The ilium is the uppermost and largest part of the hip bone, forming a key component of the pelvic structure. It supports the body and facilitates movement.
Anatomical Location
The ilium is located within the pelvic region, situated laterally to the sacrum, which is at the base of the spinal column. In adults, the ilium fuses with two other bones, the ischium and the pubis, to form a single, large, irregularly shaped bone called the hip bone, also known as the coxal bone or innominate bone. This fusion occurs at a cup-shaped socket called the acetabulum. The ilium’s prominent position makes it a recognizable landmark in the torso.
Part of the Pelvis
The ilium is a component of the pelvic girdle, the bony structure that connects the axial skeleton (spine and skull) to the lower limbs. The pelvic girdle consists of two hip bones joined anteriorly at the pubic symphysis and posteriorly by the sacrum. This ring-like structure provides support for the upper body’s weight and transfers it to the lower extremities. The ilium articulates with the sacrum at the sacroiliac joint, a largely immobile connection that contributes to the stability of the pelvis.
In children, these three components are separate and connected by cartilage, but they fuse by puberty to form a single, complex bone. This fusion creates the socket for the hip joint, where the head of the femur articulates. The pelvic girdle also encases and protects various abdominal and pelvic organs.
Key Anatomical Features and Function
The ilium presents several anatomical features, including its wide, fan-like upper part, known as the ala or wing. The superior border of this wing is the iliac crest, a curved ridge palpable at the hips. This crest extends from the anterior superior iliac spine (ASIS) at the front to the posterior superior iliac spine (PSIS) at the back. The iliac crest serves as an attachment point for various muscles, including abdominal muscles like the external and internal obliques, as well as the latissimus dorsi and erector spinae muscles.
Beneath the iliac crest, the ilium features the iliac fossa, a shallow, concave depression on its internal surface. This fossa provides an origin for the iliacus muscle, which is involved in hip flexion. The ilium also contributes to the greater sciatic notch, a large indentation on its posterior border that allows the passage of the sciatic nerve and other structures. The ilium’s functions include bearing body weight, protecting internal pelvic organs, and providing surfaces for the attachment of muscles that facilitate movement of the trunk and lower limbs.
Ilium vs. Ileum: Clarifying Common Confusion
A common point of confusion arises from the similar spellings and pronunciations of “ilium” (the bone) and “ileum” (a part of the small intestine). Despite their phonetic resemblance, these two terms refer to entirely different anatomical structures with distinct locations and functions. The ilium is a hard, bony component of the skeletal system, specifically the uppermost part of the hip bone.
In contrast, the ileum is a soft, muscular, tubular structure that forms the final and longest section of the small intestine. It is situated in the abdominal cavity, where its primary role is to absorb nutrients, including vitamin B12 and bile salts, from digested food before it passes into the large intestine. The ileum is part of the digestive system, while the ilium is part of the musculoskeletal system.