Introduction: The S2 alar iliac screw (S2AIS) is used in the corrective surgery of adult spinal deformities. It provides biomechanical support and acts as a strong caudal anchor; however, it passes through the sacroiliac joint, which has a range of motion, and its effect on the joint is unclear. The sacroiliac joint has rotational movements; therefore, it is difficult to stabilize by just one screw and S2AIS loosening might be inevitable. In this study, we investigated the effect of S2AIS loosening on the sacroiliac joint.

Methods: This retrospective study included 38 patients who underwent corrective surgery from the thorax to the sacrum for adult spinal deformity and at least 2 years of follow-up. S2AIS loosening was assessed via the radiolucent zone of computed tomography. The patients were divided into two groups (loosening and nonloosening groups), and the two groups were compared in terms of the sagittal parameters of standing radiographs, SRS-22, arthritic change, and pain in the sacroiliac joint.

Results: S2AIS loosening was observed in 32 (42%) patients. There was no significant difference between the two groups in terms of full spine standing radiographs and SRS-22. Arthritic change was observed in five joints (11%) in the nonloosening group and five joints (6%) in the loosening group. Sacroiliac joint pain was observed in 5 joints (11%) in the nonloosening group and 4 joints (13%) in the loosening group.

Conclusions: Loosening of the S2AIS occurred in 42% of the patients. There was no difference between the two groups in terms of the occurrence of arthritic changes or joint pain in the sacroiliac joint. Loosening of the S2AIS does not cause arthritic changes and joint pain in the sacroiliac joint for a minimum of 2 years after adult spinal deformity corrective surgery.