Sacrum

Study entry

Parts

Wedge-shaped bone, usually formed by five fused sacral vertebrae (fusion begins after age 20). Base above (the upper surface of S1, with the promontory in front); apex below, with an oval surface for the coccyx. Pelvic surface smooth and concave, with four transverse lines of the fusions and the anterior sacral foramina; dorsal surface rough and convex, with five crests, the posterior foramina, the hiatus and the sacral cornua. Inside, the sacral canal, with the cauda equina. At the side, the auricular surface.

Joints

Lumbosacral (base with L5: body through the disc and superior articular processes with the inferior ones of L5); sacroiliac (auricular surface with the ilium); sacrococcygeal (apex with the coccyx).

Clinical

The promontory is an important obstetric landmark.

Source: cap. 2, «Vértebras: sacro» y «Anatomía de superficie de las vértebras lumbares, el sacro y el cóccix»

Other structures in this region

• Typical cervical vertebra (C3–C6)
• Atlas (C1)
• Axis (C2)
• Vertebra prominens (C7)
• Typical thoracic vertebra (T1–T12)
• Typical lumbar vertebra (L1–L5)
• Coccyx