Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported.
Evaluation of a hot-wire hair removal device compared to razor shaving
Article first published online: 5 JUN 2013
Copyright © 2013 Wiley Periodicals, Inc.
Lasers in Surgery and Medicine
Volume 45, Issue 5, pages 283–295, July 2013
How to Cite
Biesman, B. S. (2013), Evaluation of a hot-wire hair removal device compared to razor shaving. Lasers Surg. Med., 45: 283–295. doi: 10.1002/lsm.22144
- Issue published online: 20 JUN 2013
- Article first published online: 5 JUN 2013
- Manuscript Accepted: 17 APR 2013
- Tria Beauty, Inc.
- hair removal;
- hair reduction;
Background and Objectives
We describe a blinded, controlled, prospective clinical study of a hot-wire device promoted for hair removal and the reduction or delay of hair regrowth (no!no!, Radiancy, Inc., Orangeburg, NY) compared to a shaving control.
Study Design/Materials and Methods
Twenty-two subjects were treated by trained clinical staff with the hot-wire device according to its Instructions for Use on the lower leg two times per week for 8 weeks. An adjacent site was shaved with a razor blade on the same schedule to provide a control. Subjects were followed for 3 months after the last treatment to study the durability of the results. Standardized high-resolution photographs were made at baseline, once a week during treatment, and monthly during the post-treatment follow-up period. Micro-tattoos were used to ensure treatments and photographs were reliably made in the same anatomical location from visit to visit. Both active and control sites were shaved prior to baseline and allowed to regrow for a fixed period of time before first treatment to provide a consistent and well-defined baseline hair condition. Quantitative hair counts were made by a third party from the photographs and standard statistical analysis was performed to look for differences between the active and control sites. Visual assessments and quantitative analysis was also performed on the photographs to see if there were any differences in hair thickness (diameter) and hair color between the active and control sites.
The results show that shaving and the hot-wire device are indistinguishable in short-term or long-term effect, based on both visual assessment of the photographs and statistical analysis of the hair counts. The control (shaving) had a mean baseline hair count of 79.4, which remained stable (74.8–84.3) during the 8 week-treatment phase and climbed substantially after stopping treatment to 98.8, 100.1, and 104.6 at 1, 2, and 3 months post-treatment, respectively. The active (hot-wire device) had a mean baseline hair count of 86.0 which remained fairly stable (81.7–95.1) during the treatment phase and then climbed substantially after stopping treatment to 104.0, 106.4, and 109.0 at 1, 2, and 3 months post-treatment, respectively. The difference in hair counts between the control and shaving showed that (a) in the treatment phase, shaving was slightly more effective at hair removal than the hot-wire device with weak statistical significance (P < 0.05 at 5 of 7 time points) and (b) in the follow-up phase, shaving and the hot-wire device were statistically indistinguishable (P = 0.252, 0.0972, and 0.230 at 1, 2, and 3 months, respectively). Likewise, the difference in percentage change from baseline in hair counts (which normalizes to baseline values) between the shaving control and hot-wire device is close to zero at every time point (−4.9% to +4.9%) and the t-test P-values are high (0.154< P < 0.890 over all the time points in the study and 0.360 < P < 0.890 during the 1, 2, and 3 month follow-up period), indicating no detectable difference between shaving and the hot-wire. In terms of hair characteristics, no difference in hair color or hair thickness was seen between the shaving control and the hot-wire sites in the treatment or follow-up period.
Relative to shaving, the hot-wire (no!no!) device does not produce lessened hair density, decreased hair re-growth rate, greater duration of effect, nor induce changes in hair thickness and color. We conclude that the hot-wire device does not offer any benefit as compared to shaving. Lasers Surg. Med. 45:283–295, 2013. © 2013 Wiley Periodicals, Inc.
The no!no! (Radiancy, Inc., Orangeburg, NY) is a hot-wire device marketed to consumers for hair removal. The device is used by rolling it across the skin's surface, causing a heated wire in the device to move into position about half a millimeter above the skin, where it can come into contact with surface hair. The device has been the subject of one prior journal article, which suggested the device was clinically efficacious . While it is logical that the hot-wire device is capable of severing hairs and thereby affecting temporary surface hair removal similar to a standard razor, the ability of this device to induce long term changes in hair follicle growth has not been clearly established since the prior work had a number of limitations, including no shaving control, poorly controlled initial hair count conditions, unclear hair counting methodology, variations in or poor quality photography, unclear means of indexing the treatment locations, limited reporting of the raw data, lack of statistical analysis, minimal follow-up, low sample sizes, non-quantitative assessment methods, lack of blinding, and similar defects. Thus, the benefits of the hot-wire device beyond that of a shaving effect cannot be inferred from these studies.
The purpose of the present study is therefore to perform a scientifically rigorous, independent measurement of the efficacy of the hot-wire device.
This was a prospective, single-center, controlled clinical study using objective hair-counts. Its objective was to measure the efficacy of a commercially available hot-wirehair removal device versus razor-blade shaving control. Endpoints assessed included short-term hair removal and long-term suppression, reduction, or delay in hair regrowth, and evaluation of the color and thickness of any recurrent hairs. The protocol was approved by an institutional review board (IRB), and the study was performed in accordance with the current international standards for good clinical practice, including the International Conference on Harmonization Good Clinical Practice Consolidated Guideline, Declaration of Helsinki, and US federal regulations 21 CFR Part 312 and Part 812.
The willing subjects were recruited from the local community. Enrollment was targeted at 25 with an objective of 20 or more completing the protocol. Completion was defined as undergoing the baseline visit, all treatment visits, and at least two of the four follow-up visits including the final follow-up visit at 12 weeks post final treatment. The sample size was chosen, as, on the basis of previous experience, it was determined that the N value was high enough that the study would be sufficiently powered from a statistical perspective.
Primary inclusion criteria required subjects to be 18–55 years of age, to have sufficient hair density in the study area (the lower leg) for accurate hair counts, to be willing to have hair removal in this area, to have sufficient contrast between skin and hair color for good photography, and to agree to abstain from shaving or using any other hair removal method in the study area during the entire study period. Primary exclusion criteria were cuts, abrasions, infections or other potential confounding or unsafe skin conditions in the study area; plucking, tweezing, waxing, chemical depilatories, or any method of temporary hair removal other than shaving in the study area within 12 weeks of the study start; any prior laser hair removal, electrolysis, or any other permanent hair removal methods in the study area; bleaching or any other hair lightening product in the study area within 6 months of enrollment; a history of keloid scar formation; having ever taken spironolactone or undergone chemotherapy or expecting to do so within the study duration; and being pregnant or expecting to become so within the study duration.
The device studied was the current version of the no!no!hot-wire hair removal system Model 8800 including the device body and its replaceable “Thermicon” tips that contain the hot-wire component, Instructions for Use (IFU, or, User Manual), instructional video, power supply, skin buffing pads, and other miscellaneous components that come in the retail packaging. For sanitation purposes and to ensure that these materials were not overused, each subject had a unique tip and buffing pad that were carefully stored at the study site between visits.
Photography was performed with a Nikon D300 high-resolution (4,288 × 2,848 pixels) digital SLR camera. To provide consistent, high-quality standardized photographs, the camera was mounted to a custom fixture that consists of a transparent acrylic plate that is positioned directly against the skin and a camera mount that fixes the camera 25 cm above the plate such that the camera images the skin through the plate. Images were saved on the camera flash memory cards, the study site computer, and a password-protected, encrypted on-line repository.
Other relevant materials included: micro-tattoo ink (SkinCandy Tattoo Supply, Burbank, CA) and needles with applicators (SofTap Inc., Livermore, CA); electric hair trimmer with 2 mm trimmer heads (Oster); disposable razors and shaving gel; and alcohol prep pads for skin cleansing and skin moisturizer (Cetaphil).
The study schedule was as follows:
- Visit 0: Consent, screening, enrollment, and pre-treatment procedures
- Visits 1–16: Treatments and photography
- Visits 17–20: Follow-up visits and photography
At the initial visit and after informed consent, screening, enrollment, and pregnancy testing, the study staff identified two generally symmetric 3 cm × 3 cm areas on the inside lower leg, each having a minimum of 15 hairs, as the study sites. Sites were adjacent but separated by 3 cm. The upper area (closer to the knee) was delineated as the control (shaving) site and the lower or distal area as the active (treatment) site. To provide visit-to-visit reproducibility, the sites were marked at this visit with two invisible ink (visible under black light only) microdots located at the two extreme corners of the treatment site. A rigid template was used to precisely locate these two microdots. At each treatment visit, the two microdots were visualized with black light and then marked with visible ink. A rigid template was then used to mark the corners of the two sites based on the two reference microdots.
Following the tattooing and site marking, both sites were (a) photographed to document the pre-study condition; (b) clipped if necessary to achieve a hair shaft length of 2 mm for consistent photographs and accurate hair counts and in accordance with the no!no! hot-wire instructions, (c) photographed to document the post-clipping condition, (d) shaved to provide a controlled baseline condition for both sites, and (e) photographed again to document post-shaving skin conditions. The baseline hair counts (for the control site and the active site) were taken from the pre-treatment photos at the subsequent visit, which occurred 4 days later. This ensured that both control and active sites had a well-defined initial hair state for all subjects, with currently telogen (dormant) hairs shaved flush with the skin and currently anagen (active) hairs having 4 days of stubble or regrowth, such that the effect of subsequent shaving and hot-wire treatments can be accurately compared without bias (See Discussion Section).
Twice a week treatment visits then occurred for the next 8 weeks. Each of these treatment visits had identical procedures, as described below, except that photographs were taken only once a week. At each treatment visit, the study sites were (a) evaluated for adverse events caused by the previous treatment, (b) located and marked using the rigid template indexed with the black-light micro-tattoos, and, if photographs were being taken, (c) clipped if necessary to 2 mm hair length for consistent photographs and in accordance with the no!no! hot-wire instructions, and (d) photographed to provide the pre-treatment hair count. Following this, the clinical staff shaved the upper (control) site using a disposable razor and performed the hot-wire treatment on the active (lower) site. If a photography day, digital photographs were then taken again to document the post-treatment skin condition. The study sites were then cleaned with an alcohol prep pad to remove the visible ink and a pea-sized amount of moisturizing lotion was applied to each site.
The hot-wire treatments were performed according to the device's Instructions for Use and the following procedures: the “wide” tip was used (as recommended for large areas like the leg); the treatment level was set to “high” (as recommended for best results); the device was slid across the skin from a starting point at least 5 cm below the marked site to ensure that the filament wire had heated up prior to reaching the treatment site; four passes over the site were made (the IFU recommends multiple passes); and, the treatment area was buffed with the supplied buffing pads to remove any remaining crystallized or singed hairs (as recommended in the IFU).
At the follow-up visits, which occurred 4 days, 4, 8, and 12 weeks after the last treatment, the same steps were used as in the treatment visits, however, no shaving or hot-wire treatments were performed. The visits were solely to observe for adverse events, locate and mark the active and control sites, and photograph them for later assessment of hair count and characteristics.
The hairs were counted from the photographs by a trained, independent third party. A careful methodology was followed whereby the raw images were set to consistent size using Microsoft Picture Manager software so that the 3 cm × 3 cm square fills the program window from top to bottom (60% zoom). Manual counting was conducted in Microsoft Paint software by transferring screenshots from Picture Manager. Each hair was marked with the yellow pencil tool as it was identified and tallied with a digital counter. A 19″ monitor with 1,280 × 1,024 resolution was used. An example photograph printed at the counting resolution is shown in Results Section for the reader's reference.
The photographs were also analyzed for visual changes in the hair fineness (or thickness or gauge) and color intensity (or lightness or darkness) as well as apparent density. The same third party graded the hairs for fineness and color intensity by comparing image pairs consisting of a baseline photo and a follow-up photo (at 4 days and 3 months after the last treatment), where the two photos were randomly positioned on the “left” or “right”. The image pairs were created for both the control (shaving) and the active (hot-wire) sites. The grader was asked to select which image of the pair had hair that was thinner and lighter. Answers were “left”, “right”, or “cannot tell”.
In addition, the photographs were digitally analyzed by Canfield Scientific to measure the diameter, color value, and number of hairs at the same two time points (4 days post-treatment and 3 months post-treatment) compared to baseline.
Efficacy was assessed by quantitative hair counts based on the photographs at each time point, as well as visual assessment of the photographs for changes in hair gauge (fineness or thickness) and color intensity (lightness or darkness). The primary efficacy endpoint was the percent change in hair counts from baseline, which is defined as %cfb = (count − baseline)/baseline × 100. The same assessments were made for the control and active sites and results are reported for control, active, and the difference between the two. Using a difference metric is a standard statistical approach for comparing active and control in this type of study, because it reduces intra-subject variability and provides greater sensitivity, since the difference is calculated within subject (i.e., the difference between control and active is calculated for each subject, showing the relative effect for that subject, and then the statistics are computed for the subjects as a group).
Statistics were analyzed by a professional, independent statistician using industry-standard statistical methods and the commercial SAS package.
Twenty-five subjects were enrolled in the study and 22 completed the study. Two subjects were discontinued after the third treatment due to inability to comply with the required study visits, and one subject was discontinued at the 4 weeks follow-up visit due to inappropriate behavior as judged by the principal investigator (BSB). No subject missed more than two visits, and all subjects had at least 14 treatments. The sample size at all measurement time points thus was no less than twenty. The small number of dropped subjects and missed visits did not meaningfully bias the results.
The demographics of the sample are shown in Table 1. As can be seen in the table, 18 of 25 (72%) were women, the median age was 39 with a minimum of 19 and a maximum of 54, the ethnicity breakout was 92% Caucasian, 4% African-American, and 4% Other.
|African American||1 (4%)|
|Other: Bi-racial||1 (4%)|
As discussed in Methods, high-resolution standardized digital photographs indexed by UV-ink micro-tattoos were taken at regular intervals throughout the study. An example photograph for one subject is shown in Plate 1 for the baseline visit. The photograph shows the control site (shaving) on the left and the active site (hot-wire) on the right. The micro-tattoos are located in the top left and bottom right corners (not visible in the regular lighting used in the photograph) and were used to mark the 3 cm × 3 cm treatment squares visible in the photograph. As is evident even in the small reprint in Plate 1, the hairs are clearly visible and countable. The large field-of-view is also useful for verifying that the subject did not perform unsanctioned shaving, in that visibly longer hair appears in regions adjacent to the treatment site. Plate 2 shows a subsection of Plate 1, reprinted at the actual resolution used for the hair counts for the reader's reference.
Plate 3 shows a complete time history of photographs for one typical subject, including baseline, pre-tx 3 (meaning just prior to treatment 3 and 4 days after treatment 2), pre-tx 5, pre-tx 7, pre-tx 9, pre-tx 11, pre-tx 13, pre-tx 15, 4 days post-tx (meaning 4 days after the 16th and final treatment), 4 weeks post-tx, 8 weeks post-tx, and 12 weeks post-tx. Again, the control site is shown on the left and the active site on the right. Even at the small size required for Plate 3, it can likely be appreciated that there is no discernible difference between the hot-wire and shaving sites at any time point for this subject.
Photographic results at key time points are shown in Plate 4 for three randomly-selected subjects. The plate shows results at baseline, 4 days after the last (16th) treatment, and at 3 months after the last treatment. Comparing the control site on the left and the active site on the right, it can be seen qualitatively that there is no meaningful visible difference between them in the number or characteristics of the hairs. The number of hairs present immediately after completing the 8 weeks course of treatments appears to be the same with shaving or hot-wire. Further, the number of hairs present 3 months after completing the 8 weeks course of treatment is also qualitatively the same between shaving and hot-wire. Analysis of hair length 12 weeks post treatment revealed substantially identical outcomes between control and active treatment zones. Lastly, it can be seen (see also Plate 5 for a zoomed view for one subject) that the hair is qualitatively the same color and the same thickness in both the control and active sites and appears unchanged in these characteristics from the baseline. This suggests that use of the hot-wire device does not produce finer or lighter hair in either the short-term (while treating or just after a treatment course) or long-term (1–3 months post-tx). As will be shown in the statistical section below, these conclusions exemplified in Plate 4 apply to the sample as a whole and in a quantitative manner, not just the three subjects discussed qualitatively in this section.
Quantitative Hair Counts
Quantitative hair counts were taken from the photographs and statistically analyzed as described in Methods Section. The results are listed in Table 2 and plotted in Figures 6-9. The tabulation lists the mean hair counts for the control, active, and difference. Also tabulated are the percent change from baseline in hair counts for the control, active, and difference. The differences are defined throughout as control minus active. The table includes the sample size (N and %), mean and standard deviation (SD), median, min, and max as well as t test and Wilcoxon Signed Rank for conveying statistical significance.
|VISIT||Parameter||Raw score (N = 23)||Percent change from baseline (N = 22)|
|Baseline||N (%)||22 (95.7%)||22 (95.7%)||22 (95.7%)|
|Mean (SD)||79.4 (36.08)||86.0 (38.93)||−6.6 (18.62)|
|Pre Tx 3||N (%)||23 (100.0%)||23 (100.0%)||23 (100.0%)||22 (100.0%)||22 (100.0%)||22 (100.0%)|
|Mean (SD)||79.5 (35.60)||86.3 (40.23)||−6.9 (19.40)||2.1 (9.72)||1.4 (13.27)||0.8 (12.76)|
|Pre Tx 5||N (%)||21 (91.3%)||21 (91.3%)||21 (91.3%)||21 (95.5%)||21 (95.5%)||21 (95.5%)|
|Mean (SD)||80.0 (35.18)||90.9 (41.74)||−10.9 (21.37)||3.4 (12.85)||5.4 (17.94)||−2.0 (14.51)|
|Pre Tx 7||N (%)||22 (95.7%)||22 (95.7%)||22 (95.7%)||21 (95.5%)||21 (95.5%)||21 (95.5%)|
|Mean (SD)||80.8 (33.33)||90.2 (38.51)||−9.4 (19.43)||3.3 (16.45)||5.6 (20.69)||−2.3 (16.56)|
|Pre Tx 9||N (%)||22 (95.7%)||22 (95.7%)||22 (95.7%)||21 (95.5%)||21 (95.5%)||21 (95.5%)|
|Mean (SD)||74.8 (33.77)||81.7 (36.35)||−7.0 (17.86)||2.2 (17.10)||1.4 (17.31)||0.8 (14.69)|
|Pre Tx 11||N (%)||23 (100.0%)||23 (100.0%)||23 (100.0%)||22 (100.0%)||22 (100.0%)||22 (100.0%)|
|Mean (SD)||80.6 (34.72)||91.4 (40.12)||−10.8 (20.70)||7.7 (39.70)||10.8 (28.39)||−3.1 (23.62)|
|Pre Tx 13||N (%)||23 (100.0%)||23 (100.0%)||23 (100.0%)||22 (100.0%)||22 (100.0%)||22 (100.0%)|
|Mean (SD)||81.3 (34.79)||92.3 (40.82)||−11.0 (20.59)||6.2 (17.29)||11.5 (22.70)||−5.3 (16.92)|
|Pre Tx 15||N (%)||23 (100.0%)||23 (100.0%)||23 (100.0%)||22 (100.0%)||22 (100.0%)||22 (100.0%)|
|Mean (SD)||83.8 (35.46)||93.1 (39.73)||−9.3 (22.98)||11.7 (25.88)||14.1 (25.14)||−2.3 (19.03)|
|4 Day FU post||N (%)||23 (100.0%)||23 (100.0%)||23 (100.0%)||22 (100.0%)||22 (100.0%)||22 (100.0%)|
|Mean (SD)||84.3 (32.60)||95.1 (38.10)||−10.9 (22.52)||14.4 (31.02)||19.3 (31.74)||−4.9 (15.56)|
|4 Week FU Post||N (%)||22 (95.7%)||22 (95.7%)||22 (95.7%)||21 (95.5%)||21 (95.5%)||21 (95.5%)|
|Mean (SD)||98.8 (35.24)||104.0 (35.31)||−5.1 (20.46)||41.0 (40.77)||40.5 (41.33)||0.6 (17.96)|
|8 Week FU Post||N (%)||21 (91.3%)||22 (95.7%)||21 (91.3%)||20 (90.9%)||21 (95.5%)||20 (90.9%)|
|Mean (SD)||100.1 (40.54)||106.4 (44.67)||−7.5 (19.81)||28.0 (33.52)||30.9 (38.55)||1.4 (26.50)|
|12 Week FU Post||N (%)||22 (95.7%)||22 (95.7%)||22 (95.7%)||21 (95.5%)||21 (95.5%)||21 (95.5%)|
|Mean (SD)||104.6 (38.25)||109.0 (43.86)||−4.4 (16.86)||37.2 (32.25)||32.2 (31.74)||4.9 (24.18)|
The mean raw hair counts are plotted in Figure 6 for the control and active sites. The control (shaving) site has a mean baseline hair count of 79.4, which remains stable (74.8–84.3) during the 8 week treatment phase and then climbs substantially after stopping treatment to 98.8, 100.1, and 104.6 at 1, 2, and 3 months post-treatment, respectively. The active (hot-wire) site has a mean baseline hair count of 86.0 which remains fairly stable (81.7–95.1) during the 8-week treatment phase and then climbs substantially after stopping treatment to 104.0, 106.4, and 109.0 at 1, 2, and 3 months post-treatment, respectively. Thus, shaving and hot-wire exhibit identical effects, namely that they maintain the baseline hair count while used regularly (2×/wk) but that substantial and equivalent regrowth occurs beginning immediately after cessation of use. This increase in post-treatment hair counts for both the treatment and shaving sites is readily explainable by reference to the hair grown cycle. Only anagen hairs (i.e., hairs in active growth phase) would have been apparent and counted at baseline (since telogen hairs would have been shaved off) and throughout treatment (since hairs that transitioned to telogen during the treatment phase would have been shaved off or removed with the hot-wire device). However, once treatment ceased, the existing hairs would remain as they transition to telogen phase while new hairs would be transitioning from telogen to anagen phase, thus increasing the total hair counts over the 3-month post-treatment assessment period.
To better compare the two techniques, Figure 7 plots the mean of the difference between the control and active sites at each time point, in a box and whiskers plot. As can be seen, the mean difference is near zero at every time point in the study, indicating that no meaningful difference is discernible between the control (shaving) and active (hot-wire) therapy. This can be appreciated visually by noticing that the mean falls well within the box at every time point, the box defining where 50% of the data lie and providing an indication of the measurement range. The fact that the mean difference is almost exclusively negative indicates that the hot-wire sites generally had more hair than the control sites. This matched the study staff's anecdotal observations that it was generally more difficult to obtain “a perfect shave” using the hot-wire device than the razor and that multiple passes and buffing were required, as suggested in the IFU and as permitted by the study protocol. Based upon the t test, the control and active are statistically indistinguishable (P > 0.05) at seven out of 12 time-points. For the five points with statistical significance, the significance is weak (P = 0.018–0.034) but the data indicates that the control (shaving) is actually superior to the active (hot-wire) treatment. These points all occur during the active treatment phase. In the follow-up phase, the p values are 0.252, 0.0972, and 0.230 at 1, 2, and 3 months, respectively, which indicates that shaving and hot-wire are not statistically different from one another in terms of regrowth at any point in time during the post-treatment phase of the study.
Figure 8 plots the mean percentage change from baseline in hair counts for the active and control sites, that is cfb (%) = (count − baseline)/baseline × 100, which normalizes the data. It can be seen from the figure that
- The control and active results overlay each other to a high degree at all time points.
- The degree of difference at any time point is small compared to the measurement range indicated by the boxes at that time point,
- shaving is generally slightly more effective than the hot-wire device in removing hair during the treatment period but the difference is small,
- both shaving and hot-wire essentially maintain the shaved baseline hair counts throughout the treatment period, although there is a slight increase in hair in the later phases of treatment,
- there is a substantial increase in hairs in the follow-up period after cessation of shaving and hot-wire treatments,
- there is no measurable difference between shaving and the hot-wire device in the follow-up period.
The percentage change from the baseline is generally not statistically significant for either the control or active treatment during the treatment phase, but after treatment has stopped, the hair regrows promptly and the percentage change from baseline is strongly significant (generally, P < 0.001) for both shaving and the hot-wire device, indicating significant regrowth immediately after stopping treatment.
To better compare the normalized active and control to each other, Figure 9 plots the difference in percentage change from baseline in hair counts between the active and control sites. This definitive figure shows that the mean normalized difference between the treatments is very close to zero at every time point, ranging from only −4.9 to 4.9% points, and that any excursions from zero are small compared to the measurement range. The t-test P-values are high, ranging from P = 0.154–0.890 over all the time points in the study and P = 0.360–0.890 during the 1, 2, and 3 month follow-up period. Thus, we can safely conclude that there is no statistically significant difference in hair counts between the hot-wire device and shaving in this study, and, thus, that the hot-wire device did not suppress, inhibit, or reduce hair regrowth, change the rate of or otherwise delay hair regrowth, or effect permanent or long-term hair reduction.
It is also evident from Table 2 and Figures 6-9 that the results cannot be explained on the basis that only some persons will respond to the hot-wire treatment. In some cases, the mean can be misleading because certain types of treatment work well on responders but poorly on non-responders. The data from the present study shows a similarly even distribution of results between the shaving control and the hot-wire device as evident from the similar medians and the similarity of the box and whisker plots. The qualitative results were corroborated by quantitative digital hair counts determined by analysis of digital photographs. The results of this analysis are provided in Table 4 which lists the mean percentage change from baseline in hair count for the control (shaving) and active (hot-wire) sites at the two time-points, as well as the P-value describing the statistical significance of any differences between the control and active. As can be seen, the shaving and hot-wire sites both show a substantial increase (23–52%) in hair counts after cessation of treatment. The P-values are high at both time points (0.2398 and 0.7148). Thus, the quantitative analysis further confirms that shaving and hot-wire have no statistically significant differences in the rate of regrowth of hair, either at the end of 8 weeks of treatment or at 3 months after the last treatment.
Visual Hair Characteristics
As noted, the photographs were also analyzed for visual changes in the hair fineness (or thickness or gauge) and color intensity (or lightness or darkness) as well as apparent density. The results of the subjective visual assessments are listed in Table 3, which shows that the blinded grader was unable to discern any significant difference in either hair thickness or color between baseline and post-treatment in the vast majority of the image pairs (73–89%). The differences that were reported between image pairs were distributed between the control and active with no statistical significance (χ2 > 0.05 in all cases). That is, sometimes the control (shaving) was felt to be thicker or darker and sometimes the active (hot-wire) was felt to be thicker or darker. Therefore, there was no overall difference between the treatment (hot-wire) and control (shaving) groups either at the end of the 8 weeks treatment period or the 12 weeks post-treatment follow-up. This matched the study staff and principal investigator's live observations of the subjects, in which they could not observe any difference in the hair characteristics (or hair count) between the shaved control site and the hot-wire sites. Note that because the sites were adjacent (only a 3 cm gap), the live comparison is quite simple to perform and any cosmetically meaningful differences, had they existed, would have been readily apparent.
|Can not tell any difference||Control (shaving) judged more||Active (hot-wire) judged more||χ2||Statistically significant difference?|
|4 days post-tx|
|3 months post-tx|
Again, the results of the quantitative analysis of the digital photographs corroborate the qualitative analysis. The results are provided in Table 4 which lists the mean percentage change from baseline in thickness and intensity for the control (shaving) and active (hot-wire) sites at the two time-points, as well as the P-value describing the statistical significance of any differences between the control and active. As can be seen, the shaving and hot-wire sites show similarly insignificant changes in hair thickness (−1–8%) and hair color (−1–4%) from baseline. The P-values are high for both parameters at both time points (0.2398 and 0.7148). Thus, the Canfield Scientific analysis confirms that the hot-wire device has no statistically significant impact on hair thickness or hair color, either at the end of 8 weeks of treatment or at 3 months after the last treatment.
|Hair count||Hair size||Hair intensity|
|4 days post||12 wks post||4 days post||12 wks post||4 days post||12 wks post||4 days post||12 wks post||4 days post||12 wks post||4 days post||12 wks post|
|Mean change from baseline (%)||23||52||32||49||3||5||−1||8||−1||2||−1||4|
|t test P-value (control:active)||0.2398||0.7148||—||—||0.2202||0.3420||—||—||0.8723||0.3428||—||—|
Two adverse events were reported during the study: (1) one subject had bruising on the lower leg outside the treatment area from bumping into a table leg, and (2) one subject had breast augmentation. Neither was judged to be related to the study and thus there were no adverse device events related to treatment observed by the study staff or reported by subjects. While burning of the skin by the hot-wire device has been reported on websites such as Amazon.com, these reports are unconfirmed and burns were not encountered in this study.
Photographic analysis of the treated sites using several parameters including number of hairs present at the end of the treatment period, number of hairs present at the end of the 12-week follow-up period, and hair color and thickness generated data from which we can draw several conclusions about the differences between the treatment and control methods. First, the data indicates that the hot-wire device provides a shaving-like benefit in the short-term as there was no difference in hair counts at the end of the treatment period. That the hair counts were similar at the 12 week post-treatment follow up indicates that the hot-wire device did not inhibit the rate of regrowth or disable hair follicles compared to shaving. That hair length was substantially identical at the 12 week follow up point further indicates that the hot-wire device did not inhibit the rate of regrowth. Finally, analysis of hair color and thickness found no difference between treatment and control groups, suggesting that use of the hot-wire device does not produce finer or lighter hair in either the short-term (while treating or just after a treatment course) or long-term (1–3 months post-tx).
The current results differ markedly from the only prior published study . This can be readily explained by problems and limitations in the prior study. Some of these limitations are as follows. First, the prior study was uncontrolled so it cannot be reliably concluded that the hot-wire device was responsible for the observed effect. Without a control, any number of systematic or random factors other than the treatment device (e.g., occasional shaving by study subjects, difference in photography quality, or difference in anatomical location) could have caused the observed reduction in hair counts. Second, there are undefined initial hair count conditions, such that some anatomic sites may have had recent hair removal and others none. In fact, the paper describes that clipping was, at least sometimes, required to reduce the hair length to 3–4 mm, which indeed indicates that the hair at baseline had not been treated for a substantial period of time. This is a problem because it means that the baseline hair count would include both anagen and telogen hair whereas the post-treatment (for a razor or for hot-wire) would leave just anagen hair (stubble) and therefore the count would be just anagen hair. Thus, while it may appear that the treatment reduced the hair count, in reality it only severed dormant telogen hairs at the surface (just like shaving with a razor) that would not regrow until the follicle changed phase. Since a significant amount (∼50%) of hair is in telogen at any given time, this effect alone could be substantial, and indeed at least one article indicates that such a shaving bias can still be substantial at 12 weeks post-shaving [2, 3]. Third, this artifact is partially built-into the protocol because of the 2-week hair removal “washout”, whereby the baseline hair counts would have at least 2 weeks of growth compared to the first post-treatment measurement which was made immediately after treatment, almost guaranteeing a “reduction” in hair counts at that time point. Fourth, there is no description of how the treatment locations were indexed from visit-to-visit, and despite the concerted effort made to reproducibly identify treatment locations throughout the study, in fact the images shown in the paper suggest that the positions were not repeatable. Since there can be meaningful variations in hair density at different anatomical locations, this can cause error. Lastly, there are a number of other potential limitations in the prior study that contribute to it being inconclusive, including small sample size, no description of the hair counting methodology, poor camera resolution and variable photographic technique, unsupervised treatments, and no discussion of how it was verified that no shaving occurred during the study.
It is also interesting to compare the present hot-wire device to alternative home-use light-based hair removal systems. At present, there are three FDA-cleared home-use light-based systems that have an indication for permanent hair reduction: the TRIA laser (TRIA Beauty, Dublin, CA), the Silk'n or Sens-epil flashlamp (HomeSkinovations, Yokneam, Israel), and the iLight flashlamp (Remington, Wadsworth, OH), as well as systems in the European Union such as the Lumea flashlamp (Philips, Eindhoven, NV) and Smooth Skin flashlamp (Boots, Nottingham, UK). In terms of mechanism of action, the hot-wire device purportedly delivers heat to the follicle via conduction down the hair shaft from the heat deposited when the moving hot-wire severs the surface hair. In contrast, the light-based systems heat the follicle by delivering light energy directly into the skin where it is absorbed by melanin throughout the pigmented hair shaft and follicle to thermally injure the follicle. These light-based systems have limitations, such as only working on brown or black hair and being safe only for lighter skin types, but are known to deliver a substantial amount of energy to the follicle and have a growing body of peer-reviewed literature establishing safety and efficacy for indicated users (for example, [4-8]).
As to whether conducting additional treatments beyond the 8 week period of this study (which matches the longest period recommended in the hot-wire device's instructions for use) might improve the clinical results achieved by the hot-wire device, the current study cannot address this question conclusively. However, the study does show that there is no evidence for an inhibition in regrowth rate after the initial regimen, and it seems unlikely that, if 16 treatments produce no effect whatsoever, 24 or 48 or some other number would deliver the profound results claimed by the manufacturer. Follicular injury is presumably necessary to affect a structural and/or functional change on the follicle such that the regrowth rate is impacted. As has been demonstrated theoretically and experimentally in electrolysis and light-based hair removal, (for example, [9-12]) such follicular injury is presumably a threshold effect, that is sufficient energy must be delivered to the follicle to at least induce minimal injury that may then accumulate over time in repeated minimal injuries. If 16 treatments demonstrate no detectable changes to follicular function, it suggests that insufficient energy is being delivered to the follicle to produce even a minimal injury and therefore that additional treatments are not likely to eventually accumulate to a meaningful benefit. It also seems unlikely that the average consumer would sustain treatments for such an extended period in any case, given that the treatments are more time-consuming and cumbersome than shaving and do not produce any noticeable benefit over shaving during a long initial regimen.
Lastly, the above results and discussion are not to say that this device might not be a good alternative to shaving for some users. For example, for women who have unwanted hair on their face and who are stigmatized by the thought of using a razor blade and unwilling or unable to perform waxing, bleaching, laser hair removal or other alternatives, the hot-wire device may be a viable option.
This clinical investigation of the hot-wire device demonstrated it to be a novel hair removal device that produces short-term surface hair removal comparable to that achieved using a standard razor. The hot-wire device did not induce long-term reduction in hair counts, slow or delay hair regrowth, alter the thickness or color of hairs that do regrow, or offer any long-term benefit relative to shaving.
Special thanks to Tobin Island, PhD for his assistance in manuscript preparation.
- 1Clinical evaluation of a handheld self-treatment device for hair removal. J Drugs Dermatol 2007; 6(8):788–792..