Sacrum

Introduction

Labeled illustration of the sacrum

Sacral anatomy and relationship to spinal column

Normal anterior posterior sacral radiograph

Normal lateral sacral radiograph

General

• Large bone at the terminal end of the vertebral canal
• Forms the posterior aspect of the pelvis
• Aids in supporting and transmitting the weight of the body

Description

• Fusion of the five sacral vertebrae
• Inverted, triangular concave shape
• Consists of base, apex and four surfaces

Bony Landmarks

• Base: articulates with the fifth lumbar vertebrae, associated with intervertebral disc
• Apex: abuts the coccyx inferiorly
• Auricular surfaces: located laterally, articulates with the ilium
  • Named auricular because the surface is shaped like an ear
• Anterior/Posterior surface: act as bony landmarks for ligaments, muscles
• Central canal: continues along the core of the sacrum, ends at the 4th sacral foramen and sacral hiatus

Dorsal Surface

• Median sacral crest: formed by fusion of spinous process of first three sacral vertebrae
  • Attachment site of the supraspinous ligament
• Intermediate sacral crest: formed by fusion of sacral articular process
  • Attachment site of posterior sacroiliac ligaments
  • Note that L5-S1 is not fused
  • S5 (sacral cornu) articulates with the coccygeal cornu
• Transverse processes are fused to form the lateral sacral crests
  • Attachment for sacrotuberous ligament
  • Fusion is incomplete, allows sacral nerve fibers to exit via four pairs of posterior sacral foramina

Pelvic Surface

• Transverse lines: there are 4, remnants of fused sacral intervertebral discs
• Sacral promontory: superior projection, forms posterior margin of pelvic inlet
  • Continuous with margin of the ala of the sacrum, arcuate line of the ilium, and the pectin pubis and pubic crest of the pubic bone

Muscular Attachments

• Anterior: Piriformis, Coccygeus, Iliacus
• Posterior: Multifidus lumborum, Erector spinae

Spinal Cord

• Central canal hosts the sacral fibers of the cauda equina
• Filum terminale: continuation of meninges continuous inferiorly

Autonomic Nervous System

• Sympathetic trunks run along the pelvic surface of the sacrum
• Medial to sacral foramina
• Each trunk has 4 ganglia

Vasculature

• Median Sacral Artery
• Lateral Sacral Arteries

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Clinical Significance

• Sacroiliac Joint Pain

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See Also

Back

• Back Pain Main
• Physical Exam Back
• Back Anatomy Main

Hip

• Hip Pain Main
• Physical Exam Hip
• Hip Anatomy Main

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