Sacrum Documentation - Anatomy Software


- The sacrum is a large, triangular bone, situated in the lower part of the vertebral column and at the upper and posterior part of the pelvic cavity.


- Consists of a base, apex and three surfaces.


-Its base projects forward and forms the prominent sacrovertebral angle when articulated with the last lumbar vertebra.


-It consists of usually five initially unfused vertebrae which begin to fuse between ages 16–18 and are usually completely fused into a single bone by age 30-35.


  1. Pelvic Surface

-The pelvic (anterior) surface is concave .

-Faces anterior and inferior.


    1. sacral promontory

·         Anatomical term for the superior most portion of the sacrum (S1).

·         Anterior edge of S1 vertebra.

·         It marks part of the border of the pelvic brim.

·         Making an angle of 30 degrees anterior from the horizontal plane.


    1. 4 pairs of sacral foramen

·         At the ends of the raised transverse ridges.

·         Located lateral to the articular processes (intermediate crest).

·         Usually four in number on either side.

·         Give exit to the anterior divisions of the sacral nerves and entrance to the lateral sacral arteries.


    1. 4 raised transverse ridges/lines

·         The middle part/pelvic surface of sacrum is crossed by four transverse ridges.

·         Fusion of the bodies of the sacral vertebra.


  1. Dorsal Surface

-The dorsal surface is convex.

-Typically faces posterior and superior.


    1. superior articular facets (S1)

·         Are the superior articular processes of the first sacral vertebra (S1).

·         Are large and oval in shape.

·         Their facets are concave from side to side.

·         Face posterior and medial.

·         Articulate with the facets on the inferior processes of the fifth lumbar vertebra (L5).


    1. sacral hiatus

·         The lamina of the fifth sacral vertebra, and sometimes of the fourth, fails to meet behind, and thus a hiatus or deficiency occurs in the posterior wall of the sacral canal.


    1. sacral cornua

·         Tubercles which represent the inferior articular processes of the fifth sacral vertebra (S5) are prolonged downward as rounded processes.

·          Connected to the cornua of the coccyx.


    1. median crest

·         Represents the fusion of the spinous processes of the upper three or four sacral vertebra.


    1. intermediate crest

·         Represents the fusion of the articular processes of the upper three or four sacral vertebra.


    1. lateral crest

·         Represent the fusion of the transverse processes of the sacral vertebra.

·         Situated on the lateral side of the posterior sacral foramina.


            Lateral Surface

·         Is broad above, but narrowed into a thin edge below.

·         The thin lower half of the lateral surface gives attachment to the sacrotuberous and sacrospinous ligaments, some fibers of the gluteus maximus behind, and to the Coccygeus in front.


a.      auricular surface

·         Ear-shaped surface.

·         Usually extends the length of three sacral vertebra (S1-S3).

·         Covered with hyaline cartilage.

·         Articulation with the ilium to form the synovial part of the sacroiliac joint.


b.      attachment site for interosseous SI ligament (sacral tuberosity)

·         Rough and pitted surface.

·         Posterior and superior to the auricular surface.

·         Attachment for the interosseous SI ligament.


c.       inferior lateral angle (ILA)

·         Is the lower projected part of the lateral surface.