In this study including 522 participants with a median age of 9.9 years, we found a mean pelvic incidence of 36.6° (SD 8.0), with a normal distribution, without any difference between boys and girls. The vertebral concavity ratio indicated mild concave vertebrae at all levels, with lower ratios for boys compared to girls. Both a larger pelvic incidence angle and a larger vertebral concavity ratio were associated with a decreased disc height, and a smaller pelvic incidence was associated with less disc bulging. A higher fat-free mass index was associated with a smaller pelvic incidence, while none of the body composition measures were associated with the vertebral concavity ratio.
Spinopelvic alignment develops during childhood, and the morphology of the spinopelvic setting is fixed at the end of skeletal growth . The pelvic incidence found in the present study population was somewhat smaller compared to previous studies in children within the same age range [16, 17]. This difference might be explained by the difference in position of the participants: the MRI’s in the present study were performed in supine position, while participants in the other studies underwent standing lateral radiographs. Although the pelvic incidence is described to be independent of measurement position, an overestimation of the pelvic incidence on standing radiographs or an unstable sacroiliac joint in certain patients has been suggested . The vertebral shape measure used, i.e. vertebral concavity ratios, showed a mild concave shape of the vertebrae at all lumbar levels in the present study, which indicates that this is a normal vertebral shape in children aged nine years old. We found significant differences in the vertebral concavity ratios between boys and girls, which is in contrast to studies in adults that showed no differences in vertebral concavity between sexes [19, 20]. The children in our study population were in their growth, with different growth patterns for boys and girls. This seems reflected in cervical vertebrae measures  and may disappear after further development towards adulthood. With the follow-up measurements of the present longitudinal cohort study, future analyses will enable us to get more insight into these differences of development between sexes.
In literature, a lower pelvic incidence or smaller lordosis is associated with more disc degeneration in adults . In our study, no association between pelvic incidence and disc degeneration was found. This difference is likely due to the fact that we studied a group of children in whom these abnormalities are relatively rare and it might be too early in the development to identify these associations. However, a larger pelvic incidence is associated with decreased disc height. An increased pelvic incidence indicates an increase in lumbar lordosis. This may cause more compression and shear forces on the intervertebral discs, resulting in decreased disc height. However, this is purely suggestive, since our analyses are based on cross-sectional data and no causal relationship can be determined. A negative association was found between a lower pelvic incidence and the presence of disc bulging, which indicates less posterior disc bulging in children with a more straight spine. So these findings indicate that spinopelvic alignment already seems to play a role in the presence of structural spinal abnormalities in childhood.
The positive association of the vertebral concavity ratio with the presence of a decreased disc height found in our study population seems logical, as a greater ratio indicates the vertebra to be more flat or even somewhat convex, which results in a smaller disc height of the adjacent intervertebral disc. As no other associations were found between any of the structural abnormalities and vertebral shape, the role of the vertebral shape in the presence of structural spinal abnormalities seems unlikely.
Associations of body weight and composition with structural spinal abnormalities have been described in adults, adolescents, and children [8, 23,24,25]. Jankowicz-Szymanska et al. (2019), recently showed that excessive body weight is associated with the sagittal shape of the lumbar spine in children aged 10–12 years . In the present study, we found an association of the BMI-SD score with the pelvic incidence, but this association did not remain after adjustment for potential confounders. However, the association of the fat-free mass index with the pelvic incidence did remain significant after adjustment. It is difficult to interpret and explain the mechanism behind and meaning of this association, especially since we did not find any associations of the other body composition measurements with the pelvic incidence. The association we found might be a coincident finding. However, it might also be the case that especially the fat-free mass, and not the fat mass, plays a role in the spinal development in children. Therefore, further research is needed to investigate how this relation between the fat-free mass and pelvic incidence develops towards adulthood.
Strengths and limitations
The major strength of this study is the large sample size of children from a general population, in which we were able to assess the children’s spines on MRI and additionally retrieved extensive information on their body composition. Additionally, we were able to adjust the analyses for the skeletal age. However, some limitations should be taken into account. First, to reduce the risk of inaccuracy of the MRI measures, annotations were only made if the landmarks were clearly visible. Unfortunately, this caused missing values in some of the images, but these are likely to be random and therefore not introducing any bias. Also, the calculated pelvic incidence and vertebral concavity ratios were very sensitive for small variations in the manual annotations. These variations were limited by following the standard protocol and only two people working on the annotations. Furthermore, the subsample of the study cohort we used included more children with a Dutch ethnicity and less children with overweight or obesity compared to the total study cohort, even though the selection was intended to be random . Therefore, caution should be made in the interpretation of the study results, as the study sample is not fully representative of the Dutch and worldwide population of children.
In conclusion, the pelvic incidence of the 9-year-old children in this study follows a normal distribution with a mean of 36.6° on supine MRI images, and a slightly concave shape of the lumbar vertebrae is seen. Spinopelvic alignment seems to be associated with structural spinal abnormalities, already at this young age, and might itself be influenced by the children’s body composition. Longitudinal studies are needed to further assess the relations between body composition, pelvic incidence, and structural abnormalities, and their development and influence on complaints later in life.