Vertical growth in mono‐and dizygotic twins: A longitudinal cephalometric cohort study

Abstract

Objective The aims of this longitudinal analysis of untreated monozygotic and dizygotic twins were to investigate vertical changes of the craniofacial structures during growth, to determine the concordance between genetically twins and to assess the genetic component for the various aspects of vertical growth. Settings and sample population The sample consisted of 34 pairs of untreated monozygotic twins (23 male, 11 female) and 30 untreated dizygotic siblings of multiple birth (8 male, 8 female and 14 mixed) from the Forsyth Moorrees Twin Study (1959‐1975); lateral cephalograms taken from 6 to 18 years of age were analysed at 3‐year intervals. Materials and Methods Cephalograms were traced, and longitudinal changes between twins in six angular and proportional vertical cephalometric variables (SN‐NL, ML‐NL, SN‐ML, y‐axis, PFH/AFH and LAFH/AFH) were analysed with intraclass correlation coefficients and linear regression modelling. Results The concordance between monozygotic/dizygotic twins at 18 years of age was moderate to high with intraclass correlation coefficient values between 0.51 and 0.66. Additionally, sex differences in concordance at 18 years of age were found for three variables. High heritability (66%‐79%) was observed for 5 of the 6 variables (LAFH/AFH, ML‐NL, y‐axis, SN‐ML, PFH/AFH), while SN‐NL showed limited heritability (34%). Conclusions Although monozygotic/dizygotic twins share at least part of their genetic material, differences in the vertical dimension were found. This supports the complex developmental mechanism of the human face and the varying influence of genetic and environmental factors.