Correlation of the anatomical sacral slope with pelvic incidence in female patients with developmental hip dysplasia: a retrospective cross-sectional study

BACKGROUND: The anatomical sacral slope is considered as an anatomical pelvic parameter independent of femoral head centers for measurement of anatomical sacral slope and was previously described to strongly correlate with pelvic incidence on a two-dimensional examination of healthy subjects. However, the correlation between anatomical sacral slope and pelvic incidence was unclear in patients with developmental dysplasia of the hip. This study aimed to examine the correlation between anatomical sacral slope and other spinopelvic parameters by analyzing plain radiographs of female patients with developmental dysplasia of the hip. METHODS: Eighty-four women with developmental dysplasia of the hip were examined. Lumbar lordosis, thoracic kyphosis, pelvic incidence, sacral slope, and anatomical sacral slope (the angle formed by the straight line of the S1 superior endplate and a line at a right angle to the anterior pelvic plane) were determined by analyzing plain radiographs. The correlations were examined by Pearson’s correlation coefficients, and intra- and inter-rater intraclass correlation coefficients were evaluated for reliability. RESULTS: A strong correlation was observed between pelvic incidence and anatomical sacral slope (r = 0.725, p < 0.001). In addition, the correlation between anatomical sacral slope and lumbar lordosis was similar to that between pelvic incidence and lumbar lordosis (r = 0.661, p < 0.001, and r = 0.554, p < 0.001, respectively). The intra-rater intraclass correlation coefficient values were 0.869 and 0.824 for anatomical sacral slope and pelvic incidence, respectively. Furthermore, the inter-rater intraclass correlation coefficient values were 0.83 and 0.685 for anatomical sacral slope and pelvic incidence, respectively. CONCLUSIONS: We observed that the strong correlation between anatomical sacral slope and pelvic incidence in patients with developmental dysplasia of the hip was equal to that in normal healthy subjects. The correlation between anatomical sacral slope and lumbar lordosis was equal to that between pelvic incidence and lumbar lordosis. Additionally, the intraclass correlation coefficient values for the anatomical sacral slope were slightly higher than those for pelvic incidence. Thus, we conclude that anatomical sacral slope can be considered as a helpful anatomical pelvic parameter that is a substitute for pelvic incidence not only in normal healthy subjects, but also in patients with developmental dysplasia of the hip.