The Fractional Photothermolysis (FP) theory was first proposed by Dr. Rox Anderson, the founder of the laser theory foundation of the Wellman Laboratory at Harvard University, and then he led the team to develop the world's first non-ablative lattice based on this theory. laser.
Fractional Photothermolysis originally meant "focal photothermolysis". In the early days, clinicians in Hong Kong and Taiwan had not seen Fraxel equipment and technology. From the general idea of the 2004 paper, they learned that it was through "partial, "multiple" and "fragmented", etc. The term Fractional Photothermolysis technology is called focal photothermolysis, and it is still used clinically. Now it is more called fractional laser in clinical practice.
After 2007, a variety of ablative and non-ablative fractional lasers with different wavelengths have been gradually applied to the clinic, and the research and use of fractional laser clinical treatment has started. , hemangioma, percutaneous drug delivery and other fields, the application of laser physical energy is linked with the skin healing mechanism, which provides a new field for the clinical application of laser.
Fractional laser irradiates only a small part of the skin, creating a tiny three-dimensional columnar thermal damage zone in the tissue, which is called "Microscopic Thermal Zone" (MTZ). The surrounding tissue remains intact, and rapid repair is achieved through the migration and growth of surrounding active cells. Fractional lasers mainly use water as the target base, rather than skin pigment groups (such as melanin and hemoglobin), so when we analyze the mechanism of fractional action and penetration depth, we should first consider the absorption coefficient of water in this band of lasers.
Fractional laser classification
Fractional lasers of different wavelengths will produce different tissue responses. A very important factor in this is the difference in the absorption rate of water to these bands.
Fractional lasers can be divided into two categories according to the hydrophilicity of different laser wavelengths: Non-Ablative Fractional Resurfacing and Ablative Fractional Resurfacing.
The wavelength laser with high water absorption rate is exfoliation laser, including erbium yttrium aluminum garnet laser (Er: YAG, 2940nm), carbon dioxide laser (CO2, 10600nm); laser with moderate water absorption rate is non-ablative laser such as erbium Fiber laser 1550nm, thulium fiber laser 1927nm, semiconductor laser 1440nm, neodymium yttrium aluminum garnet laser (ND: YAG, 1064nm).
Histological studies have found that non-ablative fractional laser can degenerate the epidermis and dermis columnar, disrupt the junction between the epidermis and the dermis, that is, create subepidermal fissures within the "mini-thermal damage zone", while the stratum corneum remains intact. (Subepidermal fissures are normal tissue manifestations of non-ablative lattices and cannot be named as micro-ablative lattices because of their presence.) Heat-damaged tissue is replaced by surrounding migrating keratinocytes within 24 hours. The Microscopic Epidermal Necrotic Tissue (MEND) formed in the dermis is an area of tissue coagulation necrosis that is cleared and covered by migrating keratinocytes.
The ablative fractional laser is to form ablation of tiny treatment cavities, the surface of the cavity is covered with a layer of eschar, and the depth of the cavity and the thickness of the coking inner wall vary with the laser pulse and wavelength. An annular region of collagen regeneration is created around the cavity. The stratum corneum is absent after ablative fractional laser irradiation. Over the next 48 hours, re-epithelialization of the collagen-regenerating zone occurred.
In general, the histological differences between exfoliative and non-exfoliative lattices lie in the integrity of the stratum corneum, whether the dermal injury area is degenerated or a gasified cavity, and the mechanism of tissue repair.
01 light spot size
The microbeam diameter of the lattice laser is usually between 100 μm and 500 μm. According to the design pattern and a certain density of microbeam spots, the spot of the lattice laser is formed. The diameter of the laser micro-beam mainly affects the healing status of the micro-treatment area. If the diameter is too large, the inflammatory response of wound healing will be aggravated, the healing period will be prolonged, and the probability of scar healing will increase; if the diameter is too small, the treatment area will be small, and the tissue trauma and stimulation will be insufficient. produce a therapeutic effect.
For a certain wavelength of laser light, under the condition of a certain output power, since the amount of tissue vaporization per unit time is relatively fixed, there is a certain variable relationship between the size of the microbeam diameter and the tissue penetration depth. Clinical studies have shown that the optimal microbeam diameter can make Laser penetration is deeper.
Due to the large number of laser wavelengths, different laser output modes, and complex tissue effect characteristics, it still requires a lot of clinical practice and experience for the operating doctor to master the optimal microbeam diameter. The internal automatic adjustment to the optimal microbeam diameter greatly reduces the difficulty of operation and achieves the best curative effect.
02 Pulse Energy
Refers to the laser energy carried by a single pulse in joules (J). The magnitude of the pulsed laser energy output reflects the ability of the laser to treat.
However, in actual clinical use, the energy density of some laser equipment is more meaningful than the energy. Energy density refers to the light energy per unit area, commonly expressed as "J/cm2". In Fraxel DUAL, the spot size is automatically adjusted to the optimal diameter according to the energy, so the Fraxel pulse energy is proportional to the width and depth of the MTZ. As the energy increases, the width and depth of the MTZ also follow increase and vice versa.
03Treatment coverage
The size and number of each MTZ are different between our different devices or even within the same device, which makes it impossible for us to unify the parameters through the shape and density of the spot or the distance between the spot and the spot. Therefore, we need a more uniform standard to evaluate the density of skin lesions. Currently, we use the area percentage of treated lesions as the density unit to be more uniform and accurate.
The coverage rate refers to the percentage of the area of the treatment injury to the area of the treatment area, coverage rate = total treatment point density x point area/treatment area area (%).
Treatment coverage is equal to the treatment intensity level (Treatment Level), the higher the treatment intensity level, the intensity and repair period of its side effects (such as edema and erythema) will also increase, usually in type III, IV skin coverage rate generally does not exceed 20% , so as to ensure that epidermal regeneration can be completed within 24 to 48 hours. Relevant experiments show that when using fractional laser to treat acne scars and large pores, the parameter setting method of high energy, low density, low energy and high density has better treatment effect.
To sum up, in the clinical use of traditional lasers, only the laser output mode and tissue absorption characteristics are usually considered. The wound area formed during treatment is related to the size of the disease, and relatively little consideration is given to the healing result of the wound after treatment, which is usually limited to Whether it can heal itself, whether scars are left, etc.
After the emergence of the dot matrix technology, as a new form of laser use, non-invasive or minimally invasive healing has become its design starting point. The diameter of the microbeam is related to the skin healing mechanism of the treated tissue, and the laser energy irradiation area is no longer rigid. Due to the size of the original disease, the laser treatment area can cover the lesion, but the actual treatment area only accounts for a few percent of the lesion area. The organic combination of the physical properties of the laser and the skin healing mechanism is realized, and the challenge revealed to us is not only the traditional laser. It also includes a series of issues such as further research on the skin healing mechanism in disease states, which is another progress in the history of laser clinical application.
Post time: Aug-22-2022
