O2086: 3D CT Modeling Demonstrates the Anatomic Feasibility of S1AI Screw Trajectory for Spinopelvic Fixation in Neuromuscular Scoliosis

Background: The S2 alar iliac(S2AI) screw trajectory is safe and effective however pelvic anatomy can be distorted in pediatric neuromuscular scoliosis making placement of an S2AI screw challenging. Additionally, screw head prominence can lead to pressure related injuries with devastating consequences. Here we use 3D CT modeling to demonstrate the anatomic feasibility of an S1 alar iliac screw (S1AI) in a population of patients with neuromuscular scoliosis and compare it to S2AI screws.

Methods: This retrospective study used CT scans of 14 patients with neuromuscular scoliosis. We used CT-based 3D reconstructions of the lumbar spine and pelvis to model the trajectories of bilateral S2AI and S1AI screws. S1AI screws were modeled with the start point superolateral to the first dorsal foramen angled inferiorly and laterally above the sciatic notch(Figure1). We compared feasibility of insertion, length & diameter of the screw that could be obtained, as well as the potential for implant prominence between the two trajectories.

Results: We modeled 28 S1AI and 28 S2AI screws across 14 patients with pending spinal deformity surgery. Patients with neuromuscular scoliosis with a mean age of 14.42 (range: 8-21), a mean major coronal cobb angle of 84.75(range: 54-141) & a mean pelvic obliquity of 27.75(range:4-51). The maximum screw length that could be obtained through the S1AI and S2AI trajectories was similar; 78.4±15mm versus 75.5±18mm respectively P=0.343. Maximum diameter was slightly smaller for S1AI screws versus S2AI screws 7.6±5.5mm versus 7.9±1.3mm P=0.039. S2AI screws were on average 6.3±5 mm more prominent than S1AI screws relative to the iliac crests. S2AI screws had a feasible entry trajectory in all patients however in 2 patients the S1AI trajectory was not feasible because the entry pathway was blocked by the L5 and L4 lamina.

Conclusion: In this cohort of neuromuscular scoliosis patients the S1AI trajectory offers comparable screw size. The entry point requires less dissection versus S2AI as it is more cranial. The S1A1 screw head was less prominent in most cases compared to S2AI screws. In some patients the S1AI screw may not be feasible to place because the entry trajectory may interfere with the posterior elements of the level above.