Primary indications for pelvic fixation include high-grade lumbosacral spondylolisthesis (Meyerding Grade III or IV). For high-grade spondylolisthesis, the reduction of lumbosacral kyphosis to restore spinopelvic sagittal alignment can result in high mechanical complication rates due to the significant shear forces at the lumbosacral junction. Biomechanically, fixation extending to the pelvis helps offset the cantilever forces exerted for correction of high-grade spondylolisthesis [32].

A common indication for pelvic fixation is a long construct fusion for adult spinal deformity, particularly in the revision setting where distal fixation in the sacrum has previously failed and incorporation of the pelvis is required. Historically, what constitutes a long construct has been controversial. Some surgeons consider it to be one that extends to L2, while others contend that a long fusion is one that crosses the thoracolumbar junction [18, 33–35]. It is our experience that for most adult patients, fusion that extends proximally to L2 or higher creates sufficient biomechanical stresses at the lumbosacral junction that pelvic fixation is required.

Other conditions that may require pelvic fixation include paralytic kyphoscoliosis and neuromuscular kyphoscoliosis and congenital scoliosis. Lumbosacral deformities warrant pelvic fixation to offset the biomechanical stresses imposed on the construct to maintain deformity correction.

Rather than a specific level, more important is the concept that ending the distal construct in the lumbar spine could result in residual coronal imbalance or sagittal kyphosis that could progress over time. Multiple authors therefore advocate extending long spinal arthrodeses to the pelvis and augmenting it with anterior L5–S1 interbody fusion to prevent the development of flat back syndrome [36–38]. In the revision setting, the presence of pseudarthrosis at the L5–S1 junction with loose S1 screws is another indication for extension of fusion, with inclusion of pelvic fixation.

Degenerative spinal deformities involving the lumbosacral junction are common indications for sacropelvic fixation, including oblique take-off of L5, adult degenerative scoliosis, revision decompression surgery, and postlaminectomy flat back syndrome. Along with advanced degeneration of the L5–S1 motion segment, these conditions cause lumbosacral instability, exerting huge biomechanical stresses on the construct. In such deformities, extending the fusion to the pelvis is a prerequisite to achieving and maintaining the correction [39].

In the setting of adult spinal deformity, sacropelvic fixation is indicated with the use of corrective osteotomies to correct coronal and sagittal malalignment. A three-column osteotomy or multiple posterior column osteotomies, when utilized to recreate lumbar lordosis, may require extension of fusion to the pelvis to maintain the correction. It is our recommendation that a minimum of six points of fixation are required distal to the osteotomy site; however, if pelvic anchors are used, four points of distal fixation in addition to two pelvic anchors may be adequate to prevent excessive motion and pseudarthrosis at the lumbosacral junction.

Other indications include long segment fusions in the setting of osteoporotic or traumatic fractures. We had a case at our institution of a 52-year-old female, with severe osteoporosis, who developed a sacral fracture and sagittal malalignment 6 months after posterior spinal fusion L3–S1 for degenerative scoliosis at an outside institution. Our approach for this case was to remove all previous instrumentation and use larger size, bicortical, pedicle screws at L3–S1 for sufficient purchase. We then proceeded to place two S2AI screws bilaterally, referred to as a dual screw technique, and perform a sacral osteotomy to achieve correction in the sagittal plane, as well as translation (Fig. 16.6).

In all of these cases, the purpose of sacropelvic fixation is to provide structural support to partially unload S1 and/or S2 screws until fusion has occurred, thereby preventing fixation failure and progressive deformity. In particular, sacral insufficiency fractures complicating long fusions can pose a significant challenge. The authors believe that rigid spinopelvic fixation is the best prophylactic measure and serves at the same time as the treatment of choice for these fragility fractures [34]. Traumatic lower lumbar vertebra or sacral fractures with spinopelvic dissociation similarly demand spinopelvic fixation, a mechanically stable construct that allows weight bearing.

Pelvic fixation is also used in pathologies involving destruction of the sacrum, such as from neoplasm, infection, or sacral fractures leading to spinopelvic disassociation. The sacroiliac joint plays a critical role in load transmission from the axial skeleton to the lower limbs, and therefore lumbopelvic stabilization should be considered in the case of sacrectomies above the S1 foramina and total sacrectomies. If it is not possible to salvage the sacral pedicle for screw fixation, options include a bridging bone graft from the L5 transverse process to the sacrum or an anterior lumbar interbody fusion at L5–S1 for caudal support [40].

To preface, failure with S1 pedicle screw fixation as the sole means of distal fixation of a long construct can be as high as 44 % [8, 15]. The key factors that play a role in distal sacral fixation alone include screw length and diameter, as well as bi- or tricortical screw placement. Studies have also confirmed the relationship between bone mineral density (BMD) and screw fixation strength [41]. Failure of the S1 pedicle screw largely occurs from insufficient sacral bone stock, whether by virtue of a small S1 sacral body or osteoporotic bone, or incorrect direction or depth of the screw.

Options for S1 pedicle screws include bicortical fixation with an entry point either in the anterior sacral cortex or the S1 superior endplate, or tricortical fixation incorporating the apex of the sacral promontory (posterior sacral cortex). Tricortical technique involves directing the screw toward the medial sacral promontory, allowing purchase of the dorsal cortex, the anterior cortex, and the superior endplate cortex (Fig. 16.7). This technique therefore provides three potential points of fixation. Studies have confirmed the biomechanical advantage of tricortical versus bicortical S1 screws, based upon the ability to insert longer screws with the tricortical purchase trajectory, and doubling the insertional torque of the bicortical screw inserted parallel to the S1 endplate [42]. This latter configuration is thought to improve pullout strength and to increase load to failure, both of which are particularly important concepts in osteoporotic bone.

Although the combination of S1 and S2 pedicle screws is stronger than S1 screws alone, the S2 pedicle screw remains dorsal to the lumbosacral pivot point limiting its effect on overall strength of the lumbosacral fixation construct [12]. Thus, S2 screws add little to the overall biomechanical strength of the construct in resisting flexion at the lumbosacral junction. Zindrick et al. noted that, compared with other sacral screws, medially directed S2 screws had the worst pullout strength and that screws inserted at a 45° lateral angle into the ala and medially into the first sacral pedicle were the strongest [11].

Sacral alar screws are placed into the lateral anterior cortical bone of the sacrum and are aimed laterally 30° to 45° [30]. The safe zone is narrow, with potential for injury to the lumbosacral trunk, the internal iliac vein, and the sacroiliac joint. Screw lengths average 38 mm with 30° lateral angulation and 44 mm with 45° lateral angulation [30]. Poor clinical results and high pseudarthrosis rates have been noted when these screws were used for long fusions to the sacrum [8].