Biophysics of lasers for face polishing

The concept of selective photothermolysis allows the surgeon to choose the length of the laser wave absorbed by the target tissue component as much as possible by the tissue chromophor. The main chromophore for carbon dioxide and erbium: YAG lasers is water. It is possible to construct a curve that reflects the absorption by water or other chromophores of laser energy at different wavelengths. One must remember about other chromophores that can absorb a wave of this length. For example, at a wavelength of 532 nm, laser energy is absorbed by oxyhemoglobin and melanin. When choosing a laser, it is necessary to take into account the possibility of competitive absorption. The additional effect of a competitive chromophore may be desirable and undesirable.

In modern lasers, used for epilation with the target chromophore, is melanin. These waves can also be absorbed by hemoglobin, which is a competitive chromophore. Absorption of hemoglobin can also lead to damage to the blood vessels supplying the hair follicles, which is undesirable.

The epidermis is 90% water. Therefore, water serves as the main chromophore for modern laser-grinding lasers. In the process of laser resurfacing, intracellular water absorbs laser energy, immediately boils and evaporates. The amount of energy that the laser transfers to the tissues, and the duration of this transfer determine the volume of the evaporated tissue. When polishing the skin, the main chromophore (water) must be evaporated, while transferring to the surrounding collagen and other structures the minimum amount of energy. Collagen type I is extremely sensitive to temperature, denaturing at a temperature of +60 ... +70 ° C. Excessive thermal damage to collagen can lead to undesirable scarring.

The energy density of laser radiation is the amount of energy (in joules) applied to the tissue surface (in cm2). Therefore, the radiation density is expressed in J / cm2. For carbon dioxide lasers, the critical energy for overcoming the tissue ablation barrier is 0.04 J / cm2. To restore the surface of the skin, lasers with an energy of 250 mJ per pulse and a spot size of 3 mm are usually used. In the intervals between the impulses the tissues cools. The time of thermal relaxation is the time necessary for complete cooling of the tissue between pulses. With laser polishing, very high energy is used to evaporate the target tissue almost immediately. This makes it possible to make the pulse very short (1000 μs). Consequently, unwanted thermal conductivity to adjacent tissues is minimized. The specific power, usually measured in watts (W), takes into account the integral energy density, the pulse duration and the area of the treated area. A common misconception is that the lower energy density and specific power reduce the risk of scarring, whereas in fact the lower energy boils the water more slowly, causing more severe temperature damage.

In the histological examination of biopsy specimens taken immediately after laser resurfacing, a zone of evaporation and ablation of the tissue is revealed, under which lies the basophilic zone of thermal necrosis. The energy of the first pass is absorbed by the water of the epidermis. After penetrating the dermis, where there is less water capable of absorbing laser energy, heat transfer causes more thermal damage for each subsequent passage. Ideally, a larger ablation depth with a smaller number of passes and less conductive thermal damage is accompanied by a lesser risk of scarring. Prir research of the ultrastructure in the papillary layer of the skin reveals collagen fibers of smaller size, united in large collagen beams. After laser resurfacing, as collagen is produced in the papillary layer of the dermis, molecules associated with wound healing, such as the tenascin glycoprotein, accumulate.

Modern erbium lasers can emit two beams simultaneously. In this case, one bundle in the coagulation mode can increase the damage to surrounding tissues. Such a laser results in more thermal damage due to an increase in pulse duration and therefore slower heating of tissues. Conversely, too much energy can cause deeper evaporation than required. Modern lasers damage collagen with heat generated by grinding. The greater the thermal damage, the greater the synthesis of the new collagen. In the future, grinding lasers well absorbed by water and collagen can be clinically used.

[1], [2], [3]