Pelvic obliquity in adolescent idiopathic scoliosis planned for posterior spinal fusion: A preoperative analysis of 311 lower limb axis films

To analyze the incidence, pattern, and contributing factors of pelvic obliquity among Adolescent Idiopathic Scoliosis (AIS) patients who will undergo surgery. 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. 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. 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