INTRODUCTION: The aims of this longitudinal analysis of untreated monozygotic twins were to investigate the change of the facial soft tissues during growth, to determine the concordance of soft tissue growth patterns between genetically identical twins, and to assess the genetic component of soft tissue development.
METHODS: The sample consisted of 33 pairs of untreated monozygotic twins (23 male, 10 female) from the Forsyth Moorrees Twin Study (1959-1975); lateral cephalograms taken from 6 to 18 years of age were analyzed at 3-year intervals. Cephalograms were traced, and longitudinal changes in the soft tissue profile between twins were analyzed with intraclass correlation coefficients and linear regression modelling.
RESULTS: The concordance between monozygotic twins at 18 years of age was moderate to high with intraclass correlation coefficients values between 0.37 and 0.87. Additionally, female twins showed higher concordance at 18 years of age than did male twins for all included variables. However, about 10% to 46% of the twin pairs had large differences in their soft tissue parameters, even after the growth period.
CONCLUSIONS: Although monozygotic twins possess the same genetic material, differences in the soft tissues were found. This supports the complex developmental mechanism of the human face and the varying influence of genetic and environmental factors.