The selection of the lowest instrumented vertebra (LIV) is crucial in adolescent idiopathic scoliosis (AIS) surgery to optimize functional and radiological outcomes. Despite established guidelines, the selection strategies vary widely among surgeons. This study aimed to assess how often prone uninstrumented intraoperative frontal radiographs influence LIV selection. The main hypothesis is that uninstrumented intraoperative radiographs lead to a more conservative selection of the LIV level during adolescent idiopathic scoliosis surgery.

A total of 139 patients undergoing posterior spinal fusion for AIS were prospectively analyzed. Preoperative fusion levels were determined, by the staff, six months before surgery using 3D EOS low dose stereoradiography (standing and lateral bending) and supine frontal radiographs. The final LIV chosen during surgery was recorded and compared to the planned one. Frontal radiological parameters (preoperative and intraoperative) were compared between the group “no change” (NC) and the group “change” (C), in which the surgeon changed his decision based on the intraoperative radiograph.

The final LIV matched the preoperative plan in 57% of cases, but changes occurred in 43%, particularly for Lenke 1A curves (47.5%). Two radiographic parameters significantly differed between groups: LIV-CSVL (Central Sacral Vertical Line) (p = 0.04) and LIV + 1-CSVL (p = 0.02). Logistic regression indicated Lenke lumbar modifiers A and B significantly increased the likelihood of intraoperative LIV changes (p = 0.008 and p = 0.01, respectively).

Prone uninstrumented intraoperative frontal radiographs substantially impact LIV selection in AIS surgery, especially in Lenke 1A and B curves where selective thoracic fusion is considered.

III; retrospective cohort.