Usefulness of modified S-line for upper instrumented vertebra selection in adolescent idiopathic scoliosis Lenke type 2 curves

Abstract

No validated systems exist for selecting the upper instrumented vertebra (UIV) for optimal postoperative shoulder balance in Lenke type 2 adolescent idiopathic scoliosis (AIS). This study evaluated a new method for shoulder balance prediction using the modified Shinshu line (MSL) for UIV selection in AIS Lenke type 2 curves. Fifty-five consecutive AIS patients receiving posterior spinal fusion (PSF) for a Lenke type 2 AIS curve were retrospectively analyzed according to several UIV determination models. Shoulder imbalance was judged as absolute radiographic shoulder height ≥ 10 mm at the 2-year observational endpoint. The MSL was the line between the center of the spinous process of C7 and that of the lowest instrumented vertebra. The vertebral body first touched proximally by the MSL was defined as the MSL vertebra (MSLV) and recommended as the UIV. The group with the UIV matching the MSLV had a significantly lower prevalence of shoulder imbalance of 23% (odds ratio 4.08, 95% CI 1.22-13.7, P = 0.02). Setting the MSLV as the UIV in PSF for AIS Lenke type 2 may reduce the prevalence of postoperative shoulder imbalance.

Figure 1

The MSL is the line connecting the center of the spinous process of C7 with that of the LIV. The vertebral body first touching the MSL proximally is defined as the MSLV. (A) A relatively mild PT curve. (B) For severe PT curves, the line may deviate from the vertebral column, in which case we redefined the MSLV as the most proximal vertebral body first touching the MSL. PT proximal thoracic, LIV lowest instrumented vertebra, UIV upper instrumented vertebra, MSL modified Shinshu line, MSLV MSL vertebra.