Lumbar curve correction is durable at 5 years after thoracic fusion in patients with open triradiate cartilage: a cohort comparison to mature patients

Purpose: To evaluate the behavior of the unfused lumbar spine 5 years after thoracic fusion (TF) in adolescent idiopathic scoliosis (AIS) patients with open triradiate cartilage (OTRC) compared to patients with closed triradiate cartilage (CTRC). Methods: A multicenter AIS database was queried for patients who underwent posterior TF (lowest instrumented vertebra L1 or cephalad) for a primarily thoracic curve (i.e., Lenke 1–3), with a B or C lumbar modifier. Patients were grouped by skeletal maturity into OTRC and CTRC (Risser 4 or 5) cohorts and compared for radiographic outcomes, inclinometer measurements, and SRS-22 scores. Radiographic measures included thoracic curve magnitude (TCM), thoracic kyphosis, lumbar curve magnitude (LCM), lumbar apical vertebral translation (AVT), coronal balance, and adding on. Generalized estimating equation models were utilized to estimate and evaluate differences in means. Results: The OTRC group included 33 patients (mean age 11.9 years). The CTRC group included 131 patients (mean age 15.3 years). LCM decreased from preoperative to the first postoperative visits in both CTRC and OTRC groups, with no further change at 1-year and 5-year postoperative in both groups. Lumbar AVT improved from 1-year to 5-year postoperative in both groups. Coronal balance deteriorated immediately postoperatively in both groups but improved by 1 year and remained stable through 5 years. In all postoperative periods, there was no difference between OTRC and CTRC groups in LCM, lumbar AVT, coronal balance, and SRS-22 scores. Thoracic and lumbar rotation via inclinometer improved comparably in both groups. The OTRC group had a higher rate of distal adding-on (48.5% vs 22.1% at 5 years), but this was not associated with worse coronal balance, reoperation, or SRS-22 scores. Conclusion: TF in younger adolescents with OTRC results in similar balanced spine with maintained lumbar correction at 5-year follow-up compared to more skeletally mature patients and remains a reasonable surgical option for compensatory lumbar modifier B and C curves.