Chapter Clinical Summary
Academic synthesis, diagnostic methodology, and surgical recommendationsSpinal deformities and long fusion constructs extending to the sacrum (L5-S1) face immense mechanical cantilever forces that cause high rates of S1 screw loosening, sacral insufficiency fracture, pseudarthrosis, and loss of lumbopelvic lordosis. Sacropelvic fixation was developed to transfer mechanical loads directly into the robust cortical bone of the pelvis, shielding the S1 pedicle screws and sacral promontory. While traditional iliac screws achieved excellent biomechanical anchoring, they required extensive lateral soft-tissue dissection, separate offset connectors, and caused high rates of prominent hardware pain and wound breakdown over the posterior superior iliac spine (PSIS). The S2 Alar-Iliac (S2AI) screw technique solved these challenges by placing screws starting at the S2 sacral lamina, crossing the sacroiliac joint, and entering the dense cortical column of the ilium toward the anterior inferior iliac spine (AIIS). The S2AI entry point aligns naturally with the proximal lumbar/thoracic pedicle screw line, eliminating the need for offset connectors and placing the screw head deeply recessed beneath the sacral musculature.
Detail the anatomical landmarks, biomechanical rationale, fluoroscopic 'tear-drop' orientation, and step-by-step execution of the S2 Alar-Iliac (S2AI) screw fixation technique. The reader will master entry point selection, trajectory angles, navigation/fluoroscopy protocols, and complication prevention (sciatic notch breach, acetabular penetration, sacroiliac joint pain).
Long constructs terminating at S1 have pseudarthrosis rates exceeding 30-40% due to the massive flexion bending moments at the lumbosacral junction. Pelvic fixation provides a long lever arm in the iliac bone that neutralizes these forces. S2AI screws offer pullout and construct rigidity equivalent to traditional iliac screws, but with a 15 mm deeper head profile and in-line rod connectivity (Figure 79.1).
Entry point: Located 2–4 mm lateral and 2–4 mm distal to the S1 dorsal sacral foramen (or at the midpoint between S1 and S2 dorsal foramina on the lateral sacral alar margin). Trajectory: Angled 35°–40° laterally in the axial plane and 25°–30° caudally in the sagittal plane, aiming directly toward the superior border of the greater trochanter and anterior inferior iliac spine (AIIS).
Under fluoroscopy, the trajectory is guided by the iliac 'tear-drop' view (obturator oblique inlet view). The screw must pass through the narrow cortical isthmus above the greater sciatic notch and below the iliac crest (Figure 79.2). Crossing the sacroiliac joint into the ilium provides bicortical/tricortical purchase.
Step 1: Cortical entry opening at the S2 alar landmark. Step 2: Advancing a long gearshift probe (65–90 mm length) across the SI joint into the ilium with continuous tactile checking. Step 3: Ball-tip feeler palpation of all walls. Step 4: Undertapping. Step 5: Placement of a large-diameter (8.5–10.5 mm) and long (75–100 mm) polyaxial pelvic screw directly in-line with the L5/S1 pedicle screw heads.
Complications include greater sciatic notch violation (injuring the superior gluteal artery and sciatic nerve), acetabular or intra-articular hip penetration, anterior cortical breach into the pelvis (internal iliac vessels), and SI joint arthralgia. Preoperative 3D CT trajectory mapping and intraoperative fluoroscopy/navigation prevent cortical breaches.
In clinical practice, S2AI fixation is indicated in adult spinal deformity (ASD) long fusions extending to the pelvis, high-grade spondylolisthesis (grade III-V), sacral tumors, neuromuscular scoliosis, and revision lumbosacral pseudarthrosis. Preoperative thin-slice CT must measure the iliac corridor width (typically >10-12 mm) and determine optimal screw length (usually 80–90 mm in adults). Intraoperatively, the entry point requires no lateral fascial dissection; the soft tissue exposure remains strictly subperiosteal along the sacrum. During probe advancement across the sacroiliac joint, the surgeon will feel a firm tactile 'give' as the probe traverses the joint cartilage into the dense iliac bone. Connecting the S2AI screws directly to the longitudinal rods without offset connectors reduces operative time, blood loss, and hardware failure.