Iliac and S2 Alar-Iliac Screw Fixation

Radiologic Assessment—Key Factors to Consider on X-ray, MRI, CT

Assess pelvic obliquity. Lumbar scoliotic deformity can change the orientation of the pelvis and alter the necessary screw trajectory.
Assess pelvic morphology. The normal adult pelvis should have a wide bony channel for screw placement, running roughly between the anterior inferior iliac spine (AIIS) and the posterior superior iliac spine (PSIS). Pediatric neuromuscular or congenital deformity can result in a significantly narrower pelvic brim and may result in a shorter screw trajectory.
Prior iliac crest bone harvest can potentially compromise iliac screw placement. A CT scan showing the iliac crests may assist with preoperative planning in this regard.

Positioning

The patient is prone on a Jackson table. Specific positioning information is described in the chapter on posterior lumbar fusion.
In cases with severe pelvic obliquity because of a rotary scoliosis, bolsters can be strategically placed under the anterior superior iliac spine in order to help reduce the rotation and correct the deformity.
- Intraoperative distal femoral pin traction may occasionally be considered in severe, neuromuscular cases with substantial obliquity.

Anesthesia/Neuromonitoring Concerns

None specific to iliac or S2 alar-iliac (S2AI) fixation.

Localization of Incision

The incision is midline, centered over the spinous processes. The distal incision will extend to the level of the S2 body. The proximal extent of the incision will depend on the extent of the planned thoracolumbar fusion.

Approach

A standard midline lumbar approach is made using electrocautery.
The dissection is carried distally to the S2 body, with enough caudal extension to expose the S1 dorsal foramen. There is a large venous plexus around foramen, and care should be taken not to enter it with the cautery.
The muscle and soft tissue are elevated laterally off of the sacral ala. Care should be taken not to enter the sacroiliac joint with the cautery.
If an iliac screw is planned, the dissection is continued up onto the PSIS for a distance of 5 to 10 mm, until the bony prominence of the PSIS is easily palpated.
- The start site is just proximal to the most distal point, or “termination” of the PSIS. Dissection should therefore continue onto this most distal point where the PSIS “terminates.”
If an S2AI screw is planned, the lateral dissection can terminate at the sacroiliac (SI) joint and need not carry out laterally onto the PSIS.
The S1 pedicle screw head location should be determined in order to help align the iliac screw to facilitate rod placement.

Retractor Placement

An assistant uses a large handheld retractor to assist in elevating the soft tissue envelope laterally.
When dissecting the ilium for iliac screw placement, once the outer table has been dissected with cautery, a Cobb elevator is placed over the outer table, levered up, and held in place by the assistant in order to maintain visualization.

Instrumentation/Fusion Techniques

Placement of the iliac fixation generally proceeds after S1 screws have already been placed. Placing the S1 screws first allows the surgeon to try and place the iliac fixation inline with the S1 screws, which greatly facilitates subsequent rod placement.
The difference between iliac screws and S2AI screws is primarily a difference in starting points and trajectory, with specific details outlined later.

Iliac Screw Fixation (Table 27-1)

Table 27-1 ▪

Technique	Pros	Cons
Iliac screw (Figures 27-1, 27-2, 27-3, 27-4, 27-5, 27-6)	Straightforward trajectory	Potentially more prominent screw head
	Entire path is cancellous, and thus the “feel” of placing the screw is similar to a large pedicle screw	Requires more lateral soft tissue dissection
		If the starting point is too lateral it may require the addition of a lateral offset connector or difficult rod bending

Figure 27-1 ▪ Preoperative anteroposterior and lateral 36 inch standing scoliosis films of a 65-yearold female with moderate sagittal and coronal plane deformity because of degenerative scoliosis.

Figure 27-2 ▪ Postoperative anteroposterior and lateral 36 inch standing scoliosis films after T10 pelvis reconstruction with posterior Smith-Petersen-type osteotomies. The pelvic fixation was completed with iliac screws.

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