Lasers

Lasers are now widely used for treating numerous cutaneous lesions, scar revision (hypertrophic and keloid scars), skin resurfacing, skin remodeling, and for fractional pho­tothermolysis (wrinkles removal) [24]. For these treatements, lasers are used to generate heat. The modulation of the effects (volatilization, coagulation, hyperthermia) is obtained by using different wavelengths and laser parameters. The heat source obtained by conver­sion of light into heat can be very superficial and intense if the laser light is well-absorbed (far-infrared: CO2 or Er:YAG lasers), it can be much deeper and less intense if the laser light is less absorbed by the skin (visible or near – infrared). This heat source will always transfer its energy to surrounding tissues, and whatever the laser used, a 45° C-50° C tem­perature gradient will be always obtained in the skin (Fig. 19.4). If a wound-healing pro­cess exists, it cannot be induced by the dead cells, but only by live cells reacting to this low temperature increase.

The importance of temperature in the wound-healing process has been already recog­nized as a novel way in which to manipulate the wound-healing environment [25]. The use of heat to treat disease goes back to ancient times. Hippocrates (460-370 BC) wrote “What medicines do not heal, the lance will; what the lance does not, fire will”, while Parmenides (510-450 BC) stated “Give me a chance to create fever and I will cure any disease”.

The biological effects of far-infrared ray (FIR) on whole organisms remain poorly under­stood. However, this generated supraphysiologic level of heat is able to induce a heat – shock response, which can be defined as the temporary changes in cellular metabolism. These changes are rapid, transient, and characterized by the production of a small family of proteins termed the heat shock proteins (HSP). In this context, recent experimental studies have clearly demonstrated that HSP 70 which are overexpressed following laser irradiation could play a role with consequently a coordinate expression of other growth factors such as TGF-beta which is known to be a key element in the inflammatory response and the fibro- genic process [24,26,27]. This thermal effect induced by FIR is also known to increase microcirculation [28].

Besides, their use for skin resurfacing, skin remodeling, or rejuvenation and for frac­tional photothermolysis, lasers are now proposed for surgical scar-healing improvement.

Capon et al. have demonstrated that an 800 nm-diode laser could accelerate wound heal­ing with increased tensile strength (30-58 % greater than in control groups at 7 and 15 days), and could lead to a slender scar if applied almost immediately after conventional skin suture. Histological examination has revealed a much earlier continuous epidermis and dermis, and a reorientation of collagen fiber and elastin network along the skin incision. This observation was particularly interesting, since the most significant difference between normal tissue and scar tissue is due to collagen deposition and alignment during dermal wound healing [29].

In this study, it was confirmed that laser irradiation led to a moderate increase of tissue tem­perature (<50° C) insufficient to create a thermal damage but high enough to activate HSP 70

Wound healing

Non ablative remodeling L Resurfacing

Carbonization (>200°C)

Vaporization (>100°C) Coagulation (55 – 85 °С)

Protein

Denaturation (45 – 55 °С) Heat Shock Response (40 – 45 °С)

Figure 19.4 Range of thermally dependent interactions from a typical thermal laser.

which was markedly induced in skin structures examined after laser exposure[30,31]. The results observed in this experimental study show that the healing process for the skin occurs with regeneration (in opposition to reparation), a known phenomenon by fetus. Interest­ingly, some authors have also reported that predominant expression of TGF-B3 compared with TGF-^1 and TGF-^2 induces a “scarless healing” [32,33]. Figures 19.5 and 19.6 sum­marize the cascade of the wound-healing process and the role of TGF-B1, which could be induced by an elevation of temperature [24].

With a different laser (a 595 nm pulsed dye laser) and a different timing (treatment of surgical scars starting on the day of suture removal), two different studies have shown that the final cosmetic appearance of scar was significantly better for the laser-treated scars when compared to untreated scars [34,35]. In both studies, each scar was divided at the midline into two fields, with half receiving laser-treatment in order to eliminate any bias due to the comparison of different scars. In the laser-treated scar, the fibroblast number was

similar to normal skin, the collagen alignment possessed normal multidirectionality, and more elastin fibers were present in the treated sides. Treated halves showed more preserva­tion of normal tissue architecture with more of an elastin tissue network present, whereas the untreated scars had more extensive, visible scarring with decreased elastin tissue net­works. To the authors, the 595 nm pulsed dye laser was a safe and effective option to improve the cosmetic appearance of surgical scars in skin types I-IV starting on the day of suture removal.

19.3 Conclusions

Several light-based systems have been proposed to promote wound healing. For LLLT and LED, despite numerous experimental papers, their efficacy in humans needs to be demonstrated in well-designed clinical studies. These systems may increase certain aspects of healing in the early stages, but not to such a degree as to be clinically undisputable. The principle of action of this low level light (photomodulation) is still debated.

Concerning lasers, where the principle of action is based on the generation of low tem­peratures, numerous studies have demonstrated that lasers play an indisputable role in the wound-healing process, in particular for incisional scars, scar revision, laser-assisted skin closure, laser remodeling, fractional photothermolysis, and laser resurfacing. For these techniques, lasers are used to generate heat. The modulation of the effects (volatilization, coagulation, hyperthermia) is obtained by using different wavelengths and laser parame­ters. The heat source obtained by conversion of light into heat can be very superficial and intense if the laser light is well-absorbed (far-infrared : CO2 or Er:YAG lasers); it can be much deeper and less intense if the laser light is less absorbed by the skin (visible or near­infrared). This heat source will always transfer its energy to surrounding tissues, and what­ever the laser used, a 45-50° C temperature gradient will be always obtained in the skin. If a wound-healing process exists, it cannot be induced by the dead cells, but only by live cells reacting to this low temperature increase. This generated supraphysiologic level of heat is able to induce a heat-shock response, which can be defined as the temporary changes in cellular metabolism.