Iliac Fixation

Overview

The lumbosacral junction of the spine is a high stress area; it is the transition from the highly mobile lumbar spine to the rigid pelvis. As a result of this high stress, the lumbosacral junction is prone to instrumentation failures and pseudarthrosis. When performing a lumbar fusion, the lumbar–sacral junction then becomes a transition from two rigid areas of the axial skeleton, increasing the risk of pseudarthrosis. Failure of instrumentation of S1 pedicle screws from pseudarthrosis or before fusion has led to the use of iliac screw fixation for long fusions. Long fusion to the sacrum, which is considered to L2 or higher, is an indication for pelvic fixation as reported by some authors. The indications for iliac screws has been extended to pathology or fusion constructs; this places high stresses at the L5–S1 motion segment when fusion is desired at that level.

The first modality for fixation to the pelvis was body casting, which had numerous complications and was poor at stabilizing the pelvis. This was followed by Harrington rod fixation to the sacrum with hooks. Both techniques had pseudarthrosis rates near 50%. Using the next development in spinal fixation, Luque instrumentation, King attached the longitudinal construct to the posterior superior iliac spine (PSIS) using rods with a 90 degree bend at the distal ends and threaded tips of the rods that were passed through the ilium and secured with washers and bolts to the outer table; however, pseudarthrosis rates continued to be high.

The next advance in instrumentation was the Cotrell-Dubousset system with alar and iliosacral screws, yet pseudarthrosis rates of 33% and instrumentation-related complications of 44% continued. Iliosacral screws are placed by starting on the outer cortex of the ilium, crossing the inner table, and entering the S1 pedicle above the sacroiliac joint. A connector attaches near the tip of the screw and links the screw to the longitudinal rods.

Iliosacral screws were replaced by the Galveston technique, which is currently used by some surgeons. The Galveston technique provides a more rigid fixation across the lumbosacral junction by inserting a contoured rod at the PSIS in the ilium. This technique decreased instrumentation-related complications and prominence; however, the technique is technically demanding.

Currently the most widely used technique for fixation into the pelvis is the iliac screw or bolt. It is derived from the Galveston technique; however, a screw is placed in the ilium as opposed to a contoured rod. Iliac screws are placed by inserting a polyaxial screw into the ilium between the inner and outer tables. The use of iliac screws and cross connectors has reduced the technical demands of pelvic fixation and has allowed for variability in screw diameter and length and in pullout strength from the screw threads. Published pseudarthrosis rates of L5–S1 with the use of iliac screws have been as low as 5%.

Recently, the S2 alar iliac screw has been used in place of the standard iliac bolt. The main advantages of this technique are decreased hardware prominence and no longer needing a cross connector to attach to the longitudinal rods. The screw is inserted through the sacrum into the ilium starting inferior and lateral to the S1 dorsal foramen ( Fig. 54-1 ). Early studies show good results without disruption of the sacroiliac joint; however, long-term studies have not been published.

Indications

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Long fusions to the sacrum, as in neuromuscular scoliosis treatment
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Flat back deformity that requires corrective osteotomy ( Fig. 54-2 )

Figure 54-2

Preoperative and postoperative radiographs of flat back deformity correction using T4–pelvis posterior spinal fusion and instrumentation with L3 pedicle subtraction osteotomy.
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Correction of pelvic obliquity
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Grade 3 or higher spondylolisthesis as a result of the high stress at the S1 screws
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Sacral fractures with spinopelvic dissociation
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Lumbosacral fusions in patients with osteoporosis, as in adult scoliosis

Surgical Techniques

Equipment

The equipment required for placement of iliac bolts starts with a well-padded radiolucent bed that allows prone positioning of the patient and should include the necessary instruments for standard midline exposure of the spine, including Bovie cautery, Cobb elevators, towel clips, and retractors. A large Leksell rongeur and sharp Cobb elevator are needed to prepare the insertion site. A T-handled or Steffee broach, ball-tip probe, and of course the implant with an appropriate tap are also needed. Crosslinks are also required to attach the iliac bolt to the longitudinal rod.

Patient Positioning and Incision

The patient is positioned prone on the operating table and is padded appropriately before sterile prep and draping; the feasibility of obtaining intraoperative radiographs or fluoroscopy should be assessed. Radiographic guidance is not needed for iliac bolt insertion by this technique; however, it can be used for trajectory verification until the surgeon is comfortable with the technique. If preforming an osteotomy, the ability to position the lumbar spine over the break of the operating room table and use the table controls to obtain correction should also be taken into consideration.

A midline incision is made followed by subperiosteal dissection of the soft tissues overlying the spinal column with exposure of the transverse processes in the lumbar spine. Once dissection and possibly placement of lumbar instrumentation is achieved, dissection for placement of the iliac bolts can be addressed. The incised midline fascia is towel-clipped back together using one or two clips. Dissection in the plane superficial to the fascia is carried out from the midline, using palpation of the PSIS to guide the level and amount of dissection, which should be as minimal as possible ( Fig. 54-3 ). Using palpation, the fascia is then incised with Bovie cautery over the PSIS approximately 5 cm, and the periosteum is dissected off the PSIS both laterally and medially ( Figs. 54-4 and 54-5 ). The medial dissection is carried out until it communicates with the midline dissection. The lateral dissection is carried out to the edge of the outer table, at which point a sharp Cobb or periosteal elevator can be used to expose the outer table along the trajectory of the iliac bolt. This allows clinical guidance of the broach and screw during placement; an arcade of bone can be palpated, and the screw trajectory should follow this arcade ( Fig. 54-6 ).