Does the upper line of the sacral ala approximate a horizontal line on pelvic radiographs of idiopathic scoliosis?

Abstract

Background: Lower instrumented vertebra (LIV) in adolescent idiopathic scoliosis (AIS) corrective surgery is selected using the center sacral vertical line on supine side-bending radiographs. However, a horizontal line reference is not possible on supine radiographs. Therefore, we aimed to determine the pelvic reference line that best reflects the horizontal line, which can be used in determining the LIV on supine side-bending radiograph in patients with AIS.

Methods: Patients with AIS (n = 258) were evaluated. On whole-spine standing anteroposterior radiographs, three lines were selected (the upper sacral line [USL], sacroiliac joint line [SIL], and upper iliac line [UIL]) and compared with the horizontal line to investigate tilt angles. Patients were allocated into thoracic (T; n = 90), lumbar (L; n = 61), and double (D; n = 107) curve groups based on thoracic and lumbar Cobb angles.

Results: The mean USL; SIL; and UIL tilts were 2.3°, 4.7°, and 3.8°; 1.6°, 2.0°, and 2.0°; and 1.4°, 2.2°, and 1.8° in the T, L, and D groups, respectively. The USL was significantly tilted in all groups, without significant difference between the SIL and UIL tilts. The USL tilted significantly more from the horizontal line in the L and D groups than in the T group; the UIL tilted more in the L group compared with the T group. The SIL tilt was comparable between the groups. The Cobb angle on the thoracolumbar or lumbar curve significantly, albeit weakly, correlated with the USL and UIL tilt.

Conclusions: On whole-spine standing anteroposterior radiographs, the USL had the greatest tilt (mean: 3.5°), whereas the SIL had the smallest tilt (mean: 1.8°) without significant variation across scoliosis types. Independent of Cobb angle and scoliosis type, the SIL was the most stable and reliable reference line for horizontal alignment, making it the preferred reference to LIV selection in patients undergoing AIS corrective surgery.