Formation and anatomical position
The sacrum (os sacrum) is a massive unpaired bone resulting from the physiological fusion of five sacral vertebrae (this bone consolidation process is usually fully completed by the age of 13). The bone has a clear orientation: its smooth, concave pelvic surface faces forward, the wide base (basis) is directed upward, and the narrowed apex (apex) points downward.
It is on the lateral surfaces of this bone that the structures responsible for contact with the pelvic ring are located. The lateral parts (partes laterales) are the upper-outer sections of the sacrum formed by the fusion of the transverse processes of the initial sacral vertebrae. On the outer side of these lateral masses lies the auricular surface (facies auricularis). It articulates with the corresponding articular facet on the ilium.
Topography of the sacral surfaces
To understand pelvic biomechanics, it is important to know the surroundings of the auricular surface and the relief of the bone itself:
- Pelvic surface (facies pelvica): concave and smooth. It shows transverse lines (lineae transversae), which are traces of the fusion of the bodies of four vertebrae. Four pairs of anterior sacral foramina (foramina sacralia anteriora) also open here, through which the anterior rami of the spinal nerves emerge.
- Dorsal surface (facies dorsalis): convex and ridged. The midline features the median sacral crest (crista sacralis mediana), formed by fused spinous processes. Medial (intermediate) crests—the result of the fusion of articular processes—run along its sides.
- Lateral sacral crests (cristae sacrales laterales): a series of tubercles formed by the fusion of transverse processes. Between the intermediate and lateral crests are the posterior sacral foramina (foramina sacralia posteriora), which are smaller than the anterior ones and serve for the exit of the posterior rami of the nerves.
Ligamentous apparatus and sacral wings
To ensure the stability of the pelvic ring, articular contact alone is not sufficient. Therefore, posterior and somewhat medial to the auricular surface lies a pronounced roughness — the sacral tuberosity (tuberositas ossis sacri).
This zone is critically important for pelvic stability as it serves as the site for the strong attachment of the interosseous sacroiliac ligaments. A similar tuberosity is present on the hip bone, allowing the ligaments to stretch between the two bones, firmly fixing the joint.
Above the auricular surface, the lateral part transitions into the upper triangular area. This is the wing of the sacrum (ala ossis sacri) — the most prominent zone of the lateral parts. The anterior border of the sacral wing smoothly transitions into the pelvic brim.
Connection with the vertebral canal
The auricular surfaces transmit the weight of the entire torso to the lower extremities. This weight initially arrives at the base of the sacrum (basis ossis sacri) — a transverse-oval depression articulating with the body of the 5th lumbar vertebra.
- The anterior margin of the base projects forward into the pelvic cavity, forming the promontory (promontorium), which bears the main load of the spinal column.
- Projecting from the posterior part of the base are the superior articular processes (processus articulares superiores) of the 1st sacral vertebra. Their articular facets are directed posteriorly and medially.
Inside the bone runs the sacral canal (canalis sacralis), a curved continuation of the vertebral canal, wider at the top and narrowed at the bottom. It contains the cauda equina (cauda equina). The canal terminates inferiorly as the sacral hiatus (hiatus sacralis), flanked on either side by downward-projecting sacral horns (cornua sacralia).
Sexual dimorphism in structure
The structure of the sacrum and the spatial orientation of its articular surfaces exhibit distinct sex differences directly related to the reproductive function of the female body.
| Sign | Male sacrum | Female sacrum |
|---|---|---|
| Shape | Long, narrow | Short, wide |
| Curvature | More pronounced | More flattened |
| Indication | Optimization of support function | Enlargement of the lesser pelvis volume |
The auricular surface resembles an ear (auricula) that "listens" to the ilium, and the tuberosity behind it is an "earring" to which strong interosseous ligaments attach.
Class and exam questions
Where exactly is the auricular surface located?
It is located on the outer side of the lateral parts of the sacrum and also has an absolutely symmetrical counterpart on the ilium of the pelvic bone.
What is the function of the sacral tuberosity?
The sacral tuberosity (tuberositas ossis sacri), located posterior and medial to the auricular surface, serves as the attachment site for powerful interosseous sacroiliac ligaments.
From what structures do the lateral parts of the sacrum develop?
The lateral parts (partes laterales) are formed during ontogenesis by the fusion of the transverse processes of the sacral vertebrae.
How does the shape of the sacrum affect the female pelvis?
In women, the sacrum is shorter, wider, and less curved. This spreads the pelvic bones apart and increases the volume of the lesser pelvis, which is necessary for childbirth.
What else to study on this topic
- Structure of the sacral canal and its contents (cauda equina)
- The sacral promontory (promontory) and its biomechanical role
- Topography of the anterior and posterior sacral foramina
- Sacrococcygeal joint and features of the sacral apex
- Median, intermediate, and lateral sacral crests
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