Evidence of secular trends in a collection of historical craniofacial growth studies.

BACKGROUND/OBJECTIVES Although secular trends have been well documented in the craniofacial region, there is no evidence to suggest that these temporal changes exist in samples used for orthodontic research. The aim of this study is to determine the effect of secular trends on craniofacial growth in a series of longitudinal birth cohorts that are frequently used in orthodontic research. MATERIALS/METHODS Cephalometric data from serial lateral headfilms of 138 adolescents (total of 1252 cephalograms) were collected from the Craniofacial Growth Legacy Collection, which includes nine historical growth studies that were mostly conducted throughout the past century. Mixed-effects linear models were used to test the effect of 'year of birth (yob)', 'age', and their interaction on six sagittal (SNA, SNB, ANB, S-N, Co-A, Co-Po) and two vertical (N-Me, ANS-Me) cephalometric measurements. RESULTS Five of the eight cephalometric variables showed a significant (P ≤ 0.017) 'yob' effect, with four of these indicating an increase over time (SNA, ANB, S-N, Co-A) and one indicating a decrease (SNB). Highly significant (P < 0.001) interactions between 'age' and 'yob' were found for the measurements SNA, ANB, Co-A, and S-N. LIMITATIONS Some of the limitations of the present study include the use of a small, non-random sample of the original large-scale growth studies. CONCLUSIONS/IMPLICATIONS Secular trends were found in the craniofacial growth records of the 138 participants derived from the longitudinal growth studies. These secular trends are likely to have important clinical implications for the findings of controlled clinical trials in orthodontics. More research is needed to establish the presence of secular trends in other historical collections.