Effects of low intensity laser irradiation during healing of infected skin wounds in the rat.

Background and objective: Low intensity laser irradiation remains a controversial treatment for non-healing wounds. This study examines the effect of low intensity light on healing of infected skin wounds in the rat.

Materials and methods: Wounds on the rat dorsum were inoculated with Pseudomonas aeruginosa. Wounds were irradiated or sham-irradiated three times weekly from day 1 to 19 using 635-nm or 808-nm diode lasers delivering continuous wave (CW) or intensity modulated (3800 Hz) laser radiation, all at radiant exposures of 1 and 20 J/cm². Wound area and bacterial growth on the wound surface were evaluated three times a week. Histological and immunohistochemical analyses were performed at day 8 and 19.

Results: Wounds that were irradiated using a wavelength of 635 nm (1 and 20 J/cm²) or intensity modulated 808-nm laser light at 20 J/cm² were smaller in area at day 19 than the sham-irradiated controls (achieved significance level=0.0105-0.0208) and were similar to controls in respect of bacterial growth. The remaining light protocols had no effect on wound area at day 19 although they increased Staphylococcus aureus growth across the time line compared with controls (p<0.0001 to p<0.004). CW 808-nm light at 20 J/cm² significantly delayed half-heal time. Histological and immunohistochemical analyses supported wound closure findings: improved healing was associated with faster resolution of inflammation during the acute phase and increased signs of late repair at day 19. Significant inflammation was seen at day 19 in all irradiated groups regardless of radiant exposure, except when using 635 nm at 1 J/cm².

Conclusions: Red light improved healing of wounds. Only one 808-nm light protocol enhanced healing; lack of benefit using the remaining 808-nm light protocols may have been due to stimulatory effects of the light on S. aureus growth.