Iliopsoas muscle

Composition and origin

Anatomically, the m. iliopsoas is a combined complex comprising the iliacus muscle (m. iliacus), as well as the major and minor psoas muscles (m. psoas major et minor).

• Iliac fascia (fascia iliaca) originates from the inner lip of the iliac crest (crista iliaca) and the lateral surface of the lumbar vertebral bodies.
• Fascial sheath covers the m. iliacus and m. psoas major/minor. Laterally, the fascia fuses with the inguinal ligament (lig. inguinale), forming the iliopectineal arch (arcus iliopectineus). This arch divides the subinguinal space into two lacunae: the muscular and vascular lacunae.

Topography and adjacent structures

Throughout its course, the muscle is in contact with several important anatomical structures of the abdominal cavity and thigh:

• In the abdominal cavity — it lies adjacent to the retroperitoneal adipose tissue along with the quadratus lumborum and transversus abdominis muscles; posteriorly, the m. iliopsoas is adjacent to the ascending colon.
• Muscular lacuna (lacuna musculorum) is the lateral compartment of the subinguinal space through which the m. iliopsoas passes into the thigh. Together with it, the femoral nerve (n. femoralis) and the lateral femoral cutaneous nerve (n. cutaneus femoris lateralis) pass here.
• The hip joint — the anterior "weak spot" of the joint between lig. iliofemorale and lig. pubofemorale is covered by the fascial sheath of the iliopsoas muscle. Between the articular capsule and the muscle lies the iliopectineal bursa (bursa iliopectinea), which communicates with the joint cavity in 10% of cases.

Biomechanics and descending tracts

The muscle is the primary hip flexor.

• Function — provides hip flexion with its lateral rotation. When the lower extremities are fixed, it allows raising the torso from a supine or sitting position.
• Organization of movements — muscle activity is associated with the lateral system of descending spinal cord tracts: the pyramidal, rubrospinal, and reticulospinal tracts. They exert a facilitatory influence on flexor motor neurons. Peak muscle activity occurs during the swing phase of the step, when it is necessary to lift the leg and swing it forward.

Clinical significance: neuropathies

A lesion accompanied by impaired iliopsoas function leads to motor disorders:

• High lesion — causes impaired hip flexion and the inability to raise the torso from a supine or sitting position. Walking, running, and especially climbing stairs are difficult. The patient avoids flexing the leg and fixes the limb in a position of excessive knee extension.
• Lesion below the inguinal ligament — the key difference is that iliopsoas paresis is absent: hip flexion is preserved or only slightly impaired.

Surgical infection and abscess tracts

The process is usually secondary in nature. Sources of infection can include acute appendicitis, osteomyelitis of the pelvic bones and spine, inflammatory kidney diseases, and perforation of the ascending or descending colon.

• Main pathway of spread — pus spreads along the posterior surface of the m. iliopsoas through the muscular lacuna to the lesser trochanter (trochanter minor). Due to the unyielding inguinal ligament, the swelling has an «hourglass» shape.
• Alternative pathways — infection can spread medially between the pubic bone and the m. pectineus or along the femoral vessels into the adductor canal.
• Psoas sign — a sign of inflammation within the retroperitoneal space itself. Due to pressure from the abscess, reflex contraction of the m. iliopsoas occurs: the patient keeps the leg flexed, slightly adducted, and inwardly rotated. An attempt to straighten the leg causes increased pain.

Class and exam questions

What else to study on this topic

• Muscular and vascular lacunae
• Femoral nerve (n. femoralis)
• Femoral triangle
• Fasciae of the lower extremity

Other topics in the "Anatomical Structures" section

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