Flexor retinaculum of ankle

The flexor retinaculum (laciniate ligament; internal annular ligament) is a strong fibrous band, extending from the medial malleolus above to the margin of the calcaneus below (and plantar aponeurosis), converting a series of bony grooves in this situation into canals (termed together as the tarsal tunnel) for the passage of the tendons of the Flexor muscles and the posterior tibial vessels and tibial nerve into the sole of the foot. This retinaculum forms the roof of the tarsal tunnel, beneath which pass the tendons of the tibialis posterior, flexor digitorum longus, and flexor hallucis longus, as well as the posterior tibial artery, vein, and tibial nerve.

The flexor retinaculum is a localized thickening of the deep fascia of the leg, composed of parallel collagen fiber bundles arranged in 2–3 layers, with minimal loose connective tissue and a notable absence of elastic fibers. It is densely packed and contains numerous nerve fibers and corpuscles, reflecting its proprioceptive role.

It is continuous by its upper border with the deep fascia of the leg, and by its lower border with the plantar aponeurosis and the fibers of origin of the Abductor hallucis muscle.

On imaging, it appears as a low-signal-intensity band on MRI, with a mean thickness of approximately 0.9–1 mm.