Facial soft tissue growth in identical twins

Abstract

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.

Sub-clustering in skeletal class III malocclusion phenotypes via principal component analysis in a southern European population

“…The position of the lower lip with respect to the lower incisor was analyzed in several studies that found a close relationship between the two 23,24 , even observing a more significant relationship in the position of the lower incisor with respect to the lower lip compared to the relationship between the upper incisor and the upper lip 23 . This study found that in 5 of the 6 sub-groups, the position of the lower lip coincided with the position of the lower incisor.…”

“…The relationship between the position of the lower incisor with the NB plane or A-Pg plane with respect to the position of the lower lip with the S line and/or E line was observed in C5, C4, C3, and C2, but not in C1. Individual characteristics may lead to different soft tissue behaviors with respect to the bones and/or teeth [23][24][25] , with different correlations between the two components. The method used for the analysis of soft tissues should also be taken into account, since recent studies support the need to conduct this analysis using facial photographs, since skeletal cephalometry and soft tissue cephalometry lead to different diagnoses of the patient's facial characteristics 23 .…”

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

“…Since half of the examined variables (LAFH/AFH ratio, y‐axis and ML‐NL) demonstrated high heritability, they can be used as predictors of unfavourable growth. Finally, no post hoc sample size calculation was done for this study, which is an extension of a previous study on MZ twins . As however the first study included a sample size calculation and the current study simply doubled the originally adequately powered sample, we do believe that this study is also adequately powered.…”

“…Classical methods of analysis have been based on comparisons of the differences within pairs of identical or fraternal twins, whereas the extent of the concordance or difference, respectively, being taken as an indication of the relative genetic contribution . However, only few studies up to now have assessed genetic components of craniofacial structures by following longitudinally MZ and DZ twins during the growth process …”

Authors' response

“…[8][9][10][11][12] Most recent orthodontic investigations have used only 1 examiner. [13][14][15][16][17] The current error study was adequately performed and showed random errors within acceptable limits and no significant systematic errors. 7,18 The sagittal and vertical effects of posterior discrepancy explained by Sato's theory were based on 2dimensional views.…”