KeywordsIliumSacroiliacS2AIFixationDeformityIliac boltS2 alar iliacLumbopelvic
Pelvic instrumentation in the form of iliac bolts  or S2AI screws provides distal fixation to long-construct fusions for deformity or scoliosis . The techniques, while similar, provide unique challenges, such as necessitating cross-connectors for iliac bolts, or breaching the sacroiliac (SI) joint in S2AI. Anatomically, the axial spine consists of vertebrae from cervical to lumbar, ending atop the sacrum distally. The sacrum articulates with the ilia bilaterally forming a flat sacroiliac (SI) joint with minimal motion. Both the ilium and the sacrum provide distal fixation points for instrumented spinal fusions. Studies have shown a high pseudoarthrosis rate at the lumbosacral junction when distal fixation ends at S1 [3–5]. Several biomechanical studies have shown increased rigidity at the lumbosacral junction with the addition of iliac fixation [6–9]. Alternatively, pelvic fixation can serve as an adjunct to internal fixation or definitive treatment for comminuted sacral fractures and pelvic ring injuries with spinopelvic dissociation.
The ilium provides a bony corridor from the posterior superior iliac spine (PSIS) to the anterior inferior iliac spine (AIIS) that can hold one or two large diameter screws. The sacroiliac joint is a wide, flat joint between the sacrum and the ilium bilaterally. This joint has two areas, the inferior half of which is lined with cartilage and does allow minimal motion. S2AI screw trajectory is such that placement does not always penetrate this area of articulation  and the long-term effects of such are not well-studied.
Radiographically, this corridor of bone is visualized on the obturator outlet view and known as the teardrop. The teardrop is the confluence of three points: the posterior superior iliac spine, the sciatic notch, and the anterior inferior iliac spine . This starting point can be entered from the PSIS with trajectory toward the AIIS or via a sacral start point in S2 and directed across the sacroiliac joint toward the AIIS. Trajectory should be confirmed on the iliac outlet view to ensure the greater sciatic notch is not penetrated. Alternatively, placement of these screws can be performed with the assistance of CT-guided navigation .