Sacrum and Coccyx¶

Definition¶

The sacrum is a large triangular bone formed by the fusion of five sacral vertebrae (S1–S5), situated between the lumbar spine and the coccyx. It forms the posterior wall of the pelvis and articulates with the iliac bones at the sacroiliac joints. The coccyx (tailbone) consists of 3–5 fused rudimentary vertebrae at the terminal end of the vertebral column.

Anatomy¶

Sacrum¶

The sacrum is a keystone structure transmitting the weight of the spine to the pelvis:

Promontory — the anterosuperior margin of S1, a key obstetric and surgical landmark
Sacral ala (wings) — lateral expansions of S1 that articulate with the iliac bones
Sacral canal — continuation of the vertebral canal, containing the cauda equina and filum terminale
Sacral foramina — four pairs of anterior and posterior foramina transmitting the sacral nerve roots
Sacral hiatus — the opening at the caudal end of the sacral canal (absent S5 laminae), used for caudal epidural access
Sacral cornua — small bony projections flanking the sacral hiatus

Sacroiliac Joints¶

The sacroiliac (SI) joints are synovial joints anteriorly and syndesmotic (ligamentous) joints posteriorly:

Transmit axial load from the spine to the lower extremities
Surrounded by the strongest ligaments in the body
Common site of inflammatory sacroiliitis (ankylosing spondylitis)

Coccyx¶

Composed of 3–5 fused segments
Articulates with the sacrum at the sacrococcygeal joint (often partially fused)
Serves as an attachment point for pelvic floor muscles and ligaments
Highly variable in morphology; may be curved anteriorly

Clinical Pearl

The sacral hiatus is an important landmark for caudal epidural steroid injections. It is identified by palpating the sacral cornua. On imaging, absence of the posterior elements at S5 (and sometimes S4) is a normal finding, not a defect.

Imaging Findings¶

Radiography¶

AP pelvis — evaluates sacral alignment, SI joints, and sacral fractures
Lateral sacrum — assesses sacral curvature, sacrococcygeal junction, and coccyx
Ferguson view — AP view angled cephalad 30° for dedicated sacral visualization

CT¶

CT is the primary modality for evaluating:

Sacral fractures (especially insufficiency fractures — characteristic "H" pattern)
Sacroiliac joint pathology
Sacral tumors (chordoma, giant cell tumor)
Sacral canal dimensions

MRI¶

Finding	T1 Signal	T2 Signal	Significance
Normal sacral marrow	Bright (fatty in adults)	Intermediate	Complete fatty conversion by adulthood
Sacral insufficiency fracture	Low	High (STIR)	"H" or "Honda sign" pattern on coronal STIR
Sacroiliitis (acute)	Low	High	Periarticular bone marrow edema
Sacral chordoma	Low–intermediate	Very high	Classically T2-bright, midline, destructive
Tarlov cyst	Dark (CSF)	Bright (CSF)	Perineural cyst, usually incidental

Key Points¶

The sacrum is formed by fusion of five vertebrae and is the keystone of the pelvic ring
The sacral canal contains the cauda equina and terminates at the sacral hiatus
Sacral insufficiency fractures show an "H" pattern on coronal STIR MRI
The sacroiliac joints are a key site for inflammatory sacroiliitis
The coccyx is highly variable in morphology and commonly injured by direct trauma

References¶

Sattar MH, Guthrie ST. Anatomy, Back, Sacral Vertebrae. [Updated 2023 Jul 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026. Available from: https://www.ncbi.nlm.nih.gov/books/NBK551653/
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Woon JTK, Perumal V, Maigne JY, Stringer MD. CT morphology and morphometry of the normal adult coccyx. Eur Spine J. 2013;22(4):863-870. doi:10.1007/s00586-012-2595-2. PMID: 23192732; PMCID: PMC3631051.
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Poilliot AJ, Zwirner J, Doyle T, Hammer N. A Systematic Review of the Normal Sacroiliac Joint Anatomy and Adjacent Tissues for Pain Physicians. Pain Physician. 2019;22(4):E247-E274. PMID: 31337164.
Al-Mnayyis A, Obeidat S, Badr A, Jouryyeh B, Azzam S, Al Bibi H, et al. Radiological Insights into Sacroiliitis: A Narrative Review. Clin Pract. 2024;14(1):106-121. doi:10.3390/clinpract14010009. PMID: 38248433; PMCID: PMC10801489.
Fujii M, Abe K, Hayashi K, Kosuda S, Yano F, Watanabe S, et al. Honda sign and variants in patients suspected of having a sacral insufficiency fracture. Clin Nucl Med. 2005;30(3):165-169. PMID: 15722819.