CorpusHuman Anatomy
SkeletalBone

Sacrum

Os sacrum

Also known as: sacral vertebrae, S1–S5

A large triangular bone formed by fusion of five sacral vertebrae, wedged between the hip bones. It forms the posterior wall of the pelvis and transmits body weight to the pelvic girdle through the sacroiliac joints.

Show on body map Muscles — Posterior View

Overview

Fused vertebrae5 (S1–S5)
End of dural sacS2
Female sacrumShorter, wider, less curved

The sacrum has a broad base above (S1 body, with the sacral promontory projecting anteriorly — the posterior limit of the pelvic inlet) and an apex below articulating with the coccyx. Lateral to the S1 body extend the alae (wings). The concave pelvic surface shows four transverse ridges (fused intervertebral discs) and four pairs of anterior (pelvic) sacral foramina transmitting the ventral rami of S1–S4. The convex dorsal surface carries the median sacral crest (fused spinous processes), intermediate crests (fused articular processes), lateral crests (fused transverse processes) and four pairs of posterior sacral foramina (dorsal rami).

The ear-shaped auricular surfaces on the lateral parts articulate with the ilia (sacroiliac joints); behind them the rough sacral tuberosity anchors the massive interosseous sacroiliac ligaments. The sacral canal (continuation of the vertebral canal) contains the cauda equina and ends at the sacral hiatus (absent laminae of S5, ± S4), flanked by the sacral cornua — the landmark for caudal epidural anaesthesia. The dural sac ends at S2 (level of the posterior superior iliac spines).

Location

Posterior pelvis, between the two hip bones, below L5.

Parts & subdivisions

  1. Sacral promontory
    Anterior lip of S1; landmark for the obstetric conjugate.
  2. Alae
    Lateral wings; lumbosacral trunk and obturator nerve cross their anterior surface.
  3. Anterior and posterior sacral foramina
    Four pairs each; ventral and dorsal rami of S1–S4.
  4. Median sacral crest
    Fused spinous processes.
  5. Auricular surface
    Articulates with the ilium (sacroiliac joint).
  6. Sacral hiatus and cornua
    Inferior opening of the sacral canal; caudal epidural block.
  7. Sacral canal
    Contains the cauda equina; dural sac ends at S2.

Anatomy

Articulations

L5 (lumbosacral joint and disc), ilia (sacroiliac joints), coccyx (sacrococcygeal symphysis).

Ligaments

Anterior, interosseous and posterior sacroiliac ligaments; sacrotuberous and sacrospinous ligaments; iliolumbar ligament.

Ossification

Each segment has the three primary centres of a typical vertebra plus costal-element centres in the alae; the segments begin to fuse at puberty from below upward and complete fusion by ≈25–30 years.

Function

  • Transmits body weight from the vertebral column to the pelvic girdle
  • Forms the posterior wall of the pelvis
  • Protects sacral nerve roots
  • Attachment for gluteus maximus, piriformis, erector spinae and pelvic ligaments

Clinical significance

Caudal epidural block

Local anaesthetic injected through the sacral hiatus (palpated between the sacral cornua) anaesthetises the sacral and coccygeal roots — used in paediatric surgery and obstetrics.

Sacral insufficiency fracture

Osteoporotic fracture of the alae in elderly women, presenting with low-back/buttock pain after minimal trauma; "Honda sign" on bone scan.

Lumbarisation and sacralisation

Transitional vertebrae: S1 separated from the sacrum (lumbarisation) or L5 fused to it (sacralisation) — important for correctly numbering levels before spinal surgery.

Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.