The L1 spino-pelvic (L1SP) angle: a simplified approach for the assessment of the PI-LL mismatch in hip surgery.

Introduction: Pelvic incidence - lumbar lordosis (PI-LL) mismatch is often considered when assessing spinopelvic alignment in the sagittal plane. The mismatch is conventionally obtained by measuring 2 separate angles on lateral spinopelvic radiographs. This study describes a simplified approach for assessing spinopelvic mobility and measuring the PI-LL mismatch through the evaluation of the L1-spinopelvis angle (L1SP).

Methods: 96 standing lateral radiographs were obtained from consecutive patients presenting for total hip arthroplasty between November 2020 and July 2021. 3 operators were recruited to annotate landmarks on digital radiographs. Correlation analysis and error analysis were applied. Measurement reproducibility was assessed using intraclass correlation coefficient (ICC).

Results: The correlation coefficients of the 3 variables were respectively 0.87 for PI, 0.94 for LL, and 0.96 for L1SP. The normalised root mean square error between the 2 measurement sets was 9.96% for PI, 5.97% for LL, and 4.41% for L1SP. The absolute error was 3.49° ± 4.63° for PI, 3.23° ± 3.78° for LL, 2.68° ± 3.19° for PI-LL conventional, and 2.35° ± 2.88° for PI-LL via L1SP, respectively. In terms of reproducibility, measurement of L1SP outperformed that of PI and LL (ICC = 0.97 versus 0.83 and 0.93, respectively).

Conclusion: The simplified L1SP method, through the measurement of a single angle, produced similar measurements to the conventional PI-LL method. The measurement repeatability between operators was improved using the L1SP method. From a clinical practice perspective, both methods are equivalent. The new method is readily reproducible using commercially available PACS software during preoperative templating.