Iliopsoas

Introduction

Introduction to Iliopsoas

The iliopsoas is the primary flexor of the hip joint and the most powerful muscle of the anterior thigh region. It is a composite muscle consisting of two distinct parts: the psoas major (and sometimes psoas minor) arising from the lumbar spine, and the iliacus arising from the iliac fossa. Together, they form a fusiform muscle that passes deep to the inguinal ligament and inserts onto the lesser trochanter of the femur. The iliopsoas is essential for walking, running, and maintaining an erect posture by controlling the lumbar lordosis. Its deep location makes it clinically significant in psoas abscess, iliopsoas bursitis, and as a surgical landmark in anterior approaches to the hip and lumbar spine.

Study

1. Composition and General Features

The iliopsoas complex consists of the psoas major, psoas minor (when present), and iliacus muscles. The psoas major is a long, thick, fusiform muscle that lies along the sides of the lumbar vertebral column and the pelvic brim. The iliacus is a flat, triangular muscle that fills the iliac fossa. The two muscles converge beneath the inguinal ligament, pass anterior to the capsule of the hip joint, and insert by a common tendon onto the lesser trochanter of the femur. The iliopsoas is classified as an inner hip muscle, distinct from the anterior thigh muscles.

2. Psoas Major

The psoas major originates from the anterior surfaces of the transverse processes, the lateral borders of the vertebral bodies, and the intervertebral discs from T12 to L5. The muscle descends inferolaterally, passes along the pelvic brim, and runs deep to the inguinal ligament. The psoas major is innervated by direct branches from the ventral rami of the lumbar spinal nerves L1, L2, and L3. Its blood supply is derived from the lumbar branches of the iliolumbar artery and segmental lumbar arteries.

The psoas minor is a slender muscle present in approximately 40–60% of individuals. It lies on the anterior surface of the psoas major and inserts onto the iliopubic eminence and the arcuate line. It is a weak flexor of the lumbar spine and is innervated by a branch from L1. Its tendon may be mistaken for the psoas major tendon during imaging or surgery.

3. Iliacus

The iliacus arises from the upper two‑thirds of the iliac fossa, the inner lip of the iliac crest, the anterior sacroiliac ligament, and the upper surface of the lateral part of the sacrum. Its fibres converge inferiorly and medially to join the lateral side of the psoas major tendon. The iliacus is innervated by the femoral nerve (L2, L3, L4), receiving branches while still within the pelvis. Its blood supply comes from the iliac branch of the iliolumbar artery and branches of the medial circumflex femoral artery.

4. Relations

Within the abdomen, the psoas major is related anteriorly to the peritoneum, the ureter (which crosses it obliquely), the gonadal vessels, and the genitofemoral nerve which pierces and descends on its surface. Posteriorly, it lies on the lumbar transverse processes and the sacroiliac joint. Medially, it is related to the lumbar vertebrae and the great vessels. The iliacus lies lateral to the psoas, within the iliac fossa, related medially to the femoral nerve which lies in the groove between the psoas and iliacus.

In the femoral triangle, the iliopsoas tendon lies in the lateral part of the floor, lateral to the pectineus muscle, and is separated from the femoral artery by the femoral nerve. A bursa, the iliopectineal (iliopsoas) bursa, lies between the tendon and the hip joint capsule. This bursa is the largest synovial bursa in the body and communicates with the hip joint in approximately 15% of individuals.

5. Actions

The iliopsoas is the strongest flexor of the hip joint. It also assists in lateral rotation of the femur. When the lower limb is fixed, the iliopsoas flexes the trunk against gravity, as in sitting up from a supine position (the sit‑up muscle). It is active during the swing phase of gait to initiate hip flexion. The psoas major also has an important role in maintaining the normal lumbar lordosis by pulling the lumbar vertebrae anteriorly. Weakness of the iliopsoas results in difficulty climbing stairs, rising from a chair, and walking upstairs. A tight iliopsoas, conversely, can cause a hip flexion contracture, contributing to an increased lumbar lordosis and low back pain.

6. Clinical Relevance

Psoas abscess: A collection of pus within the psoas major sheath, often secondary to tuberculosis of the lumbar spine (Pott's disease) or pyogenic vertebral osteomyelitis. The abscess tracks along the psoas sheath and may present as a swelling below the inguinal ligament in the femoral triangle.

Iliopsoas bursitis: Inflammation of the iliopectineal bursa, causing deep groin pain and a palpable mass in the femoral triangle. It may communicate with the hip joint. Snapping hip syndrome (internal type) occurs when the iliopsoas tendon snaps over the iliopectineal eminence or the lesser trochanter during hip movement.

Hip flexion contracture: A tight iliopsoas leads to a fixed hip flexion deformity (Thomas test positive), often seen in cerebral palsy, prolonged sitting, or after hip surgery. Iliopsoas release or lengthening may be required.

Surgical approaches: The iliopsoas tendon is a key landmark in the anterior (Smith‑Petersen) and anterior‑lateral approaches to the hip. It is released in certain procedures for hip osteoarthritis. The psoas major is used as a surgical plane for retroperitoneal approaches to the lumbar spine.

Summary

Summary of Iliopsoas

The iliopsoas is the primary hip flexor, composed of the psoas major (innervated by L1–L3) and iliacus (innervated by the femoral nerve). It originates from the lumbar spine and iliac fossa and inserts on the lesser trochanter. The muscle is vital for hip flexion, trunk flexion, and maintaining lumbar lordosis. Clinical conditions include psoas abscess, iliopsoas bursitis, snapping hip, and hip flexion contracture.