The sacrum is a distinctive bone at the base of the spine, forming the lower, posterior part of the vertebral column. Positioned between the two hip bones, it is integral to the pelvic structure.
The Sacrum’s Distinctive Shape
The sacrum is a large, irregularly shaped bone, resembling an inverted triangle or wedge. Its widest part, the base, is at the top, tapering to a narrower apex at the bottom. It is concave on its anterior (front) surface and convex on its posterior (back) surface. The anterior concavity contributes to the pelvis’s bowl-like shape, while the posterior convexity gives it a rounded appearance.
The sacrum’s triangular outline results from the gradual decrease in size of the sacral vertebrae as they descend and fuse. This shape allows it to fit precisely within the pelvic girdle. When viewed from the side, its curve becomes apparent, contributing to the overall S-shape of the human spine. The superior portion is broader and more robust, reflecting its role in connecting with the lumbar spine.
Key Visible Features
The sacrum forms from the fusion of five individual sacral vertebrae, labeled S1 through S5, a process completing by early adulthood. Evidence of this fusion is visible on the anterior surface as four transverse ridges, remnants of original intervertebral discs. Both anterior and posterior surfaces feature prominent holes, sacral foramina. Four pairs of these openings are on the anterior side, and four pairs on the posterior, allowing for sacral nerve passage.
The broad, upper lateral parts are the alae, resembling wings extending from the first sacral vertebra. On the sides, particularly in its upper half, are the auricular surfaces. These ear-shaped, roughened areas connect the sacrum with the hip bones. At the superior anterior edge of the first sacral vertebra (S1) is the sacral promontory. This projection marks the anterior lip of the S1 body and is a landmark.
The posterior surface features several longitudinal ridges or crests. Along the midline, the median sacral crest forms from the fusion of the spinous processes of the upper sacral vertebrae, often appearing as three or four small tubercles. Lateral to the median crest are the intermediate sacral crests, resulting from the fusion of the articular processes. Further laterally are the lateral sacral crests, formed by the fused transverse processes. At the bottom of the posterior sacrum is the sacral hiatus, typically inverted U or V-shaped and bordered by bony sacral cornua.
Its Place Within the Pelvis
The sacrum’s wedge shape allows it to fit snugly between the two hip bones, the ilia. This arrangement forms the posterior wall of the pelvic girdle, creating a strong, supportive ring. The auricular surfaces on its sides articulate directly with the ilium, forming the sacroiliac joints. These connections are integral to the overall structure of the pelvis.
Superiorly, the sacrum connects with the fifth lumbar vertebra (L5), forming the lumbosacral joint. This articulation occurs at its broad base, where the L5 vertebral body sits atop the S1 segment. Inferiorly, the sacrum articulates with the coccyx, or tailbone, completing the lower end of the vertebral column. This series of connections highlights the sacrum’s role as a central component, integrating the spine with the pelvic structure.