The Fate of the Unfused Lumbar Curve After Thoracolumbar Scoliosis Correction in Children Versus Adults

BACKGROUND AND OBJECTIVES: The unfused lumbar curve (ULC) distal to a thoracolumbar fusion may respond differently in pediatric vs adult scoliosis patients because of age-related differences in flexibility and compensatory capacity. However, no previous study has directly compared age-related postoperative ULC behavior between these cohorts to better understand differences in coronal balance restoration. METHODS: Four hundred fifty-one patients undergoing thoracolumbar fusion for idiopathic scoliosis with a thoracic upper instrumented vertebra and lower instrumented vertebra between L1-L4 were included: pediatric aged 8 to 17 years (n = 346), and adults aged 18 years and older (n = 105). Pediatric ULC (pULC) and adult ULC (aULC) were defined as coronal Cobb angle between lower instrumented vertebra and S1, if >10°. Radiographic parameters and outcomes (Scoliosis Research Society-22, mechanical complications) were assessed preoperatively, 6 weeks (early), and 2 years (late). Global coronal malalignment (GCM) was central sacral vertical line distance. RESULTS: Pediatric (14.9 ± 2.2 yo) and adult (34.5 ± 15.1 yo) patients had similar major curve magnitude and correction (pediatric: preop 57.7° ± 12.8° vs 55.3° ± 17.6°, 6-week 20.3° ± 7.5° vs 19.9° ± 12.4°; P > .2). Preoperative pULCs were larger (22.8° ± 6.5° vs 19.9° ± 7.3°; P < .001) and experienced greater correction at 6 weeks (Δ18.2° vs Δ12.9°; P < .001). pULC and GCM changed significantly at 6 weeks and remained stable at 2 years, whereas adult GCM changed over 2 years (P < .001). Age was associated with lower preoperative ULC and higher residual ULC at 6 weeks and 2 years (all P < .001). pULC remodeled directionally to improve GCM early postoperatively (P = .03), whereas aULCs remodeled to improve GCM late postoperatively (P = .037). Larger residual pULC was associated with lower Scoliosis Research Society-22 pain and function; residual pULC and aULC were not associated with mechanical complications (P = .21) or reoperations (P = .06). There were few caudal extension fusions (3 adults, 0 children) in this cohort. CONCLUSION: Pediatric patients experience greater magnitude correction of the ULC compared with adults after thoracolumbar fusion for scoliosis. ULC remodels to improve GCM early in children and late in adults. Residual ULC magnitude was not associated with mechanical complications.