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How I Do It: Ultrasound-Guided Sacral Lateral Branch Blocks
Sacroiliac joint- (SIJ-) related pain has traditionally been diagnosed and treated with intra-articular injections; however, the increased interest in radiofrequency ablation techniques targeting the posterior joint innervation (sacral lateral branches) has underlined the need for diagnostic blocks specifically targeting those nerves. Although fluoroscopy guidance has been validated for this purpose, it is seldom used because of the 17 injections (and nine skin entry sites) required. A simplified, ultrasound-guided technique, however, requires only three injections.
Although the anterior aspect of the joint is innervated by the ventral rami of L4 and L5, a posterior sacral neural network (PSN) innervates the dorsal aspect of the joint and posterior ligaments (posterior sacral complex). The results of a cadaveric study demonstrated that the PSN received contributions from the S1 and S2 lateral branches in 100% of specimens, from S3 in 88% of cases, and rarely from the L5 and S4 levels in 8% and 4%, respectively. When a contribution from the L5 dorsal ramus was present, it merged with the PSN and did not innervate the joint directly.
Most of the nerves contributing to the PSN cross the lateral sacral crest (LSC) between the second and third transverse sacral tubercles; however, a superior lateral branch can also innervate the SIJ at the S1 level in up to 40% of subjects. Although the LSC is difficult to identify using fluoroscopic imaging, it is a reliable sonographic landmark and constitutes the injection target when performing an ultrasound-guided sacral lateral branch block (SLBB).
Figure 1: Posterior view of the sacrum on a skeletal model. Left side of the model depicts the injection points for an ultrasound-guided sacral lateral branch block on the lateral sacral crest between the S2 and S3 transverse sacral tubercles (1); on the lateral crest immediately above the S2 transverse sacral tubercle (2); and on the lateral crest at the level of the S1 transverse sacral tubercle (3). Right side of the model depicts the individual scan lines corresponding to the sonograms in Figures 2A–2F
Transverse (short axis) scan and technique. This view is used for needle placement and injection. Place a C5-2 MHz curved transducer on the lower sacrum and perform a systematic scan, first identifying the sacral hiatus in the midline, then moving over to the target side keeping the median sacral crest in view. Then move the probe cephalad to visualize the posterior sacral foramina (S4, S3, S2, and S1) and LSC (Figure 1 and 2).
Sagittal (long axis) scan. This view is used to confirm level and needle position before injection. The posterior foramina appear as gaps in the bony contour of the sacrum (Figure 2F).
Key landmarks are identified by scanning the sacrum in the transverse plane, cephalad from the sacral hiatus (Figure 1 and 2). Perform three injections of local anesthetic on the sacral lateral crest by advancing the needle in-plane from a medial to lateral direction: 1.5 mL between S2 and S3, 0.5 mL between S2 and S3, and 0.5 mL lateral to S1. After each needle placement, perform a sagittal scan to confirm needle position in the cephalo-caudal dimension (Figure 2F).
- In the upper sacral spine, the S1 posterior foramen can be identified by localizing the L5-S1 facet joint, then scanning caudad until the foramen is visualized. At the level of the S2 posterior foramen (S2), the caudad aspects of the SIJ and posterior superior iliac spine (PSIS) immediately cephalad to it can serve as confirmatory landmarks.
- Because of sacral angulation, the injection target at the S1 level can be more challenging to access than at other levels.
- The median sacral crest may hinder needle placement, particularly in thinner patients, necessitating a more lateral needle entry site.
- Vascular breach is common during injection procedures targeting the sacral area and can be prevented by identifying vulnerable vessels in the projected needle path using duplex color Doppler.
Figure 2: Sonographic images of the posterior sacrum depicting the various views required for the performance of an ultrasound-guided sacral lateral branch block. This view is used to confirm needle positions prior to injection (bi-planar technique). Injection points are marked by a star, and scan lines are illustrated in Figure 1. 2A: transverse sonographic view of the lower sacrum demonstrating the median sacral crest, posterior foramen of S3, and lateral sacral crest (LSC); 2B: transverse scan of the lower sacrum depicting the injection point on the LSC, midpoint between the posterior foramina of S3 and S2; 2C: transverse scan at the level of the S2 posterior foramen illustrating the caudad aspects of the sacroiliac joint and posterior superior iliac spine; 2D: transverse scan of the posterior sacrum demonstrating the injection point on the lateral sacral crest cephalad to the S2 posterior foramen; 2E: transverse scan of the sacrum illustrating the injection point lateral to the S1 posterior foramen; 2F: sagittal sonographic view of the sacrum with visualization of the posterior foramina of S1, S2, and S3
To date, one randomized controlled trial involving 40 patients with chronic low back pain compared fluoroscopy- and ultrasound-guided SLBB techniques. In the fluoroscopy group, SLBB were performed according to a previously described 17-injection technique, which involves 9 skin entry sites and the targeting of the L5 posterior root and S1–S3 sacral lateral branches. The ultrasound-guided technique consisted of the three injections described previously, and analgesic testing was done by probing the dorsal sacroiliac ligament, sacroiliac joint, and interosseous ligament with a 22-gauge block needle. Ultrasound guidance was associated with shorter performance times, fewer needle passes, and a lower incidence of vascular breach. However, the block effect (the proportional decrease in numerical rating scale between pre-block and post-block analgesic testing) was similar in both groups. Furthermore, no statistical differences were found in the proportions of patients achieving complete analgesia at each test site.
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