Overview

1. Anatomy and physiological basis of hair

Hair The hair shaft is composed of three parts: the hair root below the epidermis and the hair follicle. The hair shaft has three layers from the inside to the outside: the medulla, the cortex and the cuticle. The hair root is wrapped in the hair follicle, and the end is swollen and spherical, called The center of the hair bulb is concave, connected to the dermal papilla of the hair follicle, called the hair papilla, which contains abundant capillaries and nerve tissue, can nourish the hair bulb and has sensory function. The upper part of the hair follicle is the infundibulum and isthmus, and the lower part is the bulb. The hair follicle is divided into three layers from the inside to the outside: the inner and outer root sheaths and the connective tissue sheath. Hair is generally divided into vellus hair, terminal hair and intermediate hair. Vellus hair is short, fine and soft, usually without myelin and melanin; terminal hair is long, coarse and hard, with myelin and melanin; intermediate hair is between vellus hair and terminal hair.

The terminal hair bulb is located between the dermis and subcutaneous tissue. During the growth period, it can reach the subcutaneous tissue. It is mainly composed of immature hair matrix cells and contains a small amount of melanocytes. Hair matrix cells are just above the hair papilla. The cells of the hair matrix have a strong metabolism and are not affected by day and night. They are renewed quickly and are continuously divided into new cells. The hair follicles are the most important part of the hair. The hair follicles are the most important part of the hair. If the hair shrinks or becomes damaged, it stops growing and gradually falls out.

The hair growth cycle is generally divided into the growth phase, regression phase and resting phase. Each hair follicle undergoes cyclical changes independently, even if the adjacent hair follicles do not In the same growth cycle. The growth and shedding of hair is determined by the cyclical changes of hair follicles from the growth phase to the resting phase. In addition, nerves and various hormones (androgens, growth hormones, thyroid hormones, adrenal cortical hormones, etc.) are also involved. The growth regulation of hair follicles and hair.

2. Classification of hirsutism

Hairy condition refers to excessive growth of terminal hair, including the upper lip, chin, ears, cheeks, lower abdomen, back, chest and proximal limbs. [1] The incidence rate in women of childbearing age is 5% to 10%, and the disease may be the initial manifestation of androgen excess.

2.1 Physiological hirsutism

Physiological hirsutism is also called idiopathic hirsutism. These patients may not be affected by endocrine and tumor factors, and the amount of androgen in their serum is normal, but because hair follicles are sensitive to low levels of androgen, they may still have hirsutism[2].

2.2 Pathological hirsutism caused by metabolic and endocrine diseases 

The most common endocrine disease is polycystic ovary syndrome Polycystic ovarian syndrome (PCOS) accounts for 72% to 82% of hirsutism patients[3]. The key feature of congenital adrenal hyperplasia (CAH) is excessive production of androgens. < 5% of hirsutism patients have adrenal hyperplasia. Affected females show hirsutism from the peripubertal stage, often accompanied by irregular menstruation. Severe insulin resistance syndrome can stimulate the adrenal glands and ovaries to produce more androgens, while hyperinsulinemia inhibits the liver from synthesizing sex hormone-binding globulin, thereby increasing serum total testosterone. Concentrations of prolactin can cause hirsutism in patients [5]. Hyperprolactinemia, acromegaly, and hypothyroidism are also less common causes of hirsutism. Androgen-secreting tumors, ovarian theca cell hyperplasia, and gonadotropin-stimulating hormone (GTP) are also known causes of hirsutism. Adrenal cortical hormone cell adenomas and non-malignant ovarian diseases are also often accompanied by hirsutism due to excessive secretion of androgens.

2.3 Drug-induced 

Use of androgens such as testosterone or dehydroepiandrosterone Drugs such as danazol may be associated with the onset of hirsutism. Commonly used immunosuppressants such as cyclosporine also have one of the side effects of hirsutism. Other drugs include acetazolamide, citalopram, adrenocorticotropic hormone, and glucocorticoids. Corticosteroids, metoclopramide, methyldopa, minoxidil,

reserpine, streptomycin, sodium valproate and heavy metals, etc. [6].

2.4 Other non-endocrine diseases 

Hirsutism can also occur in patients with some systemic diseases, including porphyria Phenyriasis, anorexia nervosa, malnutrition, juvenile dermatomyositis, tuberculosis, hypothyroidism, and even paraneoplastic syndrome[7].

3. Common hair removal methods

Common temporary hair removal methods include shaving, waxing, tweezing, chemical hair removal, etc. These methods are simple and quick, and patients can perform them by themselves. The disadvantage is that the hair will regenerate quickly and it is impossible to permanently remove excess hair. In addition, improper shaving can cause trauma, waxing can cause pain or even burns, and depilatory creams contain thioglycolate, which can irritate the skin. ) and thermolysis. Both methods use electrodes to act on hair follicles to destroy them, thereby achieving permanent hair removal. However, the operation is cumbersome and time-consuming, causing pain to the patient, and there is a risk of scarring, so it cannot be used for large-area hair removal. .

Intense pulsed light (IPL) is a high-energy incoherent light of a specific wavelength. The commonly used output wavelength range is 420 to 1200 nm. Clinical The filters used for hair removal are mainly 560nm, 590nm, 645nm, 695nm, 755nm, etc. By adjusting the pulse width and releasing energy in a pulsed manner, the target tissue can be continuously heated up, and the selective photothermal effect is applied to the hair follicles, so that the hair follicles are The purpose of hair removal is to destroy the hair follicles. Transparent cold gel is also used during treatment to reduce the temperature of the epidermis. The parameters of intense pulsed light can be adjusted flexibly, and appropriate treatment parameters can be selected according to skin color, hair follicle size and depth [8]. Transient adverse reactions include erythema, edema, blisters, and pigmentation in the treated area[9].

4. Advantages of laser hair removal and commonly used equipment

Laser hair removal is based on the principle of selective photothermal action. The laser penetrates deep into the dermis, using the abundant melanin in the hair follicles and hair shafts as the target color base, selectively absorbing the heat generated by the laser The hair follicles are instantly heated to a high temperature, and the heat energy is transferred from the hair shaft to the stem cells of the hair bulb sheath, destroying the germinal layer of the hair follicles, thereby achieving the purpose of accurately destroying the hair follicle structure. The thermal relaxation time (TRT) of the hair follicle is 40 Current research suggests that the blood vessels and nerves in the hair papilla during the growth period, the hair matrix cells with strong differentiation ability in the hair bulb (called matrix above the hair papilla), and the hair follicle stem cells in the bulge of the hair follicle are the three key factors for permanent hair removal. Important target tissue. The pulse width of long-pulse laser is in the millisecond level, and the thermal diffusion effect on melanin is currently an effective and safe non-invasive hair removal method. Compared with traditional hair removal methods, it has the advantages of clear target color base, controllable operation, less pain, and less likely to cause scars. When using the optimal parameters for hair removal, about 15%~30% of the hair follicles are completely destroyed each time. Thus, permanent hair removal is achieved.

The selection of laser wavelength needs to consider two factors: the depth of laser penetration and the selectivity of target absorption. To penetrate deep, a laser with a longer wavelength and deeper penetration should be used. In the visible light and some infrared bands, the laser penetration depth increases with the increase of wavelength. The spectrum with a wavelength of 600 to 1100nm can penetrate to the depth where the hair follicles are located and Absorbed by melanin, it is the ideal wavelength range for laser hair removal. Currently, the commonly used laser hair removal equipment is semiconductor laser, with wavelengths of 800nm and 810nm. It has deep penetration and melanin absorbs better in this band [10]. Can remove hair at different depths. Semiconductor lasers use a low-energy multi-pulse mode, which can not only ensure heating, but also reduce pain and adverse reactions caused by thermal damage [11]. Long pulse width 755nm alexandrite laser has the characteristics of high energy, long pulse width and large spot size. , has strong penetrability, and the energy accumulated in the dermis is higher than that in the epidermis, which is safer for dark-skinned people and Asians [11]. Melanin has a lower absorption rate to the long-pulse width 1064 nm Nd:YAG laser than the first two. However, it has deeper penetration, longer pulse width, and a wide range of selectable energy densities, making it suitable for all skin types, especially safe for laser hair removal for those with darker skin. Currently, the long pulse widths of 755nm and 1064nm on the market are widely available. The dynamic cooling system (DCD) can protect the epidermis, relieve pain, and reduce the incidence of adverse reactions such as blisters and scars caused by thermal injury[12].

Principles and operation methods of laser hair removal

1.Semiconductor laser hair removal

1.1 Principle 

Semiconductor laser is one of the most commonly used hair removal devices, with a wavelength of 800-810 nm, moderate melanin absorption, and can penetrate deep into the skin. Hair follicles can be safely used for hair removal treatment of Fitzpatrick skin types II to V [13].

In hair removal treatment, lower energy is used Density and longer action time are conducive to continuous thermal action on stem cells, achieving long-term effects. Immediately after treatment with lower energy, the epidermis and stratum corneum were structurally normal, but significant edema was observed in the superficial dermis and hair shaft, indicating structural changes and inflammatory infiltration [14]. Vacuolization occurred in the hair follicles, which appeared similar to degeneration. The hair follicles showed basophilic masses, polarity changes of hair follicle cells, and cell structure was difficult to identify; ultrastructural observation under electron microscope also showed mild edema of hair follicle cells, and some hair follicle cells were separated. Immunohistochemical staining of hair follicle cell proliferation cell nuclear antigen PCNA (-), only part of p53 (+) [15].

Currently, semiconductor lasers are mainly divided into 3 Types: ① low temperature and sliding treatment mode (super hair removal, SHR, i.e. freezing point hair removal); ② negative pressure suction mode (light sheer duet, i.e. vacuum hair removal); ③ fixed point treatment mode. In recent years, due to the development of semiconductor technology, semiconductor lasers of different wavelengths can be integrated into one device to output lasers simultaneously, achieving the effect of treating hair follicles of different depths and diameters. For example, lasers of 755nm, 810nm, and 1064nm wavelengths can be integrated into The same semiconductor laser is used for sliding treatment[16]. There are also treatments for darker skin by integrating 810nm, 940nm, and 1064nm wavelength lasers[17] and using a stamping treatment. More clinical and basic research verification.

1.2 Methods, operation steps and treatment course 

The treatment parameters of semiconductor laser hair removal should be adjusted according to the color of the patient's skin and hair. The treatment endpoint is mild edema of the hair follicles or mild local congestion. For people with light skin and dark hair, medium to high energy and narrow pulse width can be selected. . If you have light skin and light hair, choose higher energy and narrower pulse width. If you have dark skin and dark hair, choose lower energy and wider pulse width. In addition, the thicker the hair, the shorter the pulse width. The treatment interval is usually 4 to 6 weeks, and 80% of the hair can be removed after 6 treatments.

Operation steps: ① SHR treatment ② Negative pressure suction mode (vacuum hair removal): commonly used energy density 5 ~ 11 J/cm², pulse width 30 ~ 100 ms. Sequential treatment at the treatment site, laser irradiation spot overlap rate 20% ~ 30%, only one treatment. ③ Stamping mode: commonly used energy density 10 ~ 60 J/cm², pulse width 5 ~ 100ms, parameter adjustable range is large, pay attention to preoperative testing. Apply a thin layer of coupling gel before treatment, and stamp-style sequential treatment once during treatment, and try not to overlap the spots.

2. Alexandrite laser (long pulse width) hair removal

2.1 Principle

Alexandrite laser (long pulse width) hair removal is one of the most effective laser hair removal devices and is currently widely used in clinical practice. After the melanin in the hair follicle tissue selectively absorbs energy, it heats up rapidly in a short period of time, causing the hair follicle tissue to degenerate, atrophy, and destroy, thereby achieving the purpose of hair removal. After each laser treatment, 15% to 30% of the hair follicles will be completely destroyed [18], but it is ineffective for white, golden or gray hair, and cannot achieve the ideal effect for some red hair. The hair removal effect of alexandrite laser is not significantly different from that of semiconductor laser [19], but because it has less competition with epidermal melanin and has a lower risk of pigmentation or burning sensation, alexandrite laser is recommended for type IV skin [20]. The main adverse reactions include erythema, edema, and pigmentation, which are related to the competition of epidermal melanin for laser energy. The use of a cooling system can not only reduce pain during treatment, but also protect the epidermis and avoid inflammation and post-inflammatory pigmentation [21].

Currently, there are a variety of available alexandrite lasers, including Ultrawave II ~ III and GentleMax Pro, which have two wavelengths of 755nm and 1064nm, suitable for patients with various skin types. GentleLase and GentleMax Pro are equipped with a dynamic cooling system (DCD) to protect the epidermis. Apogee laser pulse width ranges from 0.5 to 300ms, energy density is up to 50 J/cm², and air cooling or contact cooling is used for epidermal protection. Epicare is equipped with a SmartScreen software package to assist in recording diagnosis and treatment plans. Epitouch Alex is equipped with a scanner option.

2.2 Methods, operation steps and treatment course

The general treatment parameters recommended for long-pulse 755 nm alexandrite laser hair removal are a spot diameter of 12-18 mm, a pulse width of 3 mm, and an energy density of 6-14 J/cm². Among them, relatively low energy density can be used for lip hair and upper limbs, while 10-15 J/cm² is required for armpits. The interval between each treatment is 4-6 weeks, and the total number of treatments is 4-6 times. The alexandrite laser equipped with DCD can provide relatively high energy density at a spot size of 12-15 mm, and is a safe and efficient device[22]. The doctor selects the appropriate parameters based on the patient's skin type (Fitzpatrick classification), skin condition and treatment area. During treatment, the rangefinder should be in horizontal contact with the skin, the handle should be at a 90° angle to the skin surface, and the laser pulses should overlap by 20% to 30% to completely cover the hair removal area.

3. Nd:YAG laser (long pulse width) hair removal

3.1 Principle

The principle and advantages of long pulse width 1064 nm Nd: YAG laser hair removal are as follows: ① Deep penetration; ② High penetration rate. The absorption spectrum shows that at this wavelength of 1.0 μm, the skin color target (melanin and hemoglobin) has a low relative absorption rate, and the 1064 nm wavelength energy is not absorbed by the target, which can enhance the penetration rate in the tissue, the epidermal pigment absorption rate is low, and there are fewer adverse reactions. For dark skin (IV-VI types), higher laser energy can be tolerated and there are fewer complications. The long-pulse 1064nm Nd:YAG laser is the first laser approved by the U.S. Food and Drug Administration (FDA) for hair removal and has been proven to be safe and effective [23].

3.2 Methods, operation steps and treatment course 

Select laser treatment parameters according to the patient's skin color, hair color, location and tolerance. The spot and frequency are relatively fixed at 10-12mm and 2-4Hz. The energy density and pulse width are mainly set. The energy density can be adjusted from 20 to 100 J/cm², and 30 to 60 J/cm² is often selected. Test spots can be made behind the ears, on the inner side of the upper arm, etc. If there is no obvious redness or swelling, increase the energy density by 10% for treatment. According to the Fitzpatrick skin classification, the pulse width is selected, which is 10ms (type I/II), 20ms (type III/IV), and 30ms (type V/VI). After 3 treatments, with an interval of 4 to 6 weeks, the average reduction of facial hair was 41% to 46%, and that of trunk hair was 48% to 53%, with no scar formation [24,25].

Some scholars have provided references for hair removal parameters for different parts: for armpit hair and bikini hair, the spot size was 5 mm, the pulse width was 50 ms, and the energy was 50, 80 or 100 J/cm², with no obvious adverse reactions [26]. For lip hair, 40 to 50 J/cm², 10 to 15 ms; for limbs and trunk, 50 to 60 J/cm², 20 ms; for hairline, 40 to 45 J/cm², 30 ms.

Indications and contraindications of laser hair removal

1. Indications

Congenital hirsutism, such as pigmented piloepidermal nevus and spina bifida [27,28]; acquired hirsutism, such as polycystic ovary syndrome, patients who take large amounts of glucocorticoids for a long time [29] and healthy people who require hair removal for beauty.

2. Contraindications[30]

2.1 Absolute contraindications

①Unrealistic expectations; ②Poor general condition, mental illness or emotional instability, or patients with immunodeficiency diseases; ③Pregnancy and lactation; ④History of photosensitivity; ⑤Skin sensitivity, such as contact dermatitis, eczema and other allergic skin diseases or active herpes simplex, impetigo and other infectious skin diseases, or unhealed wounds at the hair removal site.

2.2 Relative contraindications 

① Patients who have undergone waxing, physical plucking, chemical agents and other forms of hair removal (various types) within 6 weeks; ② Minors (< 18 years old) with Fitzpatrick skin types V and VI.

The efficacy and influencing factors of laser hair removal

1. The influence of hair on the efficacy of hair removal

1.1 The relationship between hair texture and hair removal effect

The optical properties of the patient's skin and hair color are one of the important factors affecting the effect of laser hair removal. The laser cannot distinguish the melanin in the hair follicles and the epidermis. Therefore, laser hair removal is most effective for patients with fair skin and dark hair, while laser hair removal is poor for patients with dark skin and white or brown hair [31]. Secondly, the larger the hair diameter, the higher the energy absorbed and the better the hair removal effect.

1.2 The relationship between hair growth cycle and hair removal effect 

The hair growth cycle is divided into growth phase, resting phase, and regression phase. Only hair in the growth phase is easily damaged by laser because it contains more melanin [32,33]. Therefore, laser hair removal requires multiple treatments to achieve satisfactory results. Different hairs have different growth cycles. The longer the growth phase, the shorter the hair removal interval, and the better the hair removal effect.

2. The influence of the location on the effect of hair removal

At any time point, almost 10% to 15% of the hair may be in the regression phase or resting phase [34]. Studies have shown that the average interval between hair removal for pubic hair is 60 days, while for male beards and hairlines it is about 30 days, and for other parts it is mostly 40 to 50 days [35]. The ratio of hair in the growth phase to the resting phase varies in different anatomical parts, so their response rates to laser hair removal treatment may vary. It is generally believed that the effect of the armpits and belt area is better than that of the legs, arms and chest [34]. The effect of hair location on hair removal effect also varies depending on the characteristics of hair in different parts. For example, the armpits with thick and black hair shafts require relatively fewer laser treatments than the upper lip hair with fine and soft hair. Some hair is significantly affected by hormone levels, which also means that more treatments may be required, such as male beards. The armpits and hairline generally require 3 to 6 treatments, with an interval of 4 to 6 weeks; the limbs and trunk require 3 to 8 treatments, with an interval of 4 to 6 weeks; the lip hair and cheek hair require 5 to 8 treatments, with an interval of 4 to 6 weeks.

3. Effect of laser equipment on hair removal efficacy

Ruby laser (wavelength 694 nm) is effective for dark hair on light skin (skin types I to III) [36]; alexandrite laser (wavelength 755 nm) is more suitable for skin types I to III and light hair [37]. For darker skin, long-pulse semiconductor laser (810 nm) and long-pulse Nd: YAG laser (1064 nm) are the first choice [38].

There is no significant difference in efficacy between long-pulse semiconductor laser and alexandrite laser. The latter has a relatively shorter pulse width and is more suitable for the treatment of fine hair [39]. Long-pulse Nd: YAG laser (1064nm) has better penetration in the dermis, less damage to the epidermis, and very little competitive absorption of epidermal melanin, making it suitable for all skin types [40], especially for dark skin (skin types IV to VI), and is considered the best laser for hair removal in patients with dark skin [41].

In general, the effect of laser hair removal treatment depends on the laser wavelength, energy, spot size, number of treatments, and individual factors (such as skin type, hair type, and hormone levels).

Complications of laser hair removal and postoperative care

1. Normal reactions and care after laser hair removal

After laser hair removal, local skin will experience pain, erythema, edema around the hair follicles, etc. [42], which are normal treatment reactions. The severity of the symptoms is proportional to the treatment energy. Erythema and edema around the hair follicles will disappear on their own after 2 to 7 days. Cooling the skin surface before, during and after surgery can relieve pain ; Immediate cold compress after surgery, and topical glucocorticoid cream or ointment when necessary [43] can help reduce erythema and edema reactions. To avoid irritating the local skin, try to avoid using chemical detergents within 1 week after hair removal surgery, avoid using water that is too hot when washing, and protect the treated area from the sun and moisturize it after surgery.

2. Complications and treatment

Laser hair removal is generally safe, but postoperative complications occur from time to time, which are mostly related to excessive energy density, dark skin color, and improper skin care after treatment. The most common adverse reactions are pigment changes, including pigmentation and hypopigmentation. These changes are usually transient, generally lasting 6 to 9 months, and improve over time. Permanent hypopigmentation may also occur. Topical depigmenting agents (such as hydroquinone), oral tranexamic acid, sunscreen, and moisturizing can improve pigmentation. Topical calcineurin inhibitors (such as tacrolimus and pimecrolimus) help accelerate pigmentation. Recovery of reduction. Scarring is a rare adverse reaction that may occur when too high energy is used or severe secondary infection occurs after surgery. Occasionally, there are reports that patients induce hair growth after laser hair removal, that is, abnormal hair stimulation growth. This situation is more common in female patients with dark skin and abnormal androgen levels of skin types III to VI. Therefore, sub-therapeutic energy should be avoided in clinical treatment of such patients, and patient screening should be done before surgery. Complications such as livedo reticularis and urticaria have also been reported. The former does not require special treatment, and the latter can be improved by oral antihistamines.

Foremed Legend

Suzhou Foremed Legend The core founding team of Medical Technology Co., Ltd. comes from well-known universities at home and abroad such as Peking University. Foremed Legend focuses on the design, development and application of high-end medical beauty optoelectronic equipment based on compliance and product strength, and is committed to becoming a leading enterprise in the field of high-end medical beauty optoelectronic equipment, a provider of integrated intelligent solutions for diagnosis and treatment, and a pioneer of medical beauty data integration platform.

Through tackling a series of underlying key technologies, Foremed Legend has independently developed a number of high-end medical equipment such as picosecond laser therapy devices, long pulse laser therapy devices, intense pulsed light therapy devices, photoacoustic imaging skin detection devices and cold air therapy devices, and continues to deepen the research and development of core product technologies, using better technical solutions to benefit the vast number of beauty seekers.

Adhering to the principle of technology for good, Foremed Legend will work with industry and ecological partners to bring more safe and effective medical beauty optoelectronic equipment and integrated diagnosis and treatment solutions to the global medical beauty market.