Lowest Instrumented Vertebra Selection in Adolescent Idiopathic Scoliosis With Concomitant Spondylolysis

Study DesignRetrospective Cohort Study.ObjectivesAdolescent idiopathic scoliosis (AIS) with a concomitant pars defect poses the surgical challenge of balancing stress avoidance at the lytic segment, deformity correction, and mobility preservation. Data on lowest instrumented vertebra (LIV) selection are limited. This study aimed to identify a safe LIV in posterior spinal fusion (PSF) to minimize pain and slip progression.MethodsRetrospective review of AIS patients (10-18 years) with spondylolysis or spondylolisthesis who underwent PSF (2016-2023) with ≥ 2-year follow-up. Variables included demographics, curve characteristics, Meyerding grade, LIV selection, and mobile segments between LIV and lytic level. Primary outcome was back pain (VAS). Secondary outcomes included slip progression, subjacent curve, mechanical complications, and revision. Between-group comparisons used Mann-Whitney U; regression assessed effects of mobility preservation and subjacent curvature on pain.ResultsOf 462 AIS patients, 29 met inclusion criteria (mean age 15.5 ± 1.5 years; 24% spondylolisthesis). Fusion ended at or above L3 in 76% and below in 24%. Follow-up VAS pain was higher in patients with <3 mobile segments between LIV and lytic level (4.5 ± 1.4 vs 2.8 ± 1.5; <i>P</i> = .003) and in fusions extending below L3 (5.0 ± 1.3 vs 3.0 ± 1.4; <i>P</i> = .004). Regression showed each additional mobile segment decreased pain (β = -0.77, <i>P</i> = .001), while greater subjacent curvature increased pain (per 10°, β = 0.92, <i>P</i> = .043). Two patients progressed to grade 1 slip; three were indicated for revision with distal junctional failure or persistent pain.ConclusionsIn AIS with coexisting spondylolysis, selecting LIV proximal to L3 minimizes pain while preserving mobile segments, without excessive slip progression.