Makindo Medical Notes

The sacrum is a large, triangular bone located at the base of the spine, forming the posterior portion of the pelvic girdle. It is situated between the lumbar vertebrae and the coccyx (tailbone). In humans, the sacrum consists of five fused vertebrae (S1-S5) that form a single solid structure.

Key Anatomical Features of the Sacrum

• Base of the Sacrum: The broad upper part of the sacrum, which articulates with the fifth lumbar vertebra (L5) through the lumbosacral joint.
• Apex of the Sacrum: The narrow, lower part that connects to the coccyx (tailbone).
• Anterior Surface (Pelvic Surface): The concave surface facing the pelvic cavity. It has four pairs of anterior sacral foramina, which allow for the passage of the anterior branches of sacral nerves.
• Posterior Surface: Rough and convex, featuring ridges formed by the fusion of the sacral vertebrae. It includes the median sacral crest (a ridge formed by the fusion of spinous processes), sacral tubercles, and posterior sacral foramina for the exit of the posterior branches of the sacral nerves.
• Sacral Promontory: The prominent upper border of the sacrum, important in obstetrics as it defines the inlet to the pelvic cavity.
• Auricular Surface: Located on the lateral sides of the sacrum, this region articulates with the ilium of the pelvis, forming the **sacroiliac joints** which are critical for transferring weight from the spine to the pelvis.
• Sacral Canal: A continuation of the vertebral canal that runs through the sacrum, carrying the sacral nerve roots (part of the cauda equina). The canal terminates at the sacral hiatus, an opening at the lower end of the sacrum.

Function of the Sacrum

• Supports the weight of the upper body when standing or walking, transferring it to the pelvis and lower limbs.
• Forms the posterior wall of the pelvic cavity, contributing to the stability of the pelvic girdle.
• Provides passage for the sacral nerves, which innervate the pelvic organs, lower limbs, and parts of the perineum.
• Plays a key role in the sacroiliac joint, which is critical for movement and stability of the pelvis during walking or standing.

Clinical Relevance

• Sacral Fractures: Often result from trauma, such as falls or motor vehicle accidents. They can cause pain and nerve damage.
• Sacroiliac Joint Dysfunction: A condition causing pain in the sacroiliac joint due to inflammation or misalignment, often leading to lower back or leg pain.
• Sacralization: A congenital anomaly where the fifth lumbar vertebra fuses with the sacrum, which can lead to altered biomechanics and lower back pain.
• Sacral Tumors: Tumors in the sacrum can cause nerve compression, pain, and neurological deficits.