16.1 Introduction

Indications for sacropelvic fixation include long spinal fusions for scoliosis, high-grade spondylolisthesis, pelvic obliquity correction, sagittal plane deformity correction, sacral fractures, and lumbosacral fusions in patients with poor bone quality and osteoporosis. Each of these indications requires strong distal fixation to resist significant flexion moments and cantilever forces in this transitional area. Despite many advances and developments in spinal instrumentation techniques, fixation failure at the lumbosacral junction continues to be a challenge.

Due to the significant biomechanical forces across this junction and relatively poor sacral cancellous bone quality, fusion at the lumbosacral junction has been associated with rates of pseudarthrosis as high as 33 to 39%, ^{1 ,​ 2} loss of lordosis, ^{1 ,​ 2} and instrumentation failure, especially with historical methods such as body casting and Harrington and Luque instrumentation. Cotrel–Dubousset (CD) instrumentation and the Galveston technique were introduced in the 1980s, and although pseudarthrosis rates were lower, there were instrumentation-related complications with the CD system ³ and technical difficulties with rod contouring, insertion in the ilium, and rod loosening with the Galveston technique.

16.2 Biomechanics

McCord et al pointed out the importance of the lumbosacral pivot point, which was defined as the point at the middle osteoligamentous column between the last lumbar vertebra and the sacrum. The farther the implants extend anterior to this point, the greater the stiffness of the construct. Various instrumentation models (iliac fixation, S1 fixation, and S2 fixation) were tested. The two constructs that withstood the greatest load before failure had caudad iliac (rod or screw) fixation, and they concluded that crossing the sacroiliac (SI) joint is warranted if instrumentation extends anterior to the pivot point. ⁴

O’Brien identified three distinct zones of the sacropelvic region: zone 1 consists of the S1 vertebral body and the cephalad margins of the sacral alae; zone 2 consists of the inferior margins of the sacral alae, S2, and the area extending to the tip of the coccyx; and zone 3 consists of both ilia. Fixation strength improves progressively from zone 1 to zone 3. Zone 3 offers the greatest biomechanical fixation strength to counter the pullout forces and bending moments at the lumbosacral junction. Also, in agreement with McCord, iliac fixation with pelvic screws or iliac rods allows placement of implants more anteriorly beyond the lumbosacral pivot point than any other implant type. ⁵

Lebwohl and colleagues performed a biomechanical comparison of lumbosacral fixation in a calf spine model and noted that supplementary fixation distal to S1 pedicle screws provides a benefit over S1 fixation alone, and iliac fixation was superior to a second point of fixation in the sacrum. ⁶

Cunningham et al studied ex vivo porcine spines biomechanically to ascertain the value of anterior column support compared with that of iliac fixation in lumbosacral fusions. When tested to failure, the authors found that iliac screws significantly reduced lumbosacral motion, particularly with axial rotation, flexion–extension, and lateral bending. Iliac fixation was found to be more protective of S1 screws and more resistive of motion than anterior interbody cages. ⁷

However, other investigators have argued in favor of anterior column support, particularly at L4–L5 and L5–S1, in long fusions to the sacrum. ^{8 ,​ 9} Anterior lumbar interbody fusions place the bone graft ventral to the instrumentation and the lumbosacral pivot point, and the graft is placed in compression to optimize fusion and stability. ⁹ This procedure improves the overall chance of fusion, decreases the strain on caudad pedicle screws, and has a definite role in long fusions to the sacrum, especially in adults and other patients at risk for pseudarthrosis.

Various contemporary techniques of sacropelvic fixation have been described to achieve better fusion rates, improve bone purchase, neutralize forces, avoid pullout, ease difficulty, and reduce complications. In this chapter, we will cover the key components of surgical techniques for the majority of iliac fixation options. This will include the following: (1) Galveston and unit rod technique; (2) iliac bolts/screws with and without offset connections to the longitudinal rods; (3) double iliac bolts; (4) iliac screws in an alternative, anatomic pathway; and (5) S2 alar iliac (S2AI) screw fixation

16.2.1 Galveston and Unit Rod Technique

The Galveston technique allows for the incorporation of the ilium into the foundation of the construct via the insertion of rods between the inner and outer tables of the ilium, which provides a broader base and a more biomechanically advantageous position. ^{1 ,​ 4} The transverse portions of the rods are inserted under a large muscle flap and enter the ilium at the posterosuperior iliac crest. The orientation is approximately 30 to 35 degrees caudally and 20 to 25 degrees laterally. The rods may cross the SI joint, and contouring can be difficult. ¹⁰ The technique lowers the pseudarthrosis rate of long fusions to the sacrum, ^{11 ,​ 12} but it is also associated with a moderate incidence of loosening secondary to micromotion at the rod tips within the ilium, despite lumbosacral fusion. ¹³ Radiographically, this is described as a windshield-wiper effect and may be associated with pain and the need for implant removal, ^{13 ,​ 14} but in our long-term experience of unit rod fixation in patients with cerebral palsy, this has not been a common symptomatic issue requiring reoperation. ¹⁵ Lonstein and his colleagues published their results and complications of 93 patients with cerebral palsy and scoliosis who underwent posterior-only spinal fusion with Luque–Galveston instrumentation with an average follow-up of 3.8 years. ¹⁴ Coronal curve correction was 50% and pelvic obliquity correction was 40% at latest follow-up. The late complication rate was 47% and included the windshield-wiper sign, junctional kyphosis, pseudarthrosis (7.5%), and implant problems including breakage, dislodgement, and prominence. Seven patients required reoperation, most commonly for pseudarthrosis and/or failed implants.

The unit rod, by virtue of its precontoured unibody construction, provides rigid control of spinal deformities involving pelvic obliquity and allows for a cantilever mechanism to correct the pelvic obliquity and the scoliosis simultaneously. ¹⁶ See Fig. 16‑1 for an example of a patient with severe thoracolumbar scoliosis and pelvic obliquity treated with a unit rod. The rods are available in various lengths with corresponding thoracic, lumbar, and pelvic contours. The specific technique of unit rod insertion follows. ¹⁷

At the inferior margin of the incision, the outer wing of the ilium is subperiosteally exposed down to the sciatic notch and sponges packed out over the pelvis to maintain hemostasis. The right and left drill guides for the unit rod are placed in the respective sciatic notch; care should be taken to ensure that the drill guide is as inferior as possible along the posterosuperior iliac spine (PSIS). The handles of the drill guide are the reference points for alignment: the lateral handle should be parallel with the pelvis and the axial handle parallel with the sacrum. The drill hole is next made utilizing the guide using a 3/8-inch drill to the predetermined depth directed toward the anteroinferior iliac spine (AIIS); the hole is then palpated with a ball-tipped feeler to confirm that there has been no breach of the cortical bone of the inner or outer pelvic table. Alternatively, after establishing the landmarks, the pedicle gearshift can be used to cannulate the cancellous bone of the iliac pathway, either freehand or with fluoroscopic guidance. Gelfoam should be inserted into the drill holes to control cancellous bone bleeding. After the proper length unit rod is selected, the pelvic limbs of the rod are crossed and inserted into their respective drill holes. Each limb should be advanced alternatively in 1-cm increments with an impactor. Care must be taken to maintain control of the rod and insure that it does not penetrate either table of the pelvis. In the setting of hyperlordosis, the marked anterior inclination of the pelvis increases the risk of the pelvic limb perforating the inner cortex during insertion. The pelvic ends of the rod need to be directed in a more posterior direction to accommodate this angulation; rod placement is facilitated by manual correction of the lordosis prior to rod insertion. In instances of marked lordosis, the pelvic limbs of the rod may be cut and inserted separately and then attached to the rod with rod-to-rod connectors.

In our series of surgical correction of scoliosis in pediatric patients with cerebral palsy using unit rod instrumentation, ¹⁵ 241 patients were observed for more than 2 years and had an average coronal curve correction of 68% and pelvic obliquity correction of 71% with a very cost-effective implant system. Intraoperative complications with pelvic fixation occurred in 17 patients; sagittal plane deformities, especially lumbar hyperlordosis, were a risk factor. Late postoperative complications occurred in 12 patients: 3 pseudarthroses, 3 deep infections, and 6 prominent proximal implant issues.

Although the unit rod provides excellent correction of pelvic obliquity and resistance to flexion moments, there is little resistance to axial pullout and torsion by virtue of the rods being smooth and immediate micromotion of the iliac construct after insertion. These concerns can be mitigated by adding a transverse connector distally to improve torsional rigidity and adding lumbar pedicle screws distally at L5 to significantly improve axial pullout, strength, and stiffness. ¹⁸

16.2.2 Iliac Screws—Standard Technique

The difficult learning curve associated with rod contouring and insertion into the ilium using the Galveston technique has been resolved with the use of iliac screws, which permits screws of variable length and diameter to be inserted into each ilium separately. Simpler fixation to the pelvis with screws may also mitigate the potential complications of the Galveston technique, which was up to 62% in Gau et al’s series ¹¹ and 47% in Lonstein et al’s series. ¹⁴ The screws can then be connected to the main construct by various connectors. This technique has simplified the process of obtaining iliac fixation, especially in patients with significant pelvic asymmetry or hyperlordotic lumbar deformities while improving pullout strength through better interdigitation of the threaded implant within the iliac cortical and cancellous bone. ^{19 ,​ 20} See Fig. 16‑2 for an example of a patient with severe thoracolumbar scoliosis and pelvic obliquity treated with segmental instrumentation and iliac screws.

When placing iliac screws in lieu of smooth Galveston pelvic fixation, the starting point is similar—1 to 2 cm inferior to PSIS. To decrease implant prominence, a notch in the ilium can be used to bury the head of the screw below the contour of the cortical bone. A pedicle gearshift or drill is used to cannulate the cancellous bone between the inner and outer tables of the ilium, directed toward the AIIS, 1 cm above the sciatic notch. The pathway is then palpated with a ball-tipped probe and tapped to increase purchase and size the diameter and length of the screw. The exposure of the PSIS can be made through the same incision, elevating the paraspinal lumbosacral musculature and soft tissues, with a subperiosteal dissection of the PSIS and a portion of the outer table or through a separate, small oblique fascial incision by retracting the skin over the iliac crest. Alternatively, using a technique described by Wang and colleagues ²¹ with the Viper Screw System (DePuy Synthes Spine, Raynham, MA), iliac screws can be inserted with a fluoroscopically guided percutaneous technique to avoid the morbidity and complications associated with such a large muscle dissection and soft-tissue devitalization.

Every effort should be made to insert the largest diameter, longest screws that can be safely accommodated to achieve favorable biomechanics. Screws should extend past McCord’s sacropelvic pivot point at the anterior edge of the L5/S1 disk at a minimum. ⁴ The farther anteriorly and laterally the screws extend, the better control of pelvic flexion, extension, obliquity, and rotation. Even in smaller patients, we have been successful in implanting screws of 7.5 to 8 mm in diameter and 65 to 80 mm in length. Larger patients can accommodate screws of up to 10 mm × 100 mm. Large taps are available to enlarge the entry site and size the screws by using insertional torque. These various options are all available in the Expedium Neuromuscular and Pelvic Fixation Set (DePuy Synthes Spine, Raynham, MA) in closed and open screw head options along with offset connections that will allow modular, rigid connections to the longitudinal rods up to the spinal implants. Since the typical iliac pathway starting at the PSIS is 1 to 2 cm lateral to the pedicular line, offset connectors are frequently needed and options in the set are fixed or variable axis, open or closed connectors that can be customized for length, or short-throw rod contour and attached to the rod with slip strengths equal to Expedium pedicle screw connections. Alternatively, the longitudinal rods can be contoured laterally in the coronal plane distal to the L5 or S1 screws to connect directly to the iliac screws either prior to rod insertion or with in situ benders.

A retrospective, single-center cohort study comparing two groups of 20 patients (flaccid and spastic paralytic scoliosis) each with Luque–Galveston constructs and iliac screws showed similar maintenance of pelvic obliquity and scoliosis correction, but the iliac screw techniques avoids the complex lumbosacral three-dimensional rod bends and had less haloing around the pelvic implants with minimal implant complications. The Galveston group had four broken rods and two reoperations and the iliac screw group had one broken screw and no reoperations. ²²

A larger multicenter retrospective study of 157 patients with virtually equal distribution compared the unit rod to “custom-bent” rods with iliac screw fixation and found that although the unit rod had better pelvic obliquity correction, mean surgical time, blood loss, hospital stay, infection rate, and proximal fixation problems were significantly higher in the patients with unit rods. ²³

Related posts:

19 Osteotomies: Ponte and Vertebral Column Resection 18 Halo-Gravity Traction: An Adjunctive Treatment for Severe Spinal Deformity 17 Comparison of Unit Rods with Modular Constructs in Cerebral Palsy 20 Growing Spine Options for Neuromuscular Scoliosis 21 Anterior Approaches to the Spine for Neuromuscular Spinal Deformity 8 Surgical Treatment of Spinal Deformity in Myelomeningocele

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