Concerning inter-observer variations, the Pearson correlation coefficient and ICC were 0.935 (P <0.001) and 0.890 (P <0.001), respectively. On the contrary, the Pearson correlation coefficient and ICC were 0.973 (P <0.001) and 0.894 (P <0.001), respectively, in the intra-observer variations. Notably, all values indicated excellent reliability.
Characteristics of the participants
The characteristics of participants and the result of one-sample t-test on comparison with Chinese standards are presented in Table 2.
In this study, according to the IOTF cut-off points of BMI (for thinness, overweight, and obesity) by gender and age, which is based on international data and linked to the widely acceptable adult cut-off points of BMI (18.5, 25, and 30 kg/m2 for male and female) (Table 1), participants were classified into the following groups: thinness group (102, 7.7%), normal-weight group (796, 59.8%), overweight group (excluding children with obesity) (286, 21.5%), and the obese group (146, 11.0%).
The basic information of the study participants is shown in Table 3. The sample consisted of 1330 participants (730 males, 54.9%; 600 females, 45.1%), and no significant difference in CA, SA, and relative SA was observed between males and females; however, the body height, body weight, and BMI were significantly higher in males than females (P <0.01). The results of the paired-samples t-test showed that, overall, the SA was significantly lower than CA in the thinness and normal-weight groups; however, SA was significantly higher than CA in the overweight and obese groups (P <0.01), excluding females with overweight.
Skeletal maturation and BMI levels
As shown in Table 3 and Figure 2, the trend of relative SA increased with BMI in both gender groups. The one-way ANOVA revealed significant differences in relative SA between BMI groups in both sexes (P <0.05).
Next, their significance was determined using the least significant difference (LSD) post hoc test. In males, the change in mean relative SA was -0.5, -0.3, 0.2, and 0.4 years in the thinness, normal-weight, overweight, and obese groups, respectively. The relative SA distributions in each BMI group are shown in Figure 3. Although there was no significant difference in the relative SA between the overweight and obese groups, and between the normal weight and thinness groups, the relative SA was significantly greater in both obese and overweight groups than in normal and thinness groups (P <0.01) (Figure 2a). In females, the post hoc test indicated significant differences in relative SA among all groups in the following order: obese (0.5 years) > overweight (0.2 years) > normal (-0.2 years) > thinness (-0.5 years) (P <0.01) (Figure 2b).
The relationship between SA and CA is illustrated in Figure 4. In the thinness and normal-weight groups, SA tended to be lower than CA: 69.6% of children with thinness (29 males, 42 females) (Figure 4a) and 64.7% of children with normal-weight (284 males, 231 females) (Figure 4b) had a SA lower than CA. On the contrary, SA tended to be higher than CA in the overweight and obese groups: 58.0% of children with overweight (103 males, 63 females) (Figure 4c) and 61.0% of children with obesity (52 males, 37 females) (Figure 4d) had a SA greater than CA. Significant differences in the ratio of SA to CA were observed between the BMI groups (male, x2 = 48.73, df = 3, P <0.001; female, x2 = 26.22, df = 3, P <0.01); however, no differences were observed between males and females. A scatter plot was plotted to visually determine the linear relationship between SA and CA; the regression lines and equations are superimposed on the graph. According to the linear regression model, a greater difference between SA and CA was observed with an increase in CA in the obese group.
Accelerated skeletal maturation
Table 4 describes the ratio of accelerated skeletal maturation in gender, age, and BMI groups. In this study, 12.9% of males and 13.0% of females had accelerated skeletal maturation. Among the BMI groups, 7.8% of children with thinness, 7.0% of children with normal-weight, 22.0% of children with overweight, and 30.8% of children with obesity had accelerated skeletal maturation (x2 = 89.442, df = 3, P <0.01). In the age groups, the percentage of accelerated skeletal maturation was 3.8% (3.5 years), 13.1% (4.0 years), 10.5% (4.5 years), 13.5% (5.0 years), 16.9% (5.5 years), and 27.8% (6.0 years), respectively (x2 = 43.417, df = 5, P <0.01). No significant differences were observed in the gender groups by the Chi-square test.
In addition, logistic regression analysis was used in the assessment of the effects of age, gender, and BMI groups on the accelerated skeletal maturation of the participants. Consequently, the obtained logistic model had a statistical significance (x2 = 97.86, P <0.01). According to the Hosmer-Lemeshow goodness-of-fit statistics (P = 0.812), the estimated model appropriately fitted 87.1% of the occasions when predicting accelerated skeletal maturation. The results of binary logistic regression analysis of accelerated skeletal maturation are presented in Table 5. After adjusting the two other independent variables (gender and age) in this model, compared with the normal-weight group, participants with overweight/obesity had an increased risk of accelerated skeletal maturation (Overweight, OR = 3.27, 95% CI: 2.20-4.87; Obese, OR = 4.73, 95% CI: 2.99-7.48).