Effect of Different Pulse Duration on Hair Removal Efficiency with Long Pulse Alexandrite: A Comparative Study of a Split Face
Introduction. Laser hair removal is widely used by a large number of patients complaining of unwanted hair. However, little is known about the effect of changing pulse width on clinical outcomes and side effects. This study was aimed at investigating the effectiveness of hair removal using an alexandrite laser with different pulse durations.
Methods. Fifty patients with facial hirsutism underwent an alexandrite laser hair removal procedure using pulses of 3 and 10 milliseconds on each side of the face every 5 weeks for three sessions. Before the treatment and one month after the operation, photographs were taken and the amount of hair was checked.
Results: One month after laser treatment, the elimination rate was 56% with a pulse duration of 3 and 10 milliseconds. There were no significant differences in clinical efficacy or side effect profile.
Conclusion. The use of a 755 nm alexandrite laser for hair removal is an effective and safe method of delaying hair regrowth, and this delay does not significantly change when the pulse duration is increased from 3 to 10 milliseconds.
Introduction
Unwanted hair growth, which is observed in some pathological situations, such as hypertrichosis or hirsutism, is one of the most common complaints of patients in dermatological clinics. Over the past decade, hair removal lasers have been widely used for long-term hair removal and their technology has advanced very rapidly. In fact, the invention of these tools has revolutionized the hair removal industry and there are many different laser systems available today.
Melanin, the natural chromophore of these types of lasers, absorbs the laser light and converts the energy into heat. The speed and extent of this heat is determined by the energy density and duration of exposure. Each of the pulsed light sources generates pulses of different durations, spot sizes, and energy densities that provide different clinical responses. An astute clinician with precise selection of parameters can lead to the desired damage to chromophore-limited tissues. The main targets of laser hair removal are the dermal papilla and the bulge.
Selective thermal damage to these targets occurs when sufficient energy density at a given wavelength, which is more absorbed by the chromophore, is released in a period of time not exceeding the thermal relaxation time (TRT) of the target.
Many studies have been conducted to investigate the effect of different spot sizes and energy flux densities on the clinical improvement and side effect profile of various hair removal lasers, but to the best of our knowledge, there are not many studies investigating these factors, although it has been shown that this parameter is important. The pulse duration (or pulse width) must be long enough for the epidermal melanin to dissipate heat and therefore be safe from thermal damage, but short enough to store enough energy in the hair follicle to destroy it.
In previous studies, the TRT for the epidermis was 3-10 milliseconds, while that for the hair follicle, depending on its diameter, was approximately 10-40 milliseconds. Therefore, from a rational point of view, the ideal must be somewhere between these two values.
This prospective study was designed to investigate the effect of different pulse widths on the clinical efficacy and side effects of laser hair removal using a 755 nm alexandrite laser system.
Materials and methods
Study Design
This was a single center prospective clinical study that was conducted over a period of 9 months from September 2018 to May 2019 at Razi Hospital, the main dermatology clinic in our country.
The study included 50 women with facial hirsutism and Fitzpatrick skin types II-IV.
Patients with the following features were excluded from the study: a history of epilation treatment for facial hirsutism, severe photosensitivity, a tendency to keloid or hypertrophic scarring, recent sun exposure, immunosuppression, and pregnancy.
After being explained to the study participants, an informed written consent was signed by each patient. Then their demographic data was collected.
Clinical assessment and setting
For study purposes: a detailed history regarding the onset of hirsutism and rate of progression, family history, past history, medication history, and any evidence of polycystic ovary syndrome (PCOS) or other forms of hormonal dysfunction were obtained from each patient at the first session. Then, areas of unwanted hair growth were identified, which were noted in two areas of the face measuring 2 × 2 cm on each side of the face. The digital camera took pictures at baseline and 1 month after the last laser treatment using standard equipment and lighting.
Patients were instructed not to shave or remove hair from the treated area for at least 1 week prior to treatment, and were also asked to avoid waxing, threading, bleaching, or plucking for at least one month prior to study entry. No specific preoperative topical skin lightening or other skin care products were prescribed.
At each treatment session, the face was rinsed with water and dried. Local anesthesia was not used. Hair quadrants were trimmed or shaved so that the hair shaft was shorter than 1 mm above the skin surface.
The treatment was started from safe parameters in a conservative manner, especially for darker skin types: using the manufacturer’s recommended methods and settings, one side of each patient’s face (randomly determined) was treated with a 10-millisecond pulse, and the other side with a 3-millisecond pulse with alexandrite. laser.
The remaining parameters (Fluence, DCD at 40/30, spot size 18 mm and repetition rate 1 Hz) were the same for both sides of the face.
Treatment started at 14 J/cm2 for type II skin, 13 J/cm2 for type III skin, and 12 J/cm2 for type IV skin. In the following sessions, the energy flux density decreased or increased by 1 J/cm2, depending on the presence of side effects, as well as patient tolerance. We have achieved the desired results with an energy density of 14 to 18 J/cm2.
Subjects received three treatment sessions at regular intervals of 5 weeks.
The doctor counted the number of growing hairs and determined the rate of elimination in relation to the presence of baldness on the visual analogue scale (VAS) as follows:
Patients without any changes and the degree of clearance is less than 25%, from 25% to < 50%, from 50% to 75% and more than 75%.
An effective treatment was defined as more than 50% hair reduction.
Any side effects have also been documented, including erythema, hypo- or hyperpigmentation, and scarring.
Statistical analysis was performed using SPSS version 23. A P value < 0.05 was considered significant. Chi-square and McNemar tests were used to identify correlations.
Results
Patient characteristics
Fifty patients with facial hirsutism aged 20 to 59 years (mean: 34.7 years) completed the study. 6% (n=3) of patients had skin type II, 64% (n=32) had skin type III, and 30% (n=15) had skin type IV.
Sixteen percent of subjects had a history of PCOS and 84% had no history of PCOS or other hormonal disorders. None of the patients discontinued treatment during the study.
Based on hair density recorded at the start and one month after the last treatment session, we found that 96% of subjects showed some degree of hair reduction at 3 milliseconds and 94% at 10 milliseconds. Effective treatment (defined as a clearance rate greater than 50%) was observed in 56% of patients with both pulse durations.
The difference between pulse durations of 3 and 10 milliseconds in each group was not statistically significant (P-value for each group: 1.000).
We also compared treatment efficacy with these two pulse widths in patients with and without PCOS. Using a chi-square test, we found no significant difference between PCOS and non-PCOS patients at 3 and 10 milliseconds (P-value: 0.441 and 1,000, respectively).
Based on Fitzpatrick skin phototypes, we found that a clearance rate of >50% was observed in 40% of skin type IV subjects and in 42% of skin type II or III subjects with a pulse duration of 3 milliseconds (P value = 0.851).
With a pulse duration of 10 milliseconds, a clearance rate above 50% was achieved in 33% of skin type IV patients and in 42% of skin type II or III patients (P value = 0.529). No adverse events were noted during the study.
The discussion of the results
One of the most effective methods of delaying the growth of unwanted hairs is the use of an alexandrite laser with a wavelength of 755 nm. The principle of selective photothermolysis predicts selective thermal damage to the chromophore by light and is the basis for safe and effective treatment using different types of lasers. Therefore, first of all, we must identify important targets, and in relation to hair removal, the main target sites seem to be the germ cells of the hair, matrix and papilla or bulb and / or follicular stem cells found near the straightening saws. Laser radiation with an appropriate energy density, released at a given wavelength and for a period of time not exceeding the TRT of the target, is preferably absorbed by the target chromophore. With longer pulses, more damage to tissues with broad targets, including hair follicles, should be expected because the laser beam has enough time to absorb their targets. Theoretically, for the best result, the pulse duration will be between the TRT for the epidermis (3-10 milliseconds) and for hair follicles (10-100 milliseconds for hair follicles with a diameter of 100-300 µm).
Depending on the manufacturer, the alexandrite laser is available with pulse durations from 2 to 40 milliseconds. In our study, the pulse duration was 3 and 10 milliseconds, which corresponds to or near the TRT of the epidermis, which allows the epidermis to be protected during air cooling.
Several studies show the effectiveness of the alexandrite laser with different pulse durations for hair removal. A study by Finkel et al showed good results in the treatment of whole body hypertrichosis with a 2-millisecond long pulse alexandrite laser. A similar study by Raulin and Greve used a larger pulse width alexandrite laser, which proved to be a good treatment for unwanted hair. Their patients achieved a mean recovery rate of 75% after an average of 8 facial hirsutism treatments. However, only a few studies have compared the effect of different pulse durations on the efficiency of the alexandrite laser. To the best of our knowledge, this is the first controlled clinical trial with a split face,
According to theoretical considerations, changing the pulse width should change both the results and the side effects. For example, an elongated PD may cause less destruction of epidermal melanosomes compared to shorter PDs because small targets such as epidermal melanosomes are more vulnerable to damage by short pulses. Ataie-Fashtami et al. modeled the effect of different laser parameters on heat distribution and thermal injury pattern of hair and epidermis and found that longer PDs were associated with less thermal injury to the epidermis, but an excessive increase in PD frequency was accompanied by undesirable thermal injury to the hair and adjacent dermis. They proposed to use a diode laser with longer TDs up to 400 milliseconds, to achieve the best clinical results and avoid side effects. Regarding the relationship between PD and clinical outcomes, contrary to theoretical considerations, our study showed that prolonged PD with a long-pulse alexandrite laser does not necessarily lead to better results. In other words, the effect of pulse duration on the clinical outcome of alexandrite laser hair removal treatment may be less than previously expected. In fact, the results of our study are consistent with some previous studies using various PDs. Nanni et al. investigated the effect of PD on the clinical outcomes of long-pulsed alexandrite laser hair removal on various body regions using pulse widths of 5, 10, and 20 milliseconds.
In another study, Bossa et al. used sampling rates of 2 and 20 milliseconds. Similar observations were observed in different parts of the body after a 6-month follow-up. In contrast, McDaniel et al. found better results with longer pulse duration.
Based on our results, hair growth one month after the last laser treatment was not significantly different when PD increased from 3 to 10 milliseconds. The fact that all PDs resulted in equivalent hair reduction may be the result of the short follow-up period. Therefore, further studies with longer follow-up periods and larger sample sizes are recommended to elucidate the exact effect of PD on the effectiveness of laser hair removal.
Although we could not detect any difference between the two pulses of different durations (3 and 10 milliseconds) used in this study, in terms of safety profile, theoretically one would expect a partial discharge of 10 milliseconds to cause less damage to the epidermis due to relative savings of tiny epidermal melanosomes. It should be kept in mind that most of this study was conducted in the fall and winter, which may be an important factor in reducing the incidence of side effects associated with sun exposure.
Several studies have reported a reduction in post-inflammatory hyperpigmentation following the use of a 20-millisecond PD due to this epidermal preservation phenomenon with longer pulses.
An interesting finding from this study was that there was no difference in hair loss relative to the history of PCOS in our patients. McGill et al. demonstrated that women with PCOS may benefit less from laser hair removal than expected, but a recent study showed the efficacy of a 755nm alexandrite laser with a pulse duration of 3ms compared to intense pulsed light (IPL) in this group of patients.
Further research should also consider whether a positive history of PCOS may affect the clinical outcome of hair removal treatment using different types of lasers.
Finally, there was no significant difference in clinical response regarding skin type, which was consistent with previous studies.
In conclusion, the results of this study further support the effective and safe use of the 755 nm alexandrite laser with a pulse duration of 3 and 10 milliseconds for hair removal in patients with skin phototypes II-IV.
Despite the success of this study, some limitations must be considered, including the lack of devices to objectively measure clinical improvement, as well as the limited number of treatment sessions due to some financial issues related to the study budget. Moreover, although the number of participants in our study was comparable to other similar studies, the inclusion of more patients makes it easier to achieve a more reliable result in terms of both clinical efficacy and the occurrence of side effects.
Given these limitations, further studies with larger sample sizes, longer follow-up periods, using devices to quantify hair thickness, or even histological examination of treated areas are suggested to fully understand the effect of pulse width on laser hair removal.