The overall hairline shape differs between men and women. Men have a hairline with a pronounced fronto-temporal peak, whereas women tend to have a hairline with a round fronto-temporal angle. Because an oval hairline accentuates the beauty of the feminine face, the demand for hair transplantation for hairline correction (HTHC) has increased in women with an M-shaped, "masculine," forehead.

Hairs may be classified according to their diameter: thick hair, >80 µm; thin hair, between 60 µm and 80 µm; and fine hair, <60 µm1. Swinehart2 mentioned that a normal hairline is not a line but rather a zone in which hairs become finer and sparser. This diminution in diameter and density is most prominent in younger patients, along temporal hairlines, and in female patients. Therefore, fine single hairs at foremost zone of hairline are critical for a natural hairline design in HTHC in female patients. However, conventionally, hairs from the occipital scalp, which has the thickest diameters, are used in hair transplantation. Moreover, a large proportion of Asian patients tend to have hairs with substantially wider shaft diameters than those usually found in Caucasians, which results in a denser posttransplant appearance than ordinarily expected. Therefore, fewer grafts or sessions may produce satisfactory results, as with African American hairs (although not to the same degree). Additionally, special effort must be made to use the finest hairs for the most anterior rows3. To overcome this problem, a few methods have been suggested, including the grafting of bisected hair follicles2 and follicular unit transplantation of body4 or leg5 hairs. However, their use is limited by many practical problems such as poor yield, a longer operation time, the need for a highly skilled specialist, variations in hair angulation or quality, and insufficient body hair. Recently, Jung et al.6 suggested a refined hairline correction technique involving the sorting of one-hair follicular units with a small diameter (60~70 µm thick) from the excision of an occipital hair strip, and then transplanting them onto the front-most hairline. Naturally thinner hairs from the postauricular zone or the nape of the neck can also be harvested and transplanted. However, there are few studies on a nonsurgical revisionary method for improving an unnatural foremost hairline with thick donor hairs resulting from a previous conventional HTHC. Therefore, on the basis of the idea that hairs surviving a hair removal laser (HRL) procedure becomes thinner, the authors investigated the efficacy and safety of using HRL to create fine hairs in Asian women with thick donor hairs.

This retrospective study was approved by the internal institutional review board of Seoul National University Boramae Hospital (IRB No. 06-2012-68). A search of the medical record database revealed that 101 female adult patients received laser procedures at Modelo Clinic for creating fine hairs after HTHC between 2007 and 2010. Among them, 77 patients were excluded because of female pattern hair loss (n=6), receiving HRL treatment other than long-pulse Neodymium-Doped:Yttrium Aluminum Garnet (Nd:YAG) laser (n=3) (CoolGlide; Cutera, San Francisco, CA, USA), and insufficient data (n=68). Other exclusion criteria included pregnancy, menopause, regular medication, and chronic wasting disease (n=0). Therefore, we finally included 24 patients in the study. A retrospective chart review was done, and factors including age, HRL parameters, hair diameter (measured by a micrometer [Mitutoyo, Kawasaki, Japan] before and after the procedures), number of procedures, subjective results of treatment, and adverse effects were investigated. Foremost anterior hairs were divided into four segments along the hairline. One hair from each segment was randomly selected and measured by using a micrometer. The average of four values was calculated and considered the hair diameter. The rate of hair diameter reduction was calculated by using the following equation:

The data were statistically analyzed with IBM SPSS Statistics ver. 20.0 (IBM Co., Armonk, NY, USA) and a p-value of <0.05 was considered to be significant. The hair diameters measured before and after the procedures in one patient were analyzed with a paired t-test. The correlation between parameters was analyzed by using a bivariate correlation test.

The overall shape of the hairline and the proper ratio of the forehead to the entire face are important factors for a balanced and attractive face7,8. A hairline with a prominent fronto-temporal recession is associated with a masculine facial appearance, whereas a round hairline is associated with a feminine face. Scalp reduction surgery was first introduced to correct an M-shaped forehead8. It can allow a high density of hair growth. However, it often leaves a permanent linear scar along the hairline, results in an artificial-looking straight hairline, and causes numbness in the scalp7,8,9. On the other hand, hair transplantation can result in a more natural-looking hairline without scars. Therefore, hair transplantation surgery became the treatment of choice for hairline correction. However, hair transplantation also has some problems, including sparse hair density and unnaturally thick hairs at the foremost zone of hairline. Fine hairs at the foremost hairline are a unique feature in women; however, transplantation of hairs from the occipital area, where hairs are the thickest, create an unnatural hairline. In a recent study in Korea, normal women aged between 22 and 49 years had the thickest hairs on the occipital area (84.2 µm)10.

The creation of a natural-looking hairline thus must include a deliberate attempt to create finer-caliber hair in the most frontal part of the hairline5. A few methods, including transplantation of selected single hairs11, creating fine hairs by using damaging hair bulbs2,12, and follicular unit transplantation of fine leg hairs, were previously suggested. However, their uses are limited because they are time consuming, require high skills, and are troublesome processes that may have low reproducibility. Furthermore, in Asian women with thick donor hairs, the simple method of single-hair transplantation is not very useful. Recent methods such as the use of vellus-like one-hair follicular units approximately 60~70 µm thick from the excision of an occipital hair strip, or the harvesting of naturally thinner hairs from the postauricular zone or the nape of the neck can be valuable options for HTHC in Asian women, resulting in a natural hairline6.

In some circumstances, a method with HRL can be an alternative. For example, HRL is useful when patients do not want an additional surgery to revise the unnatural hairline resulting from a previous conventional HTHC, or when naturally thin hairs are insufficient. HRL does not directly damage the follicular epithelium by targeting melanin as a chromophore. However, according to the extended theory of selective photothermolysis, laser heats the abundant melanins in the hair shaft and then the diffused heat from the shaft damages the surrounding follicular epithelium13. On the basis of this theory, we devised a new method for creating fine hairs that deliver partial damage to the surrounding follicular epithelium by applying laser irradiation with a short pulse width. That is why we selected the parameters of 35~36 J/cm² energy and 6 ms pulse duration for a long-pulse Nd:YAG laser, instead of using the conventional parameters for thick hair removal (i.e., 50~60 J/cm² and 30~40 ms).

On the basis of our data, the hair diameter (-25.7%) was effectively reduced after the HRL procedures. Furthermore, more HRL sessions led to a larger reduction of the hair diameter. Also, we observed foremost hairs partially damaged by HRL grew shorter (data not shown). Considering the reduction of hair diameter and hair length, a significant reduction of hair volume in foremost zone of hairline can be expected. The reduction of hair diameter seen in our study was stable and persistent at 6.3 months after the last session of HRL. However, considering that the growth cycle of human scalp anagen hair takes 2~3 years, and occasionally much longer, a long-term follow-up is required. We believe that any HRL can induce similar partial damage if the appropriate parameters are applied. It would be safe to try a small test area first, and then wait for 3 months to check if the hairs do regrow thinner before treating the whole area. This method can also be tried for other types of hair transplantation in which thin hairs are required, such as in the transplantation of eyebrows, sideburns, and eyelashes. A segment of the donor zone is irradiated with HRL, and hairs growing back thinner after a few months can be harvested and transplanted. However, further studies are required to prove the above hypotheses and to expand the indications.

Although there were no serious complications after creating fine hairs with a long-pulse Nd:YAG laser, careful setting of parameters is required because focal alopecia was observed in a few patients. However, adverse effects such as focal alopecia and folliculitis may be related not only to HRL but also to HTHC itself. Additionally, the thin hairs created by the laser procedure might have accentuated the sparsely transplanted areas.

In conclusion, HRL with long-pulse Nd:YAG can create fine hairs in Asian female patients with thick donor hairs. It can be a useful alternative method when patients do not want additional surgery to revise their hairline resulting from a previous conventional HTHC. It can reduce the diameter of foremost hairs and increase the patients' satisfaction with HTHC.

The limitation of this study is its retrospective and uncontrolled nature with no long-term follow-up data. Future prospective trials with a long-term follow-up and larger sample size are required.