Anatomy Atlases: Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal System: Numerical Variation in Vertebral Column

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

Numerical Variation in Vertebral Column

Ronald A. Bergman, PhD

Adel K. Afifi, MD, MS

Ryosuke Miyauchi, MD

Peer Review Status: Internally Peer Reviewed



The usual grouping formula of 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal vertebrae is found in only about 20% of individuals studied. The cervical region is reported to be the most constant, the coccygeal the most variable.

The number of elements of the vertebral column has been reported to vary between 32 and 35. Addition to a group is frequently seen, which occurs through the reduction in number of vertebrae of an adjacent group, the total number being unchanged. In this variation, the vertebra added is usually intermediate in form between the adjacent groups. The location of such a vertebra is predominately at the ends of the column and at the levels of transition between its regions. Thus, sacralization of the fifth lumbar, lumbar-like articular processes in the eleventh thoracic, and thoracic costal facets on the seventh cervical are observed.

The levels of transition may be shifted cephalad, resulting in 23 mobile vertebrae, or shifted caudad, resulting in 25 presacral vertebrae. Such variations may occur in 2-11% of the population.

In one study of 748 vertebral columns, 717 had 17 thoracolumbar vertebrae, 26 had 18, and five had 16. Of the 717 columns, four had a lumbar increase and a thoracic decrease while five had a lumbar decrease with a compensating thoracic increase.

The number of vertebrae comprising the sacrum may be increased to six, resulting from the fusion of the first coccygeal (50% in whites, 30% in Negroes) or, less often, of the last lumbar (sacralization) (8% in whites, 11% in Negroes); or it may be increased to seven, resulting from the fusion of the first coccygeal and the last lumbar (4% in whites, 1.5% in Negroes). The number may be reduced to four, apparently by the lumbarization of the first sacral (0.4% in whites, 1.5% in Negroes) (Trotter and Lanier).

The junction between the lumbar and sacral parts of the column is occasionally made by an element presenting the characteristics of a lumbar vertebra on one side and of a sacral on the opposite (hemisacralization, hemilumbarization).

The auricular surface may extend over no more than the first two vertebra or on to the fourth sacral vertebra. Accessory articular facets on the lateral sacral crest at the level of the first or second dorsal sacral foramen have been found unpaired and bilateral, meeting in articulation with extra facets on the ilium.

The sacral canal may be open dorsally to a greater degree than is usually the case, because of failure of laminae preceding the fourth to fuse. In 1% of a series of 1,225 sacra all the laminae failed to fuse in the middle line, resulting in a completely open bony dorsal wall of the sacral canal; this occurred almost exclusively in the males and twice as frequently in whites as in Negroes.

Coalescence of the coccyx and sacrum takes place less often and later in life in the female than in the male.

Absence of the sacrum and coccyx, in part or complete, is reported; in some instances with remarkable functional adjustments (Zeligs; Sinclair, Duren and Rude).

Rudiments of an arch, chevron bones, on the pelvic surface have been interpreted as vestiges of the hemal arch of lower vertebrates. Five cases have been reported; four of the first and one of the second coccygeal vertebra (Schultz).

Vertebral Numerical Variation

References

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