Biophysics of facial resurfacing lasers

The concept of selective photothermolysis allows the surgeon to select the laser wavelength that is maximally absorbed by the target tissue component - the tissue chromophore. The main chromophore for carbon dioxide and erbium:YAG lasers is water. It is possible to plot a curve reflecting the absorption of laser energy by water or other chromophores at different wavelengths. It is necessary to 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 consider the possibility of competitive absorption. The additional effect of a competitive chromophore can be desirable or undesirable.

In modern lasers used for hair removal, the target chromophore is melanin. These waves can also be absorbed by hemoglobin, which is a competitive chromophore. Absorption by 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 skin resurfacing lasers. During laser resurfacing, intracellular water absorbs laser energy, immediately boils and evaporates. The amount of energy that the laser transfers to the tissue and the duration of this transfer determine the volume of evaporated tissue. When resurfacing the skin, it is necessary to evaporate the main chromophore (water), while transferring a minimal amount of energy to the surrounding collagen and other structures. Collagen type I is extremely sensitive to temperature, denaturing at a temperature of +60... +70 °C. Excessive thermal damage to collagen can lead to unwanted scarring.

The energy density of a laser is the amount of energy (in joules) applied to a tissue surface (in cm2). Therefore, the energy density is expressed in J/cm2. For carbon dioxide lasers, the critical energy to overcome the tissue ablation barrier is 0.04 J/cm2. For skin resurfacing, lasers with an energy of 250 mJ per pulse and a spot size of 3 mm are usually used. The tissues cool down between pulses. The thermal relaxation time is the time required for the tissue to cool completely between pulses. Laser resurfacing uses very high energies to vaporize the target tissue almost immediately. This allows the pulse to be very short (1000 μs). Consequently, unwanted heat conduction to adjacent tissues is minimized. The specific power, usually measured in watts (W), takes into account the integrated energy density, the pulse duration, and the area of the treated area. A common misconception is that lower energy density and power density reduce the risk of scarring, when in fact, lower energy boils water more slowly, causing more thermal damage.

Histologic examination of biopsies taken immediately after laser resurfacing reveals a zone of tissue vaporization and ablation, with a basophilic zone of thermal necrosis underlying the tissue. The energy of the first pass is absorbed by the water in the epidermis. Once in the dermis, where there is less water to absorb the laser energy, heat transfer causes greater thermal injury with each subsequent pass. Ideally, greater ablation depth with fewer passes and less conductive thermal injury results in less risk of scarring. Ultrastructural examination of the papillary dermis reveals smaller collagen fibers organized into larger collagen bundles. After laser resurfacing, as collagen is produced in the papillary dermis, molecules associated with wound healing, such as the glycoprotein tenascin, accumulate.

Modern erbium lasers can emit two beams simultaneously. However, one beam in the coagulation mode can increase the damage to surrounding tissue. Such a laser causes greater thermal damage due to the increased pulse duration and therefore slower tissue heating. Conversely, too much energy can cause deeper evaporation than required. Modern lasers damage collagen with the heat generated during grinding. The greater the thermal damage, the greater the synthesis of new collagen. In the future, grinding lasers that are well absorbed by water and collagen may find clinical use.

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