Technique

The SI joint may be injected under fluoroscopic or cross-sectional imaging guidance. Computed tomography (CT) is the most common cross-sectional modality used, however magnetic resonance (MR)-guided SI joints have been described in a technical note (6). The SI joint can be one of the most difficult joints to access, and therefore there have been numerous technical descriptions, and the preferred modality is often determined on the basis of operator experience and equipment availability.

Several fluoroscopic techniques have been described. In general, the patient is placed prone on a radiolucent table. The target zone is the inferior, posterior aspect of the SI joint, about 1 to 2 cm proximal to the most caudal margin of the joint. Because the sacrum is broader anteriorly than posteriorly, two joints may be visualized as defined by a thin cortical opaque line on each side of the articulation. Under direct posteroanterior (PA) projection the more medial joint is posterior and the lateral joint is anterior. Some operators use this projection and target the medial joint often hitting the sacrum and then creeping along the margin until the needle encounters the joint space (Fig. 1). In the oblique approach (7), the C-arm is rotated medially to profile the joint with supraimposition of all the joint margins. One would then simply target these superimposed joints at their inferior aspect (Fig. 2). However, this technique often fails to allow entry into the SI joint.

FIGURE 1 Sacroiliac (SI) joint anatomy. The target zone is the inferior, posterior aspect of the SI joint about 1 to 2 cm proximal to the most caudal margin of the joint (arrowhead ). Because the sacrum is broader anteriorly then posteriorly, two joints may be visualized. Under a direct posteroanterior projection, the more medial joint is posterior (large arrow) and the more lateral joint is anterior (small arrow).

Others have developed modifications using the advantage of multidirectional C-arm fluoroscopy. The technique described by Dreyfuss et al. (8) is a modification of the oblique approach. The C-arm is rotated medially (contralateral to the affected side or side of interest) to profile the joint. This orients the anterior and posterior planes to be parallel to the central ray. A single lucent zone is produced inferiorly, which reflects the confluence of anterior and posterior joints. The C-arm is then rotated back in the opposite direction. Rotation is continued until "optimal separation" is produced. This is defined by a well-visualized posterior (medial) joint with thin sclerotic margins and an inferior hyperlucent zone. Typically it requires about 5° to 20° of rotation to produce this configuration (Fig. 3). Another modification has been described by Dussault et al. (9), again using a rotating C-arm. Initially the X-ray tube is perpendicular to the table and the skin is marked over the distal 1 cm of the SI joint. The tube is then angled approximately 20° to 25° in a cephalic direction to displace the posteroinferior portion of the

FIGURE 2 Sacroiliac joint oblique approach: (A) the C-arm is rotated medially to profile the joint with superimposition of all the joint margins (arrowheads); (B) the needle is placed in the target zone at the inferior aspect or mid-portion of the joint (arrow ).

FIGURE 3 Sacroiliac joint modified oblique approach. The C-arm is rotated medially (contralateral to the affected side or side of interest) to profile the joint. This orients the anterior and posterior planes to be parallel to the central ray. A single lucent zone is produced inferiorly, which reflects the confluence of the anterior and posterior joints. The C-arm is then rotated back in the opposite direction, and (A) rotation is continued until "optimal separation" is produced defined by a well-visualized posterior (medial) joint with thin sclerotic margins and an inferior hyperlucent zone (arrowheads). Typically it requires about 5° to 20° of rotation to produce this configuration. (B) The needle is placed in the target zone at the inferior aspect of the joint (arrow).

FIGURE 3 Sacroiliac joint modified oblique approach. The C-arm is rotated medially (contralateral to the affected side or side of interest) to profile the joint. This orients the anterior and posterior planes to be parallel to the central ray. A single lucent zone is produced inferiorly, which reflects the confluence of the anterior and posterior joints. The C-arm is then rotated back in the opposite direction, and (A) rotation is continued until "optimal separation" is produced defined by a well-visualized posterior (medial) joint with thin sclerotic margins and an inferior hyperlucent zone (arrowheads). Typically it requires about 5° to 20° of rotation to produce this configuration. (B) The needle is placed in the target zone at the inferior aspect of the joint (arrow).

joint in a caudal direction. The needle is advanced through the original site marked on the skin, but the tube is maintained in the cephalic position. The needle is advanced toward the posterior SI joint without angling of the needle (i.e., maintained perpendicular to the table). The needle is advanced until it encounters the SI joint.

In all of these techniques, SI joint arthrography is performed to confirm an intra-articular location (Fig. 4). Often a small inferior caudal recess is opacified with a variable portion of the synovial joint. It is uncommon to opacify the entire SI joint. Also common in most techniques is that the needle encounters the sacral side, avoiding being too low or too lateral because of the proximity of the sciatic notch (sciatic nerve). From here the needle is migrated into the joint until one feels a depression or a "pop," then advancing it 1 to 2 mm. When the inferior joint is

FIGURE 4 Sacroiliac (SI) joint arthrography. It is uncommon to opacify the entire SI joint. Most often a small inferior caudal recess is opacified (arrowhead ). This arthrogram also shows synovitis with interdigitating of contrast between proliferative fronds of synovium (arrows).

not accessible under fluoroscopy, then most would advocate injecting the mid-portion of the joint (more cephalad). Alternatively, a cross-sectional imaging technique can be used if attempts using fluoroscopy are not adequate.

The cross-sectional imaging technique that is used most often is CT (Fig. 5). The patient is placed prone on the gantry and axial images are obtained through the SI joint. Again the inferior or mid-portion of the joint is targeted. The needle is advanced until the tip is in the synovial portion of the joint (10). The injection of contrast material is not mandatory if one feels that there is an adequate position based on the imaging. A similar technique can be used with MR imaging. Despite the absence of ionizing radiation, the lesser availability and higher cost of MR precludes its widespread use. In addition, special consideration is needed with respect to MR-compatible equipment and needles. While many needles may be safe (i.e., no significant migration) for use in the magnet environment, imaging can be problematic unless special needles are used. In terms of modality preference, it is the author's experience to first attempt SI joint injections using fluoroscopy. If this is unsuccessful then CT is employed. Overall, the majority of SI joints should be fluoroscopy accessible if one uses one of the modified C-arm techniques described above.

The SI joint may be injected with a 22-, 23-, or 25-gauge needle. The needle length depends on the patient's body habitus. Larger needles (20 gauge) may be used for arthrocentesis if there is a suspected infection. No intravenous line, premedication, or physiologic monitoring is required. The contrast material should be suitable for intrathecal administration. The volume of contrast injected should be between 0.3 and 1.0 mL maximum. This is to allow enough residual volume within the joint for anesthetic injection. In a small number of patients, it may be impossible to obtain an arthrogram. Other flow patterns to be cognizant of are venous and extracap-sular opacification. With venous opacification, there are small tributaries identified flowing away from the SI joint. Extracapsular injections are characterized by irregular collections or a "blob" pattern. These may occur concurrently with an intra-articular injection. The needle tip may be advanced or rotated slightly in this circumstance. To best visualize the arthrogram, an anteroposterior or slight oblique projection can be used to profile the structures of interest. If the needle tip is too anterior it may hit an artery but this is extremely uncommon. Degenerative joint disease can limit the amount of contrast injected. Several extravasation patterns have been described by Fortin et al. (4). They are characterized as fascia, piriformis, sacral foramina (anterior and posterior), and lumbosacral plexus extravasation patterns. Similar to other spine injections, the injectate consists of a short-acting anesthetic and/or a corticosteroid. Injection is performed until there is a firm end point or extracapsular extravasation. The total volume should not exceed 2.5 to 3 cc.

The interpretation of this procedure is based primarily upon pain relief and not the provocative aspect. Concordant pain production during SI joint injection is not validated as a diagnostic test confirming pain of SI joint etiology. Practically, the assessment should be before the injection, immediately subsequent, and approximately 15 to 30 minutes after the injection. The anesthetic response is most often assessed using a visual analog scale (VAS) and or percent pain relieved. The result is considered positive (good) if greater than 75% of pain is relieved and negative if less than 50% of the pain is relieved. Pain reduction in the range of 51% to 74% is

FIGURE 5 Sacroiliac joint computed tomography (CT) guidance. CT image shows a needle positioned in the synovial portion of the sacroiliac joint (arrow/).

considered equivocal and may be a placebo response. Pain relief may not be expected to be total because the SI joint is usually only one component of a multifactorial reason for back pain. However, there may be complete relief regionally in an area that corresponds to the SI joint distribution (posterior low back below L5 and over the buttocks), establishing the SI joint as a nociceptor.

Patients should be observed for about 30 to 60 minutes postinjection. It is recommended that a companion be present to drive the person home in case of a complication. The patient may resume normal activity immediately in the absence of a complication. The most common complication is the subjective sensation and weakness of the lower extremities (about 10%). This may occur without extravasation and is felt to be related to loss of proprioception in the pelvic girdle. In addition, the patient may temporarily experience a wobbly or unsteady gait. Lower extremity numbness and true weakness may be secondary to anesthetic leakage around the sciatic nerve.

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