The ilium is the largest and uppermost of the three bones that fuse to form the hip bone, also known as the coxal bone. This expansive, fan-shaped structure is a significant component of the pelvic girdle, providing a firm base for the torso’s connection to the lower limbs. The ilium is a purely skeletal structure with a foundational role in human movement and posture.
Location and Major Landmarks
The ilium sits superiorly within the pelvis, extending upward and outward from the central socket of the hip joint. It connects posteriorly to the sacrum, the triangular bone at the base of the spine, forming the largely immobile sacroiliac joint. Inferiorly, the ilium joins with the pubis and the ischium at the acetabulum, the deep, cup-shaped socket that receives the head of the femur to create the hip joint.
A defining feature is the Iliac Crest, the long, curved, thickened superior border of the bone that is easily felt just below the waistline. This crest extends from the Anterior Superior Iliac Spine (ASIS) at the front to the Posterior Superior Iliac Spine (PSIS) at the back, serving as an easily identifiable surface landmark. Below these points, the ilium features the anterior and posterior inferior iliac spines, which act as attachment sites for muscles and ligaments.
The bone’s structure includes a broad, flat wing, called the ala, and a smaller, central body that contributes to the acetabulum. On the posterior margin, the greater sciatic notch forms a deep indentation that provides a pathway for the large sciatic nerve to exit the pelvis and travel down the leg. The internal surface of the wing contains the iliac fossa, a large, smooth, concave depression that helps form the side walls of the greater pelvis.
Essential Functions of the Ilium
The primary biomechanical role of the ilium is to facilitate weight transmission between the upper body and the lower limbs. The ilium acts as a bridge, receiving the weight of the trunk and spine from the sacrum and transferring that force down through the hip joints to the legs. This arrangement is necessary for maintaining an upright posture and enabling bipedal locomotion.
The wide surface area of the ilium provides extensive attachment points for numerous muscles that control posture and movement. The outer surface, known as the gluteal surface, serves as the origin for the large gluteal muscles—the gluteus maximus, medius, and minimus. These muscles are responsible for extending, abducting, and rotating the thigh, enabling powerful movements like running and stabilizing the pelvis during walking.
The iliac crest and its internal surfaces also anchor muscles related to the core and abdomen. Muscles of the abdominal wall attach along the crest, contributing to core stability and trunk flexion. The large iliacus muscle originates from the iliac fossa and is a powerful flexor of the hip joint, drawing the leg up toward the trunk. The interplay of these attached muscles and ligaments ensures the stability of the entire pelvic ring during dynamic activities.
Clinical Importance
The ilium is frequently involved in medical contexts concerning traumatic injury and diagnostic procedures. As part of the pelvic ring, the ilium can be fractured, typically requiring high-energy trauma such as a motor vehicle accident or a significant fall. These iliac wing fractures can compromise the stability of the entire pelvis and are often associated with other severe internal injuries.
The iliac crest is the most common site for obtaining samples of bone marrow for diagnostic purposes. This procedure, known as bone marrow aspiration or biopsy, is often performed on the posterior iliac crest because of its accessibility, the large volume of active marrow it contains, and its superficial location beneath the skin.
The bone’s flat shape and the thickness of its spongy interior make it a safer site for needle insertion compared to other bones.
The ilium’s readily available bone tissue also makes the iliac crest a frequent source for bone grafting procedures. Surgeons harvest pieces of the bone to repair defects or promote healing in other parts of the skeletal system. Although complications are rare, the bone marrow procedure can sometimes lead to localized pain or, in rare instances, a fracture of the ilium itself.