Influence of pelvic rotation on pelvic incidence, pelvic tilt, and sacral slope.

Abstract

Radiographical laboratory study. To analyze the influence of pelvic rotation on pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS) measurements and to present a geometrical method of calculating the angle of pelvic rotation on lateral radiographs. PI, PT, and SS may potentially be dependent on the axial rotation of the pelvis while acquiring the radiograph. However, no study investigating this problem has been published. One radiological pelvic phantom was used to obtain 1 anteroposterior and 10 lateral calibrated radiographs with axial rotation of the pelvis from 0° to 45° at 5° intervals. PI, PT, and SS were measured. The maximal angle of rotation that changed PI, PT, or SS measurements of less than 6° (compared with these measured on the radiograph with rotation of 0°) was considered as acceptable.Linear distance between the centers of the femoral heads (A) was measured on the anteroposterior radiograph. Horizontal distances between the centers of the femoral heads (B) were measured on lateral radiographs. The angles of rotation (α) of the pelvis on each radiograph were calculated: α= arcsinB/A. Agreement between the measured and the calculated angles of rotation of the pelvis was assessed. Intra- and interobserver reliability of the proposed method of calculations were tested. The maximal acceptable angle of rotation of the pelvis was 30°. There was an excellent agreement between the measured and the calculated angles of rotation of the pelvis (intraclass correlation coefficient = 0.99; median error for a single measurement [SEM] = 0.3°). Excellent intra- and interobserver reliability of the method was revealed (intraclass correlation coefficient = 0.99 with SEM = 0.5° and intraclass correlation coefficient = 0.99 with SEM = 0.7°). PI can be influenced by rotation of the pelvis. The acceptable maximal angle of rotation for reliable measurements of PI, PT, and SS was 30°. The angle of rotation of the pelvis on the lateral radiograph can be reliably calculated. 3.