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The sacrum has a base, an apex, and three surfaces – a pelvic, dorsal and a lateral surface. The base of the sacrum, which is broad and expanded, is directed upward and forward. On either side of the base is a large projection known as an ala of sacrum and these alae (wings) articulate with the sacroiliac joints.
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 wing (ala) of ilium is the large expanded portion of the ilium, the bone which bounds the greater pelvis laterally. It presents for examination two surfaces—an external and an internal—a crest, and two borders—an anterior and a posterior.
The posterior border of the ala of sacrum, shorter than the anterior, also presents two projections separated by a notch, the posterior superior iliac spine and the posterior inferior iliac spine. The posterior superior iliac spine serves for the attachment of the oblique portion of the posterior sacroiliac ligaments and the multifidus.
The arcuate line marks the border between the body (corpus) and the wing (ala) of the ilium, and, running inferior, anterior, and medial from the auricular surface to the area corresponding to the acetabulum, it also indicates where weight is transferred from the sacroiliac joint to the hip joint.
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 apple-shaped line passing through the prominence of the sacrum, the arcuate and pectineal lines, and the upper margin of the pubic symphysis. Occasionally, the terms pelvic inlet and pelvic brim are used interchangeably.
A fracture of the left iliac wing. The iliac crest has a large amount of red bone marrow, and thus it is the site of bone marrow harvests (from both sides) to collect the stem cells used in bone marrow transplantation.