A critical thoracic kyphosis is required to prevent sagittal plane deterioration in selective thoracic fusions in Lenke I and II AIS.

Thoracic hypokyphosis following AIS correction may be associated with reduced lumbar lordosis with potential adverse effects on the global sagittal balance. In the present study, we were interested in how the amount of thoracic kyphosis influences the sagittal profile and balance in selective thoracic (STF) and thoracolumbar fusions. Out of 154 patients, 86 patients had correction of AIS Lenke I or II with a side-loading pedicle screw system and completed a 2-year follow-up. Patient factors such as age, Risser grade, lowest and upper instrumented vertebra, and lumbar modifier were recorded. Coronal Cobb and sagittal parameters were measured using Surgimap. Statistical analysis according to distributions and multiple linear and logistic regressions was performed using STATA for Mac v13. In STF, logistic regression against post-operative change in SVA versus thoracic kyphosis allowed calculation of a critical thoracic kyphosis of 23° (ROC AUC 0.65, spec 0.70, sens 0.63), below which deterioration of the sagittal vertical axis is more likely (PPV 71.4%). Patients with hypokyphosis exhibited an increase in the SVA (pre-operative 7.2 ± 37.1 mm vs. 23.1 ± 27.6 mm at 2 years, p = 0.0164), whereas it was maintained from pre-operative to 2 years post-operative if thoracic kyphosis is above 23° (pre-operative 2.5 ± 28.9 mm vs. 5.4 ± 26.9 mm at 2 years, p = 0.579). A critical thoracic kyphosis of 23° and more should be aimed for in hypokyphotic patients to potentially avoid post-operative sagittal plane deterioration with mechanical and likely also clinical consequences. These slides can be retrieved under Electronic Supplementary Material.