Method and materials

Ethical approval

This study was based on a computed tomography scan with ethical approval from the University of Lincoln, United Kingdom. The authors fulfilled the standards for medical research as stated in the October 2013 principles of Helsinki by the World Medical Association and of the Nigerian Data Protection Regulations (NDPR) for public institutions.

Computed tomography imaging

In this study, computed tomography scans were performed on live subjects over 7 months, from August 2023 to February 2024, at the Uthman Dan Fodio University in Sokoto, north-western Nigeria. The computed tomography machine is Revolution Apex by General Electric (GE), a 16-slice helical scanner manufactured by Hangwei Medical Systems Co., Ltd.

All subjects were scanned in a supine position with the head centred by equal support to both the left and right sides of the face. Those that were not well-centred were rejected as the measurement could not be optimised. Each subject had axial, sagittal and coronal views analysed. The images were acquired at 2.5 mm thicknesses, and the data were manipulated to create a 1.25 mm high spatial resolution slice.

There was no additional radiation risk to any subject because the images were required for purposes unrelated to this study. Images were selected based on the exclusion criteria stated under data collection below. A shared online link was created at the University of Lincoln, and suitable images were remotely downloaded for analysis by a maxillofacial radiologist of the Uthman Dan Fodio University, Sokoto, Nigeria.

Ethnicities studied and data collection

The ethnic groups studied included the Hausa, Fulani, Yoruba, and Dakarkari, with the Hausa and Fulani being the predominant ethnic types in this part of the country. The biodata gathered from each subject included date of birth, sex at birth, ethnicity, body mass index, and date of imaging. A standardised Form F was used for each subject dataset collection. All data were anonymised, but with unique identifiable numbers. Exclusion criteria were the same as in previous studies of facial soft tissue thickness [2, 39, 43] and were equally stated on the Form F. Excluded subjects included those with a history of facial swelling, head and neck trauma, orthodontic treatment, bony abnormalities, orthognathic surgery, facial identification marks and cleft lip and palate. And also, those with peculiar tribal marks on the face, such will distort the soft tissues [43, 44].

Body mass index classification

The body mass index was determined using the World Health Organisation (WHO) formula, weight (in kg)/height^2 and classification for 18 years and above, as stated thus: less than 18.5 (underweight), 18.5–24.9 (normal weight), 25–29.9 (pre-obese), and equal to 30 or greater (obese). Since the onset of facial soft tissue thickness studies, BMI/weight has always been reported as an index [45]. In this Nigerian population, normal weight is quite prevalent rather than underweight, overweight, or obesity [46].

Facial soft tissue thickness points defined

In this study, 12 midlines and 19 bilateral facial soft tissue points were defined, making a total of 50, as a standard for each subject. These points are a reproduction of points stated in the study on the combined Nigerian male population (CNME) [43] and listed in Tables 2 and 3.

FSTT points/(Acronyms) | Definitions | Synonyms
Vertex (v) | The highest point on the head | 
Supraglabella (sg) | The most anterior midline points on the forehead | Ophryon (of)
Glabella (g) | The most prominent point between the supra-orbital ridges in the midsagittal plane | 
Nasion (n) | Midpoint of the suture between the frontal and the two nasal bones | 
Mid-Nasal (ns) | The midpoint of the nasal bone, which is between the nasion and rhinion | 
Rhinion (rh) | The end of the nasal bones at the cartilage-bone junction | End of the nasal
Midphiltrum (mp) | Midpoint of the philtral column- located by the length from the midline and midway point between prosthion and the subspinale. Measured from the deepest point (point A) of the maxillary alveolar bone | 
Upper lip border (ls) | The most anterior midpoint of the upper vermilion line is measured from the prosthion.Measured from between the central incisors at the level of the cementum–enamel junction to the vermillion borderThe prosthion is the midpoint between the upper central incisors at the level of the cementum–enamel junction. | Supradentale or alveolare or Labiale superius
Lower lip border (li) | The midpoint of the lower vermilion line.Measured from between the lower central incisors at the level of the cementum–enamel junction to the most anterior of the vermillion of the lower lip | Infradentale orLabiale inferius
Labiomental (lm) | The midpoint of the labiomental groove. The deepest midline point furrow on the mandible between the teeth and the chin prominence | Chin-lip fold sublabiale or supramentale
Mental eminence (me) | The most anterior midpoint of the chin | Pogonion
Menton (mn) | The lowest medial landmark beneath the chin | Gnathion or Beneath the chin

FSTT points/(Acronyms) | Definitions | Synonyms
Frontal eminence (fe) | Centred on pupils’ most anterior projections at both sides of the forehead.A point on the projections at both sides of the forehead (centred on the pupils/lies above the midpoint of the eyebrow). It is located as the most protruding point of the frontal bone along the axis of a line (drawn from ecthocanthion to endocanthion) that vertically bisects the orbit. | 
Supraorbital (os) | Above the orbit, it’s the highest point of the upper margin of the orbit (centred on the pupil). Located from a point midway between the ecthocanthion and endocanthion, and by drawing a perpendicular line up to the supraorbital margin | 
Infraorbital (io) | The lowest point on the lower margin of the orbit. Located from a point midway between the ecthocanthion and endocanthion, and by drawing a perpendicular line up to the Infraorbital margin. | Orbitale (or) suborbital
Supraglenoid (sg) | This is the root of the zygomatic arch. It is anterior and superior to the external auditory meatus. | 
Zygomatic arch (zy) | The most lateral point of the zygomatic arch. | Zygion
Lateral Orbit (lo) | This is an anterolateral point on the zygomatic bone. It is the lowest point on the suture between the maxillary and zygomatic bones. It is lined up vertically to the lateral orbital margin on the zygomatic bone. | 
Lateral Glabella (lg) | A point on the medial orbit at the junction of the frontal, maxillary, and lacrimal bones | 
Lateral nasal(ln) | A point on the side of the nose in line with the endocanthion. It is a point on the Frankfurt horizontal plane at the side of the bridge of the nose. | 
Inferior malar (Im) | The most protruding point of the maxilla lies beneath the zygomatic process at a point on a vertical line with the infraorbital and supraorbital points. It is centred on the vertical line from the pupil and just inferior to the zygomatic process. | Lateral maxilla
Lateral nostril (lno) | This is a point just next to the most lateral point of the alar border. | 
Supra-canine (sc) | This is a point on the superior alveolar ridge of the upper canine. It is on the horizontal level of the mid-philtrum and lines up vertically with the cheilion. It is measured from the maximum bulge of the upper canine prominence. | 
Infra-canine (ic) | This is a point on the inferior alveolar ridge of the lower canine. It is vertically lined up with the cheilion, on the horizontal level of the chin–lip fold. | Subcanine
Mid-lateral orbit (mlo) | This is a point just next to the lateral orbital border | 
Supra-M2 (sm2) | This is a point on the superior alveolar ridge of the second upper molar. | 
Infra-M2 (im2) | This is a point on the inferior alveolar ridge of the second lower molar | 
Mid-masseter (mm) | A point located halfway between the supraglenoid and the gonion, located about the middle of the masseter, (located from a coronal view) | 
Gonion (go) | The most lateral point at the angle of the mandible (located on the angle of the mandible), | 
Occlusal line (ol) | A point on the anterior part of the ascending ramus in alignment with the line where the teeth occlude anterior laterally | 
Mid-mandible (mdm) | A point on the inferior border of the mandible, vertically lined with the supra-M2 (Bulut et al., 2018) | 

These points are also replicas from previous studies, including Bulut et al. [2], Wilkinson [1], Sahni et al. [5], Auslebrook et al. [26], De Greef et al. [33], and Cavanagh and Steyn [39]. These points are illustrated in Figs. 1 and 2; the tip of the arrowhead defines each FSTT point.

Fig. 1: Midline and bilateral facial soft tissue points, defined by the tip of the blue arrowhead

Fig. 2: Facial soft tissue thickness points in a lateral bony window, defined by the tip of the blue arrowhead

Computed tomography scans processing and FSTT estimations

The software package, RadiAnt, was used to estimate all facial soft tissue thickness (FSTT) values. This software is a Medixant Company, PACS DICOM (Digital Imaging and Communications in Medicine) viewer, version 2023.1 (64-bit) [47]. This software demonstrates both the soft and hard tissue windows and is described in detail in the combined Nigerian adult male study [43].

The RadiAnt software supports all types of DICOM images, including monochromatic (e.g., chest x-ray (CXR), computed tomography, colour (for example 3D reconstructions), static images (e.g. CXR), magnetic resonance imaging, ultrasound and Positron Emission Tomography (PET). It rapidly reconstructs images in multiple orthogonal planes (sagittal, axial, coronal, oblique). There is a 3D VR (volume rendering) tool, which allows for soft or hard tissue anomalies to be detected. This software angle tool allows for facial soft tissue thickness to be determined at a 90° angle from bone to skin surface.

The protocols involved downloading each scan from the shared online drive into RadiAnt. The desired facial soft tissue thickness point was then located on the relevant orthoslice. The measurement at each point was obtained at 90° from the bone to the skin surface with the skull in a Frankfurt position, this is similar to previous studies [1, 2, 39, 40, 48]. The midline facial soft tissue thickness points were located and estimated on the mid-sagittal view using the nasal septum and dentition as guides. Bilateral FSTT points were located on the axial view orthoslice except for the mid-masseter point, which was estimated on the coronal view.

After identifying the desired facial soft tissue thickness point, the angle tool had to be manoeuvred to 90° from bone to skin, for example, at the mp (mid-philtrum), Fig. 3A. The manipulation was time-consuming, as it was impossible to pre-program the angle tool to take each measurement at 90°. Secondarily, after placing the angle tool (AT) at 90°, a measuring tool (LL) was then placed directly over the angle tool to estimate the representative distance from bone to skin surface, Fig. 3B. The angle tool could then be deleted, leaving only the linear line. A total of 3101 manual measurements were taken at these FSTT points.

Fig. 3: Use of RadiAnt in FSTT measurements. (a) Angle tool (AT, red colour) placed at 90°. (b) Linear line (LL, in red colour) placed for measurement