Distal instrumentation failure after posterior spinal fusion in adolescent idiopathic scoliosis: Incidence, risk factors, and radiographic consequences.

Abstract

To evaluate the incidence, associated factors, and radiographic consequences of distal instrumentation failure after posterior spinal fusion (PSF) for adolescent idiopathic scoliosis (AIS). We retrospectively reviewed 302 patients with AIS who underwent one-stage PSF using all-pedicle screw constructs between 2021 and 2023 and had at least 2 years of radiographic follow-up. Distal instrumentation failure was defined as axial rod slip (ARS) or distal screw fixation failure on serial radiographs. ARS was defined as a ≥2 mm change in rod position relative to the lowest instrumented vertebra (LIV) screw. Patients were divided into failure (n = 35) and non-failure (n = 267) groups. Demographic, surgical, and radiographic variables, including LIV level, rod protrusion length, LIV screw angulation, rod material, and implant system, were analyzed. Distal instrumentation failure occurred in 35 of 302 patients (11.6%), most commonly as ARS (28/302, 9.3%). Among patients with ARS, 50.0% were first detected within 3 months postoperatively, and 64.3% occurred on the convex side. The failure group had a significantly larger preoperative lumbar curve than the non-failure group. Failure incidence tended to be higher in patients with a higher Lenke lumbar modifier and with more distal LIV selection, particularly at L3 or L4. Greater inferior LIV screw angulation was significantly associated with screw fixation failure (p = 0.006). Compared with the non-failure group, the failure group showed greater correction loss at 1 year (7.0% vs. 1.1%, p = 0.007) and a higher rate of distal curve progression greater than 5° (16/35, 45.7% vs. 29/267, 10.9%, p < 0.001). Failure incidence was lower in the torque-limiting system than in the break-off set-screw system, with borderline significance (23/237, 9.7% vs. 12/65, 18.5%, p = 0.051). Distal instrumentation failure after AIS fusion was not uncommon and most commonly manifested as ARS. A larger preoperative lumbar curve, more distal LIV selection, and greater inferior LIV screw angulation were associated with failure. Early recognition of these radiographic and surgical risk factors may help preserve postoperative correction and guide preventive strategies in AIS surgery.