Composition and Origins
Anatomically, the m. iliopsoas is a combined complex consisting of the iliacus muscle (m. iliacus), along with the psoas major and minor muscles (m. psoas major et minor).
- The iliac fascia (fascia iliaca) originates from the inner lip of the iliac crest (crista iliaca) and the lateral surfaces of the lumbar vertebral bodies.
- A fascial sheath encloses the m. iliacus and m. psoas major/minor. Laterally, the fascia blends with the inguinal ligament (lig. inguinale), forming the iliopectineal arch (arcus iliopectineus). This arch divides the subinguinal space into two lacunae: the muscular lacuna and the vascular lacuna.
Topography and Adjacent Structures
Throughout its course, the muscle contacts several important anatomical structures of the abdominal cavity and thigh:
- In the abdominal cavity — it lies against the retroperitoneal fat tissue alongside the quadratus lumborum and transversus abdominis muscles; posteriorly, the ascending colon relates to the m. iliopsoas.
- Muscular lacuna (lacuna musculorum) — the lateral compartment of the subinguinal space through which the m. iliopsoas enters the thigh. Passing alongside it are the femoral nerve (n. femoralis) and the lateral femoral cutaneous nerve (n. cutaneus femoris lateralis).
- Hip joint — the anterior "weak spot" of the joint capsule between the iliofemoral ligament (lig. iliofemorale) and pubofemoral ligament (lig. pubofemorale) is covered by the fascial sheath of the iliopsoas muscle. The iliopectineal bursa (bursa iliopectinea) lies between the joint capsule and the muscle and communicates with the joint cavity in 10% of individuals.
Biomechanics and Descending Tracts
The muscle is the primary flexor of the hip.
- Function — produces hip flexion combined with external rotation. When the lower extremities are fixed, it allows the trunk to be raised from a lying or sitting position.
- Motor organization — muscle activity is linked to the lateral descending motor systems of the spinal cord: the corticospinal (pyramidal), rubrospinal, and reticulospinal tracts. These exert a facilitatory influence on flexor motor neurons. Peak muscle activity occurs during the swing phase of gait when the leg must be lifted and carried forward.
Clinical Significance: Neuropathies
Lesions impairing iliopsoas function lead to specific motor deficits:
- High-level lesions — cause impaired hip flexion and an inability to raise the trunk from a supine or seated position. Walking, running, and particularly climbing stairs are difficult. The patient tends to avoid hip flexion and fixes the limb in excessive knee extension.
- Lesions distal to the inguinal ligament — the key differentiator is that iliopsoas paresis is absent; hip flexion is preserved or only minimally impaired.
Surgical Infections and Abscess Spread
Infections involving this compartment are generally secondary. Sources include acute appendicitis, pelvic and vertebral osteomyelitis, inflammatory renal disease, and perforation of the ascending or descending colon.
- Primary pathway of spread — pus tracks along the posterior surface of the m. iliopsoas through the muscular lacuna toward the lesser trochanter (trochanter minor). Due to the unyielding inguinal ligament, the resulting swelling often takes an "hourglass" shape.
- Alternative pathways — infection may spread medially between the pubic bone and the pectineus muscle (m. pectineus) or along the femoral vessels into the adductor canal.
- Psoas sign — a clinical indicator of inflammation within the retroperitoneal space. Irritation causes reflex spasm of the m. iliopsoas: the patient holds the hip flexed, slightly adducted, and internally rotated. Attempted passive leg extension provokes significant pain.