Inferior extensor retinaculum of ankle

The inferior extensor retinaculum (IER; cruciate crural ligament; lower part of anterior annular ligament) is a Y-shaped band placed in front of the ankle-joint

The stem of the Y (frondiform ligament) is attached laterally to the upper surface of the calcaneus, in front of the depression for the interosseous talocalcanean 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. Other originates from the lateral part of the sinus tarsi and are called the “Lateral root of inferior extensor retinaculum”.

• The deep layer divides into the “intermediate root of inferior extensor retinaculum” and “medial root of inferior extensor retinaculum” of the IER and attaches to the floor of the tarsal sinus and canal. 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 and a broad medial component.

From the medial extremity of this sheath the two limbs of the Y diverge:

• The oblique superomedial band is directed upward and medialward, to be attached to the medial malleolus, passing over the Extensor hallucis longus and the vessels and nerves. In the region of the anterior tibial tendon, the oblique superomedial band splits into superficial and deep layers, encasing the anterior tibial tendon and its tendon sheath in a fibrous tunnel. The deep layer is usually relatively thick compared to the superficial layer.

• The oblique inferomedial band extends downward and medialward, to be attached to the border of the plantar aponeurosis, and passes over the tendons of the Extensor hallucis longus and Tibialis anterior and also the vessels and nerves.

Some authors describe an oblique superolateral band that is a non-constant, thin band that extends superolaterally from the stem, connecting to the lateral aspect of the ankle and sometimes used in surgical procedures such as the Broström-Gould augmentation, though its mechanical contribution is limited.