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The sacrum (pl.: sacra or sacrums [1]), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1–S5) between ages 18 and 30. [ 2 ] The sacrum situates at the upper, back part of the pelvic cavity , between the two wings of the pelvis .
The linea terminalis or innominate line consists of the pubic crest, pectineal line (pecten pubis), the arcuate line, the sacral ala, and the sacral promontory. [1]It is the pelvic brim, which is the edge of the pelvic inlet.
The sacrum is curved upon itself and placed very obliquely, its base projecting forward and forming the prominent sacrovertebral angle when articulated with the last lumbar vertebra. It is also known as the "lumbosacral angle".
The pelvic brim is an approximately butterfly-shaped line passing through the prominence of the sacrum, the arcuate and pectineal lines, and the upper margin of the pubic symphysis. The pelvic brim is obtusely pointed in front, diverging on either side, and encroached upon behind by the projection forward of the promontory of the sacrum.
The sacroiliac joint or SI joint (SIJ) is the joint between the sacrum and the ilium bones of the pelvis, which are connected by strong ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side. The joint is strong, supporting the entire weight of the upper body.
The number of vertebrae in a region can vary but overall the number remains the same. In a human spinal column, there are normally 33 vertebrae. [3] The upper 24 pre-sacral vertebrae are articulating and separated from each other by intervertebral discs, and the lower nine are fused in adults, five in the sacrum and four in the coccyx, or tailbone.
Anatomy [ edit ] The anterior longitudinal ligament extends superoinferiorly between the basiocciput of the skull and the anterior tubercle of the atlas (cervical certebra C1) superiorly, and the superior part of the sacrum inferiorly; [ 1 ] inferiorly, it ends at the sacral promontory . [ 2 ]
With the sacral promontory, it makes up the linea terminalis. [ 2 ] The Iliopectineal line divides the pelvis into the pelvis major (false pelvis) above and the pelvis minor (true pelvis) below.