, thermal safety, and oxidative stress modulation.
2. Laser-Skin Interactions: The Impact of Skin Color and Tissue Thickness
The interaction of laser light with skin is a critical determinant of LLLT efficacy, as skin color and tissue thickness significantly influence light transmittance and reflectance. Souza-Barros et al. (2018) conducted a comparative study to examine the effects of 635 nm (red) and 808 nm (near-infrared) lasers on transmittance, reflectance, and skin temperature in 40 healthy volunteers. The researchers measured skin color and skin-fold thickness at a standardized site near the elbow and applied energy doses ranging from 2 to 12 Joules, adhering to American National Standards Institute (ANSI) safety guidelines for irradiance.
The results revealed that skin color and wavelength were key factors affecting reflectance. Darker skin exhibited decreased reflectance, with a more pronounced reduction for the 635 nm red laser compared to the 808 nm near-infrared laser (Souza-Barros et al., 2018). Transmittance was significantly higher for the 808 nm laser than the 635 nm laser; however, this advantage was diminished in darker skin and with increasing tissue thickness. Notably, while dose had a significant effect on skin temperature (0.7-1.6°C increase across 6, 9, and 12 J doses), the effects of wavelength, skin color, and tissue thickness on temperature were negligible in comparison. Importantly, participants were not aware of the temperature increase, and absorption remained unchanged with higher energy doses and elevated temperatures (Souza-Barros et al., 2018). These findings emphasize the need to account for skin color and tissue thickness when selecting LLLT energy doses to ensure therapeutic effectiveness at the target tissue, particularly when using different wavelengths.
3. Therapeutic Potential of LLLT in Spinal Cord Injury Recovery
Spinal cord injury (SCI) is a devastating condition characterized by sensorimotor deficits, autonomic dysfunction, and neuropathic pain, with limited effective treatment options. Svobodova et al. (2019) investigated the effects of a Multiwave Locked System (MLS) laser, combining 808 nm continuous emission and 905 nm pulsed emission, on functional recovery and tissue repair in a rat model of SCI. The laser treatment was administered to the injury site 15 minutes after SCI induction and continued for 10 consecutive days.
Functional assessments using the Basso, Beattie, and Bresnahan (BBB) test, Beam walking test, MotoRater kinematic analysis, and Plantar test demonstrated significant improvements in locomotor function, coordination, and thermal sensitivity in laser-treated rats compared to controls (Svobodova et al., 2019). Histopathological analysis revealed enhanced preservation of gray and white matter in the cranial and caudal regions of the lesion, reduced soleus muscle atrophy, and a shift in microglial/macrophage polarization toward the anti-inflammatory M2 phenotype. This polarization was confirmed by increased CD68+/CD206+ double-labeled cells and significant downregulation of the M1 marker Cd86, along with non-significant upregulation of the M2 marker Arg1 (Svobodova et al., 2019). Additionally, gene expression analysis showed significant downregulation of pro-inflammatory and apoptotic genes (Fgf2, Casp3, Cd86, Vegf) in laser-treated animals. These results indicate that the combination of 808 nm and 905 nm wavelengths is a promising non-invasive therapy for improving functional recovery and tissue sparing after SCI, likely through modulation of inflammation, oxidative stress, and cellular metabolism.
4. Thermal Effects of Therapeutic Lasers on Human Skin
The thermal safety of LLLT is a critical consideration, especially when treating individuals with different skin colors. Joensen et al. (2011) evaluated the thermal effects of 810 nm (200 mW) and 904 nm (60 mW) lasers on skin temperature in 40 healthy volunteers with varying skin color, age, and gender. Using a thermographic camera, skin temperature was measured at irradiated and non-irradiated control sites across six energy doses (2-12 J).
In accordance with World Association for Laser Therapy (WALT) guidelines, recommended doses for musculoskeletal and inflammatory conditions resulted in negligible thermal effects (<1.5°C increase) across light, medium, and dark skin (Joensen et al., 2011). However, higher doses revealed wavelength-specific thermal responses. The 904 nm laser produced significantly higher temperatures in dark skin (5.7°C ± 1.8°C at 12 J) compared to light skin, though no participants requested treatment termination. In contrast, the 810 nm laser induced three to six times more heat in dark skin, with a maximal temperature increase of 22.3°C, leading 8 out of 13 dark-skinned participants to discontinue treatment due to discomfort (Joensen et al., 2011). These findings underscore the importance of selecting appropriate wavelengths and doses based on skin color to avoid excessive thermal effects, particularly when using high-power 3B lasers.
5. Modulation of Oxidative Stress in Diabetic Wound Healing
Diabetic wounds are often complicated by impaired healing due to increased oxidative stress, which disrupts cellular function and tissue repair. Denadai et al. (2017) investigated the acute effects of 660 nm LLLT (100 mW, 6 J/cm², spot size 0.028 cm²) on oxidative stress levels in diabetic rats with skin wounds. Thirty-six rats were divided into four groups: non-diabetic non-irradiated (NDNI), non-diabetic irradiated (NDI), diabetic non-irradiated (DNI), and diabetic irradiated (DI). Malondialdehyde (MDA) levels, a marker of oxidative stress, were measured in wound tissue after treatment.
Results showed a significant reduction in MDA levels in irradiated tissues compared to non-irradiated controls, both in diabetic and non-diabetic rats (Denadai et al., 2017). Specifically, the diabetic irradiated group (DI) exhibited significantly lower MDA levels than the diabetic non-irradiated group (DNI), indicating that LLLT effectively mitigates oxidative stress in diabetic wounds. These findings suggest that 660 nm LLLT may enhance diabetic wound healing by reducing reactive oxygen species (ROS) production and improving antioxidant capacity, highlighting its potential as an adjuvant therapy for diabetic wound management.
6. Discussion and Future Directions
This review integrates key findings from four studies to highlight the multifaceted roles of LLLT in skin interactions, SCI recovery, thermal safety, and diabetic wound healing. Collectively, these studies demonstrate that LLLT efficacy is influenced by wavelength, energy dose, skin color, tissue thickness, and the underlying pathological condition. For example, 808 nm and 905 nm wavelengths show promise in SCI recovery by modulating inflammation and promoting tissue repair, while 660 nm LLLT effectively reduces oxidative stress in diabetic wounds. Skin color and tissue thickness significantly impact laser transmittance, reflectance, and thermal responses, emphasizing the need for personalized treatment protocols.
Future research should focus on optimizing LLLT parameters (wavelength, dose, duration, frequency) for specific conditions and patient populations, including individuals with diverse skin types and comorbidities. Additionally, further studies are needed to elucidate the molecular mechanisms underlying LLLT effects, such as its impact on mitochondrial function, cytokine production, and cell signaling pathways. Long-term clinical trials are also warranted to validate the therapeutic potential of LLLT in human SCI, diabetic wound healing, and other conditions, ensuring its safety and efficacy in clinical practice.
7. Conclusion
Low-Level Laser Therapy (LLLT) is a versatile and promising non-invasive therapeutic approach with diverse applications in healthcare. The findings reviewed herein demonstrate that LLLT interacts with skin in a wavelength- and skin color-dependent manner, promotes functional recovery and tissue sparing after spinal cord injury, exhibits wavelength-specific thermal effects that require careful dose adjustment based on skin type, and modulates oxidative stress to enhance diabetic wound healing. By understanding these multifaceted effects and optimizing treatment protocols, LLLT has the potential to improve outcomes for a wide range of medical conditions, from neurorehabilitation to wound care. Continued research and clinical validation will be key to unlocking the full therapeutic potential of LLLT.
References
- Denadai, A. S., Aydos, R. D., Silva, I. S., Olmedo, L., Cardoso, B. M. S., Silva, B. A. K., & Carvalho, P. T. C. (2017). Acute effects of low-level laser therapy (660 nm) on oxidative stress levels in diabetic rats with skin wounds. J Exp Ther Oncol, 11(2), 85-89. https://pubmed.ncbi.nlm.nih.gov/28976129/
- Joensen, J., Demmink, J. H., Johnson, M. I., Iversen, V. V., Lopes-Martins, R. Á. B., & Bjordal, J. M. (2011). The thermal effects of therapeutic lasers with 810 and 904 nm wavelengths on human skin. Photomed Laser Surg, 29(3), 145-153. https://pubmed.ncbi.nlm.nih.gov/21219241/
- Souza-Barros, L., Dhaidan, G., Maunula, M., Solomon, V., Gabison, S., Lilge, L., & Nussbaum, E. L. (2018). Skin color and tissue thickness effects on transmittance, reflectance, and skin temperature when using 635 and 808 nm lasers in low intensity therapeutics. Lasers Surg Med, 50(4), 291-301. https://pubmed.ncbi.nlm.nih.gov/29178437/
- Svobodova, B., Kloudova, A., Ruzicka, J., Kajtmanova, L., Navratil, L., Sedlacek, R., Suchy, T., Jhanwar-Uniyal, M., Jendelova, P., & Machova Urdzikova, L. (2019). The effect of 808 nm and 905 nm wavelength light on recovery after spinal cord injury. Sci Rep, 9(1), 7660. https://pmc.ncbi.nlm.nih.gov/articles/PMC6529518/