Laser hair removal uses the principle of selective photothermal action of hair follicle tissue: the laser penetrates the epidermis into the dermis, and is selectively absorbed by the melanin in the hair shaft and matrix to produce a photothermal effect. The generated heat is conducted to surrounding tissues. Destroy hair follicles and dermal papilla to achieve the effect of long-term hair removal. (Main mechanism)
Light energy is easily absorbed by the hemoglobin in the dermal papilla nutrient blood vessels in the hair follicle, which quickly produces a photothermal effect. The dermal papilla nutrient blood vessels are destroyed by hemoglobin coagulation (secondary mechanism)
To obtain the selective photothermal effect of hair, three basic conditions are required:
① The laser must penetrate deeply to the target tissue and be preferentially absorbed by the melanin in the target tissue;
②The pulse width of the laser must be less than or equal to the thermal relaxation time of the target tissue (3ms for epidermis and 40-100ms for hair follicles);
③There is enough energy density to cause damage to the target tissue.
In addition, the light spot should be greater than the penetration depth of light into the tissue (5-10 mm).
Energy selection
In order to achieve permanent hair removal or long-lasting inhibition of hair growth, sufficient energy density should be used to completely destroy the hair growth area. The increase in energy density is directly proportional to the actual hair reduction after laser treatment. However, if the energy is too large, it will damage the epidermis and other tissues. Therefore, the energy density must be selected reasonably according to the patient's skin color, hair color, hair diameter, hair removal site and tolerance. Laser hair removal treatment is most effective when choosing the highest energy density that the skin can tolerate. The effect of hair removal is cumulative. The higher the energy density and the more treatments, the better the final curative effect will be.
The skin color is slightly darker, the hair is thicker, and the more sensitive parts should choose low energy density, because the epidermis contains more melanocytes, which can easily absorb energy and cause side effects such as pain, pigment changes and blisters.
The skin tone is whiter and the hair is finer, so you can set a higher energy density.
When treating areas with high hair density, such as beards or upper arm hair, in order to avoid heat damage caused by heat transfer from adjacent hairs, a lower energy density should be selected for the initial treatment.
Due to the difference in the depth of hair follicles, for the deeper parts of the hair follicles, a higher energy density can achieve better results.
The initial energy is determined by the spot test of the treatment site (slight erythema and peripilar edema appearing in a few minutes) and the patient's pain tolerance. Later, as the hair diameter continues to decrease, the energy density will increase by 2 to 4 J per treatment. cm2, until the end of treatment (the skin at the treatment site is red and hot, and there are scattered red papules around the pores)
Sep 13, 2021
