The medial arcuate ligament is a thickened arch of the psoas fascia connecting the body and transverse process of L1, arching over the psoas major muscle as the posteromedial diaphragmatic attachment. The sympathetic trunk and the least splanchnic nerve pass posterior to the medial arcuate ligament as they enter the abdomen from the thorax.
Forms the medial diaphragmatic attachment arch over the psoas; transmits diaphragmatic tension to the lumbar vertebra; provides the medial boundary of the hiatus through which the aorta, thoracic duct, and azygos vein pass (though this is specifically the aortic hiatus, not the medial arcuate ligament itself).
The medial arcuate ligament is relevant in retrocrural approaches to the celiac plexus and in thoracolumbar surgery where the diaphragmatic crural attachments must be divided for exposure. Entrapment of the sympathetic trunk between the medial arcuate ligament and the vertebral body produces autonomic dysfunction at the thoracolumbar junction. In median arcuate ligament syndrome (MALS), the median arcuate ligament (a separate structure connecting the two crura over the aorta) compresses the celiac axis — the medial and lateral arcuate ligaments are distinct from the median arcuate ligament.
Thoracolumbar approach to the T12-L1 vertebrae requires detachment of the diaphragm from its crural and arcuate ligament attachments; the medial arcuate ligament is divided close to the L1 transverse process to release the psoas fascia and create the diaphragmatic window for lateral spinal access without full diaphragm takedown.
