The medial arcuate ligament is a thickening of the fascia over the upper psoas major, arching from the side of the body of L1 or L2 to the front of the transverse process of L1. It gives origin to part of the posterior diaphragm, and psoas major passes behind it from the thorax into the abdomen. The sympathetic trunk also passes behind it. It is distinct from the median arcuate ligament, which links the two crura in front of the aorta.
| Origin | Side of the body of L1 or L2 |
|---|---|
| Insertion | Front of the transverse process of L1 |
Provides a fibrous arch over the psoas major through which the diaphragm attaches posteriorly, allowing the psoas to pass from the thorax to the lumbar spine without penetrating the diaphragm.
The medial arcuate ligament is a landmark in retroperitoneal surgery and lumbar sympathectomy, as the sympathetic trunk passes deep to it at the thoracolumbar junction, and it is detached during thoracolumbar approaches to the T12-L1 vertebrae. Because the psoas fascia passes beneath it, a psoas abscess from tuberculous thoracolumbar spondylitis can track down from the thorax along the muscle into the groin. Median arcuate ligament syndrome (coeliac artery compression) involves the separate median arcuate ligament, not this one.
Pus from thoracolumbar spinal infection, classically tuberculosis, tracks beneath the medial arcuate ligament within the psoas sheath and may point in the groin, treated with drainage and antimicrobials.
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