The ankle is the joint which connects the foot to the lower limb. It is a hinged synovial joint with a joint capsule; the capsule of which thickens medially to form the deltoid ligament and laterally to form the lateral ligament (which has three parts). The ankle also includes the distal tibiofibular joint which is a fibrous joint (syndesmosis). There are many ligaments which stabilise the ankle and subtalar joint.
The ankle joint is made up of three bones – the distal fibula, the distal tibia and the dome of the talus. The talus itself does not have any muscular attachments, these pass from the proximal tibia/fibula, across the talus to insert distally. The articular surface of the ankle joint (tibiotalar joint) is made up of hyaline cartilage, this is relatively thin on the talar dome leading to future issues with OA in traumatic ankle injuries.
The attachments for the capsule proximally are the distal tibia and fibula. Distally the capsule attaches to the talus, navicular and calcaneus. Thickenings in the capsule give rise to the lateral ligament (laterally) and the deltoid ligament (medially).
The dorsal or anterior aspect of the ankle joint is covered superficially to deep by: skin, subcutaneous fat, the long saphenous vein, the saphenous nerve, the extensor retinaculum (superior and inferior), the extensor tendon sheaths and the following extensor tendons from medial to lateral: Tibialis anterior, EHL, EDL, and peroneus tertius. In between EHL and EDL lies the dorsalis pedis artery (arising from the anterior tibial artery) and the deep peroneal nerve.
Posterior to the lateral malleolus are found the following structures: peroneus longus and, underneath it, peroneus brevis (distally p. brevis lies in front of p. longus). These structures are held in place by the superior and inferior peroneal retinaculum. The superior retinaculum runs from the lateral malleolus to the lateral calcaneus. The inferior retinaculum runs from inferior extensor retinaculum and attaches to the lateral calcaneus also. Distally the two tendons have separate sheaths, proximally they share one sheath.
General development: Limb buds develop from the end of the 4th week (upper limb 1-2 days before lower limb), broadly governed by HOX genes. They arise from the venterolateral wall mesenchymal cells. This is mesoderm covered by ectoderm. The end of the limb has an apical ectodermal ridge (AER) which causes mesenchyme to grow. The mesenchyme farther away from this ridge becomes muscle/cartilage.
At 6 weeks old the limb bud flattens distally and the AER undergoes apoptosis around week 8 to create the ‘gaps’ between the toes (same applies for fingers). Hyaline cartilage begins to form and condenses in areas where future joints will exist – this is the beginning of synovial joints.
In week 7 the limbs rotate. The lower limbs rotate ~90 degrees medially (upper limbs rotate 90 degrees laterally) which explains why the extensors are ‘anterior’ in the lower limb but ‘posterior’ in the upper limb.