The ilium is a significant part of the human skeletal system, contributing to the structure of the pelvic region.
Where the Ilium Is Located
The ilium is the largest and uppermost bone of the hip bone, which in adults (also known as the innominate bone or os coxae) is formed by the fusion of three separate bones: the ilium, ischium, and pubis. These three bones unite during puberty, primarily at a cup-shaped socket called the acetabulum. These hip bones, along with the sacrum and coccyx, collectively form the pelvic girdle. The pelvic girdle serves as a stable ring that connects the axial skeleton (spine) to the lower limbs. Each ilium is positioned on either side of the sacrum, forming the sacroiliac joints.
Anatomy of the Ilium
The ilium has a broad, fan-like upper part, referred to as the ala or wing. Its narrower lower part, known as the body, contributes to the formation of the acetabulum, which is the hip socket. A prominent feature is the iliac crest, the curved superior border of the ilium.
Iliac Crest and Spines
Along the iliac crest are several bony projections. These include the anterior superior iliac spine (ASIS) and anterior inferior iliac spine at the front. At the posterior end are the posterior superior iliac spine (PSIS) and posterior inferior iliac spine.
Iliac Fossa and Sacroiliac Joint
The ilium also features a large, concave inner surface called the iliac fossa, which provides an attachment site for muscles. Posteriorly, the ilium articulates with the sacrum at the sacroiliac joint.
What the Ilium Does
The ilium serves a primary role in weight bearing, transferring forces from the upper body to the lower limbs. This bone helps support the spine when the body is upright, contributing to overall stability.
Numerous muscles of the trunk, hip, and thigh attach to the iliac crest and its various spines. For example, abdominal muscles like the external and internal obliques, and transversus abdominis, attach to the iliac crest. Muscles such as the sartorius and tensor fasciae latae originate from the anterior superior iliac spine, facilitating hip and thigh movement. The ilium also provides attachment for gluteal muscles, which are important for hip extension and rotation. The broad structure of the ilium helps protect internal pelvic organs.
Common Conditions Involving the Ilium
Iliac crest fractures can occur due to direct trauma, leading to pain, swelling, and difficulty bearing weight. These fractures often result from impacts to the hip area.
Apophysitis, particularly common in adolescents, involves inflammation of the growth plate, such as iliac crest apophysitis or apophysitis of the anterior superior iliac spine. It typically arises from repetitive stress or overuse in young athletes. The sacroiliac joints, where the ilium meets the sacrum, can also experience issues like sacroiliitis. This condition involves inflammation of the joint, potentially causing localized pain or discomfort that may radiate to the lower back and legs.