Sagittal Balance in Children: Reference Values of the Sacral Slope for the Roussouly Classification and of the Pelvic Incidence for a New, Age-Specific Classification

Background: The Roussouly classification, based on the functional parameter sacral slope
(SS), describes the normal sagittal balance in adults and has proved useful for surgery. Reference
values in children should be defined, since they are an important treatment target of conservative
treatment. Moreover, during growth, there are few correlations between sagittal parameters, and we
hypothesize that a new classification based on the anatomical parameter pelvic incidence (PI) could
also be useful. We performed a cross-sectional study to identify the reference values for the Roussouly
classification during growth and to develop a new classification based on PI in children. Methods:
Correlations between sagittal parameters and age were searched in 222 healthy subjects at the first
consultation (6–18 years old). A new classification, based on PI, and comprising three types, is defined
and compared to the Roussouly classification. Results: With age, correlations among sagittal balance
parameters increase, as well as SS and PI, but with different cut-offs. The distribution of Roussouly
types do not correspond to that in adulthood; thus, we defined new reference cut-offs. We defined a
PI-based classification in three types, not overlapping Roussouly’s. We found a uniform and balanced
distribution of cases among the nine possible combinations. Conclusions: In children, we need to
use new thresholds for the Roussouly types. The new classification based on PI is correlated with the
Roussouly classification, but it is also clearly different. Future studies will determine its validity.