General structure of the sacrum: base and apex
In adults, the sacrum is a single, massive bone formed by the natural fusion of five sacral vertebrae (vertebrae sacrales S₁–S₅). This bone performs a vital weight-bearing function as it directly participates in the formation of the posterior pelvic wall.
Anatomically, it has two terminal parts:
- Base of the sacrum (basis sacri) — faces upward and bears the body of the first sacral vertebra (corpus vertebrae sacralis primi). On the anterior margin of this body lies the promontory (promontorium) — a characteristic anteriorly projecting point that plays a key role as an obstetrical landmark. Flanking the body are the wings of the sacrum (ala ossis sacri). On the posterosuperior surface of the base, one can find the superior articular processes (processus articulares superiores), which are necessary for articulation with the fifth lumbar vertebra (L₅).
- The apex of the sacrum (apex sacri) is the narrowed inferiorly facing part that articulates with the coccyx.
Relief of the sacral surfaces
The sacrum has two distinct surfaces with completely different surface relief due to their functions:
- Pelvic surface (facies pelvica). It is concave and relatively smooth. It clearly displays transverse lines (lineae transversae), which are remnants of the fusion of the sacral vertebral bodies (typically four lines). Located here are four pairs of anterior sacral foramina (foramina sacralia anteriora), through which the anterior rami of the sacral spinal nerves enter the pelvic cavity.
- Dorsal surface (facies dorsalis). It is convex and extremely rough, characterized by prominent longitudinal bone crests:
- Median sacral crest (crista sacralis mediana) — formed by completely fused spinous processes.
- Intermediate sacral crests (cristae sacrales intermediae) — located laterally to the median crest, resulting from the fusion of articular processes.
- Lateral sacral crests (cristae sacrales laterales) — lie even more laterally, originating from rudimentary transverse processes.
Opening onto this same surface are four pairs of posterior sacral foramina (foramina sacralia posteriora) for the exit of posterior nerve rami.
Lateral parts and the sacral canal
On the lateral masses of the sacrum lies the articular auricular surface (facies auricularis), which articulates with the corresponding surface of the hip bone. Immediately posterior to it is the sacral tuberosity (tuberositas ossis sacri), a massive, rough area serving as the attachment site for powerful stabilizing ligaments.
Running vertically through the center of the bone is the sacral canal (canalis sacralis), which is a direct anatomical continuation of the vertebral canal. Inferiorly on the dorsal surface, this canal terminates in the sacral hiatus (hiatus sacralis). Flanking this hiatus inferiorly are the paired sacral cornua (cornua sacralia).
Anatomy of the coccyx
The coccyx (os coccygis) is an evolutionary remnant (rudiment) of the caudal spine. In humans, it is formed by the fusion of two to four small coccygeal vertebrae (vertebrae coccygeae Co₁–Co₄).
The first coccygeal vertebra is the largest and bears rudimentary articular processes on its superior surface called coccygeal cornua (cornua coccygea). They project superiorly and articulate with the sacral cornua.
Although the coccyx contributes minimally to body support, it holds significant biomechanical importance as a secure attachment site for the ligamentous apparatus and a portion of the pelvic floor muscles.
Briefly: answers to topical questions
What passes through the anterior and posterior sacral foramina?
The branches of the sacral spinal nerves pass through the anterior and posterior sacral foramina.
- Anterior sacral foramina (foramina sacralia anteriora) — serve as the exit points for the anterior rami of the sacral spinal nerves.
- Posterior sacral foramina (foramina sacralia posteriora) — serve as the exit points for the posterior rami of the sacral nerves.
What are the anatomical contents of the sacral canal?
The anatomical contents of the sacral canal include the cauda equina, venous plexuses, and adipose tissue.
- Cauda equina (cauda equina) — located directly within the canal cavity.
- Internal vertebral venous plexuses (plexus venosi vertebrales interni) — run within the epidural space down to the lower end of the sacral canal.
Additionally, the sacral epidural space contains adipose tissue.
Which muscles originate on the pelvic surface of the sacrum?
Only one muscle mentioned in the sources originates on the anterior (pelvic) surface of the sacrum: the piriformis.
- Piriformis muscle (m. piriformis) — originates on the anterior surface of the sacrum, exits the pelvic cavity through the greater sciatic foramen (foramen ischiadicum majus), and inserts into the apex of the greater trochanter.
What anatomical structures limit the sacral hiatus?
The sacral hiatus is anatomically bounded by the dorsal surface of the sacral apex and the sacral cornua.
- Apex of the sacrum (apex ossis sacri) — the dorsal surface of the apex forms the boundary of the canal outlet.
- Sacral cornua (cornua sacralia) — located lateral to the sacral hiatus, pointing inferiorly and bounding it.
Externally, this hiatus is closed by the superficial dorsal sacrococcygeal ligament (sacrococcygeal membrane).
To easily remember the origin of the crests on the dorsal surface of the sacrum, relate them to the structure of a typical vertebra from center to lateral: Median — Spinous, Intermediate — Articular, Lateral — Transverse (M-S, I-A, L-T).
Class and exam questions
What is the sacral promontory and why is it clinically important?
The sacral promontory (promontorium) is an anteriorly projecting point on the anterior margin of the body of the first sacral vertebra. It serves as a crucial obstetric landmark when assessing the true conjugate and overall pelvic dimensions.
Where do the transverse lines on the pelvic surface of the sacrum come from?
Transverse lines (lineae transversae) are bony seams representing the remnants of the fusion of originally separate sacral vertebrae into a single bone. There are typically four of them.
What passes through the sacral foramina?
The anterior rami of the sacral spinal nerves emerge through the anterior sacral foramina, while their posterior rami emerge through the posterior sacral foramina, respectively.
What is the function of the sacral and coccygeal cornua?
These are rudimentary articular processes of the corresponding vertebrae. The sacral cornua bound the sacral hiatus laterally and articulate with the coccygeal cornua, forming the sacrococcygeal joint.
What else to study on this topic
- Mechanism of formation and boundaries of the posterior pelvic wall
- Topography and branching of the sacral spinal nerves
- Anatomy of the articulation between the auricular surface and the hip bone
- Ligamentous apparatus of the sacral tuberosity
- Characteristics of the sacrococcygeal joint and its mobility in females
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