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Other Terms: Os iliacum, Os ilii, Os ilium
The body, the thick base of this bone, is a thick mass forming the superior portion of the acetabulum. Its upper margins are the arcuate line internally and the superior rim of the acetabulum externally. The ala radiates from the body as a fan-shaped plate of bone. The medial or sacropelvic surface is concave while the lateral or gluteal surface is slightly convex. The top of this expanded bone forms the arched crest terminating at both ends as the anterior superior and posterior superior spines. At the posterior end of the crest the internal surface becomes exceedingly rough providing attachment for the sacrum.
This term has had varied meanings over anatomical time. The ancient Romans used ilia which means the flanks to designate the wing-like portion of the pelvic skeleton on the soft flanks of the body. Vesalius initially referred to the hip bone as the os ilium. Some earlier writers used the term ilium and ileum interchangeably. Some etymologists use this fact to suggest that the bone name arises from its juxtaposition to that portion of the small intestine.
The ilium articulates with two bones: the femur and sacrum. The ilium forms the upper two-fifths of the bowl-like acetabulum that articulates with the rounded head of the femur. The medially directed, rough surfaces on the posterior portion of the ilium articulate with the sacrum by forming rough syndesmotic articulations and a smoother synovial surface, the auricular surface of the ilium.
The ilium ossifies from a primary center that first arises during the eighth prenatal week. This center appears just cranial to what will become the greater sciatic notch in the future body. At puberty two secondary centers appear in the iliac crest. These secondary centers fuse with the primary centers anywhere between the fifteenth and twenty-fifth year.