Perspective Chapter: Sacral Alar Iliac Screws

Abstract

Often during the reconstruction of Adult and Pediatric Spinal Deformities pedicle screw fixation is insufficient and a distal anchor with pelvic fixation is of paramount importance. Various techniques regarding the methodologies of Spinopelvic fixation (SPF) and placement of Sacral-1 Alar Iliac (S1AI) and Sacral-2 Alar Iliac (S2AI) screws has been described in the literature. While there is some consensus among the various techniques and angles utilized, these are often difficult to reproduce in an operative setting. Recently navigation systems have allowed for the successful navigation of the Sacro Alar Iliac corridor without screw complications, however not every facility has this technology on hand. Therefore, proficiency with the safe and accurate placement of Sacroalar iliac (SAI) screws using standard fluoroscopic imaging becomes a vital technique when circumstances demand ample distal fixation. To our knowledge, a definitive step by step review of the sacroalar iliac technique warrants description in order to delineate the insertion point and exact fluoroscopic imaging which can help confirm the safe placement of SAI screws through the anatomical corridor.

1. Introduction

During revision surgery or reconstructive procedures spine surgeons are confronted with the need for sufficient distal spinopelvic fixation. There are several conventional approaches towards addressing sacro-pelvic fixation, each facing distinct challenges in the operative setting. Often in order to perform sacroalar iliac (SAI) screw fixation there are certain factors which must be taken into consideration. Careful deliberation over the exact number of SAI screws needed for the construct should be contemplated. Bone density and the pullout strength of any prior hardware needs to be taken into account. Screw prominence also plays a role and must be considered when evaluating patients’ body habitus and relevant soft tissue coverage [1, 2]. Imaging and hospital equipment needs to be planned well in advance if computer navigation or robotic instrumentation needs to be accounted for. The goal of this chapter is to introduce the relevant anatomical entry points, bony morphology and fluoroscopic imaging required to safely place Sacral Alar Iliac Screws in the bony anatomic corridor in efforts towards achieving sufficient distal fixation for all spinal constructs.

2. Surgical technique and pearls

2.1 Positioning

The patient is positioned prone on a radiolucent table and all bony prominences are padded. The following anatomical landmarks should be palpated, marked, and recognized: The anterior inferior iliac spine (AIIS), posterosuperior iliac spine (PSIS), the posteriorinferior iliac spine (PIIS), the median sacral crest, the lateral sacral crest and the greater trochanter. Initially the base of the fluoroscopic image intensifier should be positioned on the opposite side of the table as the surgeon. The midline is marked and the area should be prepped and draped in standard fashion with careful placement of the drapes as distally as possible along the intergluteal cleft.

2.2 Bony anatomy and morphology

Morphologically the entry point for the SAI screws lies near the intersection of the lateral sacral crest and a line drawn midway between the outermost lateral rim of the S1 and S2 dorsal sacral foramina (Figure 1). A distinct osseous landmark is often difficult to visualize attributing to the overhang from the PSIS and the upward slope of the lateral sacral crest. Dissection near the lateral sacral crest may stir up bleeding from the foraminal arterial branches of the lateral sacral artery. There will also be filiform nerve fibers from the dorsal branches of the sacral nerves [3]. The literature describes a wide range of specific insertion points and trajectories for the SAI screws which can be difficult to reproduce in the operative theater [3, 4, 5]. In light of this the following technique uses fluoroscopic imaging to help confirm the safe placement of SAI screws in vivo.

Figure 1.

Entry point for SAI screws lies near the intersection of the lateral sacral crest and a line drawn midway between the outermost lateral rim of the S1 and S2 dorsal sacral foramina.

2.3 Instrumentation and implants

An oscillating drill with a 3.5 mm threaded guide wire or a curved awl is used to cross the sacroiliac joint and along its course will ultimately penetrate the two cortices of the sacrum and the outer iliac cortex in a trajectory towards the anterior inferior iliac spine (Figure 2A and B). For ease of ultimate screw placement a long threaded guidewire can be inserted and then confirmed radiographically. If the imaging demonstrates a trajectory that is not ideal in any plane, a second guidewire can be situated next to the initial trajectory in order to provide any needed course correction, then the first guidewire can be removed (Figures 3 and 4). Consideration at this point should be made if there are one or two SAI screws being placed in order to allow sufficient clearance as the trajectories are being set and defined.

Figure 2.

(A) 3.5 mm Threaded guide wires, (B) Oscillating drill.

Figure 3.

In order to achieve optimal placement a second guidewire can be inserted in a different plane from the initial guide wire.

Figure 4.

Each guide wire can be visualized fluoroscopically, then the optimal trajectory can be selected based on fluoroscopic imaging. Once proper trajectory has been confirmed the second guidewire can be removed with the oscillating drill.

3. Fluoroscopic imaging

3.1 Entry point

The obturator outlet view (OOV) renders an inclined teardrop outline along the involved hemipelvis. This teardrop structure itself represents the superimposition of anterior and posterior columns. This bony teardrop reflects the sacroiliac osseous corridor wherein the SAI screw will reside (Figures 5 and 6). The tubular shaped corridor resembles the geometric configuration of a frustrum (Figure 7). In order to obtain a proper OOV/teardrop view starting from a neutral Anteroposterior (AP) position, the fluoroscopic intensifier should be angled approximately 20–30° cephalad in the coronal plane, and then rotated over the involved hemipelvis 20–30° until the widest possible teardrop can be visualized while leaving a sufficient rim of the anterior inferior iliac spine’s outer ridge (Figures 8–10). On the fluroscopy view optimal placement of the teardrop should sit above the femoral head and the acetabular sulcus (Figures 5 and 10).

Figure 5.

Fluoroscopy view demonstrating the bony teardrop along the sacroiliac osseous corrido, with the tip of the guidewire in the northern corner of the teardrop.

Figure 6.

Animation reflecting the bony teardrop visualized during the obturator outlet view.

Figure 7.

In geometry a frustrum is the three dimensional shaped object enclosed between the two planes of another solid, it is similar to a cone with the pointed tip of the cone removed. This pink tubular shaped bony corridor represents the channel through which the SAI screws must traverse.

Figure 8.

Step 1: In order to obtain the obturator teardrop view (OOV), first the fluoroscopic intensifier should be angled approximately 20–30° cephalad in the coronal plane.

Figure 9.

Step2: Next the intensifier should be rotated over the involved hemipelvis another 20–30° until the widest possible teardrop can be visualized while leaving a sufficient rim of the anterior inferior iliac spine’s outer ridge.

Figure 10.

Step3: Rotate the Carm over the involved hemipelvis until a distinct teardrop is visualized while also leaving a sufficient lateral rim of the anterior inferior iliac spine. Final placement should center the teardrop over the femoral head and acetabular sulcus.

Once the entry point has been established, another threaded guide wire can be placed parallel to the first wire in order to accommodate two screws in the SAI corridor. Alternatively, if the construct necessitates only a single screw then at this point the threaded guide wire can be advanced to assure positioning over the sciatic notch and within the margins of the iliac wing. Ideal placement within the teardrop for a single screw has been found to be at the intersection of lines formed by the longest possible horizontal/transverse line which can be drawn along the width of the teardrop, and a vertical line dropped from the apex of the teardrop itself [6]. Final correct placement of the screw should demonstrate the entire screw lies securely within the confines of the bony teardrop which represents the frustrum spanning the entire bony sacroalar iliac corridor (Figures 11–14).

Figure 11.

Inlet view: This caudad projection of the pelvic bony rim is obtained by tilting the fluoroscopy 35–45° cephalad.

Figure 12.

Inlet view demonstrating the threaded guidewire within the bony confines of the sacroalar iliac corridor. On this view ensure the guide wire has not violated the pelvic rim medially.

Figure 13.

Outlet view: This cephalad projection is perpendicular to the plane of the sacrum and is obtained by aiming the C-arm 35–45° caudad.

Figure 14.

This outlet view provides further confirmation the threaded guidewire is within the bony constraints of the sciatic buttress and pelvis.

3.2 Advancement

In order to confirm safe advancement within the frustrum sequential radiographic views must be obtained. First a standard AP View can be utilized to help identify the S1 Endplate, the S1 and S2 Foramen. While in this position a manual palpation of the bony prominences of the Anterior Inferior Iliac Spine (AIIS) and the greater trochanter can serve as relevant landmarks for screw trajectory. The guide wire can be meticulously advanced on the AP view to check the cranio caudal angle as the pin needs to pass 35 mm inside sacral bone and then within the ilium it would need to traverse another 35–65 mm of cancellous bone [3].

The next imaging that should be obtained is an inlet and outlet view which can confirm accurate placement within the sciatic buttress and corroborate that no medial violation has occurred towards the neurovascular structures and/or bladder (Figures 11–14).

As a final view a true lateral image should be obtained where both sciatic notches are perfectly superimposed (Figure 15). This will serve as the primary view for guidewire insertion and screw final advancement. On the lateral image great care should be taken to identify the acetabulum and the greater Sciatic Notch, while ensuring the wire is headed towards the AIIS. Ideally the shaft of the screw should reside 1–2 cm above the acetabulum and the threads of the screw should graze the cortico-cancellous junction of the bone adjacent to the sciatic notch.

Figure 15.

On a true lateral view confirmation of proper guidewire placement above the acetabulum and sciatic notch can be confirmed.

3.3 Screw placement

The cannulated screw itself can be placed in line with the threaded guidewire and advanced under fluoroscopy. The guidewire can be removed and visualized on fluoroscopy, prior to final screw placement. Alternatively, if an awl is used then it can be removed and the path can be palpated with a ball probe prior to screw placement. Final screw dimensions are typically a width either 8–9 mm and a length ranging from 70 to 110 mm [3]. Prior to final screw placement a 6 or 7 mm tap can be inserted under fluoroscopic visualization. Final screw positioning can be confirmed on the Iliac Oblique View which provides a lengthwise lateral view of the entire screw to reconfirm placement above the acetabulum and sciatic notch.

3.4 Case study

61 year old male presents with intense lumbar complaints which are 50% bilateral leg pain and 50% low back pain. The radiculopathy in the legs were rated 6–8 out of ten. The radicular pains were more severe in the right leg. The patient has tried and failed a course of conservative measures inclusive of transforaminal epidural injections, physical therapy, bracing, acupuncture, and Nonsteroidal anti inflammatories. Lumbar MRI and CT scans demonstrated a considerable amount of hardware loosening, and significant lucency around the hardware (Figures 16 and 17). The decision was made to remove the prior hardware, secure proximal fixation, and obtain distal Sacral alar-iliac hardware fixation (Figures 18–19). Postoperatively the patient noted a resolution of both back pain and radiculopathy and returned to regular activities including work within three months. He remained asymptomatic at his one year postoperative appointment.

Figure 16.

Lumbar CT Sagittal 3month postoperative views demonstrating significant hardware lucency and the need for hardware revision with sacroiliac alar screws in order to achieve distal fixation.

Figure 17.

Lumbar CT Sagittal 3month postoperative views demonstrating significant hardware lucency around the screws and the PEEK interbody cages.

Figure 18.

Postoperative AP Demonstrating SAI screw fixation.

Figure 19.

Postoperative Lateral Imaging Demonstrating SAI screw fixation.

4. Conclusions

Sacral alar-iliac screws have been substantiated as a practical and effective means of achieving robust distal fixation when addressing situations ranging from revision surgery, fractures, tumor resection procedures, flat back deformity, paediatric and adult scoliosis. With properly indicated patients, meticulous preoperative planning, and sound surgical technique, sacroiliac screw fixation offers an excellent surgical option for many patients in need of distal fixation across the lumbosacral junction.

Conflict of interest

The author declares no current conflict of interest.