Sacrum

1. Overview

The sacrum is a large, triangular bone located at the base of the vertebral column and the posterior part of the pelvis. It forms the keystone of the pelvic girdle, anchoring the spine to the hip bones and transferring weight from the upper body to the lower limbs. The sacrum is formed by the fusion of five sacral vertebrae (S1–S5) during adolescence and early adulthood.

2. Location

The sacrum is situated between the two iliac bones of the pelvis:

• Superiorly: Articulates with the fifth lumbar vertebra (L5) at the lumbosacral joint.

• Inferiorly: Articulates with the coccyx at the sacrococcygeal joint.

• Laterally: Articulates with the ilia (hip bones) at the sacroiliac joints.

• Posteriorly: Forms the dorsal wall of the pelvic cavity.

3. Structure

The sacrum is a curved, wedge-shaped bone composed of five fused sacral vertebrae (S1–S5):

• Base: The broad superior part, which articulates with L5.

• Apex: The narrow inferior portion, which connects to the coccyx.

• Anterior (pelvic) surface: Smooth and concave, contains four pairs of anterior sacral foramina for ventral rami of spinal nerves.

• Posterior (dorsal) surface: Rough and convex, with ridges and four pairs of dorsal foramina for dorsal rami of spinal nerves.

• Sacral canal: Continuation of the vertebral canal; transmits the cauda equina.

• Sacral hiatus: An opening at the inferior end of the sacral canal due to failure of the laminae of S5 (and sometimes S4) to fuse.

• Lateral parts: Include auricular surfaces and tuberosities for articulation with the ilium and attachment of sacroiliac ligaments.

4. Function

The sacrum serves as a central structural and load-bearing component of the axial skeleton:

• Transmits body weight: Transfers weight from the upper body to the pelvis and lower limbs.

• Forms part of the pelvic girdle: Integrates with the hip bones to stabilize and support the pelvis.

• Supports spinal alignment: Maintains the curvature of the vertebral column and contributes to postural balance.

• Protects neural structures: Houses sacral nerve roots and the terminal part of the spinal canal.

5. Physiological role(s)

The sacrum supports a range of physiological roles related to movement, protection, and support:

• Stabilizes the trunk: Acts as a foundation for trunk muscles and ligaments, contributing to core stability.

• Enables locomotion: Facilitates coordinated motion of the pelvis and spine during walking, running, and lifting.

• Anchors pelvic organs: Provides attachment points for ligaments supporting the rectum, uterus (in females), and bladder.

• Neurovascular passage: Allows exit of sacral spinal nerves through the sacral foramina, contributing to motor and sensory function of the lower limbs and pelvic floor.

6. Clinical Significance

The sacrum is involved in a variety of clinical conditions and anatomical considerations:

• Sacroiliac joint dysfunction:

  • Can result from trauma, arthritis, or ligament laxity, leading to lower back or buttock pain.

• Sacral fractures:

  • Often occur due to falls, trauma, or stress injuries (common in osteoporosis); may affect nerve roots or pelvic stability.

• Sacralization of L5:

  • A congenital anomaly where the L5 vertebra is fused to the sacrum, potentially causing altered biomechanics or back pain.

• Spina bifida occulta:

  • A defect in sacral lamina fusion, usually asymptomatic but may be associated with tethered cord syndrome in some cases.

• Caudal epidural anesthesia:

  • The sacral hiatus is an access point for epidural anesthesia during childbirth or lower limb surgeries.

• Metastases and primary tumors:

  • The sacrum may be affected by metastatic lesions (e.g., from prostate or breast cancer) or primary bone tumors (e.g., chordoma).