Lucas et al. have performed a systematic review of cell studies and animal experiment with LLLT on wound healing. Manuscripts were identified by searching Medline, Embase, and SPIE (the International Society for Optical Engineering). It was assessed whether the studies showed a beneficial effect of active treatment or not. The magnitude of the effect was expressed in standardized mean difference. In-depth analyses were performed on (1) studies in which inflicted wounds on animals were irradiated and evaluated; (2) studies with primary outcome measures on dimensions with direct reference to wound healing (ranging from acceleration of wound closure to epithelialization, but excluding surrogate dimensions regarding wound healing; in this case: tensile strength); (3) animal studies with ‘true controls’; (4) studies in which animals functioned as their ‘own controls’ and (5) studies with the highest methodological quality score. The 36 included studies contained 49 outcome parameters of which 30 reported a positive effect of laser irradiation and 19 did not. Eleven studies presented exact data about the effect of active treatment and controls. The pooled effect (SMD) over 22 outcome measures of these studies was -1.05 (95% Cl: -1.67 to -0.43) in favor of LLLT. Methodological quality of the studies was poor. In-depth analysis of studies showed no significant pooled effect size in studies with highest methodological quality scores [9].
Medrado et al. investigated the effects of LLLT on the participation of myofibroblasts in the wound-healing process. Cutaneous wounds were inflicted on the back of 72 Wistar rats (punch-skin removal of 50 mm2). The wounds of two groups of animals were treated immediately after surgery with an AlGaInP diode laser (670 nm-9 mW) at a fluence of 4 J/cm2 (exposure time: 31 seconds—Group 1) or 8 J/cm2 (exposure time: 31 seconds—Group 2), while a third group consisted of untreated control animals (Group 3). On day 1, 2, 3, 5, 7, and 14 following surgery and laser treatment, fragments of skin were analyzed by histology using conventional sating, immunochemistry, and electron microscopy. Statistically significant differences in the areas of laser-treated and untreated cutaneous ulcers were observed as early as 24 hours after surgery. After 72 hours, the low level laser-treated ulcers exhibited the greatest difference when compared to untreated ulcers. This tendency for treated ulcers to be smaller than untreated ulcers was observed until the seventh day. By the fourteenth day, cutaneous wound in animals were completely healed. These gross changes were positively correlated with the microscopic findings. In treated animals, the extent of edema and the number of inflammatory cells were reduced (p < 0.05), but the amount of collagen and elastic fibers appeared slightly increased. The group that received 4 J/cm2 laser treatment exhibited significantly more action/desmin-marked cells in correlation with its more marked vascular proliferation than the group treated at 8 J/cm2. An enhanced proliferation of fibroblasts and myofibroblats was also observed.
In this study, LLLT reduced the inflammatory reaction, induced increased collagen deposition, and a greater proliferation of myofibroblasts in these experimental cutaneous wounds. However, as clearly stated by the authors, an apparent paradox was noted at the end of this study: LLLT induced a series of morphological changes, presumably favorable to the resolution of wound healing, but did not shorten healing time [10].
In another study performed by Gal et al, the purpose was to evaluate, from the histological point of view, the effect of diode laser irradiation on skin wound healing in Sprague – Dawley rats. Two parallel full-thickness skin incisions were performed on the back of each rat (n = 49) and immediately sutured. After surgery, one wound of each rat was exposed to laser irradiation (continuous mode, 670 nm, AlGaInP diode laser, irradiance: 25 mW/cm2). Each section was irradiated for 8 minutes daily to achieve the total daily dose 30 J/cm2), whereas the parallel wound was not irradiated, and served as control. Both wounds were removed 24, 48, 72, 96, 120, 144, and 168 hours after surgery and routinely fixed and embedded in paraffin sections, stained with hematoxylin and eosin, van Gieson, periodic acid Schiff + periodic acid Schiff diastase, Mallory’s phosphotungstic hematoxylin, and azur and eosin, and histopathologically evaluated. As compared to nonirradiated control wounds, laser stimulation shortened the inflammatory phase, and also accelerated the proliferative and maturation phase, and positively stimulated the regeneration of injured epidermis and the reparation of injured striated muscle. LLLT at 670 nm, used at 25 mW/cm2 -30 J/cm2, positively influenced all phases of rat-skin wound healing [11].
When comparing these two experimental studies, performed with the same AlGaInP diode laser (670 nm), wound healing was improved, but at 4 J/cm2 in the first one and 30 J/cm2 in the second one. Since it is generally accepted that the biological effect of LLLT depends on three major parameters: wavelength, irradiance (or power density), and fluence (or dose), such a discrepancy between the dose is difficult to interpret. The selection of wavelengths and treatment parameters needs to be rationalized.