Pelvico-Sacro-L1 Angle: An Index for Rapid Evaluation of Spinopelvic Sagittal Balance.

Objectives: This study aimed to evaluate the Pelvico-Sacro-L1 angle (PSLA) as a direct parameter for assessing PI-LL mismatch as a key determinant of sagittal balance. Methods: In this retrospective study, adult patients scheduled for total hip arthroplasty (THA) underwent spinopelvic lateral radiography. The pelvic sagittal line angle (PSLA) was defined as the angle between a line drawn from the center of the femoral head to the center of the superior endplate of S1 and the superior endplate of L1. The deviation from 90° was termed the modified PSLA, proposed as an alternative to the pelvic incidence-lumbar lordosis (PI-LL) mismatch. Two independent orthopaedic surgeons measured the spinopelvic parameters and repeated the assessments four weeks later to evaluate intraobserver reliability. Results: A total of 92 patients (average age: 57.4 ± 9.9 years, 50 females) were included. The Bland-Altman plot indicated no significant systematic difference between the two measurement sets, suggesting good agreement between methods. The intraclass correlation coefficient (ICC) for PSLA in both inter- and intraobserver reliability was greater than 0.9, indicating excellent agreement. Conclusion: The PSLA demonstrated a strong agreement with the PI-LL mismatch, confirming the reliability of this approach, which provides a more straightforward and easily interpretable measurement.