Optimal measurement of sacral obliquity in adolescent idiopathic scoliosis-associated sacral deformity

Adolescent idiopathic scoliosis (AIS) presents with complex 3D spinal deformities frequently accompanied by pelvic asymmetry. Sacral obliquity (SO), reflecting sacral tilt on the coronal plane, is one of the relevant parameters with clinical significance. There are several methods for measuring SO, but no standardized approach has been established. The aim of this study was to determine the most appropriate method for measuring SO by analyzing the relationship between sacral vertebral morphology and SO across different spinal curve patterns.

Consecutive patients with AIS who underwent corrective surgery from 2013 to 2023 at a single institution were assessed. Cobb angles of the major curve and thoracolumbar/lumbar (TL/L) curve, L4 tilt, C7 plumb line to center sacral vertebral line, leg length discrepancy, SO, and iliac obliquity (IO) were measured as radiographic parameters. The S1 angle, S2 angle, and S1 base angle were measured as sacral morphological parameters on multiplanar reconstructed CT. The S1 and S2 angles were defined using the respective vertebral endplates, while the S1 base angle was defined using the transition point of the sacral wing and the superior articular process of the S1 vertebra as anatomical landmarks. Patients were classified into two groups according to Lenke classification: those with TL/L major curves (TL/L+, Lenke types 4-6) and those without TL/L major curves (TL/L-, Lenke types 1-3).

After exclusions, 128 patients (122 females, mean age 16.9 ± 4.0 years) were included in this analysis, with 35 in the TL/L+ group and 93 in the TL/L- group. The S1 angle was significantly greater than both the S1 base angle and S2 angle (mean 2.8° ± 2.5° vs 1.4° ± 1.5° and 1.4° ± 1.2°, respectively; p < 0.001). The TL/L+ group had a significantly greater mean S1 angle (4.6° ± 3.2° vs 2.1° ± 1.7°, p < 0.001), whereas no significant differences were observed in the S1 base angle and S2 angle. Multivariable analysis confirmed TL/L curve presence, TL/L Cobb angle, SO, S1 base angle, and S2 angle as independent determinants of an increased S1 angle.

The S1 endplate was the most reliable landmark for evaluating SO, particularly in patients with AIS with TL/L major curves. Given the presence of asymmetry within the S1 vertebra, use of the S1 base angle might underestimate the true degree of SO. These findings support the standardized use of the S1 endplate to enhance accuracy of SO assessment and improve precision in surgical planning and bracing strategies.