Bones, Ligaments, Muscles, Tendons, Nerves, Vessels, and Lymphatics

The SIJ is formed by the C-shaped articulating surfaces of the sacrum and ilium of the pelvis. Accordingly, this synovial joint is capable of subtle uniplanar flexion and extension motion. Acting like a keystone, the sacrum translates the compressive force of the axial skeleton outward toward the pelvis. In so doing, the SIJ is subject to torsion, shearing, rotation, and tensile forces. The majority of what we consider to be the SIJ space for interventional procedures is, in fact, outside of the joint capsule, which is at the anterior-most aspect of the true joint. The posterior joint is composed of dense fibrous bands from the posterior sacroiliac and sacrotuberous ligaments. Anteriorly, the SIJ space is bounded by the anterior sacroiliac and sacrospinous ligaments. Together, these dense ligaments provide substantial stability to the SIJ (Figures 43.1 and 43.2).

Afferent innervation of the SIJ takes a wide course as it enters the spinal cord from L4–S2 roots. Specifically, the superior aspect of the joint is innervated by the ventral rami of L4 and L5 spinal nerve roots and the superior gluteal nerve. Whereas the inferior aspect of the joint is innervated by the dorsal rami of L5–S2. Overlying cutaneous innervation over the gluteal region is sensed by the superior and middle cluneal nerves, branches of the L1–3 spinal root levels. These nerves are germane to the discussion of the SIJ as they can both act as false flags during provocative physical exam maneuvers, and are subject to injury during SIJ intervention (Figure 43.3).

While the anterior aspect of the SIJ is perfused by the iliolumbar artery, the posterior aspect of the SIJ is perfused by the median and lateral sacral arteries. Both the iliolumbar and sacral arteries are branches off the common or internal iliac artery. It is also important to note that the superior and inferior gluteal arteries hook under the inferior aspect of the SIJ line.

Landmark Approach

The patient is placed in the prone position. The skin overlying the affected SIJ is cleansed and draped in sterile fashion. The posterior superior iliac spine (PSIS) is then identified with palpation as the most prominent protuberance lateral to the sacral base, roughly at the level of L5–S1. Then a sterile needle (typically 24G 3.5inch) is inserted roughly 1 cm medial from the PSIS with a 45° trajectory in a lateral direction. If a hard endpoint is encountered superficially, it is likely the PSIS itself, requiring a medial correction. Conversely, if a hard endpoint is encountered deeper, this is likely the sacral ala and requires a lateral correction. The posterior ligaments of the SIJ provide a distinct resistance to advancing the needle. After piercing through the posterior ligaments of the joint space, gentle aspiration should be performed and the therapeutic of choice can be injected. We advocate the use of a 3 ml syringe that includes an injectant composed of 0.25% bupivacaine with 40 mg triamcinolone. This syringe size provides the proceduralist with granular tactile feedback. This is of high utility, as appreciable resistance to injection may indicate that the needle tip is either in ligament (requiring slight advancement of the needle) or subperiosteal (requiring redirection into the joint). The latter of the two is often accompanied by significant patient discomfort. It is important to note, the SIJ space itself cannot accommodate a large injectant volume, and patients typically experience deep pressure sensation with even perfectly executed SIJ injections. After needle removal, a sterile dressing and ice pack are applied at the injection site.

Fluoroscopically-Guided Approach to SIJ Injection

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