Abstract
Background
Numerous nonsurgical but invasive cosmetic procedures are performed blindly in the dermis or subcutaneous fat layer of the facial skin.
Objectives
To measure the numerical skin thickness of the facial areas where dermatological procedures are performed by applying ultrasound techniques, and to make it possible to estimate the skin thickness by investigating the influence of several individual constitutional factors such as age, sex, and body mass index (BMI), so that these variables can be applied to estimate skin thickness.
Materials and methods
Skin thickness was measured at eight different facial points using an ultrasound machine (Affiniti 50; Philips Inc.). Demographic data were gathered using questionnaires. Manual BMI was calculated from the weight and height of each participant, and individual BMI measurements were performed using a body composition analyzer.
Results
In terms of whole skin thickness, the thickest point was the mouth corner, and the thinnest point was the lateral forehead. The thickest point in the epidermis was the chin, and the thinnest point was the nasolabial fold. The thickest point in the dermis was the corner of the mouth, and the thinnest was the lateral forehead. Full skin thickness and dermal thickness were mostly lower in females. Skin thickness was not significantly correlated with BMI.
Conclusion
The skin thickness at different points on the face was variable, and realistic data about skin thickness can be obtained by in vivo ultrasonographic analysis of the skin.
Keywords: age, body mass index, dermis, epidermis, skin, thickness
1. INTRODUCTION
In recent years, numerous non‐surgical but invasive cosmetic procedures have been performed. Many of these procedures are performed blindly in the dermis or subcutaneous fat layers of the skin. However, there is little objective data on the skin thickness at each site, which may be troublesome in clinical practice. Previous studies have analyzed skin thickness obtained from skin biopsies of donated cadavers or normal skin biopsy specimens. 1 , 2 However, in such studies, it is difficult to accurately measure skin thickness due to changes made during tissue processing. 3 Additionally, the number of participants included in the previous studies is extremely limited because the research method itself is strenuous and invasive. On the other hand, measuring skin thickness using in vivo techniques, such as calipers, radiography, and high‐frequency pulsed ultrasound is preferred because it is more clinically relevant and proposed to be superior in terms of reproducibility of results. Besides these, most previous studies have been conducted on Caucasian participants, and studies on other ethnicities are rare. There are limits to applying the results of studies done on Caucasians directly to other ethnic patients. Furthermore, estimating the depth of the skin and understanding the skin thickness in correlation with factors such as age, anatomic sites, body mass index (BMI), and sex is important for more effective, personalized, and safe procedures.
2. MATERIALS AND METHODS
The study protocol was reviewed and approved by the Institutional Review Board of Korea University Guro Hospital, Seoul, Korea (2017GR0224). All participants voluntarily signed an informed consent form prior to participation in the study. All the collected data were encrypted to ensure anonymity.
2.1. Study population
This study was conducted at Korea University Guro Hospital and included Korean individuals of Asian ethnicity, aged between 19 and 75 years, as eligible candidates. Study candidates with a history of facial surgery, and facial skin disease, as well as candidates with facial tumors or scars from trauma or burns, were excluded.
Demographic data were gathered using questionnaires. Upon enrollment, the weight and height of all participants were checked, and manual BMI results were calculated. Individual BMI measurements were done on the same day employing a body composition analyzer (InBody 270 machine; Biospace Inc.). BMI was classified according to the World Health Organization criteria. 4
2.2. Measurement protocol
Measurement of the skin thickness at the eight different facial points was done by the same clinician using an ultrasound machine (Affiniti 50; Philips Inc.) with the linear L18‐5 probe (50−60 Hz). The probe and image depth were set at 57 Hz and 15 mm, respectively.
Ultrasound images clearly distinguished the upper three skin layers: epidermis, dermis, and subcutaneous fat layer (Figure 1). Skin thickness was measured by drawing a straight line perpendicular to the skin surface and the subcutaneous tissue using Viewer software (Sante DICOM Viewer Pro version 11.8.1; Santesoft Inc.). Measurements of the full skin thickness of the epidermis and dermis, and the separate skin thickness of the epidermis and dermis on the eight different face points (central forehead [CF], glabella [G], lateral forehead [LF], temporal [T], malar [M], nasolabial fold [NF], mouth corner [MC], and chin [C]) of all participant were obtained and recorded. The reference points for each measurement site are shown in Figure 2.
FIGURE 1.
Ultrasound images clearly distinguished the layers (epidermis, dermis, subcutaneous tissue + muscle layer). Distances were measured by drawing straight lines perpendicular from the skin surface to the subcutaneous tissue using Sante DICOMDIR Viewer software. Approximate schematic diagram (left), actual measurement screen (big screen).
FIGURE 2.
1. Central forehead (CF): midpoint between the mid‐hairline and the glabella, 2. Glabella (G): midpoint between both eyebrows, 3. Lateral forehead (LF): the point where the vertical line drawn at the lateral end of the eyebrow meets the horizontal line drawn at the central forehead, 4. Temporal (T): midpoint between the edge of the eye and the sideburn, 5. Malar (M): the point where the malar eminence is most prominent, 6. Nasolabial fold (NF): the point where the horizontal line drawn at the nasal ala meets the nasolabial fold, 7. Mouth corner (MC): the point 1 cm above the mouth angle, 8. Chin (C): the midpoint between the lower vermillion border of the lip and the tip of the chin.
2.3. Data collection
Furthermore, the relative thickness index (RTI) measurement system was used to compare the average thicknesses of different anatomic sites. RTI was calculated in an effort to reduce inter‐observer variability by allowing each participant to serve as his or her own control. In this RTI measurement system, the value of the thinnest point was set to one on the relative thickness scale, and each subsequent measurement was expressed as multiple values.
Additionally, a survey was conducted to determine the effects of daylight exposure, water intake, amount of exercise, and sleeping hours on skin thickness.
2.4. Statistical analysis
SPSS Statistics version 24.0 (IBM Corp.) was used for statistical analysis. Mean skin thickness was calculated for each facial point. The influence of sex on skin thickness was reviewed using the Mann–Whitney U test. Subsequently, the association between age, BMI, and skin thickness was investigated using Spearman's correlation analysis. To analyze whether skin thickness significantly differed among facial points, pairwise comparisons of means were performed using the Wilcoxon test. The p‐value < 0.05 was considered statistically significant. The association of daylight exposure, water intake, amount of exercise, sleeping hours, and skin thickness was investigated using Spearman correlation analysis.
3. RESULTS
3.1. Participants
A total of 99 participants were enrolled, and their ages ranged from 19 to 71 years (Table 1). Twenty‐one male participants and 78 female participants were classified into four age groups (< 30, 30–39, 40–49, and ≥50 years). The mean (standard deviation, SD) age of the participants was 36.83 (± 11.49 years). A total of 5.1% of the participants were underweight (BMI < 18.5), 63.6% of them had normal BMI (BMI 18.50–24.99), 26.3% of them were overweight (BMI ≥ 25), and lastly 5.1% of the participants were obese (BMI ≥ 30). All participants had Fitzpatrick skin type III or IV.
TABLE 1.
Demographics of the 99 study participants.
| Characteristic | Patients, No. (%) |
|---|---|
| Sex | |
| Male | 21 (21.2) |
| Female | 78 (78.8) |
| Age | |
| < 30 | 34 (34.3) |
| 30∼39 | 25 (25.3) |
| 40∼49 | 23 (23.2) |
| 50≤ | 17 (17.2) |
| Body Mass Index (BMI) | |
| < 18.5 | 5 (5.1) |
| 18.50–24.99 | 63 (63.6) |
| 25–29.99 | 26 (26.3) |
| 30≤ | 5 (5.1) |
3.2. Skin thickness at different facial sites
Full skin, epidermal, dermal thickness, and RTI were obtained at eight different points on the face, and the mean and SD were calculated (Table 2). The thickest full skin thickness was measured at MC (mean: 1.64 mm, 95% confidence interval [CI]: 1.54–1.75), and the thinnest full skin thickness was measured at LF (mean: 1.31 mm, 95% CI: 1.20−1.42). Similarly, the RTI showed the thickest value at MC (mean: 1.61, 95% CI: 1.42−1.73) and the thinnest value at LF (mean: 1.26, 95% CI: 1.18−1.36). In addition, the thickest point in the epidermis was C (mean: 0.40, 95% CI: 0.35−0.38) and the thinnest point in the epidermis was NF (mean: 0.33, 95% CI: 0.31−0.34). The thickest point in the dermis was MC (mean: 1.30, 95% CI: 1.20−1.41) and the thinnest was LF (mean: 0.98, 95% CI: 0.87−1.08).
TABLE 2.
Result of skin thickness measurements of each designated site.
| Skin thickness (mm) | Relative thickness index (RTI) | Epidermal thickness (mm) | Dermal thickness (mm) | |
|---|---|---|---|---|
| Site | (mean ± SD) | (mean ± SD) | (mean ± SD) | (mean ± SD) |
| CF (Central forehead) | 1.353 ± 0.567 | 1.319 ± 0.680 | 0.334 ± 0.157 | 1.019 ± 0.534 |
| G (Glabella) | 1.363 ± 0.375 | 1.327 ± 0.403 | 0.349 ± 0.210 | 1.014 ± 0.289 |
| LF (Lateral forehead) | 1.309 ± 0.553 | 1.262 ± 0.472 | 0.332 ± 0.073 | 0.978 ± 0.537 |
| T (Temporal) | 1.390 ± 0.580 | 1.333 ± 0.500 | 0.341 ± 0.153 | 1.049 ± 0.544 |
| M (Malar) | 1.488 ± 0.593 | 1.419 ± 0.493 | 0.327 ± 0.080 | 1.160 ± 0.584 |
| NF (Nasolabial fold) | 1.601 ± 0.617 | 1.549 ± 0.664 | 0.325 ± 0.076 | 1.276 ± 0.593 |
| MC (Mouth corner) | 1.643 ± 0.535 | 1.613 ± 0.662 | 0.344 ± 0.082 | 1.299 ± 0.533 |
| C (Chin) | 1.674 ± 0.747 | 1.623 ± 0.756 | 0.402 ± 0.212 | 1.273 ± 0.619 |
SD: standard deviation, RTI: normalized ratios calculated by dividing each skin thickness by the thinnest value.
3.3. Skin thickness and sex
Full skin and dermal thickness were thinner in females at CF, G, LF, T, NF, and MC (p < 0.05). There were no significant differences in epidermal thickness between the sexes. The mean skin thickness for the CF was 1.48 and 132 mm for males and females, respectively (p < 0.05). The G was on average 1.52 and 1.32 mm for males and females, respectively (p < 0.05). The LF was on average 1.62 and 1.23 mm for males and females, respectively (p < 0.005) and the T was on average 1.51 and 1.36 mm for males and females, respectively (p < 0.05). The NF was on average 1.85 and 1.53 mm for males and females, respectively (p < 0.05). Lastly, the MC was on average 1.88 and 1.58 mm for males and females, respectively (p < 0.05) (Table 3).
TABLE 3.
Mean and standard deviation of the thickness of skin, epidermis, and dermis in relation to sex.
| Site | Male (mm, mean ± SD) | Female (mm, mean ± SD) | p‐Value | |
|---|---|---|---|---|
| CF | Skin thickness | 1.478 ± 0.105 | 1.320 ± 0.066 | 0.400 * |
| Epidermal thickness | 0.333 ± 0.021 | 0.334 ± 0.019 | 0.690 | |
| Dermal thickness | 1.146 ± 0.100 | 0.986 ± 0.062 | 0.049 * | |
| G | Skin thickness | 1.522 ± 0.074 | 1.321 ± 0.043 | 0.013 * |
| Epidermal thickness | 0.344 ± 0.016 | 0.351 ± 0.027 | 0.288 | |
| Dermal thickness | 1.178 ± 0.068 | 0.970 ± 0.030 | 0.010 * | |
| LF | Skin thickness | 1.619 ± 0.215 | 1.226 ± 0.036 | 0.003 * |
| Epidermal thickness | 0.351 ± 0.016 | 0.326 ± 0.008 | 0.092 | |
| Dermal thickness | 1.267 ± 0.209 | 0.900 ± 0.036 | 0.004 * | |
| T | Skin thickness | 1.508 ± 0.089 | 1.358 ± 0.070 | 0.022 * |
| Epidermal thickness | 0.321 ± 0.017 | 0.347 ± 0.019 | 0.540 | |
| Dermal thickness | 1.188 ± 0.087 | 1.011 ± 0.065 | 0.008 * | |
| M | Skin thickness | 1.558 ± 0.116 | 1.469 ± 0.069 | 0.132 |
| Epidermal thickness | 0.330 ± 0.013 | 0.327 ± 0.010 | 0.529 | |
| Dermal thickness | 1.227 ± 0.119 | 1.142 ± 0.067 | 0.189 | |
| NF | Skin thickness | 1.847 ± 0.156 | 1.535 ± 0.065 | 0.024 * |
| Epidermal thickness | 0.338 ± 0.017 | 0.322 ± 0.009 | 0.325 | |
| Dermal thickness | 1.509 ± 0.153 | 1.213 ± 0.062 | 0.033 * | |
| MC | Skin thickness | 1.877 ± 0.133 | 1.580 ± 0.056 | 0.018 * |
| Epidermal thickness | 0.361 ± 0.020 | 0.339 ± 0.009 | 0.382 | |
| Dermal thickness | 1.516 ± 0.132 | 1.241 ± 0.057 | 0.035 * | |
| C | Skin thickness | 1.767 ± 0.154 | 1.650 ± 0.086 | 0.526 |
| Epidermal thickness | 0.373 ± 0.019 | 0.409 ± 0.027 | 0.614 | |
| Dermal thickness | 1.394 ± 0.149 | 1.241 ± 0.068 | 0.411 |
p < 0.05.
3.4. Skin thickness and age
Skin thickness was not significantly different between sites across all age groups. However, the epidermal thickness of the LF, T, M, NF, and MC showed a weak positive correlation with age (Figure 3). In contrast, the dermal thickness of G and LF showed a weak negative correlation with age.
FIGURE 3.
Correlation between age and epidermal thickness at Lateral forehead (LF), Temporal (T), Malar (M), Nasolabial fold (NF), and Mouth corner (MC) using a scatter plot. The epidermal thickness of LF (+0.235), T (+0.297), M (+0.221), NF (+0.251), and MC (+0.289) showed significant but weak positive correlation on investigation using Spearman correlation.
In female participants, epidermal thickness at LF, T, M, NF, and MC showed a positive but weak correlation with age, while the dermal thickness at G was negatively correlated with age. In male participants, there was no correlation between age and any of the three groups of thickness parameters.
3.5. Skin thickness and BMI
Skin, epidermal, and dermal thicknesses were not significantly correlated with BMI. Analysis of data from males and females respectively showed a weak negative correlation between BMI and skin thickness.
3.6. Skin thickness and other factors
Daylight exposure, water intake, exercise, and sleeping hours were not significantly correlated with full skin, epidermal, or dermal thickness.
4. DISCUSSION
In vitro or histometric modalities have primarily been used for measurements of skin thickness. 1 , 2 However, the skin of cadavers, although fresh, may not accurately reflect the conditions and skin thickness of living human beings. The real skin thickness can be altered during the preparation of histological specimens. Further, the exact details of each cadaver's medical history and body habits cannot be referred to, which might affect skin thickness. For these reasons, although the most frequently used and published method of skin thickness measurement is a histometric punch‐needle biopsy, recent studies characterizing skin thickness have employed in vivo techniques such as ultrasound or magnetic resonance imaging. 3 , 5 , 6 , 7 , 8 , 9 , 10 , 11
Ultrasonography is a non‐invasive, cost‐effective, and easily accessible modality with no harm and has been used in other studies that measure human tissues. However, there are only a few facial sonographic studies for measuring skin thickness, 8 subcutaneous fat thickness, 12 and the depth and thickness of muscles. 13 The last two studies were performed to standardize the depth of injection of botulinum toxin. 12 , 13
The skin was the thickest in the MC and the thinnest in the LF. This was the same for the RTI and dermal thickness. The difference in the whole skin thickness of each part of the face was due to differences in dermal thickness. The pattern of dermal thickness and not that of epidermal thickness dictates the total skin thickness. This is remarkable, as the same finding was mentioned in the study by Chopra et al. 2
There are several reports of thinner skin thickness in women and no difference in epidermal thickness in the face for both sexes. 14 , 15 , 16 , 17 Bailey et al. 15 studied 88 participants using a non‐invasive instrument (DermaScan C 20 MHz) and found that men had an overall 10%–20% thicker facial skin (forehead, mid‐cheek, and jowl) than women. Mogensen et al. 16 found no sex‐related differences in epidermal thickness using optical coherence tomography imaging. Gambichler et al. 17 studied 83 participants using in vivo optical coherence tomography imaging, performing intra‐ and inter‐day repeatability measurements. The results showed that epidermal thickness did not differ significantly between men and women, except for the forehead skin, which was significantly thinner in older women than in men. Our findings showed that the skin and dermal thickness tended to be thinner in women in most areas of the face (CF, G, LF, T, NF, and MC). However, epidermal thickness in all facial areas showed no sex‐related differences. It can be said that the difference in dermal thickness is important in showing the difference in overall skin thickness, regardless of sex.
As a person ages, basal keratinocyte proliferation is reduced, resulting in the overall thinning of the epidermis, and the dermal‐epidermal junction appears flattened with shorter epidermal rete pegs and dermal rete ridges. The skin is also directly affected by exposure to the environment, especially ultraviolet (UV) irradiation from the sun. Chronic exposure to UV irradiation causes skin photoaging, which is superimposed with chronologic aging. 18 Sun‐induced cutaneous changes vary considerably among individuals, reflecting the inherent differences in vulnerability and repair capacity following a solar insult. Fair‐skinned individuals of Northern European descent with Fitzpatrick phototypes I to III are more prone to photoaging than individuals with colored skin (Fitzpatrick phototypes IV–VI) with melanin protecting against sun‐induced damage. 19
As in the case of intrinsic aging, published articles report that skin thinning is a consequence of the changes in skin thickness following photoaging. 20 However, there are recent reports that photoaging in Caucasians does not appear to be simple, but appears to be the accumulation of different forms of atrophic and hypertrophic photoaging and changes in skin thickness with various spectra. 21 , 22 It is also well known that photoaging affects skin differently depending on the Fitzpatrick skin type. Furthermore, Asians such as Koreans, Japanese, and Chinese culturally prefer avoiding direct sun exposure by carrying umbrellas, sitting in the shade, and applying sunblocks, unlike most Caucasians. 23 , 24 , 25 For these reasons, it is possible that there are more variations in aging‐related skin thickness in Asians than in Caucasians. 26 Considering the difference in skin thickness at each part of the face in the aspect of aging, the lateral sides of facial skin (LF, T, M, NF, and MC) tend to be thicker compared to the central area of the facial skin. The difference in skin thickness between the lateral and central areas of the face was more pronounced in women. Previously, there have been reports of epidermal thickening in early photoaging. 13 , 22 The phenomenon of early photoaging was demonstrated in this study, which may be because the participants in our study were mostly under the age of 50. Although it is expected that various factors may have affected the apparent presence of such a trend in the physical aspects of facial skin, there is a possibility that sunblock application on the lateral sides of the facial area may have been neglected by most people, given that photoaging is usually more severe in the central facial area. However, further research is needed to clarify this.
Lastly, the differences in BMI are known to be largely related to the thickness of the subcutis, and a relationship between BMI and skin thickness has not been found. In a previous report, Darraik et al. 8 reported an increase in dermal thickness according to BMI, but our study found that BMI was not related to skin, epidermal, or dermal thickness in the facial skin.
This study identified the association of several factors with skin thickness and recruited a large number of participants. This is the first measurement of skin thickness using ultrasonography in Asians. The limitation of this study is that more women were recruited than men.
These research data can be used as a reference for facial treatment when estimating skin thickness. Furthermore, our findings suggest the possibility of thickening of the facial epidermis during early photoaging, as previously mentioned, especially in the lateral regions of the face. Therefore, we should predict that the facial dermal depth is not thin but rather thicker during the dermatologic procedure, considering the epidermal thickness in middle‐aged people, especially women. We need to adjust the depth of the injection during dermatologic treatment, considering the skin is thinner in women than in men, and that BMI and skin thickness are irrelevant. These findings can help injection in the facial skin and facial subcutaneous tissue at the correct depth, minimizing adverse events such as unnecessary use of treatment materials, erythema, swelling, and pruritus. These findings can also be used to improve surgical and cosmetic outcomes from clinical treatments that require skin thickness, such as surgical incisions that leave scars or skin flaps and grafts.
5. CONCLUSION
The skin thickness at different points on the face is variable, and realistic data about skin thickness can be obtained by in vivo ultrasonographic analysis of the skin. Sex and age affect the skin, epidermal, and dermal thickness. Estimating the correct skin thickness based on data from in vivo experiments will help maximize cosmetic and medical outcomes and minimize adverse events when performing facial skin procedures.
CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.
ETHICS STATEMENT
The study protocol was approved by the institutional review board of Korea University Guro Hospital (2017GR0224) and conformed to the principles outlined in the Declaration of Helsinki. Written informed consent was obtained from all subjects before they were enrolled in the study.
ACKNOWLEDGMENTS
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF‐2019R1G1A1006544) and by the Amorepacific Grant funded by Amorepacific Corporation.
Jeong KM, Seo JY, Kim A, et al. Ultrasonographic analysis of facial skin thickness in relation to age, site, sex, and body mass index. Skin Res Technol. 2023;29:e13426. 10.1111/srt.13426
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.
REFERENCES
- 1. Ha RY, Nojima K, Adams WP Jr., et al. Analysis of facial skin thickness: Defining the relative thickness index. Plast Reconstr Surg. 2005;115:1769‐1773. [DOI] [PubMed] [Google Scholar]
- 2. Chopra K, Calva D, Sosin M, et al. A comprehensive examination of topographic thickness of skin in the human face. Aesthet Surg J. 2015;35:1007‐1013. [DOI] [PubMed] [Google Scholar]
- 3. Van Mulder TJ, de koeiker M, Theeten H, et al. High frequency ultrasound to assess skin thickness in healthy adults. Vaccine. 2017;35:1810‐1815. [DOI] [PubMed] [Google Scholar]
- 4. World Health Organization . The Asia‐Pacific Perspective: Redefining Obesity and its Treatment. WHO; 2000. [Google Scholar]
- 5. Firooz A, Rajabi‐Estarabadi A, Zartab H, et al. The influence of gender and age on the thickness and echo‐density of skin. Skin Res Technol. 2017;23:13‐20. [DOI] [PubMed] [Google Scholar]
- 6. Pellacani G, Seidenari S. Variations in facial skin thickness and echogenicity with site and age. Acta Derm Venereol. 1999;79:366‐369. [DOI] [PubMed] [Google Scholar]
- 7. Takema Y, Yorimoto Y, Kawai M, et al. Age‐related changes in the elastic properties and thickness of human facial skin. Br J Dermatol. 1994;131:641‐648. [DOI] [PubMed] [Google Scholar]
- 8. Iyengar S, Makin IR, Sadhwani D, et al. Utility of a high‐resolution superficial diagnostic ultrasound system for assessing skin thickness: A cross‐sectional study. Dermatol Surg. 2018;44:855‐864. [DOI] [PubMed] [Google Scholar]
- 9. Derraik JG, Rademaker M, Cutfield WS, et al. Effects of age, gender, BMI, and anatomical site on skin thickness in children and adults with diabetes. PLoS ONE. 2014;9:e86637. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10. Denda M, Takahashi M. Measurement of facial skin thickness by ultrasound method. J Soc Cosmet Chem Japan. 1990;23:316‐319. [Google Scholar]
- 11. Gambichler T, Matip R, Moussa G, et al. In vivo data of epidermal thickness evaluated by optical coherence tomography: effects of age, gender, skin type, and anatomic site. J Dermatol Sci. 2006;44:145‐152. [DOI] [PubMed] [Google Scholar]
- 12. Kim MS, Jeong YY, Park SG, et al. Age‐dependent facial subcutaneous fat thickness by high frequency medical diagnostic ultrasound system. Skin Res Technol. 2020;26(5):769‐771. [DOI] [PubMed] [Google Scholar]
- 13. Choi YJ, Lee KW, Gil YC, et al. Ultrasonographic analyses of the forehead region for injectable treatments. Ultrasound Med Biol. 2019;45:2641‐2648. [DOI] [PubMed] [Google Scholar]
- 14. Rahrovan S, Fanian F, Mehryan P, et al. Male versus female skin: What dermatologists and cosmeticians should know. Int J Womens Dermatol. 2018;4:122‐130. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15. Bailey SH, Oni G, Brown SA, et al. The use of non‐invasive instruments in characterizing human facial and abdominal skin. Laser Surg Med. 2012;44:131‐142. [DOI] [PubMed] [Google Scholar]
- 16. Mogensen M, Morsy HA, Thrane L, et al. Morphology and epidermal thickness of normal skin imaged by optical coherence tomography. Dermatology. 2008;217:14‐20. [DOI] [PubMed] [Google Scholar]
- 17. Gambichler T, Boms S, Kreuter A, et al. Epidermal thickness assessed by optical coherence tomography and routine histology: preliminary results of method comparison. J Eur Acad Dermatol Venereol. 2006;20:791‐795. [DOI] [PubMed] [Google Scholar]
- 18. Lavker RM, Veres DA, Irwin CJ, et al. Quantitative assessment of cumulative damage from repetitive exposures to suberythemogenic doses of UVA in human skin. Photochem Photobiol. 1995;62:348‐352. [DOI] [PubMed] [Google Scholar]
- 19. Ayer J, Griffiths CEM. Photoaging in Caucasians. In: Watson REB, Griffiths CEM, eds. Cutaneous Photoaging. CPI Group Ltd; 2019:1‐30. [Google Scholar]
- 20. Rittié L, Fisher GJ. Natural and sun‐induces aging of human skin. Cold Spring Harb Perspect Med. 2015;5:a015370. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21. Sachs DL, Varani J, Chubb H, et al. Atrophic and hypertrophic photoaging: Clinical, histologic, and molecular features of 2 distinct phenotypes of photoaged skin. J Am Acad Dermatol. 2019;81:480‐488. [DOI] [PubMed] [Google Scholar]
- 22. Chung JH. Photoaging in Asians. Photodermatol Photoimmunol Photomed. 2003;19:109‐121. [DOI] [PubMed] [Google Scholar]
- 23. Battie C, Jitsukawa S, Bernerd F, et al. New insights in photoaging, UVA induced damage and skin types. Exp Dermatol. 2014;23(Suppl 1):7‐12. [DOI] [PubMed] [Google Scholar]
- 24. Chan IL, Cohen S, da Cunha MG, et al. Characteristics and management of Asian skin. Int J Dermatol. 2019;58:131‐143. [DOI] [PubMed] [Google Scholar]
- 25. Vashi NA, De Castro Maymone MB, Kundu RV. Aging differences in ethnic skin. J Clin Aesthet Dermatol. 2016;9:31‐38. [PMC free article] [PubMed] [Google Scholar]
- 26. El‐domyati M, Attia S, Saleh F, et al. Intrinsic aging vs. photoaging: a comparative histopathological, immunohistochemical, and ultrastructural study of skin. Exp Dermatol. 2002;11:398‐405. [DOI] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.