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Variation in Chin and Mandibular Symphysis Size and Shape in Males and Females: a CT-Based Study

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Variation in Chin and Mandibular Symphysis Size and Shape in Males and Females: a CT-Based Study International Journal of Environmental Research and Public Health Article Variation in Chin and Mandibular Symphysis Size and Shape in Males and Females: A CT-Based Study Tatiana Sella Tunis 1,2,3,* , Israel Hershkovitz 1,2 , Hila May 1,2, Alexander Dan Vardimon 3, Rachel Sarig 2,3,4 and Nir Shpack 3 1 Department of Anatomy and Anthropology, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel; [email protected] (I.H.); [email protected] (H.M.) 2 Dan David Center for Human Evolution and Biohistory Research, Shmunis Family Anthropology Institute, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel; [email protected] 3 Department of Orthodontics, The Maurice and Gabriela Goldschleger School of Dental Medicine, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel; [email protected] (A.D.V.); [email protected] (N.S.) 4 Department of Oral Biology, The Maurice and Gabriela Goldschleger School of Dental Medicine, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel * Correspondence: [email protected]; Tel.: +972-3-640-7310 Received: 12 May 2020; Accepted: 11 June 2020; Published: 14 June 2020 Abstract: The chin is a unique anatomical landmark of modern humans. Its size and shape play an important role from the esthetic perspective. However, disagreement exists in the dental and anthropological literature regarding the sex differences in chin and symphysis morphometrics. The “sexual selection” theory is presented as a possible reason for chin formation in our species; however, many other contradictory theories also exist. This study’s aims were therefore to determine how chin and symphysis size and shape vary with sex, and to discuss “sexual selection” theory as a reason for its formation. Head and neck computed tomography (CT) scans of 419 adults were utilized to measure chin and symphysis sizes and shapes. The chin and symphysis measures were compared between the sexes using an independent-samples t-test, a Mann–Whitney test, and the F-statistic. The chin width was significantly greater in males than in females (p < 0.001), whereas the chin height, area, and size index were significantly greater in females (p < 0.001). Symphysis measures did not differ significantly between the sexes. Size accounted for 2–14% of the chin variance and between 24–33% of the symphysis variance. Overall, the chin was found to be a more heterogeneous anatomical structure than the symphysis, as well as more sexually dimorphic. Keywords: chin; mandibular symphysis; morphometrics; sexual dimorphism; facial attractiveness; computed tomography 1. Introduction The chin (mentum osseum) is a distinctive feature of the anterior mandibular symphysis found only in our species, Homo sapiens [1]. It is characterized by a mental protuberance, a raised vertical structure that lies along the symphyseal midline, along with paired bulbous formations placed on each lateral side of its inferior margin (mental tubercles) (Figure1A). The presence of a chin is already noticeable in the fifth fetal month [1,2] and the mandible retains this characteristic into adult life [3]. Int. J. Environ. Res. Public Health 2020, 17, 4249; doi:10.3390/ijerph17124249 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2020, 17, 4249 2 of 15 Int. J. Environ. Res. Public Health 2020, 17, x 2 of 15 FigureFigure 1. ( 1.A) (ChinA) Chin (mentum (mentum osseum). osseum). Fron Frontaltal view view of the of themandible mandible (using (using the thevolume volume rendering rendering technique);technique); chin chin borders borders (mental (mental protuberance protuberance an andd bilateralbilateral mentalmental tubercles) tubercles) are are denoted denoted by aby dotted a dottedline. line. (B) Mandibular(B) Mandibular symphysis. symphysis. Midsagittal Midsagittal section; section; symphysis symphysis borders borders are denoted are bydenoted dotted by lines. dotted lines. The mandibular symphysis isthe region where the two halves of the human fetal mandibular corpusSeveral are hypotheses fused [4] (Figure have 1beenB). As raised opposed over to the the years chin, theto elucidate mandibular the symphysis reason for is the found chin's in all appearancehominoid in mandibles humankind. and The its morphology“sexual selection” was functionally hypothesis linked is among [5]. the most common; it states that theSeveral appearance hypotheses of the havechin in been humans raised and over its the size years and toshape elucidate variation the reasonare due for to thesexual chin’s selection,appearance relating in humankind.to facial attractiveness The “sexual [6–9]. selection” hypothesis is among the most common; it states thatIn themodern appearance dentistry, of the the chin chin in and humans symphysis and its size size and and shape variationplay an important are due to role sexual from selection, the estheticrelating perspective to facial attractiveness[10]. Chin height [6– 9and]. width are considered important factors in perceiving facial attractiveness.In modern Deformities dentistry, in the chin chin height and symphysis(when greater size than and shape50% and play 58% an of important the lower role anterior from the facialesthetic height perspective in males [ 10and]. Chin females, height respectively) and width are were considered found importantto be the factorsleast attractive in perceiving facial facial characteristic,attractiveness. and Deformities consequently, in chin surgery height was (when requ greaterired thanto correct 50% and the 58% facial of theappearance lower anterior [11]. facialA squaredheight contour in males of andthe chin females, in females respectively) is considered were found unattractive to be the since least it attractivegives the facialface a characteristic,masculine appearanceand consequently, and thus diminishes surgery was the required appearance to correct of femininity the facial [8]. appearance Narrowing [11 genioplasty]. A squared to contourachieve of a femininethe chin inand females slim lower is considered face is a unattractive well-documented since it givesprocedure the face that a masculineis practiced appearance to improve and the thus facialdiminishes appearance the and appearance the lower of femininityfacial contour [8]. [12,13]. Narrowing Valenzano genioplasty et al. to[14] achieve applied a feminine morphometric and slim analysis(quantitativelower face is a well-documented analysis of procedurethe chin thatand issymphysis practiced tosize improve and theshape) facial regarding appearance facial and the attractiveness.lower facial contourHowever, [12 ,13they]. Valenzanofailed to etfind al. [14an] appliedassociation morphometric between attractiveness analysis(quantitative and sexual analysis dimorphismof the chin of and the symphysis lower face size (including and shape) the chin). regarding facial attractiveness. However, they failed to find anQualitative association differences between attractiveness in the chin shape and sexual betw dimorphismeen the sexes of are the well lower known face (including in anthropology: the chin). males possessQualitative broad di ffanderences prominent in the chinchins shape with betweena more square the sexes appearance are well knownand females in anthropology: possess pointedmales chins possess [15–19]. broad andThayer prominent and Dobson chins with[9] compared a more square the chin appearance shape between and females the possesssexes using pointed ellipticalchins [Fourier15–19]. function Thayer and analysis Dobson (EFFA) [9] compared and foun thed that chin sexual shape dimorphism between the exists sexes in using its shape. elliptical However,Fourier there function was analysis a high degree (EFFA) of and overlap found thatin the sexual chin dimorphismshape between exists males in its and shape. females However, due to there a largewas degree a high of degree variation of overlap in each in sex. the chinNeverthe shapeless, between males males possessed and females a taller due symphysis, to a large a degree more of protrusivevariation mentum in each sex.osseum, Nevertheless, and larger males tubercula possessed laterale a taller than symphysis, females did. a moreGarvin protrusive and Ruff mentum [17] usedosseum, 3D surface and largerlaser scans tubercula to show laterale that than the abso femaleslute did.surface Garvin area andandRu theff volume[17] used of 3D the surface chin are laser sexuallyscans todimorphic; show that however, the absolute the surface sex differences area and thebecame volume non-significant of the chin are when sexually they dimorphic; were standardizedhowever, the for sex size. di ffGarvinerences and became Ruff's non-significant morphometric when shape they analysis were standardizedrevealed a relatively for size. high Garvin overlapand Ru inff the’s morphometric chin shape between shape analysis the sexes. revealed Males a relativelydisplayed high relatively overlap more in the prominent chin shape mental between protruberances,the sexes. Males lateral displayed tubercles, relatively and taller more chins prominent than did mentalfemales. protruberances, Coquerelleet al. lateral [20] and tubercles, Franklin and et al.taller [21,22] chins used than the did geometric females. morphometric Coquerelleet al.method [20] and to find Franklin that the et al.sexual [21,22 dimorphism] used the geometric of the chinmorphometric and mandibular method symphysis to find that is age-dependent. the sexual dimorphism According of the to chinCoquerelleet and mandibular al. [20], symphysis
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