Anatomy Atlas

Sacrum and coccyx

Os sacrum et os coccygis
Also: Sacrum · Coccyx · Tailbone · Sacral hiatus
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The fused sacral vertebrae form the posterior wall of the pelvis and articulate with the ilia at the sacroiliac joints; the coccyx is the vestigial tail. The sacral hiatus is the route for caudal epidural block.

Core anatomy

Location

Base of the vertebral column, wedged between the hip bones; the coccyx lies below it in the natal cleft.

Classification

Fused irregular bones (5 sacral; usually 4 coccygeal).

Key features & landmarks
  • Sacral promontory: anterior upper edge of S1; a landmark for pelvic measurements
  • Anterior (pelvic) and posterior sacral foramina for the ventral and dorsal rami of the sacral nerves
  • Sacral canal (continuation of the vertebral canal) containing the lower cauda equina and filum terminale; the dural sac usually ends at about S2
  • Sacral hiatus: opening at the lower end of the sacral canal (absent laminae of S5 ± S4), flanked by the sacral cornua
  • Auricular surfaces: articulate with the ilia at the sacroiliac joints
  • Coccyx: small fused segments; attachment for the gluteus maximus, coccygeus and anococcygeal ligament
Relations

The PSIS dimples overlie the SI joints at about S2. Anteriorly: rectum, sacral plexus, median sacral vessels.

Muscle attachments

Joints

Ligament attachments

Mentioned in

Generated automatically from links in other entries.

Lumbar puncture anatomy (sagittal view)The spinal cord ends at the conus medullaris at about L1–L2; the cauda equina continues in the dural sac, which ends at about S2. A needle entering between the L3 and L4 spinous processes passes the ligaments and epidural space into the subarachnoid space below the cord. The intercristal line crosses L4.T12L1L2L3L4L5SacrumSpinal cordConus medullaris (L1–L2)← Cauda equinaDural sac ends ≈ S2Supraspinous / interspinous lig.Ligamentum flavumEpidural space → dura → CSFLP needle (L3–L4)Intercristal (Tuffier) line ≈ L4AnteriorPosterior

Swipe sideways to see the whole diagram →

Lumbar puncture anatomy. Sagittal schematic (not to scale): the cord ends at about L1–L2, the dural sac at about S2. The needle enters at L3–L4, just above the intercristal line (≈ L4), well below the conus.
Original schematic, created for this atlas.
Dorsal surface of the sacrum with muscle attachments outlined in red, including multifidus, sacrospinalis (erector spinae) and gluteus maximus
Dorsal surface of the sacrum with muscle attachments (red outlines).
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 96. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Caudal epidural block

Mechanism / lesion

Local anesthetic injected through the sacral hiatus into the epidural space of the sacral canal.

Classic presentation

Common in pediatric anesthesia and for some chronic pain procedures. The hiatus is palpated between the sacral cornua.

Coccydynia PANCE

Mechanism / lesion

Pain at the coccyx from a fall onto the buttocks, childbirth, or prolonged sitting.

Classic presentation

Localized pain over the coccyx, worse sitting (especially leaning back) and rising from sitting. Tenderness on palpation.

Pilonidal disease PANCE

Mechanism / lesion

Hair and debris penetrating the skin of the natal cleft over the sacrococcygeal region, forming a sinus or abscess; common in young, hirsute men.

Classic presentation

Painful, fluctuant swelling in the upper natal cleft, often with midline pits. Abscesses need incision and drainage.

Sacral insufficiency fracture

Mechanism / lesion

Osteoporotic fracture of the sacrum in older adults (often after pelvic radiation or on steroids).

Classic presentation

Low back, buttock or groin pain without major trauma; radiographs are often normal. MRI or CT shows the fracture.

High-yield PANCE pearls

  • Dural sac ends at about S2; sacral hiatus → caudal epidural block.
  • Coccydynia: fall or childbirth, pain with sitting.
  • Pilonidal abscess: natal cleft, young hirsute men → incision and drainage.

Imaging anatomy

X-ray

Pelvis AP and lateral sacrum/coccyx; sacral fractures are often hard to see.

CT

Sacral fractures and SI joint detail.

MRI

Insufficiency fractures (edema), sacroiliitis, sacral tumors (e.g. chordoma).

Key views & landmarks
  • Lateral sacrum/coccyx radiograph
  • MRI or CT for suspected sacral fracture

Physical exam

Surface landmarks

PSIS dimples at about S2; sacral hiatus palpable as a depression between the cornua at the upper natal cleft; coccyx tip in the natal cleft.

Palpation

Palpate the coccyx externally (and per rectum for mobility) for tenderness. Inspect the natal cleft for pits and abscess.

Self-quiz

Q1.

At approximately which level does the dural sac end in adults?

Show answer

D

The spinal cord ends at about L1–L2, but the dural sac (and subarachnoid space) continues to about S2.

Q2.

Through which opening is local anesthetic introduced for a caudal epidural block?

Show answer

B

The sacral hiatus, between the sacral cornua, leads into the sacral epidural space.

Q3.

A 20-year-old man has a painful, red, fluctuant swelling in the upper natal cleft with small midline pits. What is the most likely diagnosis?

Show answer

B

A fluctuant swelling with midline pits in the natal cleft is a pilonidal abscess; treat with incision and drainage.

Flashcards

See also