3Hair follicle destruction by light source: (1) The damage may result from direct photothermal destruction, whereby a light source emitting a wavelength in the absorption spectrum of the desired chromophore melanin (600–1100 nm) results in selective heating of the hairshaft, follicular epithelium, and hair matrix. However, the pulse duration theoretically should be equal to or shorter than the thermal relaxation time of the hair follicle (10–100 msec depending on the size) in order to maintain spatial confinement of the thermal damage. A cooling device may be used to limit thermal damage to the epidermis containing competing melanin chromophore, by means of a cooled glass chamber, cooled sapphire window, a cooled gel layer, or a pulsed cryogen spray. Available photothermal destruction methods include the normal-mode 694 nm ruby, normal-mode 755 nm alexandrite, 800–810 nm pulsed diode, long-pulsed Nd:YAG lasers, and filtered flashlamp technology.
5(2) Photomechanical destruction method relies on the damage induced by the photoacoustic shock waves generated when very short pulsed light source causes rapid heating of the melanin chromophore within the hair follicle, inducingtrauma to the surrounding bulbar melanocytes without follicular disruption.Q-switched Nd:YAG laser and carbon suspension Q-switched .Nd:YAG laser employ this method
6(3) Photochemical destruction results when a photosensitizer is used in conjunction with a light source. In photodynamic therapy, aminolevulinic acid (ALA) becomes rapidly converted to protoporphyrin IX (PPIX) by epithelia cells. A photodynamic reaction occurs when a photon-activated PPIX reacts with molecular oxygen to form singlet oxygen species that result in oxidative cell damage.
7Anagen follicles of terminal hair extend deeply into the subcutaneous fat, lying 2–7 mm below the skin surface.
8However, sometimes the actual target is not pigmented and is at some distance from a pigmented structure. For example, the follicular stem cells, which line the outer root sheath, are not pigmented and are at some distance from the pigmented hair shaft.These cells appear to be an important target for permanent hair destruction. The concept of thermal damage time (TDT) has therefore been proposed in the case of the hair follicle. Pulses longer than the thermal relaxation time of the hair shaft allow propagation of the thermal damage front through the entire volume and better damage of the follicular stem cells..(.100 msappears .
9The bulb of a telogen, or club, hair is unpigmented because of the cessation of melanin production during the catagen stage. In fact, the cessationof melanin production is the first sign of catagen. As anagen progresses, the bulb and papilla descend deeply into the dermis so that late anagen hairs may also be somewhat laser resistant to treatment. It would seem, therefore, that it is in early anagen that hair follicles are most sensitive to laser induced injury. Since any injury, even a laser injury, may induce telogen, the timing of a second laser treatment after the first laser induced telogen formation, becomes critical. As the laser resistant terminal follicles now enter an anagen growth phase, after a first treatment, the second treatment may be more effective than the first. Conversely, a second treatment given too early, or too late, would be expected to have little effect.
10Laser dermatology by GOLDBERG: Lin et al postulate that follicles treated in the telogen phase show only agrowth delay for weeks, whereas, when those follicles are treated in the anagen phase they may be susceptible to lethal damage, may have a growth delay, or may simply switch into telogen phase. This could partly explain the growthdynamics of the hair cycle. Repeated treatments could lead to a synchronization of the anagen phase by induction and/or shortening of the telogen phase,which could increase the effiveness of hair removal witheach consecutive treatment. Another explanation might be that the follicle is not destroyed immediately, but shows a growth arrest after only one (shortened) anagen cycle. Some have questioned the assumption that effective laser hair removal is determined solely by treating hairs in the anagen cycle. These investigators suggest that melanin within a hair follicle may be more important than the actual time of treatment.
11What is the definition of permanent hair loss? Based on our current understanding of hair biology, knowledge of the hair cycleand especially the length of telogen, becomes essential for determining whether laser or light source treatment of unwanted hair is ‘temporary’ or ‘permanent’. Currently, there is no agreement on a definition for treatment induced permanent hair loss Permanence, defined as an absolute lack of hair in a treated area for the lifetime of the patient, may be an unrealistic goal. Dierickx et al. have suggested a more practical approach. They defined ‘permanent’ hair loss as a significant reduction in the number of terminal hairs, after a given treatment, that is stable for a period longer than the complete growth cycle of hair follicles at any given body site..Telogen may last 3–7 months on the thighs and chest. After telogen, the follicle will then recycle into anagen. This will also last 3–7 months. Thus hair may be considerepermanently removed from these locations if there is no recurrence after thiscomplete time period. Olsen has suggested an even longer time period. She suggeststhat permanent hair reduction would be deemed to have occurred after the lapse of6 months after complete growth cycle. If no hair regrows after this time period, it.can be assumed that the growth centers have no capacity to recover from injury
12FDA definition:Permanent hair reduction is deﬁned by the FDA as stable decrease in the number of terminal hairs for a period longer than the complete hair cycle at a given site following a treatment regime, which may include multiple sessions.
13Why permanent hair removal is not possible? 1.Laser induces catagen phase2.One of the uncertainties in laser hair removal is the exact tissue target. Although not deﬁnitively proven, from the above discussion of the hair cycle and follicular stem cells, it appears that complete destruction of hair follicle without regeneration potential may occur when both the germinative cells in the bulb and the stem cells in the bulge area are destroyed. As follicular stem cells appear to be very resilient to thermal damage, this may be difﬁ cult to achieve
16What is the role of anatomic site on the outcome? It should be noted that there is a general consensus that hair removal results will always be affected by chosen anatomic site. Most investigators note a better response on chest, face, legs, and axilla.Lesser responses appear to occur on the back, upper lip, and scalp. In addition, terminal hairs, and not vellus hairs respond to laser treatment.The ideal patient has realistic expectations, normal endocrine status, with thick dark hair and light skin tones .
17There are differences in the anagen:telogen ratio depending on the area that is treated. For example the axillae and bikini areas have a higher anagen: telogen ratio than the legs, arms, and chest. Therefore, it has been suggested that these areas would respond better to laser hair removal with more noticeable difference .in hair density
19The role of parameters?It is now widely accepted that almost any laser can induce temporary hair loss.Fluences as low as 5 J/cm 2 can induce this effect. The effect tends to last 1–3 months.The mechanism of action appears to be an induction of catagen and telogen.Permanent hair reduction, occurring at higher fluences is seen in 80% of individuals and is fluence dependent. Thus, the greater the delivered fluence, all else being equal, the better are the expected results.
20Dark hair with ample amounts of eumelanin can be effectively treated by multiple devices, provided that appropriate fluence and spot size are used. Under these conditions, the average long term hair loss per treatment is about 20-30% based on studies performed with ruby lasers.Multiple treatment sessions are usually required to achieve the maximal level of hair reduction.In individuals with blonde, red, gray, or white hair, it is unlikely that laser treatment will produce the same degree of longlasting hair removal. However, such patients may be willing to undergo treatments spaced about 2-4 months apart as necessary to maintain temporary hair loss.
22While treatment can be safely performed with a shorter wavelength device (e.g. ruby laser) in fair skinned patients, it is preferable to use longer wavelength devices in darker skinned patients.
23RUBYThe first laser hair removal system employed was the normal-mode ruby laser, but the pulse duration of 0.3 ms was not efficacious longer pulse durations of ms subsequently increased its usefulness. When the pulse duration of the ruby laser was lengthened to 3 ms, permanent hair removal was demonstrated however, there was a significant risk of dyspigmentation.In general, use of the ruby laser for hair removal is limited to patients with skin types I-III, since absorption of694 nm light by epidermal melanin results in a higher risk of compli-cations (e.g. blistering, pigmentary changes) in individuals with darklypigmented skin.
24However, sometimes the actual target is not pigmented and is at some distance from a pigmented structure. For example, the follicular stem cells, which line the outer root sheath, are not pigmented and are at some distance from the pigmented hair shaft.These cells appear to be an important target for permanent hair destruction. The concept of thermal damage time (TDT) has therefore been proposed in the case of the hair follicle. Pulses longer than the thermal relaxation time of the hair shaft allow propagation of the thermal damage front through the entire volume and better damage of the follicular stem cells..(.100 msappears .
27ALEXANDRATE BOLOGNIA: Multiple studies have demonstrated effective hair removal with thelong-pulsed alexandrite laser at fluences of J/cm2 and pulse durations of ms. At fluences of Jlcm2, several investigators have reported hair reductions of % after multiple (at least 3-5) treatments. Use of the long-pulsed alexandrite in patientswith darkly pigmented skin has been reported, but side effects such asblistering and pigmentary alteration can occure in general, this laser isbest suited for hair removal in patients with skin types I-III.
29Red hair contains pheomelanin, which absorbs poorly at any wave-length, but especially beyond 800 nm. Therefore, shorter wave-lengths, such as those of ruby or alexandrite lasers are needed in these patients.
31Which laser is suitable for individuals with low contrast between hair and skin? In cases of low contrast between skin tone and hair color, such as darkly pigmented individuals or light hair in fair individuals, a signiﬁcant portion of emitted light is absorbed by the epidermal melanin. This occurs because epidermal melanin absorbs energy not only from direct exposure, but also from backscattering. As wavelengths of light increase, a greater ratio of dermal-to-epidermal deposition of energy results in greater safety in such individuals.
32DIOD Depending on the semiconductor used in the system, diode lasers can vary widely in their emitted wavelengths. Thosemanufactured using derivatives of gallium arsenide produce wavelengths between and 900 nm, whereas those utilizing indium phosphide emit light with wavelengths between and 1550 nm. Diode lasers currently used for hair removal produce 800- or 810-nm light.
33Both 800 and 810 nm wavelengths are absorbed by melanin to a slightly lesser degree compared to the alexandrite laser.Absorption is also approximately 30% less than that of the 694 nm wavelength. Decreased absorption by epidermal melanin permits safe treatment in patients with skin types I to IV .
34Although the rates of melanin absorption by diod laser is lower than the ruby and alexandrate, the rates of hair reduction with the diode laser are similar to those observed with the alexandrite laser, likely due to deeper penetration into the dermis . Treatment with the diode laser can be somewhat more painful, however, at least partly dueto greater volumetric heating of tissue. If necessary, pain can be partially alleviated by reducing the frequency of delivered pulses.
35A diode laser was safely used in several patients undergoing isotretinoin therapy without increase in adverse effect or longterm sequela,the isotretinoin use should be avoided in general up to 6 month beforehair removal.
36Nd:YAGAlthough signiﬁcantly less well absorbed by melanin compared to previously described wavelengths, light at 1064 nm is able to penetrate deep into the dermis, up to 4 to 6 mm. Originally used in Q-switched mode with or without topical application of carbon particle suspension, the new millisecond systems appear to offer good results especially for darker skin types, albeit at a price of slightly decreased efﬁcacy in lighter skinAlthough comparable in lighter skin types (I to III), the incidence of complications in skin types IV to VI is considerably lower with the long-pulsed Nd:YAG than with the other lasers.Hair removal using Nd:YAG laser has been found to be considerably morepainful than that with diode lasers, likely due to even greater volumetric heating with the former
37chromophore? What is exogenous For persons with very dark skin and blonde or gray hair, an effective permanent laser hair removal treatment is still lacking. Potentially, the exogenous chromophore approach could solve this problem. Rather than targeting endogenous melanin, an exogenous chromophore (like dyes, photosensitizers, or carbon particles) can be introduced into the hair follicle and then irradiated with light of a wavelength that matches its absorption peak. The main problem is reliable penetration of the chromophore into all depths of the hair follicle.Therefore, the technique, in its present form, is apparently inadequate for inducing permanent hair loss. However, in vitro testing has recently shown that a new method could increase the quantity and the penetration depth of dyes in .follicular ducts
38Q-switched Nd:YAG lasers have been used to target topically applied carbon particles that have previously been applied to the hair follicle.This method was one of the first available laser hair removal techniques. This short term hair removal technique has also been used without the prior application of carbon.Immediately after Q-switched nm laser irradiation of carbon coated hairs, the car bon is heated to its vaporization temperature of about 3,700 °C. Vaporization leads to a huge volume expansion with resultant supersonic proliferation of high pressure waves. These shock waves, in turn, produce mechanical damage, aswell as the development of heat. It is not clear how much mechanical and/or heat energy produced by this mechanism is required for destruction of a hair follicle. However, histologic evidence of follicular damage is seen after such a laser exposure. This results in a clinical delay of hair growth.Although high rates of purpura, up to 18%, have previously been reported following treatment with Q-switched Nd:YAG lasers , this does not appear to occur with the long-pulsed lasers.
40TechniqueLaser treatments for hair removal are generally well tolerated bypatients and do not usually require anesthesia. However, because the hair follicle is surrounded by nerve endings, topical anesthesia is necessary when treating large surfaces of the body or sensitive areas such as the upper lip and bikini area. Local infiltration of anesthetic or nerve blocks using 2% lidocaine can also be useful in selected patients.
42Pulses are delivered in a slightly overlapping mode with a predetermined spot size. The ideal immediate response is vaporization of the hair shaft, followederythema a few minutes later by the appearance of perifollicular edema andfluence and the It is generally recommended that the highest tolerablelargest spot size be used in order to obtain the best results. However, excessive fluences that cause epidermal separation or blistering should be avoided. If the device is not equipped with a cooling mechanism, a thick layer of cool gel is applied before the delivery of laser pulses.
43Epidermal cooling during treatment accomplishes two goals. First, it minimizes the risk of epidermal damage due to absorption by epidermal melanin. Such damage may result in blistering, discoloration, and scarring, and is especially important for darker pigmented individuals. Therefore, epidermal cooling allows for use of higher ﬂ uences while minimizing the risk of epidermal damage. Second, it has temporary anesthetic effect, thus reducing patient discomfort during treatment. Epidermal cooling can be separated into contact and non-contact. Contact cooling may include cold gel, chilled sapphire window, and cooled copper plate. Non-contact cooling includes cold air convection and dynamic cooling with automatic spraying of liquid cryogen, tetraﬂ uoroethane, immediately prior to laser pulse.
44After being scattered, photons closer to the center of the beam may, in fact, regain the initial direction of travel through their interaction with other photons. On the other hand, if scattered, photons at the periphery of laser beam may not be able to regain it. Therefore, the percentage of light lost to scattering is larger for smaller beam diameter, or spot size, than for a larger one. Two corollaries of this principle are that energy density, or ﬂ uence, is lowered for larger spot sizes and that a very small spot size may, in fact, not allow for sufﬁ cient depth of penetration, even with longer wavelengths. Thus, the largest spot size accommodated by the size of the treatment area may lead to better efﬁ cacy, as well as decreased overall treatment time.
45After a spot size is selected, the choice of ﬂ uence depends of the amount of chromophore in the target structure, as well as selected wavelength. Higher ﬂ uences are needed when the amount of chromophore is reduced, as represented by blonde or thinner-caliber hair. As well, as was previously discussed, higher ﬂ uences are used with longer wavelengths due to the nearly linearly decreasing absorption of light by melanin at those wavelengths. In addition, ﬂ uences diminish rapidly as a function of the depth of penetration, as progressive absorption and scattering result in decay of the original energy. Therefore, even though temporary hair removal will occur at almost any ﬂ uence, higher ﬂuences may result in longer-lasting or more permanent hair reduction.
46For all laser and light systems, the immediate treatment endpoint is vaporization of the hair shaft. This is followed in a few seconds to minutes by mild erythema and perifollicular edema.In general, these should resolve within 10 to 60 minutes. More widespread erythema or conﬂ uent edema may indicate epidermal and dermal damage, respectively. If noted, lower ﬂ uence, longer pulse duration, or better cooling should be utilized.Gray or white discoloration immediately following laser pulse and lasting a few seconds is an indication of epidermal damage and should be avoided.
47During treatment, pressure may be applied to the skin before each pulse. This maneuver leads to decreased depth of the bulb and the bulge, increased penetration of light into the dermis, and decreased amount of hemoglobin, a competing chromophore, in dermal capillaries. Recently, a pneumatic skin ﬂattening device using negative pressure was introduced in conjunction with lasers and IPL devices. This technique may reduce intra-operative pain and post-treatment erythema and edema, while providing more uniform cooling and .similar or enhanced efﬁcacy compared to .conventional photoepilation
55Research has shown that laser hair removal requires the presence of a pigmented hair shaft. Retreatment can therefore be performed as soon as regrowth appears.Regrowth is based on the natural cycle, which varies by anatomic location ,but on average, the timing is 6–8 weeks. More research regarding this is currently being conducted.
56Repeated treatments may synchronize the anagen phase by induction and/or shortening of the telogen phase which could lead to increased effectiveness of hair removal at each consecutive treatment. Alternatively, the follicle may show growth arrest after one shortened anagen cycle
57Temporary hair loss (1–3 months) always occurs after laser treatment, regardless of hair color or device used. On the other hand, the ability to induce long-lasting hair reduction is strongly correlated with hair color. Patients with dark hair are most likely to obtain longlasting hair removal. Blonde-, red-, gray-, or white-haired patients are unlikely to experience a permanent reduction, because melanin is low or lacking in the hair follicles.