Sacrum

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.

Gross anatomy

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 vertebral 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

• lumbosacral and iliolumbar ligaments: reinforce lumbosacral articulation
• ligamentum flavum (S1 to L5): closes the spinal/sacral canal posteriorly
• sacroiliac ligaments: reinforce the sacro-iliac joint
• sacrotuberous ligament
• sacrospinous ligament
• sacrococcygeal ligaments: superficial part of the posterior sacrococcygeal ligament closes the sacral hiatus

Musculotendinous

• iliacus muscle: anterosuperiorly
• piriformis muscle: anteroinferiorly
• erector spinae muscle: posteriorly
• gluteus maximus: posteriorly

Relations

• anteriorly: fascia of Waldeyer, superior rectal vessels; peritoneum (S1-S2); rectum (S3-S5); median sacral artery; lymph nodes; piriformis muscle; sacral plexus

Blood supply

Arterial supply is via numerous branches:

• 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)

Venous drainage:

• via vertebral venous plexus to:
  • median sacral vein (to either common iliac vein or IVC)
  • lateral sacral vein (to internal iliac vein) 

Variant anatomy

• lumbosacral transitional vertebrae
• sacral agenesis

Related pathology

• sacral lesions
• Tarlov cysts
• anterior sacral meningocoele
• classification of sacral fractures
• pelvic insufficiency fractures