The inferior sacral iliac line as a horizontal reference plane in paediatric pelvic radiographs

While often used, Hilgenreiner's line may not always be a reliable reference plane following triple innominate pelvic osteotomy or trauma to the triradiate cartilage. The inferior sacral-iliac line is a horizontal line connecting the sclerotic corners of the inferior sacral-iliac joints. This is a consistent landmark in the ossifying infant pelvis as well as the fully developed adult pelvis. The goal of this study was to determine if there is a difference in measurements between the inferior sacral-iliac line and Hilgenreiner's line in order to identify an alternative horizontal reference plane. 3 reviewers (fellowship-trained paediatric orthopaedic surgeon, junior orthopaedic resident, senior orthopaedic resident) reviewed 100 paediatric non-osteotomized pelvises. The difference between Hilgenreiner's line and the inferior sacral-iliac line were measured and reported. Measurements were repeated a minimum of 48 hours apart, resulting in 600 comparisons between Hilgenreiner's line and the inferior sacral-iliac line. The standard error of measurement was calculated to assess the variation in measurements between each individual observer and the group as a whole. The mean standard error of measurement between Hilgenreiner's line and the inferior sacral-iliac line was 0.44° (95% CI, ± 0.86). Reviewers 1-3 demonstrated a mean standard error of measurement of 0.38, 0.28, and 0.35 (95% CI, ± 0.74, 0.55, and 0.86) respectively. There was no statistically significant difference between reviewers ( > 0.05). Intra-observer reliability for reviewers 1, 2, and 3 was 0.64 (0.47-0.76), 0.75 (0.63-0.83), and 0.54 (0.32-0.69) respectively, with an inter-observer reliability of 0.42 (0.20-0.60) degrees. In this proof-of-concept study, the inferior sacral-iliac line was found to be an appropriate alternative to Hilgenreiner's line that is easily identifiable for all levels of orthopaedic training. Future inferior sacral-iliac line studies should demonstrate the reliability of multiple acetabular measurements, both pre- and post-osteotomy.