Iliacus

The iliacus is a large, flat, triangular muscle that fills the iliac fossa on the inner surface of the pelvis. It originates from the iliac fossa and the iliac crest, with additional fibers blending from the ala of the sacrum and adjacent ligaments such as the sacroiliac and iliolumbar ligaments. Its tendon merges with the psoas major to form the iliopsoas, which inserts on the lesser trochanter of the femur, with some fibers inserting near the base of the femoral neck.

Nerve supply is via the femoral nerve (L2–L4). The primary arterial blood supply comes from the iliolumbar artery, with smaller muscular branches from the femoral artery contributing as needed.

Functionally, the iliacus is a principal hip flexor. Acting together with the psoas major as the iliopsoas, it flexes the hip joint and assists in external rotation of the thigh. It also contributes to stabilization of the pelvis and spine, supporting upright posture. When the thigh is fixed, the muscle can aid in flexing the trunk.

Anatomical relations place the iliacus within the iliac fossa, beneath the iliac fascia, and it passes inferiorly under the inguinal ligament to join the psoas tendon. Clinically, iliacus pathology is considered in conditions involving hip flexor pain, such as iliopsoas tendinopathy or bursitis, and may arise from overuse, trauma, or inflammatory processes.