Orientation and general presentation (Figs 13.1 & 13.2)

The sacrum has a triangular shape; its base (1) is oriented proximally, whereas the apex (2) is distal. This bone is the result of the fusion of five sacral vertebrae (S1–S5); therefore, many of its characteristics are related to the features of a ‘normal’ vertebra. The median sacral crest (3) is actually the result of the fusion of the spinous processes of the primitive sacral vertebrae. This crest is comprised of at least three prominences (i.e. spinous processes – S1, SS2 and S3). The most developed process, the spinous process of S2 (SS2), is slightly above a horizontal line passing through the posterior superior iliac spine of both iliac bones (in blue; see also p. 103). Both posterior and anterior faces show four pairs of sacral foramina (4) (only a few foramina are indicated in the figures). The coccyx (5) is below the sacral bone. Other structures include the ilium (6) and femur (7). Spinous processes of both fourth and fifth lumbar vertebrae are above (3) (in green; see also p. 29).

Figure 13.1 Sacrum (3D model).

A Location in the pelvis: anterior view, slightly lateral. B Sacral bone in relation to ilia and fourth and fifth lumbar vertebrae: posterior view.

Figure 13.2 Sacrum (anatomic specimens).

A Location in the pelvis: anterior view, slightly lateral. B Posterior aspect: posterior view.

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