Correlation between the distance from the pubic symphysis to the sacrum with pelvic incidence

Introduction: The distance between the pubic symphysis and sacrococcygeal joint has been noted as an important parameter in evaluating pelvic tilt in an anteroposterior (AP) radiograph. Similarly, pelvic incidence measures the sagittal balance of the pelvis and is influenced by pelvic rotation. The relationship between these 2 parameters is unknown and could affect interpretation of pelvic AP radiographs. Methods: We reconstructed 248 cadaveric pelvises. Pelvic incidence was measured using a previously validated method. Pubic symphysis-sacrococcygeal joint (PSS) height was measured from the superior pubic tubercle to the sacrococcygeal joint. The pelvises were positioned so that the anterior pelvis lay flush with the surface. A ruler was zeroed on the pubic tubercles and a transverse projecting laser was used to measure the height to the sacrococcygeal joint. Results: A total of 248 pelvises were reconstructed. Average age of death of the specimens was 33±6.0 years. 80% of the pelvises were male, 56% were Caucasian, and 44% African American. The mean PSS height was 2.2 ± 1.4 cm and mean PI was 44.3° ± 10.6°. Multiple regression analysis found PI and PSS height were not correlated (p = 0.144). Females had a larger PSS height than males (beta = 1.17, p < 0.001) and African Americans a larger PSS height than Caucasians (beta = 0.63, p < 0.001). Conclusions: This study provides useful information for clinicians in evaluating AP radiographs of the pelvis in that it supports the use of PSS height to judge the adequacy of a radiograph even in the context of abnormal pelvic incidence.