## 4 Discussion  
This paper aimed to describe a healthy children cohort and test the Roussouly classification in children. With age, we found a different distribution of the Roussouly types with the need for growing SS and PI thresholds. We found lower correlations between sagittal balance parameters than previously published [8; 9; 12; 22], but these correlations improve with age. Finally, we suggested a new classification based on PI anatomical thresholds as possibly more valuable during growth. In the present cohort of children, SS and PI continuously increased (Figure 2). This result agrees with several studies showing that the sagittal profile of the spine changes from childhood to adolescence and adulthood [23; 24; 25]. The wide variability in the sagittal parameters can explain the very low correlation between age and SS/PI, albeit statistically significant. Given the cross-sectional design of this study, we could not observe and combine serial data to identify the age variations. As expected, we coherently found a considerable variation in the distribution of Roussouly types, favouring types 1-2 in younger-aged participants, confirming the need to define new thresholds for the Roussouly classification during growth. Our findings and previous findings are worth noting [8; 9; 12; 22], in that the youngest have the lowest correlations among sagittal balance parameters. Our explanatory hypothesis correlates with the gradually reducing body elasticity and corresponding increasing rigidity with growth [26]. The younger subjects have more sagittal compensations than \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Roussouly Classification**}} & \multicolumn{8}{c}{**New Classification**} \\ \cline{2-11}  & \multicolumn{3}{c}{**Age \textless{} 11**} & \multicolumn{3}{c}{**Age 12–18**} & \multicolumn{3}{c}{**Age \textless{} 11**} & \multicolumn{3}{c}{**Age 12–14**} & \multicolumn{3}{c}{**Age 15–18**} \\ \cline{2-11}  & \multicolumn{1}{c}{SS} & \multicolumn{1}{c}{\%} & \multicolumn{1}{c}{SS} & \multicolumn{1}{c}{\%} & \multicolumn{1}{c}{**PI**} & \multicolumn{1}{c}{\%} & \multicolumn{1}{c}{_PI_} & \multicolumn{1}{c}{\%} & \multicolumn{1}{c}{_PI_} & \multicolumn{1}{c}{\%} \\ \hline
**type 1** & \(<\)29 & 6\% & \(<\)34 & 6\% & \(<\)34 & 29\% & \(<\)39 & 34\% & \(<\)39 & 14\% \\ \hline
**type 2** & \(19\%\) & \(<\)34 & 24\% & & & & & & & & \\ \hline
**type 3** & 30–40 & 46\% & 35–45 & 49\% & **type B** & 35–45 & 39\% & 40–50 & 41\% & 40–55 & 64\% \\ \hline
**type 4** & 41\(>\) & 29\% & 46\(>\) & 21\% & **type C** & 46\(>\) & 32\% & 51\(>\) & 25\% & 56\(>\) & 21\% \\ \hline \hline \end{tabular} Abbreviations: SS = sacral slope; PI = pelvic incidence.  
\end{table} 
Table 4: Newly defined limits in the Roussouly classification of sacral slope (SS) to distinguish the different types in different age groups during growth. Limits of pelvic incidence (PI) for the new classification in different age groups during growth. Distribution of the Roussouly classification and the new classification types in the different groups. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Roussouly Classification**}} & \multicolumn{8}{c}{**New Classification**} \\ \cline{2-11}  & \multicolumn{3}{c}{**Age \textless{} 11**} & \multicolumn{3}{c}{**Age 12–14**} & \multicolumn{3}{c}{**Age 15–18**} \\ \hline
**Types** & **A** & **B** & **C** & **A** & **B** & **C** & **A** & **B** & **C** \\ \hline
**1–2** & **17\%** & 6\% & 2\% & **25\%** & 6\% & 0 & **14\%** & 14\% & 0 \\ \hline
**3** & 12\% & **23\%** & 11\% & 7\% & **29\%** & 16\% & 0 & **36\%** & 4\% \\ \hline
**4** & 0 & 10\% & **19\%** & 2\% & 6\% & **9\%** & 0 & 14\% & **18\%** \\ \hline
**Corresponding types** & \multicolumn{3}{c}{**59\%**} & \multicolumn{3}{c}{**63\%**} & \multicolumn{3}{c}{**68\%**} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Correlations between the growing age Roussouly classification and the new classification among the different groups.