M. VERDENIK et al.: A NEW METHOD OF DETERMINING FACIAL SIZE FOR THREE-DIMENSIONAL ... 25–31 A NEW METHOD OF DETERMINING FACIAL SIZE FOR THREE-DIMENSIONAL PHOTOGRAMMETRY QUANTIFICATION NOVA METODA DOLO^ANJA VELIKOSTI OBRAZA V TRIDIMENZIONALNI FOTOGRAMETRIJI Miha Verdenik 1* , Nata{a Ihan Hren 1 , @iga Kadivnik 2 , Igor Drstven{ek 3 1 Department of Oral and Maxillofacial Surgery, University Medical Center Ljubljana, Zalo{ka 2, 1000 Ljubljana, Slovenia 2 Ortotip LLC, Obrtna ulica 40, 9000 Murska Sobota, Slovenia 3 Faculty of Mechanical Engineering, University of Maribor, Smetanova ulica 17, 2000 Maribor, Slovenia Prejem rokopisa – received: 2020-06-09; sprejem za objavo – accepted for publication: 2020-07-24 doi:10.17222/mit.2020.113 Contemporary craniofacial surgery includes the pre- and post-operative optical 3D scanning of faces as a method for diagnosing and verifying the achieved results. The influence of head size in 3D scans must be excluded in order to accurately and uniformly compare different three-dimensional facial shapes in craniofacial surgery. Regarding this purpose, different head-size parame- ters must be measured to obtain the scaling factor. A special device, a so-called head ring, has been produced as a structure that can be fixed to a person’s head. Among defined points, different linear distances (head width, length and height) and volumetric parameters (lower and upper head volumes) were calculated and compared to body-size measurements. Measurements were per- formed on 3D scans of the heads of 26 healthy adults with normocclusion (12 men and 14 women) taken using the head ring set. Body mass index (BMI) statistically significantly correlates with the lower and whole-head volume in men, while in women more precisely with the upper-head volume. BMI in men does not correlate with any linear distance, while in women it is closely connected to the facial width. In men the head width and lower head volume are the main contributors to head size, while in women the crown-to-chin length and upper volume determine the size of the head. A conclusion can be made that the correlation between the head volume, the BMI and the linear head parameters exists and is gender dependent. Keywords: 3D, facial, size, photogrammetry Na podro~ju karniofacialne kirurgije so vedno bolj uveljavljene 3D-metode, med njimi tudi neinvazivni povr{inski posnetki glave in obraza. Za natan~no primerjavo 3D-posnetkov razli~nih obrazov med seboj, je potrebno izkllju~iti vpliv velikosti glave in obraza. Za namen bolj objektivne registracije, je bil izdelan poseben pripomo~ek, ki smo ga poimenovali naglavni obro~. Z njegovo uporabo smo pridobili dodatne podatke, kot so: razdalja med u{esi, razdalja med najvi{jo to~ko glave in brado, razdalja med navideznim sredi{~em in bazo nosu ter s pomo~jo markerjev izmerjeni volumen spodnje in zgornje piramide ter volumen elipsoida, ki ga markerji opi{ejo. Dodatno je mo`no, kot parameter dolo~anja velikosti obraza, uporabiti tudi indeks glave (kot analog obraznemu indeksu) izra~unan kot razmerje med obrazno dol`ino in {irino. Meritve so bile izvedene na 26 zdravih preiskovancih brez skeletnih in zobnih nepravilnosti (12 mo{kih in 14 `enskah). ITM (indeks telesne mase) je zna~ilno koreliral z volumnom spodnje obrazne piramide in celotnim volumnom pri mo{kih preiskovancih, pri `enskah pa z volumnom zgornje piramide. ITM pri mo{kih ni imel zna~ilne povezave z opazovanimi linearnimi razdaljami, pri `enskah pa smo na{li povezavo z razdaljo med u{esi. Pri mo{kih smo ugotovili, da k velikosti glave najbolj prispevata razdalja med u{esi in volumen spodnje piramide, pri `enskah pa razdalja med najvi{jo to~ko glave in brado ter volumen zgornje piramide. Ugotovljene so bile zna~ilne povezave med ITM, spolom in nekaterimi obraznimi parametri, ki smo jih dolo~ili s pomo~jo naglavnega obro~a. Klju~ne besede: 3D, obraz, velikost, fotogrametrija 1 INTRODUCTION AND BACKGROUND The tree-dimensional (3D) analysis of facial soft tis- sues is an important diagnostic and research tool in craniofacial surgery, 1 but it could also be useful for de- tecting changes during growth and treatment. There are two major problems in a comparison of two faces. First, we have to distinguish between size and shape. 2 If we want to compare just the facial shapes, we must scale the faces to the same size, which is hard to define. The cen- troid size from general procrusted analysis is mostly used, 2–4 with some disadvantages, as it used for a limited number of points for size determination, whilst 3D sur- face scanning provides us with an enormous data set. The second problem is how to orient two facial scans within a working space to achieve their optimal registra- tion. The cranial base is often used for super-imposition- ing when comparing lateral cephalograms 5 as well as dif- ferent CT scans 6 because it shows minimal changes after childhood and it is a good reference point for comparing two different faces. But facial scans represent only soft-tissue contours, so we are unable to compare them based on skeletal structures as a cranial base. There are other methods available, i.e., general procrusted analysis, iterative closest point (best-fit) 5 regional best-fit 6,7 mid-endocanthion registration, 3 but none of them is per- fect. We have used iterative closest-point registration in our 3D description of class-III faces. 8 We have also de- termined the average Slovenian facial shell 9 because of its ethically bonded facial characteristics. 10 The pre- sented research was triggered by the discrepancies when Materiali in tehnologije / Materials and technology 55 (2021) 1, 25–31 25 UDK 616-089.8:617.52:53.08 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 55(1)25(2021) *Corresponding author's e-mail: miha.verdenik@gmail.com (Miha Verdenik) calculating the differences between face shapes before and after orthognathic surgery. The results gathered in the described way were unsatisfactory because they did not reflect the actual displacements of the facial struc- tures for several reasons, among which changes in the body mass index and differences in body sizes between the averaged and the actual face seemed to be the main reasons. In order to avoid these generalisations induced by the best-fit method, two obvious possibilities are available. One is to scan the whole head’s volume, which is a tech- nically impossible process because the hairy surfaces cannot be scanned without special preparation. The sec- ond is to find a way of uniformly defining the orientation of a facial scan by introducing special markers that do not change for subsequent scans and that do not depend on personal particularities. This study was aimed at finding a correlation for dif- ferent facial, head, and body-size parameters. An innova- tive head ring was used to obtain additional data that are usually excluded during 3D surface facial scanning. The aim of our study was to find the best parameter that will be used as a descriptor of the head volume. This parame- ter can then be used as a coefficient to scale the faces of different sizes for shape analysis. 2 METHODS A special device called a head-ring has been pro- duced that consists of a structure that can be fixed to a person’s head with spherical markers attached to the holding structure in such a way that enables their move- ments to touch the characteristic points on the head, while still preserving a position relative to the origin of the whole structure, e.g., the person’s head volume (Fig- ure 1). Five spherical markers point to five characteristic points of a human head: both acoustic ducts, the crown of the head (the highest point of the head during a natu- ral head position), the nasal base, and the chin. Since these points are hard to detect while scanning, the mark- ers have been designed in such a way that enables their exact scanning in the head-space, while simultaneously marking these five points exactly on the head’s surface. This has been achieved by designing a marker that con- sists of a scanning sphere, a "skin-piece" that touches the head’s surface and a pole that connects the spherical marker to the skin piece. In this way the marker not only represents a point on the face’s surface, but also a vector that points to the origin of the head’s volume. To prove the measurement principles and usability of the head-ring, its prototype was produced from polyamide (PA12) using the selective laser sintering pro- cess. The first trials pointed out some awkward solutions in the design (fixation of markers, positioning the ring onto the crown of the head, etc.), but they did not influ- ence the facial scanning. A pilot study was performed that included 27 young adults (14 female and 13 male, average age 25±2 and 26±3 years). They were all healthy M. VERDENIK et al.: A NEW METHOD OF DETERMINING FACIAL SIZE FOR THREE-DIMENSIONAL ... 26 Materiali in tehnologije / Materials and technology 55 (2021) 1, 25–31 Figure 1: Innovated head-ring with a schematic view of the coordi- nate system Figure 2: Photos and 3D facial scan with the innovated head ring set on with normocclusion, without any dentofacial deformities. We obtained a signed, informed consent. Each participant’s weight and height were measured and 3D facial scans were (Figure 2) obtained using an Artec 3D MH scanner that uses the flying-triangulation method. 11 The scans were taken in a relaxed environ- ment, with the subjects sitting on an ergonomic chair in a straightened posture, with the gaze fixed on a determined point in the distance, so as to exclude the subject’s self-awareness of the facial muscles and reconstruct a re- alistic physiological rest position. The volunteers were asked to refrain from any movements – if possible, as well from blinking – for the period of the scanning pro- cedure (about 15–20 s). During the scanning they wore the head-ring on the head and 3D facial scans were later processed in the Artec Studio software. Furthermore, the distances were measured between the ears, between the crown of the head and the chin, and between the head volume’s origin and nasal base. The head-width as the distance between the ears, length as distance between the crown of the head and the chin and the facial depth as distance between the centre and the nose. The volumes of the upper and lower pyramids, as well as the whole head’s and ellipsoid volumes (Figure 3), were calculated using these data. Additionally, the head index, was calcu- lated from the distance between the crown of the head to the chin and, both ears distance relationships. Each of the 11 obtained parameters (Table 1)d e - scribes the facial size in its own way. In the first part the similarity and correlation had to be checked among them. The observed subjects were arranged from the smallest to tallest according to their body heights fol- lowed by graphical visualisations of how other parame- ters follow (Figure 4). Further on the Pearson correlation M. VERDENIK et al.: A NEW METHOD OF DETERMINING FACIAL SIZE FOR THREE-DIMENSIONAL ... Materiali in tehnologije / Materials and technology 55 (2021) 1, 25–31 27 Figure 3: Measured parameters (lower, upper head volume, width, length, facial depth, ellipsoid) shown on the 3D facial scan Table 1: Obtained parameter descriptions Parameter Definition – equation Unit Description Height Body height cm Weight Body weight kg BMI Weight/Height 2 kg/m 2 Distance between ears (b) Distance from left to right point, where head ring touches the external acoustic ducts. mm Head width Distance from crown of the head to the chin (a) Distance from the uppermost point on the head in the natural head position, to the chin. mm Head length Distance from centre to the nasion (c) Distance from the centre point, origin of the head volume cre- ated where vectors from other spherical markers meet, to the tip of the nose. mm Face depth Head-index (a/b) Is an analogue of facial one (the ratio of the facial height to the zygomatic width) calculated from the distances between the crown of the head to the chin and both ears. Head form Upper-volume Volume of upper pyramid between the crown of the head, both ears and the nasion. cm 3 Lower-volume Volume of lower pyramid between both ears, nasion, and the chin. cm 3 Whole-volume Sum of both volumes cm 3 Head volume Ellipsoid Calculated volume of ellipsoid defined as #, where a, b, and 2c represent the three axes of the ellipsoid. cm 3 was used to statistically assess the correlation among these data (p < 0.5). It was supposed that the head vol- ume would describe the facial size best; therefore, a lin- ear regression model was made with the head-volume as the dependent value. This model was used to check the impacts of head width, length, facial depth, body weight, and height. 3 RESULTS AND DISCUSSION In the presented study, the young adults with normocclusion were similar regarding BMI. The average BMI was 22.8 (SD 3.1), which meant that abnormal in- dexes were excluded. The number of observed partici- pants was small, but enough for statistically relevant re- sults. Both genders were separately observed because of M. VERDENIK et al.: A NEW METHOD OF DETERMINING FACIAL SIZE FOR THREE-DIMENSIONAL ... 28 Materiali in tehnologije / Materials and technology 55 (2021) 1, 25–31 Table 2: Values of all obtained parameters for all the subjects ID Height Weight BMI Dist. between ears Dist. crown of the head – chin Dist. centre – nasion Head- index Upper- volume Lower- volume Whole- volume Ellipsoid MALE 1 173 93 31.1 139.5 259.1 110.8 1.86 319.1 300.8 619.9 335.6 2 175 68 22.2 127.7 254.5 102.5 1.99 313.4 217.5 530.8 279.2 3 175 68 22.2 127.0 252.9 109.9 1.99 339.1 238.3 577.4 295.7 4 177 70 22.3 139.6 252.7 105.0 1.81 304.0 274.5 578.5 310.2 5 180 100 30.9 134.6 245.0 97.8 1.82 262.5 241.6 504.0 270.2 6 180 80 24.7 139.3 259.1 102.5 1.86 352.4 238.1 590.6 309.8 7 183 83 24.8 136.0 264.0 111.7 1.94 336.0 261.0 597.0 336.0 8 186 88 25.4 139.0 262.2 114.2 1.89 349.2 301.1 650.3 348.6 9 187 84 24.0 130.2 257.6 104.8 1.98 304.7 254.9 559.5 294.4 10 189 76 21.3 131.6 259.8 99.6 1.97 255.7 234.7 490.4 285.1 11 190 79 21.9 136.9 259.0 103.4 1.89 327.7 249.8 577.5 307.1 12 190 83 23.0 138.5 250.8 110.7 1.81 303.1 272.7 575.8 322.3 13 194 90 23.9 130.2 256.5 111.6 1.97 285.5 240.8 526.3 312.4 183 82 24.4 134.6 256.4 106.5 1.91 311.7 255.8 567.5 308.2 ±SD 7 10 3.0 4.7 5.1 5.2 0.07 30.5 25.4 45.2 23.4 FEMALE 1 156 52 21.4 127.1 233.8 102.0 1.84 267.6 216.1 483.7 2342.9 2 158 57 22.8 135.2 250.3 95.9 1.85 290.4 231.1 521.5 2931.2 3 161 53 20.4 131.8 246.5 99.9 1.87 301.2 202.9 504.0 2579.3 4 162 50 19.1 128.0 228.2 99.1 1.78 271.1 199.0 470.1 2124.5 5 163 73 27.5 135.1 253.2 103.2 1.87 325.0 224.2 549.1 3350.7 6 165 60 22.0 136.6 248.5 103.6 1.82 314.3 238.8 553.1 3477.2 7 166 66 24.0 130.1 241.8 98.9 1.86 309.7 195.4 505.1 2560.9 8 170 58 20.1 132.3 236.9 93.1 1.79 220.5 218.6 439.1 1773.5 9 171 68 23.3 137.0 235.8 94.5 1.72 289.0 198.8 487.8 2348.2 10 174 62 20.5 124.9 237.9 106.3 1.91 292.9 217.1 510.0 2717.1 11 177 64 20.4 130.2 256.7 104.3 1.97 295.3 248.6 543.9 3344.6 12 178 66 20.8 124.4 243.5 101.5 1.96 281.9 208.3 490.3 2412.2 13 178 58 18.3 122.5 234.8 107.7 1.92 281.0 209.5 490.5 2418.7 14 180 75 23.1 132.3 253.7 101.4 1.92 333.3 220.5 553.8 3409.2 169 62 21.7 130.5 243.0 100.8 1.86 290.9 216.3 507.3 2699.3 ±SD 8 8 2.3 4.7 8.7 4.3 0.07 27.8 15.7 34.1 528.9 Figure 4: Graphically visualised male parameters’ distribution Figure 5: Graphically visualised female parameters’ distribution the sexual dimorphism. 12,13 However with respect to ever more reports about ethnically conditioned differences 14,10 we had to appreciate the fact that the observed subjects were Slovenians. The measured values are shown in Table 2 and visu- alised in the graph (Figures 4 and 5). Subjects are ar- ranged from lowest to tallest according to their body heights and separated according to gender. The results of the Pearson correlation and linear multivariate analyses are presented in Tables 3 and 4. The fact that the facial size could not be defined in one definite way, led us to find the best approximation. Different parameters were used to describe it: the arith- metic mean of selected landmarks (centroid), 2–4 those different linear face distances that describe facial height and width, and the relationships between them. 15,16 There were studies for describing the volume of the head, but mostly measured by CT scans. 17,18 The CT scans as all other x-ray imaging modalities also give us information about deeper structures, such as bone and so where more obtained data brings advantages over surface scanning modalities. However, x-rays with known radiation risk are of question for study of facial growth and develop- ment. The idea of measuring the head volume using anthropometrically assessed landmarks 19 led us to de- signing the head-ring. When using it additional data was acquired for determining the head size and for checking the accuracies of standard methods. We obtained head distances that can be presented as head lengths, width and face depth. The expectation that body height would be in correla- tion with the head volume was unconfirmed. It most M. VERDENIK et al.: A NEW METHOD OF DETERMINING FACIAL SIZE FOR THREE-DIMENSIONAL ... Materiali in tehnologije / Materials and technology 55 (2021) 1, 25–31 29 Table 3: Pearson correlation showing the relationships between the observed parameters MALE Weight BMI Between ears dist Crown of the head chin dist Centre nasion dist Head index Upper volume Lower volume Whole volume Ellipsoid Height .40 .20 –.34 .29 –.01 .97 .16 .62 .05 .87 .070 .83 –.41 .19 –.07 .82 –.31 .33 .06 .86 Weight .73 ** .01 .41 .18 .481 .11 .55 .06 –.221 .49 .03 .92 .60 * .04 .38 .22 .69 * .01 BMI .444 .15 .375 .23 .50 .10 –.291 .36 .31 .33 .67 * .02 .61 * .04 .65 * .02 Between ears dist .234 .47 .21 .52 –.916 ** .00 .32 .31 .72 ** .01 .65 * .02 .72 ** .01 Crown – chin dist .10 .75 .18 .58 .25 .44 .21 .51 .39 .36 .43 .16 Centre nasion dist. –.18 .61 .36 .25 .63 * .03 .62 * .03 .79 ** .00 Head index –.22 .49 –.64 * .02 –.54 .07 –.55 .06 Upper vol- ume .25 .43 .81 ** .00 .47 .13 Lower vol- ume .78 ** .00 .85 ** .00 Whole vol- ume .82 ** .00 FEMALE Height .55 * .04 –.24 .40 –.36 .21 .16 .58 .33 .25 .51 .06 .10 .74 .08 .78 .12 .69 .12 .68 Weight .67 ** .01 .28 .34 .56 * .04 .09 .77 .28 .34 .59 * .03 .14 .64 .55 * .04 .53 .05 BMI .63 * .02 .51 .06 –.16 .58 –.11 .70 .59 * .03 .10 .73 .53 .05 .51 .07 Between ears dist .45 .11 –.55 * .04 –.53 .05 .29 .31 .28 .34 .36 .20 .39 .17 Crown – chin dist .17 .57 .53 .05 .64 * .01 .67 ** .01 .83 ** .00 .85 ** .00 Center – nasion dist. .68 ** .42 .14 .29 .32 .47 .09 .46 .10 Head index .33 .25 .38 .19 .44 .11 .44 .12 Upper vol- ume .17 .57 .89 ** .00 .84 ** .00 Lower vol- ume .59 * .03 .67 ** .01 Whole vol- ume .99 ** .00 probably only correlate (p = 0.06) with the female head index, which meant that taller women usually have nar- rower faces. The weight alone was less important and was included in the more descriptive BMI. The initial observations showed that BMI had some statistically sig- nificant correlation with the head volume in general. Specifically, it correlated with the lower head volumes and ellipsoid sizes in men, while in women there was a highly significant correlation with the upper head vol- ume that effected the whole volume. Whilst BMI in men did not correlate with any linear distance, in women it was significantly connected to facial width and to facial length (p = 0.06). Any connection to BMI was a surprise because the measured head volume was based on land- marks set on structures with less subcutaneous fat, whilst the greatest impact of higher BMI was in other areas. In men facial width and depth correlated to lower volumes, which were significantly connected to the head and ellipsoid volumes. The same correlation among the lower volumes, the ellipsoids, and the whole head vol- umes could be seen in women, but the upper volume had a higher impact on the ellipsoid. Facial length showed correlations to lower, upper, ellipsoid, and whole vol- umes, while the distances between the ears and those from the centre to the nasion did not show any signifi- cant correlation with the volumes. It seems that the head length was the more important parameter during the head size determination for the women, while the head width determined the head sizes in men. All the parameters de- scribing the head volume more frequently correlated with the lower volume for men, but to the whole head’s volumes for women. The ellipsoid was in correlation with almost all the volumes in both genders, except for the upper ones in men. These results were expected to some extent as a head’s shape can be approximated by an ellipsoid, the volume of which depends on the lengths of all three axes. In the case of the head, the distance be- tween the chin and the crown of the head represents the major axis, and the distance between the ears being one of the minor axes and the doubled distance between the nasion and the centre being the second minor axis. The linear-regression model used to observe any in- terrelationships’ influences between the head length, width, face depth, body weight and height, and the head-volume confirmed some previously stated connec- tions. In both genders the distances from the centre to the nasion (facial-depth) had the highest impacts on the head volumes. The head-width had a slightly lower influence on the heads’ volumes, but both of them were statisti- cally significant. In women, the head length had signifi- cantly positive influence on the head volumes, in con- trast to men where this relationship was insignificant. Similar studies could not be found in the literature. No differences in the influences of sizes on facial-shape between genders were reported; 12 but the study was made on the skulls of known sexes. There is no data available about volumetric propor- tions among upper and lower facial parts as in our study, which makes it difficult to compare the results with any other research. The different correlations between upper or lower volumes in women and men were very interest- ing. In Indians, through linear measurements the ratio of the anterior facial height to the total anterior facial height was found to be non-gender specific 20 and a similar phe- nomenon was reported for Sweden. 21 If there are differ- ences between them and our population they are ethni- cally conditioned, as it was confirmed that the average Slovenian male and female faces with normocclusion have more developed chin region when compared to the Welsh population. 9 Gender-specific differences found in the chin region have been described, 22,23 but they were observed independently of the upper part of the head and face. The described gender-specific differences in the lower volume compared to the upper could also be ex- plained by more masculine faces with pronounced lower parts of the face, which have been evolutionarily more M. VERDENIK et al.: A NEW METHOD OF DETERMINING FACIAL SIZE FOR THREE-DIMENSIONAL ... 30 Materiali in tehnologije / Materials and technology 55 (2021) 1, 25–31 Table 4: Linear multivariate analysis; the whole volume obtained with the head ring is the dependent value, and the parameters that describe the head in different linear perspectives, body height and width, are the independent variable MALE Non-standardised coefficients Standardised coeffi- cients t Sig. B Std. Error Beta (Constant) –813.16 694.01 –1.17 0.29 Height –1.91 1.23 –0.32 –1.55 0.17 Weight –0.681 1.50 –0.13 –0.45 0.67 Between ears dist. 4.64 1.74 0.53 2.67 0.04 Crown – chin dist 2.38 2.22 0.22 1.07 0.32 Centre – nasion dist. 5.16 2.02 0.57 2.56 0.04 FEMALE (Constant) –815.66 255.91 –3.19 0.01 Height –0.51 0.65 –0.12 –0.78 0.46 Weight 0.84 0.72 0.19 1.17 0.28 Between ears dist. 3.10 1.33 0.42 2.33 0.05 Crown – chin dist 1.75 0.57 0.45 3.09 0.02 often selected for reproduction, as explained in anthro- pologic studies. 24 4 CONCLUSIONS The head size parameters are gender-specific in Slovenians. In men the head-width and the lower head volume are the main contributors to the head size, while in women the head length and upper volume determine the women’s head sizes. BMI significantly correlates to the whole head and lower head volumes in men, while in women highly significantly influences the upper head volume. The BMI in men does not correlate with any lin- ear distance; however, in women it is closely connected to the facial width. Ethical approval and consent to participate This study was reviewed and approved by the Slovenian National Medical Ethics Committee, Approval No. 166/02/13. Signed, informed consent was obtained from The subjects before entering the study. Consent for publication We acquired subject’s written permission for the identifiable images. Competing interests The authors declare that they have no competing in- terests. Funding No source of funding must be declared. Acknowledgement Thanks to George Yeoman for his proofreading and Ivan Verdenik for support regarding statistics. 5 REFERENCES 1 M. Y. Hajeer, A. F. Ayoub, D. T. Millett, M. Bock, J. P. Siebert, Three-dimensional imaging in orthognathic surgery: the clinical ap- plication of a new method, Int. J. Adult Orthodon. Orthognath. Surg., 17 (2002) 4, 318–30 2 R. J. Hennessy, J. P. 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