Zones where reduced implant density leads to correction loss after scoliosis surgery for Lenke 1A adolescent idiopathic scoliosis: a multicenter study

Purpose: Investigate zones where implant density should not be reduced in posterior spinal fusion (PSF) for adolescent idiopathic scoliosis (AIS) Lenke type 1A curves. Methods: 126 consecutive patients (118 female and 8 male; mean age: 15.1 ± 2.2 years) with Lenke type 1A AIS who underwent PSF using pedicle screw constructs were included. Correction loss which was calculated using immediately postoperative and 2-year postoperative Cobb angle. Implant density was assessed by dividing the instrumented levels into four zones each on the concave and convex sides. The risk factors for significant correction loss were examined using logistic regression analysis. For convex apical zone, correction loss was compared among the three groups of low (0–59%), medium (60–99%), and high (100%) implant density. Results: Multivariate analysis revealed the apical zone of the convex side (Odds ratio [OR] 1.27; 95% confidence interval [CI] 1.01–1.59; P = 0.04) and the peri-apical zone of the convex side (OR 1.33; 95% CI 1.11–1.59; P = 0.002) as independent predictors of significant correction loss. In the convex apical zone, the median (interquartile range) correction loss of the low implant density, medium implant density, and high implant-density groups was 4.8° (1.5°), 5.3° (0.8°), and 2.2° (0.3°), respectively. The median difference was 2.6° (P = 0.048) between the low implant density and high implant-density group and 3.1° (P < 0.001) between the medium implant density and high implant-density group. Conclusion: In PSF for AIS Lenke 1A, low implant density in the convex apical zones were significant factors affecting correction loss at 2 years postoperatively. However, the difference in correction loss between groups may not be large enough to consider clinically meaningful. Prospective studies of longer term outcomes are needed to determine whether these results are clinically important.