Risk factors for distal construct failure in posterior spinal instrumented fusion for adolescent idiopathic scoliosis: a retrospective cohort study

Purpose To evaluate risk factors for distal construct failure (DCF) in posterior spinal instrumented fusion (PSIF) in adolescent idiopathic scoliosis (AIS). We hypothesise increased inferior angulation of the pedicle screw in the lowest instrumented vertebra (LIV) predisposes to failure and aim to find the critical angle that predisposes to failure. Methods A retrospective cohort study was performed on all patients who underwent PSIF for AIS at our institution from 2010 to 2020. On lateral radiographs, the angle between the superior endplate of the LIV was measured against its pedicle screw trajectory. Data on demographics, Cobb angle, Lenke classification, instrumentation density, rod protrusion from the most inferior screw, implants and reasons for revision were collected. Results Of 256 patients, 9 patients had DCF with 3 further failures post-revision, giving 12 cases to analyse. The DCF rate was 4.6%. The mean trajectory angle of DCF patients compared to non-DCF was 13.3° (95% CI 9.2° to 17.4°) vs. 7.6° (7.0° to 8.2°), p = 0.0002. The critical angle is less than 11° ( p = 0.0076), OR 5.15. Lenke 5 and C curves, lower preoperative Cobb angle, titanium only rod constructs and one surgeon had higher failure rates. 9.6% of rods protruding less than 3 mm from its distal screw disengaged. Conclusion Increased inferior trajectory of the LIV screw increases the rate of DCF; inferior trajectory greater than 11° predisposes to failure. Rod protrusion less than 3 mm from the distal screw increases rate of disengagement. Level of evidence III.