Sacrum

Dr Henry Knipe et al.

The sacrum is the penultimate segment of the vertebral column and also forms the posterior part of the bony pelvis. It transmits the total body weight between the lower appendicular skeleton and the axial skeleton.

The sacrum is an irregularly-shaped bone, roughly an inverted triangular, with its base superiorly and apex inferiorly. It is curved with an anterior concavity and convex posteriorly.

The sacrum consists of five fused sacral vertebral and costal segments (numbered one-to-five) that form a central sacral body and paired sacral alae (singular ala), which arise laterally from S1. Superiorly, the anterior lip of the S1 is called the sacral promontory. Posteriorly, the spinous processes are fused to form the median sacral crest.

Through the centre of the sacral body is the triangular-shaped sacral canal, which is the continuation of the lumbar spinal canal. It terminates inferiorly at the sacral hiatus and contains sacral and coccygeal nerve roots, spinal meninges (to the level of S2) and filum terminale. The spinal epidural space extends to the sacral hiatus. First to fourth sacral nerve roots exit the sacrum via four paired anterior and posterior foramen. The fifth sacral nerve root exits via the sacral hiatus.

Articulations
  • lumbosacral: superiorly, the sacrum articulates with the inferior endplate of L5 via the L5/S1 intervertebral disc; superior articular processes of S1 articulates with the L5 inferior articular facets to form the L5/S1 facet (zygapophyseal) joints
  • sacrococcygeal: inferiorly, the sacrum articulates with the first coccygeal segment
  • sacroiliac: laterally, the sacral ala articulate with the ilium
Attachments
Ligamentous
Musculotendinous
Blood supply
  • iliolumbar arteries (posterior division of internal iliac artery)
  • superior lateral sacral arteries (posterior division of internal iliac artery)
  • inferior lateral sacral arteries (posterior division of internal iliac artery)
  • median sacral artery (abdominal aorta)
Relations

Abdominal and pelvic anatomy
Spinal anatomy
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Article Information

rID: 28693
Section: Anatomy
Tags: spine, pelvis
Synonyms or Alternate Spellings:
  • Sacrum anatomy
  • Sacral anatomy

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Cases and Figures

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    Sacrum anterior v...
    Figure 1: anterior view
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    Sacrum posterior ...
    Figure 2: posterior view
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    Case 1: normal sacral x-ray (lateral)
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    Case 1: normal sacral x-ray (frontal)
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    Case 2: normal sacrum on MRI
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    Contiguous corona...
    Case 3: sacral agenesis
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