Korean J Orthod.  2019 Jan;49(1):3-11. 10.4041/kjod.2019.49.1.3.

Influence of heritability on craniofacial soft tissue characteristics of monozygotic twins, dizygotic twins, and their siblings using Falconer's method and principal components analysis

• Song J
• Chae HS
• Shin JW
• Sung J
• Song YM
• Baek SH
• Kim YH

• KMID: 2431075
• DOI: http://doi.org/10.4041/kjod.2019.49.1.3

Abstract

OBJECTIVE
The purpose of this study was to investigate the influence of heritability on the craniofacial soft tissue cephalometric characteristics of monozygotic (MZ) twins, dizygotic (DZ) twins, and their siblings (SIB).
METHODS
The samples comprised Korean adult twins and their siblings (mean age, 39.8 years; MZ group, n = 36 pairs; DZ group, n = 13 pairs of the same gender; and SIB group, n = 26 pairs of the same gender). Thirty cephalometric variables were measured to characterize facial profile, facial height, soft-tissue thickness, and projection of nose and lip. Falconer's method was used to calculate heritability (low heritability, h2 < 0.2; high heritability, h2 > 0.9). After principal components analysis (PCA) was performed to extract the models, we calculated the intraclass correlation coefficient (ICC) value and heritability of each component.
RESULTS
The MZ group exhibited higher ICC values for all cephalometric variables than DZ and SIB groups. Among cephalometric variables, the highest h2 (MZ-DZ) and h2 (MZ-SIB) values were observed for the nasolabial angle (NLA, 1.544 and 2.036), chin angle (1.342 and 1.112), soft tissue chin thickness (2.872 and 1.226), and upper lip thickness ratio (1.592 and 1.026). PCA derived eight components with 84.5% of a cumulative explanation. The components that exhibited higher values of h2 (MZ-DZ) and h2 (MZ-SIB) were PCA2, which includes facial convexity, NLA, and nose projection (1.026 and 0.972), and PCA7, which includes chin angle and soft tissue chin thickness (2.107 and 1.169).
CONCLUSIONS
The nose and soft tissue chin were more influenced by genetic factors than other soft tissues.

Keyword

Twin study; Craniofacial soft tissue characteristics; Cephalometric analysis; Heritability

MeSH Terms

Adult
Chin
Humans
Lip
Methods*
Nose
Passive Cutaneous Anaphylaxis
Siblings*
Twins
Twins, Dizygotic*
Twins, Monozygotic*

Figure

• 

  Figure 1 Landmarks and reference planes used in the cephalometric analysis. S, Sella; N, nasion; A, A point; B, B point; Pog, pogonion; Me, menton; G, glabella; N', soft tissue nasion; Pn, pronasale; Cm, columella; Sn, subnasale; A', soft tissue A point; ULA, upper lip anterior; Stm, stomion; ULI, upper lip inside; LLI, lower lip inside; LLA, lower lip anterior; B', soft tissue B point; Pog', soft tissue pogonion; Me', soft tissue menton; C, cervical point; HRP, horizontal reference plane, a horizontal plane angulated 7° clockwise to the SN line passing through Sella; ULAP, ULA perpendicular plane to HRP, a perpendicular line to the HRP passing through ULA.

• 

  Figure 2 Cephalometric variables. Facial profile: 1, G-N'-Pn (angle determined by points G, N', and Pn); 2, Cm-Sn-ULA (angle determined by points Cm, Sn, and ULA); 3, GPog'-N'Pn (angle formed by G-Pog' and N'-Pn lines); 4, N'-Pn-Pog' (angle determined by points N', Pn and Pog'); 5, GPog'-Me'C (angle formed by G-Pog' and Me'C lines); 6, G-Sn-Pog' (angle determined by points G, Sn, and Pog'); 7, HRP-N'Pog' (angle formed by HRP and N'Pog' lines); 8, N'-Pog'-ULA (angle determined by points N', Pog' and ULA'). Projection of nose and lip: 9, Pn-ULAP perp (perpendicular distance from point Pn to ULAP in mm); 10, A'-ULAP perp (perpendicular distance from point A' to ULAP in mm); 11, LLA-ULAPog' perp (perpendicular distance from point LLA to ULA-Pog' line in mm); 12, Sn-ULAPog' perp (perpendicular distance from point Sn to ULA-Pog' line in mm); 13, B'-ULAPog' perp (perpendicular distance from point B' to ULA-Pog' line in mm); 14, ULA-SnPog' perp (perpendicular distance from point ULA to Sn-Pog' line in mm); 15, LLA-SnPog' perp (perpendicular distance from point LLA to Sn-Pog' line in mm); 16, ULA-PnPog' perp (perpendicular distance from point ULA to Pn-Pog' line in mm); 17, LLA-PnPog' perp (perpendicular distance from point LLA to Pn-Pog' line in mm). Soft tissue thickness: 18, A-A' (distance between points A and A' in mm); 19, ULI-ULA (distance between points ULI and ULA in mm); 20, LLI-LLA (distance between points LLI and LLA in mm); 21, Pog-Pog' (distance between points Pog and Pog' in mm); 22, Me-Me' (distance between points Me and Me' in mm); 23, ULI-ULA/A-A' (the ratio of the distance between points ULI and ULA to the distance between points A and A'); 24, Pog-Pog'/Me-Me' (ratio of the distance between points Pog and Pog' to the distance between points Me and Me'). Facial height: 25, G-Sn (vertical height between point G and Sn in mm); 26, Sn-Me' (vertical height between point Sn and Me' in mm); 27, Sn-Stm (vertical height between point Sn and Stm in mm); 28, Stm-Me' (vertical height between point Stm and Me' in mm); 29, G-Sn/Sn-Me' (ratio of the distance between points G and Sn to the distance between points Sn and Me'); 30, Stm-Me'/Sn-Stm (ratio of the distance between points Stm and Me' to the distance between points Sn and Stm). See Figure 1 for definition of each landmark.

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