Lowest instrumented vertebra selection by the proportion of vertebral body area within the Harrington stable zone in Lenke type 1A adolescent idiopathic scoliosis: a self-paired comparison with the last substantially touched vertebra

Abstract Purpose The last substantially touched vertebra (LSTV) rule for selecting the lowest instrumented vertebra (LIV) in Lenke 1A adolescent idiopathic scoliosis (AIS) considers only the relation between the centre sacral vertical line and the pedicles. We evaluated a criterion taking the LIV as the most cephalad vertebra below the apex with at least 50% of its projected body area within the Harrington stable zone (half-area rule). Methods Seventy-nine consecutive patients with Lenke 1A AIS (mean age 14.25 ± 1.94 years) underwent posterior pedicle screw fusion (2015–2023) with the LIV chosen by the half-area rule (minimum follow-up 24 months). The LSTV was identified retrospectively on the same standing radiographs (self-paired design). Radiographic parameters were measured preoperatively, at 1 month and at 2 years; the rules were compared with paired tests, and distal adding-on and associated factors were assessed. Results The main thoracic curve improved from 54.24° ± 13.55° to 16.53° ± 7.56° at 2 years (correction 69.60%) and coronal balance from 13.49 ± 9.24 to 8.44 ± 6.28 mm (P < 0.001). The half-area rule selected the same level as the LSTV in 39.2% of patients and one level more cephalad in 60.8% (mean 0.61 ± 0.49 levels, P < 0.001) and never distal. Adding-on occurred in 11 patients (13.9%) with no revisions. Conclusion The half-area rule achieved satisfactory correction and coronal balance with a low adding-on rate while sparing one fusion level in 60.8% of patients, offering a quantitative adjunct read directly from standing radiographs.