Free hand S2AI screw placement was 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 (Fig. 1A). Additionally, the entry point for S2AI should be harmoniously located with S1 screw entry point. A 3 mm cutting burr was utilized to create posterior cortical breach, approximately 5 mm in depth. The gearshift was initially pointed dorsally to avoid penetration of the anterior sacrum (Fig. 1B). Gearshift was advanced from the desired starting point toward the sacro-iliac (SI) joint directing approximately 20° angulation caudally in sagittal plane and 30° angulation horizontally in the coronal plane connecting the posterior superior iliac spine (PSIS). After inserting the gearshift tip approximately 3–4 cm until reaching the SI joint, the gearshift was removed and confirmed 5 bony walls (medial, lateral, superior, inferior and bottom) through ball-tip probing. This step identifies the intraosseous placement of the screw trajectory. It could be hard to advance the gearshift because of the cortical wall of the SI joint. By cautious hammering the gearshift with the mallet, it could penetrate the cortical wall of the SI joint. After crossing the SI joint, the tip of the gearshift turned to face ventral (Fig. 1B). The gearshift was preceded through the cancellous channel of the ilium heading above the superior rim of the sciatic notch. It is important to make sure that surgeon to feel the bone of entire length of the tract. A ball-tip probe was utilized to palpate the 5 bony borders ensuring that the screw would be located intraosseously. At this point, if a soft tissue breach is palpated, there may be an opportunity to redirect the gearshift probe into an appropriate position into the iliac crest. If any wall including the anterior has been breached, the iliac wing crest may be salvageable. Otherwise, we place bone wax or gelfoam in the sacral hole to eliminate the bleeding and re-angle the pedicle finder with a more appropriate trajectory. The S2AI screw tract was under-tapped with a 2 mm less diameter tap than the intended screw. Following this, the tract is palpated again to make sure that the osseous borders are intact. This second palpation will often allow palpation of distinct bony ridges confirming intraosseous position and the tract length was remeasured by the length of the ball-tip probe marked with a hemostatic clamp. This measurement was compared directly adjacent to the screw to be placed to ensure appropriate screw length. Finally, the screw was inserted in the same direction until the screw head is placed in-line with S1 promontory screw head.

It is imperative that the surgeon document intraosseous placement via fluoroscopy or plain radiography. On the lateral and coronal plane radiograph, the S2AI screws should be placed above the superior rim of the sciatic notch (Fig. 1C). The most important structure to be avoided is the sciatic notch. However, rather than using a trajectory that passes far from the notch, the ideal trajectory of the S2AI screw is situated immediately above the notch so that the bottom threads of the screw will be in contact with the cortical bone forming the upper limit of the notch, providing a better pull-out strength5).

We measured caudal angle in the sagittal plane (Fig. 1A) and horizontal angle in the coronal plane connecting the PSIS (Fig. 1B).

Our criteria for accurate placement were as follows; first, after under-tapping, two surgeons confirmed the screw trajectory was located intraosseously by palpating intact 5 bony walls using ball-tip probe. Second, we checked AP and lateral C-arm fluoroscopy; the screws should not violate sciatic notch, acetabulum; the screws should not evade the pelvic cavity; the screws should locate in the ilium.