Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

Overview of the Sacrum

• The sacrum is a large triangular bone located at the base of the vertebral column.
• It forms the posterior part of the pelvis and connects the spine to the pelvic girdle.
• The sacrum articulates superiorly with the fifth lumbar vertebra and laterally with the iliac bones at the sacroiliac joints.
• The bone is broad and thick proximally and becomes thinner and narrower distally.

Posterior Surface of the Sacrum

• The posterior surface contains several important anatomical landmarks.
• The sacroiliac joint lies laterally and may be involved in sacroiliac joint dysfunction and pain.
• Posterior sacral foramina allow passage of the posterior rami of sacral spinal nerves.
• The superior articular processes articulate with the inferior articular processes of the fifth lumbar vertebra.

Sacral Crests

• The median sacral crest represents the fused spinous processes of the sacral vertebrae.
• The intermediate sacral crest represents fused articular processes.
• The lateral sacral crest represents fused transverse processes.

Sacral Canal and Hiatus

• The sacral canal is a continuation of the vertebral canal within the sacrum.
• It transmits sacral nerve roots.
• The sacral hiatus is an opening at the inferior end of the sacral canal.
• It is clinically important for caudal epidural anesthesia.

Sacral Ala and Sacral Tuberosity

• The sacral ala are the wing-like lateral expansions of the upper sacrum.
• They contribute to the formation of the sacroiliac joint.
• The sacral tuberosity is a rough area that provides attachment for ligaments associated with the sacroiliac joint.

Anterior Surface of the Sacrum

• The anterior surface is concave and faces the pelvic cavity.
• It contains anterior sacral foramina which transmit the anterior rami of sacral nerves.
• The sacral promontory forms the anterior projecting margin of the first sacral vertebra.

Superior View of the Sacrum

• The superior view resembles the pelvic inlet seen on radiographic imaging.
• Important structures visible include the sacral ala, body of the sacrum, sacral canal, and superior articular facets.
• The sacroiliac joints are also visible laterally.

Coccyx

• The coccyx lies inferior to the sacrum and forms the terminal segment of the vertebral column.
• It is commonly referred to as the tailbone.
• The coccyx usually consists of two to four fused rudimentary vertebrae.

Functions of the Coccyx

• The coccyx provides attachment sites for ligaments, tendons, and pelvic floor muscles.
• It contributes to pelvic floor stability.
• It acts as a shock absorber when sitting.

Clinical Relevance

• Excessive movement or trauma to the coccyx may cause pain known as coccydynia.
• Sacroiliac joint dysfunction can cause lower back or pelvic pain.
• The sacral hiatus is an important landmark used in caudal epidural injections.