Hair diameter measurement methods: micrometer caliper versus phototrichogram

Article information

Arch Aesthetic Plast Surg. 2023;29(2):97-101

Publication date (electronic) : 2023 April 27

doi : https://doi.org/10.14730/aaps.2022.00423

Jae Hyun Park^,1, Yeh Hong Ho², Kotchamol Manonukul³

¹Dana Plastic Surgery Clinic, Seoul, Korea

²Dana Taipei Hair Transplantation Clinic, Taipei City, Taiwan (ROC)

³Vdesign Hair by Dana International, Vibhavadi Hospital, Bangkok, Thailand

Correspondence: Jae Hyun Park Dana Plastic Surgery Clinic, 606 Gangnam-daero, Gangnam-gu, Seoul 06038, Korea E-mail: jay8384@naver.com

Received 2022 February 11; Revised 2022 June 24; Accepted 2022 October 20.

Abstract

INTRODUCTION

Hair diameter is a crucial element in deciding the treatment and predicting the prognosis of hair transplantation in patients with hair loss [1-3]. Ishino et al. [4] reported that a decrease in hair thickness, rather than a decrease in hair density, was the main finding in men with androgenetic alopecia.

Hair diameter is the most crucial prognostic factor in determining the surgical results of hair transplantation. Higher volume and coverage can be achieved with transplantation of thick donor hairs. Meanwhile, thin-caliber hairs are needed to mimic the natural texture of eyebrows, eyelashes, the female hairline, sideburns, and the temporal region [5-7]. Thus, the measurement of hair caliber has clinical importance in planning hair transplant surgery, evaluating the progression of hair loss, and determining the effectiveness of hair loss treatment.

Hair caliber is typically measured using a micrometer caliper or a phototrichogram [8,9]. In addition, Erdogan et al. [10] introduced the KEBOT system, an artificial intelligence-based comprehensive analysis system for hair transplant surgery with follicular unit excision and noted that the measurements of hair diameter converged along the length of the long axis of the hair shaft.

Bayramoglu et al. [9] compared the diameter of hair measured using a scanning electron microscope and a micrometer caliper and demonstrated that the caliper measured the length of the short axis of the hair.

The two primary methods of measuring hair caliber include the micrometer caliper, which physically and mechanically measures hair thickness, and the phototrichogram, which measures hair thickness by an imaging process. This study compared the two methods of measuring hair on various parts of the scalp.

METHODS

Ten female volunteers participated in the study. All procedures performed were in accordance with the Declaration of Helsinki (as revised in 2013). Institutional review board approval was not required as this report was not a prospective or systematic investigation of treatment. All participants were Korean and had black hair. The participants’ age ranged from 25 to 41 years with a mean of 33 years. Candidates with a history of previous hair transplantation surgery, hair disease, or face-lift surgery were excluded.

Three sites at different horizontal positions (hereafter referred to as horizontal sites) were chosen for measurement: midoccipital, mastoid, and temporal (Fig. 1). At each site, three 1 cm² boxes were marked from superior to inferior along the midline of the site, and hair samples were randomly collected from these boxes (hereafter referred to as vertical boxes). The midoccipital point at the upper border of the helical rim was set as a reference point. The point directly above the reference point was designated as the middle point (M), the point below as the inferior point (I), and the point above the middle point as the superior point (S).

Fig. 1.

Sites for hair sampling. (A) Occipital area. (B) Temporal area.

Five anagen hairs ≥10 cm long were randomly sampled from each box. The hairs were screened by a highly experienced hair surgeon to ensure they were in the anagen phase. The thickness of the collected hairs was measured at three positions: 1 cm, 5 cm, and 10 cm from the surface of the skin. There were 1,350 measurements overall: 45 hairs per person were collected (a total of 450 hairs) and measurements were made at three positions for each hair.

The diameters of the hairs were measured using an electronic micrometer caliper (Baoshishan) and a Folliscope phototrichogram (LeadM), and the resulting data were compared and statistically analyzed (Figs. 2, 3).

Fig. 2.

Use of the phototrichogram. (A) Photographing the scalp with phototrichogram. (B) Software for analyzing phototrichogram images.

Fig. 3.

Micrometer caliper.

RESULTS

The thickness of all hairs was 76.90±12.29 μm when measured with the caliper and 108.78±19.97 μm when measured with the Folliscope. There was a statistically significant difference between the two measurement methods (P<0.001).

DISCUSSION

The measurement of hair thickness is important in determining the progress of hair loss and the effect of treatment. Such measurement is also important in hair transplant surgery. Hair caliber is an important element in surgical planning, predicting the prognosis, and assessing results.

A review of existing studies revealed two primary methods for measuring the thickness of hair: a mechanical and physical method using a micrometer caliper and a digital imaging method using a phototrichogram.

Various theories have been proposed for the cause of curly hair. One representative theory is that the structural differences of cell types and their lateral segregation are the main factors driving curl formation. In general, straight hair has a rounded cross-section, whereas curly hair has a flatter, elliptical cross-section [11,12].

Thicker hair tends to be frizzier and has a greater difference between the lengths of the long axis and short axis. Bryant and Porter observed a highly significant relationship between curliness and ellipticity in African hair [13].

Erdogan et al. [10] introduced the KEBOT system, a new robotic and vision-based imaging system and artificial intelligence-based analysis approach and stated that the thickness measured by digital image processing converges along the long-axis dimension of the hair. Bayramoglu et al. [9] reported that a measurement by micrometer caliper represents the short-axis dimension of the hair. The KEBOT system has many advantages in terms of measuring density and coverage value (CV), and the entire scalp can be easily scanned by region. However, because the head must be shaved before taking measurements, the system can be used only prior to surgery and cannot be used in follow-up visits when hair has grown out or in surgical cases that do not require shaving of the donor area, such as in long-hair or non-shaven follicular unit excision. The system is also difficult to purchase.

The micrometer caliper has disadvantages in that hair strands must be cut one by one, and its measurements are largely representative of the short-axis dimension.

Measurements made using a phototrichogram are easy to calculate and do not require shaving. They are close to the long-axis dimension and more comparable to the thickness of hair as visualized by the human eye.

The present study, like that of Bayramoglu et al. [9], found that the measurements obtained with the micrometer caliper compared to those obtained with the Folliscope were significantly different, presumably due to the elliptical shape of the hair shaft cross-section.

Harris introduced the concept of the hair diameter index (HDI) [14], expressed as

HDI=HSD×(hairs/cm²)/100,

where HSD is the hair shaft diameter, measured in μm.

The HDI ranges between 1 and 200 when the HSD is measured in μm. Erdogan introduced the CV, a concept similar to the HDI [9]. The CV is calculated as follicular units per square centimeter multiplied by the number of hairs per follicle and average hair diameter. Both the HDI and CV provide a mathematical criterion for adequate coverage in hair transplantation. The CV and HDI, as measures of scalp coverage, represent coverage of the scalp as seen by the human eye. Therefore, the coverage of the scalp is more strongly related to the long-axis dimension than the short-axis dimension of the elliptical hair shaft.

In this study, both the caliper and Folliscope showed a statistically significant difference in each measurement site and in the overall sum (P<0.001) (Table 1). When directly comparing the average values, there was a difference of approximately 30%. Although there have been reports that investigated caliper and Folliscope measurements separately or showed their association with the short axis and the long axis, this study is the first to make a direct comparison of the two methods in the same patients and confirm a major difference.

Table 1.

Difference in measurements between the caliper and Folliscope

Since the degree of scalp coverage is associated with the visual effect of hair transplantation, the long axis value is clinically more significant than the short axis.

Mechanically measuring the thickness of each hair strand with a micro-caliper can be difficult and time-consuming, even more so when working with many hairs. Also, the short axis of the hair that it measures, does not accurately reflect the perceived scalp coverage.

The Folliscope phototrichogram takes a high-resolution photo of the hair shaft to visually measure its caliber. As this reflects the long axis of the hair, its measurements have a stronger correlation with scalp coverage. Moreover, the Folliscope allows for easier measurements and can be stored for follow-up visits.

Based on its advantages, the measurement of hair thickness using a phototrichogram, which is a measure of the long-axis dimension, is useful in the treatment of patients with hair loss and patients undergoing hair transplants.

In conclusion, the phototrichogram is clinically useful in the treatment of patients with hair loss and hair transplants.

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