Background: There is a paucity of studies on the iliac curvature in developmental dysplasia of the hip (DDH). Here, we examined the iliac curvature in DDH using three-dimensional computed tomography. Methods: We allocated cases with a center-edge angle of < 20° to the DDH group (55 cases) and cases with a center-edge angle of > 25° to the control group (57 cases) and measured the straight line (line A) between the anterior and posterior superior iliac spines. We examined which part of the iliac bone line A passes through and classified the results into 4 categories (type A, inside the iliac bone; type B, through the iliac bone; type C, outside the iliac bone; and type D, both inside and outside the iliac bone) to evaluate the iliac wing curvature. After measuring the area and internal surface of the iliac wing using line A, we examined the correlation between these values, the interspinous distance, the superior iliac angle, and the center-edge angle. Results: Distributions of the four types were compared between the two groups; there was no significant difference. The length of the portion of line A inside the ilium and the area formed by line A and the iliac wing, which shows the degree of iliac wing curvature, were not significantly different between the groups. There were no correlations between these values and the center-edge angle; however, there were weak positive correlations among the interspinous distance, the superior iliac angle, and the centeredge angle. Conclusions The inward nature of the iliac bone in patients with DDH is mainly due to the internal rotation of the entire iliac bone and less likely due to the curvature of the iliac bone.

Background: Patients with developmental dysplasia of the hip (DDH) are known to have abnormal pelvic morphologies; however, rotation of innominate bone features remains unclear. Thus, we investigated innominate bone rotation in patients with DDH by measuring the associated angles and distances using three-dimensional (3D) computed tomography. Methods: We defined four straight lines in pelvic 3D models: from the anterior superior iliac spine to the posterior superior iliac spine, from the anterior inferior iliac spine to the posterior inferior iliac spine, from the pubic tubercle to the ischial spine, and from the pubic tubercle to the ischial tuberosity. Similarly, we measured the angles formed by these lines using the vertical axis of the anterior pelvic plane on the horizontal plane and the horizontal axis on the sagittal plane. Additionally, we measured the distances between the femoral head centers and the acetabular centers in the coronal plane. Results: The difference in internal rotation angle between the superior and inferior parts of the iliac bone was significantly lower, by approximately 1.7°, in the DDH group than in the control group (p = 0.007); the difference between the inferior and superior parts of the ischiopubic bone was significantly higher, by approximately 1.5°, in the DDH group (p< 0.001). In the sagittal plane, the sum of the superior aspect of the iliac bone and the inferior aspect of the ischium was significantly lower in the DDH group (p = 0.001) than in the control group. The distances between the femoral heads and the acetabula were significantly greater in the DDH group than in the control group (p = 0.03, p < 0.01, respectively). Conclusions Patients with DDH had a more internally rotated ilium and ischiopubic bone than normal individuals; however, it should be emphasized that internal rotation was reduced near the acetabulum, and the acetabulum was shifted laterally. Similarly, it was shown that patients with DDH had different rotations of the ilium and ischiopubic bone in the sagittal plane.