Assessment of Lowest Instrumented Vertebra Tilt on Radiographic Measurements in Lenke ''C'' Modifier Curves Undergoing Selective Thoracic Fusion in Adolescent Idiopathic Scoliosis

Abstract Study Design Multicenter, retrospective cohort study. Objectives The purpose of this study is to determine how the amount of residual lowest instrumented vertebra (LIV) tilt correlates with radiographic measurements. Summary of Background Data When performing a selective thoracic posterior spinal fusion for adolescent idiopathic scoliosis (AIS), the LIV may be tilted into the lumbar curve or made horizontal. Methods This is a multicenter retrospective study of 33 consecutive patients with AIS, Lenke types 1 to 4, lumbar modifier C, and a minimum follow-up of 2 years, who underwent selective thoracic posterior spinal fusions. Measurements obtained from pre- and postoperative radiographs were correlated with postoperative LIV tilt. Results At final follow-up, less postoperative LIV tilt significantly correlated with less thoracic apical translation (p = .023) when controlling for the position of the LIV relative to the stable vertebra and preoperative thoracic and lumbar curve flexibility. LIV tilt was not significantly associated with thoracic Cobb angle, lumbar Cobb angle, lumbar apical translation, coronal balance, sagittal balance, or the amount of correction obtained compared to their preoperative measurements (p > .05). Conclusion Decreased LIV tilt was significantly associated with decreased thoracic apical translation. LIV tilt did not significantly correlate with coronal balance or any other radiographic measurement. We caution that these findings may only be applicable in C modifier curves and when the correct LIV is chosen. Level of Evidence Level III, Therapeutic study.