The abdominal (lumbar) portion of psoas major occupies the retroperitoneal space lateral to the lumbar vertebral bodies, forming the lateral wall of the posterior abdomen. This portion contains the lumbar plexus within its substance and is the compartment involved in psoas abscess formation.
| Origin | Transverse processes and lateral surfaces of T12-L5 vertebral bodies and intervertebral discs |
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| Insertion | Lesser trochanter of the femur (via psoas-iliacus conjoined tendon) |
| Nerve Supply | Lumbar plexus direct branches (L1, L2, L3) — not the femoral nerve proper |
| Blood Supply | Lumbar segmental arteries |
| Actions | Primary hip flexor — flexes the thigh on the trunk and the trunk on the thigh; Lateral flexion of the lumbar spine when acting unilaterally |
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The abdominal psoas major is the compartment that harbours psoas abscesses (primary from haematogenous spread or secondary from vertebral osteomyelitis or Crohn's disease). The lumbar plexus runs within this abdominal portion — making retroperitoneal and LLIF approaches technically hazardous. CT-guided psoas drainage accesses this retroperitoneal compartment. Psoas syndrome from iliopsoas bursitis or haematoma presents at this abdominal level.
Not directly palpable. Deep palpation lateral to the lumbar spine may elicit psoas tenderness. Psoas sign (pain with passive hip extension) indicates psoas compartment pathology.
Pyogenic or tuberculous infection in the retroperitoneal psoas abdominal compartment producing flank pain, fever, and hip flexion posture, tracked by CT from its vertebral or colonic origin to its psoas iliac fossa presentation, managed by CT-guided drainage.
