Hyper‐Selective Posterior Fusion is Recommended When the Modified S‐Line is Positive in Lenke 5C Adolescent Idiopathic Scoliosis

Objective Postoperative coronal decompensation and less fusion level are dilemmas and the proper selective posterior fusion (SPF) strategy should be investigated. We proposed a parameter, modified S‐line, and aimed to investigate if the modified S‐line could predict postoperative coronal decompensation in patients with Lenke 5C adolescent idiopathic scoliosis (AIS). Methods This is a retrospective radiographic study and Lenke 5C AIS patients undergoing SPF during the period from September 2017 to June 2021 were included. The modified S‐line was defined as the line linking the centers of the concave‐side pedicles of the upper end vertebra (UEV) and lower end vertebra (LEV) at baseline. A modified S‐line tilt to the right is established as modified S‐line+ (UEV being to the right of the LEV). The patients were further categorized into two groups: the Cobb to Cobb fusion group and the Cobb‐1 to Cobb fusion group. Outcomes including thoracic Cobb angle, TL/L Cobb angle, coronal balance, upper instrumented vertebra (UIV) translation, lower instrumented vertebra (LIV) translation, UIV tilt, LIV tilt, LIV disc angle, thoracic apical vertebral translation, lumbar apical vertebral translation (L‐AVT), L‐T AVT ratio, L‐T Cobb were measured at baseline, immediately after surgery, and the last follow‐up. Radiographic parameters and the incidence of both proximal and distal decompensation between the two groups were compared by chi‐square test. Results Among 92 patients, 48 were modified S‐line+ and 44 were modified S‐line−. Modified S‐line+ status was identified as a risk factor for postoperative proximal decompensation (p = 0.005) during follow‐up. In Cobb to Cobb group, a higher occurrence of proximal decompensation in individuals with modified S‐line+ status (p = 0.001) was confirmed. Also, in the Cobb to Cobb group with baseline modified S‐line+ status, patients presenting decompensation showed a significantly larger baseline of the UIV tilt and postoperative disc angle below the lower instrumented vertebra. However, In Cobb‐1 group, the incidence of decompensation after surgery showed no association with baseline modified S‐line tilt status (p = 0.815 and 0.540, respectively). Conclusion The modified S‐line could serve as an important parameter in surgical decision‐making for Lenke 5C AIS patients. Cobb to Cobb SPF is not recommended with a modified S‐line+ status, and the Cobb‐1 to Cobb fusion may serve as a potential alternative.