Optimizing the fusion level for lenke 5C adolescent idiopathic scoliosis: is the S-line a validated and reproducible tool to predict coronal decompensation?

PURPOSE: To validate the predictability of S-line in Lenke 5C patients.
METHODS: Lenke 5C patients with a minimum 2-year follow-up and with the lowest instrumented vertebra (LIV) at lower end vertebra were included. The S-line was defined as a line connecting the centers of concave-side pedicles of upper instrumented vertebra (UIV) and LIV on baseline films. The S-line tilt to right was defined as positive S-line status (S-line +) and tilt to left as S-line - status. Statistical analysis was performed between different subgroups.
RESULTS: Among the 92 patients, 69 patients had a left lumbar curve and 23 patients had a right lumbar curve. For left curves, the S-line + status had a significantly higher incidence of both proximal and distal decompensation. However, for right curves, the S-line - status was the risk factor. Thus, we modified the definition of S-line: The value of S-line tilt for right curves was opposite to that for left curves. Patients with modified S-line + showed a significantly higher incidence of both proximal and distal decompensation in Lenke 5C patients with both left and right curves (p < 0.001 and p = 0.010). In UEV group, patients with modified S-line + showed significantly higher incidence of proximal decompensation (P = 0.001). However, in UEV-1 group, the incidence of proximal decompensation was not statistically affected by modified S-line + (P = 0.281).
CONCLUSION: Modified S-line + is a validated risk factor that predisposed to post-operative coronal decompensation in Lenke 5C AIS patients. Selecting UIV at one level caudal to UEV could be a possible solution if the modified S-line was positive.