Stem of inferior extensor retinaculum

The stem of the of inferior extensor retinaculum (stem of the Y of frondiform ligament) is attached laterally to the upper surface of the calcaneus, in front of the depression for the interosseous talocalcaneal ligament; it is directed medialward as a double layer, one lamina passing in front of, and the other behind, the tendons of the fibularis tertius and Extensor digitorum longus.

These two layers of the stem (superficial layer and a deep layer) together form a strong loop around the tendons of the peroneus tertius and the extensor digitorum longus, acting as a pulley system to stabilize these tendons. The fibers of the 2 layers blend and extend laterally, forming a tight cover above the tarsal sinus:

• Some fibers of the superficial layer remain superficial, descending to the external aspect of the calcaneus or blending into the sheaths of the peroneal tendons. These fibers are called the “lateral root of the IER”.

• The deep layer divides into the “intermediate root” and “medial root” of the inferior extensor retinaculum and attaches to the floor of the tarsal sinus. The intermediate root, which is the larger of the 2 roots, inserts on the posterolateral border of the tarsal sinus. The medial root attaches closely to the neck of the talus and then divides into a thin lateral component (LC, 5.7 ± 1.1 mm in width) and a broad medial component (MC, 16.5 ± 2.4 mm in width).