Pelvic Obliquity in Adolescent Idiopathic Scoliosis Planned for Posterior Spinal Fusion: A Preoperative Analysis of 311 Lower Limb Axis Films

Purpose: To analyze the incidence, pattern, and contributing factors of pelvic obliquity among Adolescent Idiopathic Scoliosis (AIS) patients who will undergo surgery. Methods: In total, 311 patients underwent erect whole spine anteroposterior, lateral and lower limb axis films. Radiographic measurements included Transilium Pelvic Height Difference (TPHD; mm), Hip Abduction-Adduction angle (H/Abd-Add; °), Lower limb Length Discrepancy (LLD; mm), and Pelvic Hypoplasia (PH angle; °). The incidence and severity of pelvic obliquity were stratified to Lenke curve subtypes in 311 patients. The causes of pelvic obliquity were analyzed in 57 patients with TPHD ≥10 mm. Results: The mean Cobb angle was 64.0 ± 17.2°. Sixty-nine patients had a TPHD of 0 mm (22.2%). The TPHD was <5 mm in 134 (43.0%) patients, 5–9 mm in 104 (33.4%) patients, 10–14 mm in 52 (16.7%) patients, 15–19 mm in 19 (6.1%) patients, and ≥20 mm in only 2 (0.6%) patients. There was a significant difference between the Lenke curve types in terms of TPHD ( p = 0.002). L6 curve types had the highest TPHD of 9.0 ± 6.3 mm followed by L5 curves, which had a TPHD of 7.1 ± 4.8 mm. In all, 44.2% of L1 curves and 50.0% of L2 curves had positive TPHD compared to 66.7% of L5 curves and 74.1% of L6 curves which had negative TPHD. 33.3% and 24.6% of pelvic obliquity were attributed to PH and LLD, respectively, whereas 10.5% of cases were attributed to H/Abd-Add positioning. Conclusions: 76.4% of AIS cases had pelvic obliquity <10 mm; 44.2% of L1 curves and 50.0% of L2 curves had a lower right hemipelvis compared to 66.7% of L5 curves and 74.1% of L6 curves, which had a higher right hemipelvis. Among patients with pelvic obliquity ≥10 mm, 33.3% were attributed to PH, whereas 24.6% were attributed to LLD.