Analysis of the sacroiliac joint vacuum phenomenon in adolescent thoracic idiopathic scoliosis (Lenke types 1 and 2)

The sacroiliac joint (SIJ) is the largest axial joint in the human body, and the SIJ vacuum phenomenon (SIJ VP) is a common finding in computed tomography studies of the abdomen, pelvis, and lumbosacral spine in adults, with the incidence increasing with age. Adolescent idiopathic scoliosis (AIS) is an abnormal spinal curvature that appears during adolescence and places abnormal stress on the SIJs. This retrospective observational study aimed to investigate the incidence of the SIJ VP in thoracic AIS (Lenke types 1 and 2). Sixty-seven patients with AIS (age: 12–19 years) and 76 controls (age: 11–19 years) were retrospectively analyzed to investigate SIJ VP, subchondral bone cysts, and SIJ degeneration (Eno classification: type 0, no degenerative change; type 1, mild degenerative changes; type 2, substantial degenerative changes; and type 3, ankylosis). SIJ degeneration was defined as type ≥ 2. The association between SIJ VP, cysts, SIJ degeneration, and sagittal/coronal spinopelvic alignment was assessed. SIJ VP (59% vs. 35.5%, P < .01), cysts (32.8% vs. 1.3%, P < .01), and SIJ degeneration (3.2% vs. 2.6%, P = .823) differed significantly between the 2 groups. There were 0 cases of SIJ ankylosis (Eno classification type 3) in both groups. The VP was not correlated with lumbar lordosis, sacral slope, or Cobb angle. All lumbar modifier type C belonged to the VP present group, whereas none to VP absent group. Our results suggest an association between AIS and SIJ VP and SIJ cysts. SIJ VP and SIJ cysts in AIS may be caused and accelerated by abnormal mechanical stress on SIJ due to spinal deformity.