The sacrum and sacroiliac joint complex (SIJ) are sometimes de-emphasized in radiology educational resources, but merit special attention. The sacrum is important clinical in terms of pain, trauma, insufficiency fractures, and tumors. Functionally, transmits force from the spine to the iliac bones/pelvis and through the hips to the lower extremities.

The sacrum is essentially five fused vertebrae. The vertebral bodies form the body of the sacrum, pedicles extend laterally to the ala formed by fused transverse processes. The sacral canal is enclosed posteriorly by the neural arch of lamina and tiny spinous processes (which form the median crest). Neural foramina are present, but the dorsal and ventral foramen are distinct at each level. Fused 'facet joints' form ridges (intermediate crest) along the medial posterior foramina, rudimentary unfused facet joints can sometimes be seen on CT at S1-S2. Ghost outlines of intervertebral discs between the fused vertebral bodies are seen, although a rudimentary disc may be seen at S1-2 or sometimes S2-3.

The thecal sac terminates in the upper sacral canal, while the epidural space continues throughout. S5 does not fuse posteriorly, leaving an opening called the sacral hiatus. The sacral hiatus allows one route of access to the caudal epidural space.

The anatomy of the SI joint is highly complex, an interlocking jigsaw structures, but in the essential section we will consider synovial portion and interosseous ligamentous portion. In addition a dorsal ligament complex is present.

The SIJ complex is considered on of the three most common causes of axial low back pain. In patients with spondyloarthropathy, inflammatory change can be seen within the ligaments and bones surrounding the joint. These changes are usually mild or absent in patients with suspected SIJ pain without spondyloarthropathy in my experience. Recent evidence suggests that pain might arise either from the dorsal ligament complex or from the intra-articular portion in any individual patient. This prospect has implications for percutaneous treatment approaches and diagnosis because the innervation of the dorsal ligaments and synovial joint seems to differ. Specifically, the innervation of the dorsal ligaments may arise exclusively from dorsal rami of L5 and S1-S3 whereas the synovial joint may have ventral innervation as well (which would not be amenable to percutaneous nerve blocks or ablation).

Evaluation of pain in the gluteal/SIJ complex region is made more complex by numerous other potential causes of pain in this region including: referred pain from lower lumbar facet joints, superior or middle cluneal nerve entrapment, and 'deep gluteal syndromes' (which include pyriformis, gemelli-obturator, proximal hamstring, and ischiofemoral entrapment syndromes).

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Show S1 anterior and posterior foramen-LS and radiographs

Insufficiency fracture...

Fracture classification system...

References

Dreyfuss P et al. The ability of multi-site, multi-depth sacral lateral branch blocks to anesthetize the sacroiliac joint complex. Pain Med 2009;10(40:679-88

McCrory and Bell...

Riskalla J et al. Classifications in brief: the Denis classification of sacral fracturse. Clin Orthop Relat Res 2019;477(9):2178-2181

Basic Components of the sacrum and dorsal S1 foramen. The S1 vertebral body (red) pedicles (green), and transverse processes (purple) are seen. This S1 vertebrae has transitional morphology and some lumbar features. The left transverse process has a pseudoarticulation with the left sacral ala. The S1-S2 facet joints (dark blue) are visible along the medial aspect of the dorsal S1 foramina (yellow). The lamina (orange) extend to form the dorsal wall of the sacral canal. The S1 ventral rami (light blue) are seen, whereas the S1 dorsal rami are too small to visualize.

While the posterior foramen are seen as well-defined circles, the anterior foramen are less well defined-especially the inferolateral margins. The dorsal foramen are located just medial to the arcuate lines of the medial aspects of the pedicle of the vertebral body superior to the foramen.

The sacroiliac joint complex is....complex. Globally, it consists of an anteroinferiorly located synovial portion and a posterosuperiorly located ligamentous portion. The synovial portion is somewhat shaped like an ear and thus may be referred to as the auricular surface-this is the segment that is typically targeted with percutaneous injections for pain management. The joint is angled in the axial plane with the dorsal/posterior extent located medial the ventral/anterior margin. Review of any available cross sectional imaging as useful for planning access to the SI joint as the precise orientation of the posteroinferior joint varies.

The synovial portion, ligamentous portion, and the posterior ligament complex have all been implicated as potential sources of pain generation, but the synovial joint is most typically suspected and targeted with injections.

A posterior view of the SIJ shows the synovial portion inferiorly and the ligamentous portion superiorly. Because of the orientation of the joint and position of the iliac crest and posterior superior iliac spine (PSIS), the ligamentous portion is partially obstructed. A posterior oblique view shows the entire posterior extent of the joint to be advantage. An anterior view shows relatively larger synovial portion compared to the ligamentous portion.