Reliability and Validity Analysis of Pelvic Sagittal Inclination Calculated by Inverse Cosine Function Method on Pelvic Anteroposterior Radiographs

Objective: Evaluation of sagittal pelvic tilt is significant for hip surgeons. However, the accurate measurement of pelvic sagittal inclination (PSI) is still a challenge. The objective of this study is to propose a new method for measurement of PSI from pelvic anteroposterior radiograph based on the inverse cosine function obtained from individualized pelvic model. Methods: Collecting the imaging data of 30 patients with both pelvic CT and full-length spine radiographs. Establishing pelvic model by customized 3D reconstruction software. The length of three groups of longitudinal and transverse line segments (A0p and B0) were measured from full-length spine anteroposterior radiographs. The corresponding anatomical parameters, including A, B, b,.a,.., were measured and calculated on the same patient's pelvic model. The estimated PSI (ePSI) based on three groups of anatomical landmarks, including ePSI-1, ePSI-2, and ePSI-3, were calculated by equation, ePSI 1/4 arccos (A'p/ p*B) angle alpha, and compared with the actual PSI (aPSI) measured by Surgamap software. For the reliability and validation evaluation, three observers measured these parameters in two rounds. Intra-class correlation and inter-class correlation were both calculated. Bland-Altman method was used to evaluate the consistency between the estimated PSI (ePSI) and the actual PSI (aPSI). Results: ePSI-1 and ePSI-2 showed excellent intra-observer reliability (0.921-0.997, p < 0.001) and inter-observer reliability (0.801-0.977, p < 0.001). ePSI-3 had a fair inter-observer reliability (0.239-0.823, p < 0.001). ePSI-1 showed the strongest correlation with aPSI (r = 0.917, p < 0.001). Mean (maximum) absolute difference of ePSI-1, ePSI-2, and ePSI-3 is 2.62 degrees (7.42 degrees), 4.23 degrees (13.78 degrees), and 7.74 degrees (31.47 degrees), respectively. The proportion of cases with absolute difference less than 5 degrees in three groups were 86.7% (ePSI-1), 66.7% (ePSI-2), 56.7% (ePSI-3). Conclusion: This new method based on inverse cosine function has good reliability and validity when used in the evaluation of PSI on pelvic anteroposterior radiographs.