19.1 Introduction

The sacroiliac joint (SIJ) has recently gained increased attention as a common source of low back pain. In a study of 200 patients presenting to a spine clinic with low back pain, only 65% of patients were found to have pain originating from the spine alone, while 5% had pain coming from the SIJ and 15% had pain from both the SIJ and the spine. ¹ Causes of SIJ pain and dysfunction include trauma, osteoarthritis, inflammatory arthritis, infection, and tumors. Degeneration of the SIJ following lumbosacral spinal fusion is also very common. ² In fact, up to 75% of patients develop SIJ degeneration within 5 years following lumbar fusion. ³

The burden of SIJ pathology on patients’ outcomes is also significant. In a recent analysis, SIJ pain was considered to be as debilitating as other common orthopaedic conditions, including hip and knee osteoarthritis, spinal stenosis, and spondylolisthesis. ⁴ Additionally, the economic burden of the care for these patients is also significant. Nonoperative management can cost as much as $5,259 per patient per year, with the cost increasing to as much as $30,000 in those with SIJ dysfunction following lumbar spine fusion. The 5-year estimated cost to Medicare beneficiaries is $270 million, with a 3-year cost of $1.6 billion per 100,000 commercial covered lives. ^{5 ,​ 6}

Initial treatment for SIJ pathology consists of nonoperative modalities including physical therapy, nonsteroidal anti-inflammatory drugs and pain medications, intra-articular injections, and radiofrequency ablation. Unfortunately, there is little evidence that these nonoperative modalities provide long-lasting relief of SIJ-mediated pain. The only exception to this is radiofrequency ablation of the sacral nerve root lateral branches, which has shown some promise in the short term for providing relief of pain, lasting up to 12 months in some cases. ^{7 ,​ 8 ,​ 9}

While SIJ fusion was initially performed via an open approach, the surgeries were fraught with complications and approach-related morbidity, as well as poor patient-reported outcomes. ^{10 ,​ 11 ,​ 12} Currently, the standard of care for SIJ fusion includes a minimally invasive approach to the SIJ either posteriorly or laterally, with placement of bone or one of several Food and Drug Administration (FDA)-approved porous metal implants to provide stability and bone ingrowth. Our preferred technique is a laterally based approach with placement of three triangular titanium implants. Most surgeons perform the surgery though a minimally invasive approach using fluoroscopic views of the pelvis and sacrum. Now, however, with the use of intraoperative CT scan combined with navigation technology, the same procedure can be performed with increased accuracy in a minimally invasive fashion without the need for intraoperative fluoroscopy.

19.2 Indications

The indications for MIS SIJ fusion include pain attributable to SIJ dysfunction that impacts quality of life and that has failed at least 6 months of nonoperative treatment. The surgeon must perform a thorough history and physical exam to ensure that the pain is not related to another source. SIJ pain should be confirmed with at least three physical examination maneuvers that focus on the SIJ in particular. Pain relief following intra-articular injection is a good prognostic sign that the SIJ is the source of the pain.

While there is no absolute indication for navigated SIJ fusion as compared to fluoroscopy-guided techniques, there are several circumstances in which it can be beneficial. The most important of these are in patients with sacral dysmorphism, in whom abnormal sacral morphology may complicate safe implant placement. Radiographically, patients with dysmorphic sacral anatomy can be identified by multiple features, including having the superior aspect of the iliac crests at the level of the L5–S1 disk space, the presence of irregular and large sacral foramen, upper sacral mammillary processes, residual sacral disk spaces, and an acute alar slope that is not collinear with the iliac cortical density on the sacral lateral view. In these patients, the steeper, acute alar slope provides less bone in which to place implants. Additionally, the anterior aspect of the sacrum may also have a deeper groove which contains the L5 nerve root, further restricting the safe osseous pathway across the SIJ into the sacrum. While previously considered to be rare, sacral dysmorphism and upper sacral abnormalities are actually quite common, and some degree of abnormal anatomy is present in about 50% of the population. ^{13 ,​ 14} Navigation in these cases permits safe and easy passage of implants in pathways unique to the patient morphology.

Navigated techniques need not be limited to only those patients with abnormal anatomy, for even those with normal anatomy may present challenges in the operating room. The traditional MIS technique relies on radiographic interpretation of the pelvic inlet, pelvic outlet, and sacral lateral views. Doing so requires the surgeon not only to have an understanding of the anatomy and radiographs but also must be able to obtain these views intraoperatively. Studies have shown that while the anatomy is familiar to spine surgeons, it is not always well visualized. ^{15 ,​ 16} Large patients or patients with significant amounts of bowel gas may make navigating radiographic trajectories difficult, as it limits the ability to visualize the sacral foramina. Three-dimensional (3D) navigation can circumvent the need to rely on low-quality C-arm images.

Finally, revision cases may be particularly challenging. New implants may need to be placed, and there is limited bone remaining for placement. Navigation in these instances can be helpful to maximize bone purchase in compromised anatomy (see case example 2 later in the chapter).

19.3 Technique

We perform the procedure prone on a Jackson spine table. A wide field is prepped, including as much of the lateral aspect of the hips as possible including the trochanters. First, a small incision is made over the posterior-superior iliac spine (PSIS), usually on the contralateral side, and a pin is placed into the ilium and attached to a reference array. The patient is then draped and a passed into the O-arm intraoperative spin. This generates a 3D CT scan with real-time feedback from the instruments.

A skin incision is marked laterally using a reference probe to determine the appropriate location. The reference probe is then used to determine the ideal trajectory for the first implant into S1. A guidewire is drilled through the ilium, across the SIJ, and into the sacrum with navigation. A soft-tissue dilator followed by tissue protector is placed over the guidewire, and the dilator is then removed. An appropriate-size implant is determined using the navigation to help choose the ideal implant size. The guidewire is overdrilled, the path is broached, and the first implant is placed. A parallel pin guide is used to then place a second guidewire below the first. However, the second guidewire can also be placed freehand using navigation if a better location is determined, especially in cases of abnormal anatomy. The second and third implants are placed using the same technique as the first. To double check that the navigation has resulted in appropriate implant placement, a postimplant intraoperative CT can be obtained. Alternatively, pelvic inlet, outlet, and lateral fluoroscopic views can be obtained to confirm the location of the implants.

The wound is irrigated and hemostasis is achieved if any obvious bleeding is encountered. The layers are closed with Vicryl for the deeper layers and Monocryl for the skin. Patients are given a period of 3 weeks of partial weight bearing and are then progressed to full weight bearing.