The sacrum

The sacrum consists of five fused vertebrae, being broadened by the incorporation of large costal elements and transverse processes into heavy lateral masses. The lateral masses lie lateral to the transverse tubercles on the back of the sacrum and extend between the anterior sacral foramina on to the front of the bone. The auricular surface lies entirely on the lateral mass. The sacrum is triangular in shape with the apex inferior. It is wedged between the posterior parts of the innominates with which it articulates at the sacroiliac joints. The pelvic (anterior) surface is concave and relatively smooth, being marked by four transverse ridges separating the original bodies of the five sacral vertebrae. Lateral to each ridge is the anterior sacral foramen, which represents the anterior part of the intervertebral foramen. These foramina are directed laterally and anteriorly.

The dorsal surface of the sacrum is convex and highly irregular, and presents the posterior sacral foramina, medial to which the vertebral canal is closed over by the fused laminae. However, usually the spinous processes and laminae of the fourth and fifth sacral vertebrae are absent leaving the vertebral canal open. This is the sacral hiatus, an inferior entrance to the vertebral canal, which may be used, for example during labour, to introduce an anaesthetics to block the sacral nerves. Posteriorally, in the midline, the reduced spinous processes form the median sacral crest. Lateral to the dorsal sacral foramina are the prominent lateral sacral crests, representing the transverse processes. These lateral crests provide attachment for the dorsal sacroiliac ligaments, and inferiorly for the sacrotuberous and sacrospinous ligaments. Just medial to the dorsal sacral foramina are the indistinct intermediate sacral crests, representing the fused articular processes. The superior articular processes of the first sacral vertebra are large and oval, being supported by short heavy pedicles. Their facets, for articulation with the inferior articular surfaces of the fifth lumbar vertebra, are concave from side to side, facing posteriorly and medially. The tubercles of the inferior articular processes of the fifth sacral vertebral form the sacral cornua and are connected to the cornua of the coccyx.

The lateral surface of the sacrum (figure a) is triangular, being narrower below. The upper part of this surface is divided into an anterior smoother pitted auricular surface, covered in cartilage, for articulation with a similar area on the ilium. The rougher posterior area presents three deep impressions for the attachment of the powerful dorsal sacroiliac ligaments. The superior surface (figure b) faces anterosuperiorly having a central oval area which is the upper surface of the first sacral vertebra, being separated from the fifth lumbar vertebra by a thick intervertebral disc. Its anterior projecting border is the sacral promontory. On each side of the body of the sacrum is the ala, formed by the fusion of the costal and transverse processes of the first sacral vertebra. When articulated with the innominate, the ala of the sacrum is continuous with the ala of the ilium.

Ossification

Primary centres appear in the sacrum between the third and eighth month in utero; one for each centre, one for each half of each vertebral arch, and for each costal element in the upper four vertebrae. The costal elements fuse with the arches by the age of 5 years, the arches with the centre slightly later, with the two parts of each arch uniting between the ages of 7 and 10. The segments of the lateral masses fuse together during puberty, with secondary centres appearing for vertebral bodies at about the same time. Bodies and epiphyses fuse between 18 and 25 years. Several secondary centres appear at the ends of the costal and transverse processes from which two epiphyses are formed, one of which covers the auricular surface while the other completes the lower margin of the sacrum.