Selection of the Lowest Instrumented Vertebra and Relative Odds Ratio of Distal Adding-on for Lenke Type 1A and 2A Curves in Adolescent Idiopathic Scoliosis: A Systematic Review and Meta-analysis

Abstract

Objective: To examine existing literature and pool the data to determine the relative odds ratio of "adding-on" (AO) based on various reported criteria for lower instrumented vertebra (LIV) selection in Lenke type 1A and 2A curves.

Methods: Using electronic databases, studies reporting on AO and LIV selection in Lenke type 1A and 2A curves were identified. Studies were excluded if they failed to meet the following criteria: ≥ 30 patients, Lenke type 1A or 2A curves, thoracic-only fusions, and inclusion of outcome differences in AO and non-AO groups. Review articles, letters, and case reports were excluded.

Results: Six studies were identified reporting on 732 patients with either Lenke type 1A or 2A curves treated with thoracic-only fusions. Five different landmarks were used for LIV selection in these studies including the stable vertebra (SV) -1, end vertebra (EV) +1, neutral vertebra (NV), touched vertebra (TV), and substantially touched vertebra (STV) versus nonsubstantially touched vertebra (nSTV) +1. The pooled odds ratios of AO for choosing LIV at levels above the afore landmarks (i.e. , ending the construct "short") versus at the landmarks were 2.59 (SV-1), 2.43 (EV+1), 3.05 (NV), 3.40 (TV), and 4.52 (STV/nSTV+1), all at 95% confidence interval.

Conclusion: Five landmarks shared a similar characteristic in that the incidence of AO was significantly higher if the LIV was proximal to the chosen landmark. In addition, choosing STV/(nSTV+1) as the LIV have the lowest absolute risk of AO and the greatest risk reduction. If additional levels were fused (i.e. , LIV distal to the landmark), there was no statistically significant benefit in further reducing the risk of AO. Selection of the optimal LIV is a complex issue and spine surgeons must balance the risk of AO with the need for motion preservation in young patients.

Keywords: Adolescent idiopathic scoliosis; Lowest instrumented vertebra; Pediatric scoliosis; Posterior spinal fusion.

Fig. 1.

Flow chart showing the study selection process. Studies were eliminated if they did not meet the following criteria: ≥ 30 patients with Lenke type 1A or 2A curves, thoracic-only spinal fusion, inclusion of outcome differences in adding-on and non-adding-on groups. Review articles, letters, and case reports were also excluded.

Fig. 2.

Illustration of the EV, SV, NV, LTV, STV, nSTV, and nSTV+1. The STV is defined as the most proximal vertebra where the CSVL either intersects or is medial to the pedicle outline. The nSTV is defined as the most proximal vertebra where the CSVL touches the corner of the vertebra lateral to the pedicle. EV, end vertebra; SV, stable vertebra; NV, neutral vertebra; LTV, last touched vertebra; STV, substantially touched vertebra; nSTV, nonsubstantially touched vertebra; CSVL, central sacral vertical line.

Fig. 3.

Two coronal x-rays demonstrating L4 vertebral tilt in Lenke type 1A curves. (A) L4 is tilted down to the right resulting in the designation 1A-R, and (B) shows a 1A-L curve as the left side of L4 is lower.