Study design:

Retrospective multicenter study.

Objective:

This study aimed to investigate the incidence and risk factors of subjacent disc wedging (SDW) in adolescent idiopathic scoliosis (AIS) patients with Lenke type 5 curve.

Summary of background data:

SDW is frequently observed after surgery; however, data about its mechanism and relations with outcome are limited.

Methods:

Data of 59 AIS patients with Lenke type 5 curves who underwent posterior spinal fusion to L3 as the lowest instrumented vertebra (LIV) were retrospectively analyzed. The subjacent disc angle (SDA) was defined as the angle between L3 (LIV) and L4. SDW was defined as the absolute value of SDA ≥10° at 2-year post-operation. The incidence of SDW was investigated between non-selective and selective thoracolumbar/lumbar (TL/L) fusion group. In the selective group, patients with and without SDW were compared.

Results:

Among 59 patients, 11 had nonselective and 48 had selective fusion. No patients in the non-selective group showed SDW vs 13 patients in the selective group (27%) showed SDW. In the selective group, patients with SDW showed significantly greater main thoracic (MT) curve, apical vertebral translation of the MT curve, upper instrumented vertebra tilt, LIV tilt, and SDA at 2 years post-operation, while no differences were found in the coronal balance nor clinical outcome. Multivariate analysis revealed preoperative T curve and SDA as predictors of SDW occurrence. T curve >30° and SDA >0° were calculated as cutoff values based on the receiver operating characteristic curve.

Conclusions:

SDW is sometimes seen in Lenke type 5 AIS patients who underwent selective TL/L fusion. SDW seemed to occur as a compensation mechanism for progressing deformity of unfused segments (thoracic curve and residual lumbar curve) to maintain coronal alignment. Preoperative T curve > 30° and SDA > 0° (LEV as L4) were determined as risk factors for SDW occurrence.Level of Evidence: 3.