Free Hand Insertion Technique of S2 Sacral Alar-Iliac Screws for Spino-Pelvic Fixation: Technical Note, Acadaveric Study

Abstract

A rigid spino-pelvic fixation to anchor long constructs is crucial to maintain the stability of long fusion in spinal deformity surgery. Besides obtaining immediate stability and proper biomechanical strength of constructs, the S2 alar-iliac (S2AI) screws have some more advantages. Four Korean fresh-frozen human cadavers were procured. Free hand S2AI screw placement is performed using anatomic landmarks. The starting point of the S2AI screw is located at the midpoint between the S1 and S2 foramen and 2 mm medial to the lateral sacral crest. Gearshift was advanced from the desired starting point toward the sacro-iliac joint directing approximately 20° angulation caudally in sagittal plane and 30° angulation horizontally in the coronal plane connecting the posterior superior iliac spine (PSIS). We made a S2AI screw trajectory through the cancellous channel using the gearshift. We measured caudal angle in the sagittal plane and horizontal angle in the coronal plane. A total of eight S2AI screws were inserted in four cadavers. All screws inserted into the iliac crest were evaluated by C-arm and naked eye examination by two spine surgeons. Among 8 S2AI screws, all screws were accurately placed (100%). The average caudal angle in the sagittal plane was 17.3±5.4°. The average horizontal angle in the coronal plane connecting the PSIS was 32.0±1.8°. The placement of S2AI screws using the free hand technique without any radiographic guidance appears to an acceptable method of insertion without more radiation or time consuming.

Keywords: Adult deformity; Iliac fixation; S2 alar-iliac screw; Spino-pelvic fixation.

Fig. 1. A : The entry point of the S2 alar-ilac (S2AI) screw is located at the midpoint between the S1 and S2 foramen and 2 mm medial to the lateral sacral crest. Caudal angle in the sagittal plane is marked (^*). B : Left, horizontal angle in the coronal plane connecting the posterior superior iliac spine is marked (^*). Right, the gearshift is initially pointed dorsally as a safety measure to avoid anterior wall perforation of the iliac crest. After crossing the sacro-iliac joint, the tip of the gearshift turned to face ventrally. C : On the lateral plane radiograph, the S2AI screws should be placed above the superior rim of the sciatic notch.

Fig. 2. On the AP and lateral C-arm fluoroscopy after placing the S2 alar-iliac (S2AI), we confirmed that all screws did not violate the sciatic notch and acetabulum. All screws located in the ilium. AP : anterior-posterior.

Fig. 3. S2 alar-iliac (S2AI) screws were in-line with the conventional iliac wing screws with the offset connector. Black arrow indicates S2AI screw, whereas hollow arrow indicates conventional iliac wing screw with the offset connector.

Fig. 4. A : Imaginary lines for the ideal pedicle screw and S2 alar-iliac screw trajectory on the intra-operative lateral plain radiograph, which was taken after flipping a patient. These lines become a reference lines. B : Intraoperative portable plain radiograph after placing the screws shows that all screws were inserted correctly.