Sacral Alar-Iliac (SAI) screw diameter is not associated with pelvic fixation failure for neuromuscular scoliosis patients

Purpose Determine if Sacral Alar-Iliac (SAI) screw diameter is associated with pelvic fixation failure in pediatric patients with neuromuscular scoliosis (NMS) treated with posterior spinal fusion (PSF). Methods NMS patients from a single institution who underwent PSF with bilateral SAI screw fixation from 2010 to 2021 were retrospectively reviewed. Clinical parameters, SAI screw sizes, and radiographic outcomes were analyzed. Patients with greater or less than two SAI screws, > 21 years old, or with < 2 years of radiographic follow-up were excluded. Results 142 patients had 284 SAI screws placed. Mean(+/- SD) age was 13.6 +/- 2.7 years. Preoperative curve magnitude averaged 84.3 +/- 29.1 degrees. Mean patient weight was 36.4 +/- 14.1kg and BMI was 18 +/- 5.1. Radiographic follow-up averaged 4.6 +/- 2.0 years. Most screws (234/284,82.4%) were < 8.5 mm and 7.5 mm screws were most frequently used (158/248,55.6%). Mean screw diameter was 7.4 +/- 0.7 mm. Patients with greater age, weight, and BMI trended towards larger screws. Three patients had five screw complications (1 screw fracture and 4 set screw dislodgments). One screw fracture (1/284,0.4%;7.5 mm diameter) and contralateral set screw dislodgement occurred in a patient at 14 months but was not revised. One patient who had bilateral set screws dislodge 3 months after PSF underwent revision. The remaining patient was asymptomatic and was observed. Screw diameter was not associated with risk of postoperative complications (p = 0.245). Conclusion SAI screw fracture is rare after PSF in pediatric patients with NMS. Contrary to pelvic fixation in adults, smaller diameter SAI screws, which may be optimal in patients with smaller anatomy, were not associated with increased risk of screw failure. Level of evidence III.