Clinical Utility of the Pelvic Inclination Angle in Surgically Treated Adult Spinal Deformity

Retrospective cross-sectional study.

To evaluate the clinical utility of the pelvic inclination angle (PIA) in surgically treated patients with adult spinal deformity (ASD) by examining its relationship with sagittal alignment parameters and patient-reported outcomes.

Pelvic tilt (PT) is commonly used to assess pelvic compensation in ASD but is influenced by pelvic incidence (PI), limiting its interpretability across patients with different pelvic morphology. The pelvic inclination angle (PIA) has been proposed as a PI-independent parameter; however, its clinical relevance in surgically treated ASD patients remains unclear.

Consecutive patients who underwent ≥6-level fusion surgery including the pelvis were retrospectively reviewed. Sagittal parameters, including PIA, PI-lumbar lordosis (PI-LL), PT, T1 pelvic angle (T1PA), and sagittal vertical axis (SVA), were measured. Clinical outcomes were assessed using the Oswestry Disability Index (ODI). Correlation analyses were performed between sagittal parameters and clinical outcomes, with Fisher's Z-test used for comparison. Linear regression analysis was used to derive outcome-based PIA thresholds.

A total of 211 patients (mean age, 69.6 y; 88.2% female) were included. Postoperative PIA showed strong correlation with PT and moderate correlations with PI-LL and T1PA, while remaining independent of PI. PIA demonstrated a stronger association with postoperative ODI scores (r=0.43) than conventional sagittal parameters. Changes in PIA were also more closely associated with ODI improvement than changes in other parameters. Outcome-based analysis identified PIA thresholds of 8.0° and 12.0° corresponding to mild and severe disability. Patients with postoperative PIA <8.0° achieved significantly better clinical outcomes and higher rates of minimum clinically important difference.

PIA is a PI-independent parameter that reflects both radiographic deformity severity and postoperative disability in surgically treated ASD patients. Outcome-based PIA thresholds may provide a clinically useful reference for assessing pelvic compensation in ASD surgery.