Cutting Edge in Current Orthodontics

7 Years Twin Study - Influence of Heritability on the Craniofacial Hard and Soft Tissue Characteristics
Young Ho Kim

Objectives
To investigate the influence of heritability on the craniofacial cephalometric characteristics of monozygotic(MZ) twins, dizygotic(DZ) twins, and their siblings(SIB) and to find highly heritable parts of craniofacial structures and applicate the findings in the diagnosis and treatment planning.
Material and 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). Cephalometric hard and soft tissue variables were measured to describe the size and shape of the facial horizontal and vertical, dental, mandible and cranial base structure and 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).
Results
The MZ group exhibited higher ICC values for all cephalometric variables than DZ and SIB groups. The mean of the hard tissue ICC and the soft tissue ICC were similar in each group. Among hard tissue cephalometric variables, the high h2 values were shown at SNA, SNB, SN-Pog, SN-palatal plane angle, SN-mandibular plane angle, N-ANS/ANS-Me, and ANS-Me/N-Me. SN-occlusal plane angle, L1-Occlusal plane angle, and lower gonial angle, mandibular body length, cranial base angle also showed high h2 values. Among soft tissue cephalometric variables, the high h2 values were observed for the nasolabial angle, chin angle, soft tissue chin thickness, and upper lip thickness ratio.
Conclusion
The shape of facial skeletal structure and location of the occlusal plane on the craniofacial hard tissue characteristics, and the nose and soft tissue chin on the craniofacial soft tissue characteristics were more influenced by genetic factors. While orthodontic treatment, careful approach is needed prior to changing craniofacial parameters that are under strong genetic influences.