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Anatomy Atlases: Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal System: Sacrum and Coccyx

Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal Systems: Vertebral column

Sacrum and Coccyx

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD

Peer Review Status: Internally Peer Reviewed

The number of vertebrae in the sacrum may be increased by fusion of the first coccygeal, by (less often) addition of the last (fifth) lumbar, or by addition of both the last lumbar and first coccygeal vertebrae.

The number may be reduced to four by the "lumbarization" of the first sacral vertebra. In one study of 631 bodies, the sacrum was composed of five vertebrae in 77%, six in 21.7%, four in 1%, and seven in 0.2%.

The lumbosacral junction is occasionally composed of a vertebra with characteristics of a lumbar vertebra on one side and a sacral on the other (so-called hemilumbarization or hemisacralization).

The articular surface of the sacrum may extend variably, over no more than the first two vertebrae or to the fourth sacral vertebra. Accessory articular facets may occur on the lateral sacral crest at the level of the first or second dorsal sacral foramen; these have been found to be unpaired or bilateral and to articulate with appropriate facets on the ilium.

The sacral canal may open dorsally because of failure of laminar fusion. Coalescence of the coccyx and sacrum occurs more often and earlier in males.

The subdural and subarachnoid spaces extend into the sacral canal as far as the middle third of the body of the second sacral vertebra; however, in 46% of 56 cadavers, these spaces extend caudal to this level.

The female sacrum is usually broader (in proportion to length), less curved, and directed more obliquely dorsally than the male sacrum. The curvature of the female sacrum occurs in the caudal part of the bone; in the male, the curvature is distributed over the entire length of the bone. But the sex characterics cited above , however, are subject to considerable variation in different skeletons (Trotter, 1926).

The sacrum and coccyx may be absent.

The presence of a tail in humans has been reported on several occasions. In one case, the tail was 23 cm in length.

Racial differences: the human sacrum is characterized by its great breadth in comparison to its length. The proportion is expressed by the following equation: sacral index = breadth x 100/ length. The average sacral index in British males is 112; in the female, 116. Sacra in which the index is above 100 are platyhieric, as in Europeans, Negroes, Polynesians; those under 100 are dolichohieric, as in Australians, Bushmen, Andamanese (Turner, 1886).

Pubic Tubercles and Pubic Spines

Sacrum, Left Lateral View

Sacral Facet


Absence of Sacral Dorsal Arches

Accessory Iliosacral Articulations

Three Sacra

Sacrum Composted of Five and Six Vertebrae. Coccys Composed of Four Vertebrae.


Persistence of the Tail


Lumbar Sacralization


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