The sacrum (Latin for sacred) is a large triangular bone originating from separate vertebra that fuse along with the intervening intervertebral discs. One of the functions of the sacrum is to transmit the weight of the body to the hip bones.
Anatomical variations occur frequently in this region, making the sacrum the most variable section of the spine (Esses and Botsford 1997). While most sacra have five fused vertebrae, the number of fused vertebrae can vary from four to seven. The variations consist of the sacralization of the L5 vertebra (i.e. L5 fuses with S1 to become part of the sacrum), the lumbarization of the S1 vertebra (i.e. S1 remains unfused to S2 and acts as part of the lumbar section) or the incorporation of first piece of the coccyx to the sacrum (Singh et al 2015).
The sacrum articulates with four bones; the most inferior lumbar vertebra above via a disc space and facet joint complex, the coccyx below with a ligamentous attachment and occasional bone union, and on either side with the ilium via the SI joint.
The sacroiliac (SI) joint is the largest axial joint in the body and is formed by the crescent-shaped articulation between the medial surface of the ilium and the lateral surface of the sacrum. The articulating surface of the sacrum is slightly concave to fit the convex surface of the ilium. There is wide variability in the size, shape, and surface contour of the adult SI joint. The same individual may even have large disparities between their right and left SI joint (Ruch 1997). Age-related changes in the SI joint begin at puberty and continue throughout life.
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References:
Esses, S. and Botsford, D. (1997) ‘Surgical Anatomy and Operative Approaches to the Sacrum.’
In J. Frymoyer, T. Ducker, N. Hadler, et al (eds) The Adult Spine: Principles and Practice, ed 2. Philadelphia: Lippincott-Raven.
Ruch, W. (1997) Atlas of common subluxations of the human spine and pelvis. Boca Raton: CRC Press.
Singh, I., Ajita, R. and Singh, N. (2015) ‘Variation in the number of sacral pieces.’ IOSR Journal of Dental and Medical Sciences 14, 2, 106-08.