Extra-articular Portion of the Sacroiliac Joint-Between the Sacral and Pelvic Tuberosities: An Anatomical Guide for the S2-Alar-Iliac Screw Trajectory

Abstract

Study design: A novel technique for S2-alar-iliac (S2AI) screw placement was analyzed.

Objectives: Accurate confirmation of the S2AI screw trajectory with free-hand techniques is not simple, although some anatomical landmarks have been reported. To overcome the drawback, we aimed to introduce our technique for S2AI screw placement assisted with a guidewire using a new anatomical landmark.

Methods: A total of 104 S2AI screws of 52 patients who underwent S2AI screw placement were investigated. Navigation software was used to simulate S2AI screw placement preoperatively. Screw placement was performed with the nonfluoroscopic free-hand technique. In this technique, a guidewire is inserted into the ilium from the extra-articular portion of the sacroiliac joint just lateral to the ideal screw entry point toward the tip of the greater trochanter and guides the screw trajectory. If the direction of the guidewire is satisfactory, all procedures of screw insertion are performed accordingly. The screw accuracy was assessed with computed tomography.

Results: The modal size of the screw was 9.5 mm × 90 mm. The average horizontal angle was 42.0° (SD = 5.1°) on the right and 40.7° (SD = 4.7°) on the left. Of the 104 screws, 4 screws (3.9%) breached dorsally. No screw-related complication was observed.

Conclusions: Because the guidewire can be inserted at an angle according to the individual morphology of the sacroiliac joint, it will be a reliable guide for the screw trajectory. This technique with a guidewire would help improve the accuracy of S2AI screw placement.

Keywords: S2AI screw; free-hand; lumbosacral transitional vertebra; navigation; spinal deformity; spinopelvic fixation.

Figure 1.

To easily identify the tip of the greater trochanter from the body surface during screw insertion, an electrocardiogram electrode was attached (A) thereon; a plastic bottle cap was placed (B) on the electrode and secured with medical tape (C and D).

Figure 2.

A. Intraoperative photo demonstrating the guidewires (white arrow). B. Axial computed tomography image at the level between the S1 and S2 posterior foramina, which is sliced parallel to the S1 endplate. The guidewire (white long arrow) appears to be easily introduced to the ilium if it is inserted along the sacral tuberosity from the extra-articular portion of the sacroiliac joint just lateral to the ideal entry point of the S2-alar-iliac screw—between the sacral (black arrow head) and iliac (white arrow head) tuberosities. The S2-alar-iliac screw is inserted properly if it penetrates the articular portion of the sacroiliac joint (asterisk) along the guidewire. C-E. These images are created by the navigation software. The light green represents the S2-alar-iliac screw, and the pink represents the guidewire, which is inserted along the sacral tuberosity from the extra-articular portion of the sacroiliac joint.

Figure 3.

Intraoperative pelvic inlet and lateral pelvic X-ray images. A and B. After inserting the guidewires. C and D. After inserting the screws. The right guidewire is in place, but the left guidewire is somewhat away from the greater sciatic notch. The left S2-alar-iliac screw should be inserted slightly caudal to the guidewire.

Figure 4.

Castellvi IIIb in a 75-year-old man. A. Three-dimensional computed tomography (CT). B. Sagittal reconstructed image of CT myelography. C. Axial image at the level corresponding to the S1. D. Axial image at the level corresponding to the S2. E and F. Postoperative X-ray images. G. Postoperative CT image. Preoperative 3-dimensional CT demonstrates that this lumbosacral transitional vertebra is Castellvi IIIb because the bilateral transverse processes of the L5 fuse completely to the sacral alae. Although in the sagittal image (B) S2 appears to be at level (D), when compared with axial images (C) and (D), it is clear that level (C) is the true S2 anatomically. S2-alar-iliac screw placement at level (D) would be difficult. Postoperative CT shows no cortical breach of the screws.