18 Navigated Pelvic Fixation

10.1055/b-0039-172729

18 Navigated Pelvic Fixation

Haruki Funao, Brian J. Neuman, and Khaled M. Kebaish

Abstract:

Pelvic fixation serves as a rigid distal foundation in various spinal surgeries in the lumbosacral spine, such as spinal deformity, pseudarthrosis, tumor, and infections. In these settings, a secure distal foundation with pelvic instrumentation is critical for maintaining correction and enhancing fusion rates, particularly in spinal deformity requiring long spinal constructs to the sacrum. Iliac screws and S2 alar–iliac screws are the most commonly used with high fusion rates. However, serious complications with pelvic fixation can occur such as neurologic, vascular, or bowel injuries due to the misplacement of pelvic screws. Pelvic screws can be placed with freehand technique or image guidance using either intraoperative fluoroscopy or computed tomography (CT)-based navigation. CT-based navigation can provide three-dimensional (3D) visualization of anatomy not clearly evident through surgical exposure alone. Other advantages include real-time 3D visualization of the screw trajectories and reduction of intraoperative fluoroscopy usage. In the setting of spinal deformity, this may be particularly advantageous, as unusual lumbosacral morphology can make screw insertion challenging. CT navigated pelvic fixation can also make minimally invasive pelvic screw insertion safer and easier without exposing anatomical landmarks. In our review, we describe the navigated pelvic fixation techniques.

18.1 Introduction

Pelvic fixation can serve as a rigid distal foundation in various spinal surgeries in the lumbosacral spine, such as cases of spinal deformity, pseudarthrosis, tumor, and infections. ¹ ^, ² ^, ³ ^, ⁴ The sacrum is composed of mostly cancellous bone; thus, S1 screws alone may not achieve adequate fixation, especially in patients with osteoporosis. Secure distal foundation with pelvic instrumentation is critical for maintaining correction and enhancing fusion rates particularly in spinal deformity requiring long spinal constructs to the sacrum with high rates of pseudarthrosis. ⁵ ^, ⁶ Sacropelvic fixation helps resist the strong flexion moments and cantilever forces present at the lumbosacral junction, preventing fixation failures.

To date, various pelvic fixation techniques have been developed. Allen and Ferguson reported Galveston iliac fixation with advantages of an anchor in the ilium to resist flexion bending moment. ⁷ Iliac screws and S2 alar–iliac screws are most currently used with high fusion rates. ² ^, ⁸ McCord et al described the concept of a “lumbosacral pivot point” (Fig. 18‑1), a point located at the lumbosacral joint at the intersection of the middle osteoligamentous column and the lumbosacral intervertebral disk, and demonstrated that iliac screws, which cross this point, had a biomechanical advantage at resisting flexion moments with an improved pullout over the Galveston technique. ⁹ Accordingly, the pelvic screws should terminate anterior to the pivot point increasing the pullout strength and decreasing the strain on the S1 pedicle screws, reportedly leading to higher fusion rates across the lumbosacral junction. Although iliac screws have secure distal foundation, drawbacks include extensive soft-tissue dissection around the posterior-superior iliac spine and instrumentation prominence. Recently, the S2 alar–iliac technique has been developed for both adults and children requiring pelvic fixation, with the benefits of less soft-tissue disruption, low-profile instrumentation, and in-line rod connection to the proximal constructs ² (Fig. 18‑2). To date, pelvic screws have been placed with freehand technique or image guidance using either intraoperative fluoroscopy or CT-based navigation. The misplacement of pelvic screws can lead to serious complications such as neurologic, vascular, or bowel injuries. Navigated pelvic screw placement can be beneficial when anatomy is not clearly evident.

**Fig. 18.1** The lumbosacral pivot point (*black dot*) at the lumbosacral joint at the intersection of the middle osteoligamentous column and the lumbosacral intervertebral disk. Pelvic screws should terminate anterior to the pivot point to prevent pullout of S1 pedicle screws with a biomechanical advantage at resisting flexion moments.

**Fig. 18.2** (**a,b**) A 68-year-old female presented with a worsening back pain with a history of multiple spine surgeries. She underwent a posterior spinal fusion from T10 to the pelvis with bilateral S2 alar–iliac screw fixation. No significant radiographic or clinical signs of distal junctional failure or nonunion were evident on her 1-year postoperative radiographs.

18.2 Navigated Pelvic Fixation

Inadequate screw length due to a violation of the outer cortex of the ileum or other screw malpositioning, which fails to cross the aforementioned pivot point, can lead to screw failure, as the biomechanical advantages described earlier may not be achieved. Furthermore, serious complications in pelvic screw insertion, such as neurologic, vascular, or bowel injuries, can occur with misplaced pelvic screws that breach the inner or outer cortex of the ileum, acetabulum, or sciatic notch. ¹⁰ ^, ¹¹ ^, ¹² Lack of adequate visualization or lack of familiarity with pelvic configuration can be the cause of some of these complications. Several techniques have been reported to reduce the risk of pelvic screw misplacement, including lateral soft-tissue dissection of the ilium to palpate the sciatic notch, using intraoperative fluoroscopy, ¹³ and CT-based navigation ¹⁴ ^, ¹⁵ ^, ¹⁶ (Fig. 18‑3). Image guidance may provide surgeons more information than surgical exposure alone.

**Fig. 18.3** Screen captures from the Brainlab images. The navigated probe is placed at the ideal starting point of S2 alar-iliac screw with the intended trajectory in 3D visualization on the computer screen (a). “Probe’s eye,” a view through the screw, enables confirmation of the position of the navigated probe even in the narrowest part of the pelvis (b). Screw length can additionally be measured from the three-dimensional images. An S2 alar-iliac screw successfully placed without cortical breach of the sacrum and pelvis (**c,d**).

Although pelvic screws can be placed freehand or with image guidance, ¹⁴ in cases of complex lumbosacral anatomy, navigated techniques may be particularly useful. Although intraoperative fluoroscopy is useful, radiation exposure has been a source of consternation for many adopters. ¹⁷ In addition, fluoroscopy is limited as a two-dimensional representation of a 3D configuration of the pelvis. ¹⁸

Intraoperative CT-based navigation is an innovative technology in spine surgery with the safety and ability to reduce radiation exposure to the surgeon and shorten surgical time with real-time 3D feedback, especially when visibility is suboptimal or anatomic abnormality is present. ¹⁹ ^, ²⁰ ^, ²¹ Other benefits are the ability for accurate MIS placement and to measure ideal screw length and diameter. A meta-analysis of 3D navigated pedicle screws has demonstrated a screw accuracy rate of 95%. ²²