To report the sampling study design and radiography protocol of a large-sample investigation on skeletal maturation of 3 to 18-year-old children in China.

Multi-stage stratified random sampling was employed in this study. Two provinces, municipalities, or autonomous regions were randomly selected from each of the seven regions of China, including Northeast China, Northwest China, North China, Central China, East China, Southwest China, and South China. Then one rural and one urban investigation site were randomly selected from each province, municipality, or autonomous region. In total 28 sites were included. Among those sites, four residential districts were randomly selected from each urban site, and four townships from each rural site. For each residential district or township, 1-4 kindergartens, primary schools, and middle schools were chosen. Random cluster sampling was used to extract 3-<6-year-old children in kindergartens, and 6-18-year-old children in primary schools and middle schools. The investigation on skeletal maturation was sampled proportionate to the sampling of the whole study. The estimated simple size was 780 for each site, and 21 840 for all 28 sites in total. There were six groups of 3-<6-year-old children classified at 0.5-year intervals, and 12 groups of 6-18-year-old children classified at 1-year intervals. Posteroanterior position radiography of the left hand and wrist was achieved for all subjects.

The study was performed from August 26, 2019 to October 16, 2021. In total, 20 444 children received posteroanterior position radiography of the left hand and wrist, including 10 196 males and 10 248 females, 9 711 urban and 10 733 rural, respectively. The 3-<6-year-old group included 1 611 (male 819, female 792) subjects, and the 6 to 18-year-old group included 18 833 (male 9 377, female 9 456) subjects.

This nationwide investigation on skeletal maturation of 3 to 18-year-old children in seven regions of China was successfully preformed. The results of this study can provide an important reference for establishing the current evaluation criteria of bone age in Chinese children and adolescents.

Child; Age determination by skeleton; X-ray