Posterior fusion strategy of lowest instrumented vertebra at the apical vertebra of the lumbar curve in Lenke type C adolescent idiopathic scoliosis

Abstract

Objective To investigate the radiographic risk factors related to the occurrence of distal adding-on (AO) in posteriorly treated Lenke modifier C adolescent idiopathic scoliosis (AIS) patients with the apical vertebra of the lumbar curve (L- AV) selected as the lowest instrumented vertebra (LIV). Methods Seventy-three Lenke modifier C AIS patients were analyzed with a minimum of 2-year follow-up after posterior spinal fusion surgery with L-AV selected as LIV. Patients were grouped according to the occurrence of distal AO. Radiographical parameters were measured as follows: Cobb angle, curve flexibility and AV translation of the thoracic curve and lumbar curve, L-AV rotation and tilt, coronal balance, Harrington stable zone on anteroposterior (AP) film and concave bending film, L-AV derotation and L-AV/AV+1 disc opening or closing on convex bending film, etc. The Scoliosis Research Society-22 (SRS-22) score was used to evaluate clinical outcomes. Radiographic and clinical parameters were statistically analyzed between the two groups. Results There were 23 patients in AO group and 50 patients in non-AO group. Preoperatively, the AO group had proximal L-AV, lower flexibility of the thoracic curve, coronal imbalance shifted to the convex side of the lumbar curve, lower Harrington stable zone on AP film and concave bending film, and less L-AV/AV+1 disc opening on convex bending film compared to non-AO group. The logistic regression revealed that the flexibility of the thoracic curve, coronal balance, Harrington stable zone on concave bending film, and L-AV/AV+1 disc opening or closing on convex bending film were significant predictors of distal AO. Specifically, the flexibility of the thoracic curve >40.0%, coronal balance< 19.6mm, and Harrington stable zone on concave bending film >77.8% might be optimal thresholds for selecting L-AV as LIV. At the final follow-up, AO group had larger lumbar curves and lower correction rates. No difference was found in the SRS-22 between the two groups. Conclusion For Lenke modifier C AIS patients, LIV might be considered to stop at L-AV if there were good flexibility of the thoracic curves, coronal balance, L-AV/AV+1 disc opening on convex bending film, and large Harrington stable zone on concave bending film.