Diagnosis

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The diagnosis of SI joint pain is a diagnosis of exclusion. Other etiologies of pain such as spinal stenosis, herniated disc, and facet degenerative disease must first be excluded. Various physical maneuvers (e.g., Patrick's test, Gillet test, Gaenslen's maneuver, pain with pressure application to the SI ligaments at the sacral sulcus with the patient prone) have been described to diagnose SI joint pain3,16,31-32 but may be unreliable due to the lack of intraobserver and interobserver repro-ducibility.14,33,34 Additionally, many of these maneuvers also stress the lumbar spine or hip joints,35 which may confound interpretation.

Findings of sacroiliitis obtained by computed tomography (CT) include joint space widening or narrowing, juxta-articular demineraliza-tion, osteophytes, subchondral sclerosis, erosions of the cortical surfaces and subchondral bone, and ankylosis (Figure 13.2). However, imaging abnormalities on standard CT images are relatively poor predictors of which patients have pain or which patients will obtain relief from SI joint injection. Elgafy et al.36 evaluated the CT scans of 62 patients with SI joint pain who responded to SI joint injection and compared these with the CT scans of 50 asymptomatic age-matched controls. At least one CT finding suggestive of SI joint pathology (osteophytes, joint space narrowing <2 mm, subchondral sclerosis, joint erosions, or ankylosis) was seen in 57.5% of symptomatic patients and 31% of controls. In contrast, CT findings were negative in 42.5% of symptomatic patients.

Congenital Pathology Sacroiliac Joint
Figure 13.2. Axial CT image demonstrating degenerative changes in the sacroiliac joint, including osteophytosis and subchondral sclerosis.

Bone scan findings also have been determined by some authors to correlate poorly with SI joint symptoms. Slipman et al.37 demonstrated poor sensitivity (12.9%) of positive bone scan findings in patients responding to SI joint injection. In comparison with conventional bone scintigraphy, imaging by means of single-photon emission CT (SPECT) permits better differentiation of radiotracer uptake in the ventral synovial portion of the joint, suggestive of inflammatory causes of sacroili-itis, from uptake in the dorsal syndesmotic portion of the joint, more typical for bony changes due to axial loading.38,39

Magnetic resonance imaging (MRI) allows for detailed evaluation of the SI joint and adjacent soft tissues and is particularly valuable in detecting early changes in the joint in inflammatory and infectious sacroiliitis.39-45 Typically, MRI (Figure 13.3) demonstrates focal hyper-intensity in periarticular bone on T2-weighted and short tau inversion recovery (STIR) sequences.45-46 Bollow et al.42 found evidence of early periarticular erosions and contrast enhancement of the joint capsule in MRI imaging of 72% of patients with seronegative spondyloarthropa-thy and early sacroiliitis but essentially no enhancement in control patients with mechanical causes of low back pain.

Injection of the SI joint has emerged as a diagnostic test, as well as a therapeutic procedure. Diagnostic intra-articular injection was first described by Haldeman and Sotohall.47

More recent investigators have described the use of fluoro-scopic,5,6,48,49 CT,43,44,50,51 and MR52 guidance to perform the procedure, which greatly improves accuracy of injection.53 Reported clinical effectiveness of SI joint steroid injection has been variable, with some authors reporting little or only transient patient relief51 and others reporting significant decrease in low back pain.5,6,49,52

Indications for the procedure include edematous change in the SI joints on inversion recovery MR sequences52 or a positive response to stress maneuvers on physical exam in patients who fail to improve with physical therapy.49

SI Joint Injection Technique

SI joint injections are performed on an outpatient basis and can be performed with fluoroscopic, CT, or MR guidance. Intravenous sedation before or during the procedure is generally not required.

The patient is placed in the prone position and wide sterile preparation of the soft tissues over the sacrum and buttocks is performed. If C-arm fluoroscopy is to be used in imaging the joint, the x-ray beam is angled medial to lateral and is rotated until the anterior and posterior projections of the inferior third of the joint are superimposed on each other (Figure 13.4). For fixed fluoroscopy, the patient is positioned in the prone oblique position to align the x-ray beam with the inferior third of the joint. If two joint planes are seen, the more medial one should be targeted, since it most likely represents the posterior aspect of the joint. Sections measuring 3 to 5 mm are obtained for cross-sectional imaging guidance. The inferior third of the joint is identified,

Infectious Sacroileitis

Figure 13.3. MRI images of infectious sacroiliitis. (A) Axial T1-weighted image demonstrating hypointense signal along the anterior aspect and posterior iliac surface of the right SI joint. (B) STIR image demonstrating hyperintense signal representing edema in corresponding regions.

Figure 13.3. MRI images of infectious sacroiliitis. (A) Axial T1-weighted image demonstrating hypointense signal along the anterior aspect and posterior iliac surface of the right SI joint. (B) STIR image demonstrating hyperintense signal representing edema in corresponding regions.

and the skin marked to identify the entry point for the planned needle trajectory (Figure 13.5).

The overlying skin and soft tissues are infiltrated with lidocaine. A 22-gauge spinal needle is then directed along the axis of the x-ray tube

Figure 13.4. Fluoroscopically guided SI joint injection. (A) The x-ray beam is initially directed anterior to posterior; it is rotated medial to lateral until (B) the anterior and posterior projections of the inferior third of the joint are superimposed on each other.

Figure 13.4. Fluoroscopically guided SI joint injection. (A) The x-ray beam is initially directed anterior to posterior; it is rotated medial to lateral until (B) the anterior and posterior projections of the inferior third of the joint are superimposed on each other.

Joint Ray PositioningRay Guided Injection
Figure 13.4. Continued. (C) The needle is then directed down the x-ray beam axis into the joint. The tip should remain within the joint if placed correctly (D).
Joint Axial Ray

fil d and advanced into the joint. For fluoroscopic imaging, injection of 0.2 to 0.5 mL of contrast material (e.g., Omnipaque 300) can be used to confirm position. Alternatively, the C-arm or patient can be rotated to confirm position; if the needle tip is placed correctly, it should remain within the joint (Figure 13.4C,D).

Joint Injection Scans
Figure 13.5. A CT-guided SI joint injection. Axial 3 mm images are obtained through the SI joint. The inferior aspect of the joint is identified, and a 22-gauge spinal needle is directed into the joint.

In general, injection of contrast and medication will be difficult if good positioning within the joint has not been obtained. The SI joint can accommodate only a small volume ~3 mL. A mixture of 1 mL of 0.5% bupivacaine plus 40 mg of methylprednisolone acetate (Depo-Medrol, Pharmacia and Upjohn; Kalamazoo, MI) is injected with a 1 or 3 mL syringe. Alternatively, 12 mg of betamethasone acetate and betamethasone sodium phosphate suspension (Celestone Soluspan, Schering-Plough, Kenilworth, NJ) may be used as the steroid component.

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