Name of bone
The sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra; the bottom part, with the coccyx (tailbone). In children, it consists of usually five unfused vertebrae which begin to fuse between ages 16 and 18 and are usually completely fused into a single bone by age 26.
It is curved upon itself, is placed obliquely (tilted forward), and is kyphotic (concave facing forward). The base projects forward at the sacral promontory internally, and articulates with the last lumbar vertebra to form the prominent sacrovertebral angle. The central part is curved outward towards the posterior, allowing greater room for the pelvic cavity.
The sacrum articulates with four bones:
- the last lumbar vertebra above
- the coccyx (tailbone) below
- the ileum portion of the hip bone on either side
Rotation of the sacrum forward a few degrees vis-à-vis the ilia is sometimes called "Nutation" (L. "nodding"), and the reverse motion "counter-nutation."
It is called the sacrum when referred to all of the parts combined, but sacral vertebrae when referred individually.
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- The pelvic surface of the sacrum is concave from above downward, and slightly so from side to side.
- The dorsal surface of the sacrum is convex and narrower than the pelvic.
- The lateral surface of the sacrum is broad above, but narrowed into a thin edge below.
- The base of the sacrum, which is broad and expanded, is directed upward and forward.
- The apex (apex oss. sacri) is directed downward, and presents an oval facet for articulation with the coccyx.
- The vertebral canal (canalis sacralis; sacral canal) runs throughout the greater part of the bone; above, it is triangular in form; below, its posterior wall is incomplete, from the non-development of the laminae and spinous processes. It lodges the sacral nerves, and its walls are perforated by the anterior and posterior sacral foramina through which these nerves pass out.
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The sacrum arises from a total of 35 centers of ossification. Each sacral segment is formed by five ossification centers – one primary center that forms the body, and four secondary centers that form the superior and inferior epiphyseal plates and the two halves of the vertebral (neural) arch. Additionally, the first 3 sacral segments have a pair of costal elements that project anterolaterally to form the alae. Lastly, two epiphyseal plates on each side of the sacrum form the lateral aspect of the bone, including the auricular surface that articulates with the ilium.
At birth, the five sacral vertebrae resemble the developing lumbar vertebrae and are separated by intervertebral disks. It is not until the second year of life when the alae begin to form that the sacrum takes on its distinct appearance (1). The fusion of the costal elements and auricular surfaces of the sacral alae does not begin until puberty and fusion of the vertebral bodies does not commence until near the end of the second decade of life (2). Typically, the lateral epiphyses and costal elements fuse completely by age 22, the S3-S4 and S4-S5 segments fuse by age 22, the S2-S3 segment fuses by age 23, and the S1-S2 segment may not fuse until the fourth decade of life or later (3,4). Sacral morphology changes little in adult life, but the curvature may become more pronounced after age 60 (5). The sacral alae and auricular surfaces may also exhibit bilateral asymmetry favoring the left side (6).
- The sacrum, in some cases, consists of six pieces; occasionally the number is reduced to four. The bodies of the first and second vertebrae may fail to unite.
- Sometimes, the uppermost transverse tubercles are not joined to the rest of the ala on one or both sides, or the sacral canal may be open throughout a considerable part of its length, in consequence of the imperfect development of the laminae and spinous processes.
- Sacral agenesis is a rare congenital anomaly that is characterized by the partial (hypoplasia) or complete absence of the sacrum. Incidence of sacral agenesis is 2 or 3 times more likely in mother with diabetes mellitus (7).
- The sacrum also varies considerably with respect to its degree of curvature. Anterior sacral curvature of the male sacrum is typically more pronounced than that of females (8,9). The flatter sacrum of females is largely attributable to obstetric requirements to accommodate the fetal head and shoulders during childbirth (10,11).
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Figure 1. Embryology of the sacrum
Figure 2. Sacrum
1. Esses SI, Botsford DJ, Huler RJ, Rauschning W. Surgical anatomy of the sacrum: A guide for rational screw fixation. Spine 1991; 16:283-288.
2. Cheng JS, Song JK. Anatomy of the sacrum. Neurosurgery Focus 2003; 15:1-4.
3. McKern TW, Stewart TD. Skeletal age changes in young American males, analyzed from the standpoint of identification. Headquarters Quartermaster Research and Development Command, Technical Report EP-45, Natick, MA. 1957.
4. Johnston FE. Sequence of epiphyseal union in a prehistoric Kentucky population, Indian Knoll. Human Biology 1961; 33:66-81.
5. Plochocki JH. Morphometric age-related change in the adult human sacrum Thesis. Wichita, KS: Wichita State Univ., 1999.
6. Plochocki JH. Directional bilateral asymmetry in human sacral morphology. International Journal of Osteoarchaeology 2002; 12:349-355.
7. Moore KL, Persaud TVN. The developing human, clinically oriented embryology (7th edn). Philadelphia, PA: WB Saunders Co, 2003, p. 178.
8. Flander L B. Univariate and multivariate methods for sexing the sacrum. American Journal of Physical Anthropology 1978; 49: 103-110.
9. Plochocki JH. Sexual dimorphism of the anterior sacral curvature. Journal of Forensic Sciences 2011; 56:161-164.
10. Abitbol MM. Evolution of the sacrum in hominoids. American Journal of Physical Anthropology 1987; 74:65--81.
11. Rosenberg K, Trevathan W. Birth, obstetrics and human evolution. BJOG: An International Journal of Obstetrics & Gynaecology 2002; 109:1199-1206.