The Effect of Equal Daily Dose Achieved by Different Power Densities of Low-Level Laser Therapy at 635 and 670 nm on Wound Tensile Strength in Rats: A Short Report

Introduction

I t has been well documented previously that red lasers at a dose of either 4 J/cm² or 5 J/cm² accelerate wound closure, increase collagen deposition, and reduce inflammation during wound healing. ^{1 –4} On the other hand, Kana et al. revealed a deceleration in healing by increasing the dose to 20 J/cm². ¹ Similarly, in wounded fibroblasts exposed to 5 J/cm² of He–Ne laser irradiation, an increase in cell proliferation, migration, and viability was observed, whereas higher doses (10 and 16 J/cm²) were found to be inhibitory. ^5,6

Do Nascimento et al. showed in their histological study that low-level laser therapy (LLLT) at 670 and 685 nm using a dose of 10 J/cm² was more effective when higher intensity was combined with a shorter wavelength and/or lower intensity with a higher wavelength. ⁷ Hence, it may be suggested that an inverse relationship exists between wavelength and intensity. This study was evaluated histologically. Since tensile strength (TS) measurement has been shown in numerous experimental studies ⁸ to be an effective method of wound-healing evaluation, we used it to evaluate the effect of LLLT.

The relationships between basic LLLT parameters and their effects on wound healing are not yet fully clarified. Therefore, in the present experiment we compared two different power densities (4 vs. 15 mW/cm²) and wavelengths (635 vs. 670 nm) achieving an equal daily dose of 5 J/cm² to extend our knowledge about LLLT and its impact on skin wound healing.

Materials and Methods

Results

Figure 1 shows the results of wound TS measurements. The lowest wound TS results were measured in SIC rats (10.5 ± 2.8 g/mm²). Slightly higher wound TS results were measured in group L-670/15 (11.5 ± 2.5 g/mm²) and group L-635/4 (11.7 ± 4.3 g/mm²), while significantly higher results were found in group L-670/4 (15.8 ± 4.4 g/mm²) and group L-635/15 (15.9 ± 4.8 g/mm²). The differences were significant between certain groups (p < 0.01: SIC vs. L-635/15, SIC vs. L-670/4; p < 0.05: L-635/4 vs. L-635/15, L-635/4 vs. L-670/4, L-635/15 vs. L-670/15, L-670/4 vs. L-670/15).

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FIG. 1.

The lowest wound TS results were measured in SIC rats, with slightly higher wound TS results found in groups L-670/15 and L-635/4, and significantly higher wound TS results found in groups L-670/4 rats and L-635/15 (*p < 0.05; **p < 0.01).

Discussion

Interestingly, an inverse relationship between wavelength and intensity in LLLT at 670 and 685 nm was suggested by do Nascimento et al. in their preliminary wound-healing investigation. ⁷ The results from our current investigation support this suggestion. However, in our study this effect was observed at a lower dose and a different wavelength. In contrast to do Nascimento's investigation at 670 nm, we found a similar effect at 635 nm, while their observation at 685 nm was recorded in our study at 670 nm. Therefore, further detailed investigations need to be performed to clarify this discrepancy.

Previously, by evaluating the effect of LLLT at 635 nm during the proliferative phase, we observed that epithelization and collagen synthesis were significantly accelerated in a power-density-dependent manner, ⁴ which explains, from histological point of view, our present results at 635 nm. On the other hand, in contrast to do Nascimento's results, Erdle et al. recorded in their investigation that LED light at 670 nm stimulates wound healing in the same manner as was observed in our current study. ^7,10 It was suggested that the possible biostimulatory mechanism may be based on the fact that a lower density of light delivers the energy for a longer time period and more continuously. However, the question of why LLLT at 635 nm improves wound healing at a higher density remains open.

Conclusion

Our study demonstrates that both tested red lasers are able to increase wound TS and thereby accelerate wound healing. Whereas radiation at 635 nm improves wound healing by using higher tested power density, radiation at 670 nm improves healing by using lower power density. Since a shorter time is needed to achieve the daily dose of 5 J/cm² when higher power density is used, it may be suggested that LLLT at 635 nm and 15 mW/cm² might have a better clinical outcome.