Articular surfaces and ligaments
The ankle is a mortise-and-tenon joint: the talus is the tenon, and the tibia and fibula make the socket around it. The roof of the socket is the plafond, the concave lower surface of the tibia. Its medial wall is the medial malleolus of the tibia and its lateral wall is the lateral malleolus of the fibula, which reaches about a centimetre lower and sits further back.
The trochlea (dome) of the talus is wider in front than behind. In dorsiflexion the broad front part wedges between the malleoli and the joint is at its most stable; in plantarflexion the narrow back part lies in the mortise, which allows a little side-to-side play. That is why most sprains happen with the foot pointed and turned in.
Ligaments
- Deltoid (medial collateral) ligament: a strong triangular sheet from the medial malleolus. Its superficial fibres fan out to the navicular tuberosity, the spring ligament and the sustentaculum tali of the calcaneus; its deep fibres run to the medial side of the talus. It is so strong that the medial malleolus usually avulses before it tears.
- Lateral ligaments: three separate bands from the lateral malleolus. The anterior talofibular ligament (ATFL) runs forward to the neck of the talus, the calcaneofibular ligament (CFL) runs down and back to the lateral calcaneus, and the posterior talofibular ligament (PTFL) runs almost horizontally back to the posterior process of the talus.
- Syndesmosis: the anterior and posterior inferior tibiofibular ligaments, the interosseous ligament and the inferior transverse ligament bind the fibula to the tibia and keep the mortise from widening.
Inversion and eversion do not happen at the ankle. They take place at the subtalar and transverse tarsal joints below it.
Medial column: navicular, medial cuneiform and first metatarsal
The medial column is the chain of bones in front of the talus that carries load to the great toe and forms the front of the medial longitudinal arch.
- Navicular: boat-shaped, with a deep concave back surface for the head of the talus and three facets in front for the cuneiforms. Its medial tuberosity is the main insertion of tibialis posterior.
- Medial cuneiform: the largest cuneiform, with its wedge set base-down, unlike the other two. Tibialis anterior inserts on its medial and plantar surface; fibularis (peroneal) longus reaches its lateral plantar corner. The Lisfranc ligament runs from its lateral side to the base of the second metatarsal.
- First metatarsal: the shortest and thickest metatarsal. Its base shares the insertions of tibialis anterior and fibularis longus; its head rests on two sesamoid bones in the tendons of flexor hallucis brevis, with flexor hallucis longus running between them.
Tibialis anterior pulls this column up from the medial side and fibularis longus pulls it from beneath after crossing the sole, so the two tendons act as a sling under the arch.
Relations
No muscle crosses the ankle by attaching to the talus, so every tendon, vessel and nerve passes the joint in one of four groups.
| Side | Structures, in order |
|---|---|
| Front, under the extensor retinacula (medial to lateral) | Tibialis anterior, extensor hallucis longus, anterior tibial artery, deep fibular nerve, extensor digitorum longus, fibularis tertius |
| Behind the medial malleolus, under the flexor retinaculum (front to back) | Tibialis posterior, flexor digitorum longus, posterior tibial artery and veins, tibial nerve, flexor hallucis longus |
| Behind the lateral malleolus | Fibularis brevis (against the bone) and fibularis longus under the superior fibular retinaculum; sural nerve and small saphenous vein superficial to them |
| Behind the joint | Calcaneal tendon, with plantaris medial to it and a fat pad in front of it |
In front of the medial malleolus, in the subcutaneous tissue, lie the great saphenous vein and the saphenous nerve.
Blood supply and innervation
The ankle takes its blood from malleolar branches of the three leg arteries and its nerves from those that cross it. The anterior tibial artery gives anterior medial and lateral malleolar branches; the posterior tibial and fibular arteries supply the back and sides. The capsule is innervated by the deep fibular nerve in front, the tibial nerve behind and medially, and the saphenous and sural nerves at the sides.
Clinical relevance
Ankle injuries are graded by which side of the mortise fails and at what level.
- Inversion sprain: the ATFL tears first, then the CFL. The PTFL rarely tears except in a dislocation.
- Weber classification of lateral malleolar fractures: type A lies below the syndesmosis, type B at its level, and type C above it, with the syndesmosis torn and the mortise unstable.
- Maisonneuve injury: a medial malleolar fracture or deltoid tear with a fracture high in the fibula, the syndesmosis and interosseous membrane torn between them. Always examine the whole fibula.
- Bimalleolar and trimalleolar fractures: the third 'malleolus' is the posterior lip of the tibial plafond.
- Ottawa ankle rules: an ankle radiograph is needed if there is pain in the malleolar zone with tenderness at the posterior edge or tip of either malleolus, or the patient cannot bear weight for four steps. Tenderness at the navicular or the base of the fifth metatarsal calls for foot views.
A talus that has shifted laterally in the mortise, even slightly, signals an unstable injury that needs reduction and fixation.
On the specimen
The station shows the bones of the ankle and medial foot, so the task is to name each bone and side it. The medial malleolus is short, broad and belongs to the tibia; the lateral malleolus is longer, thinner and lower, with a pit (the malleolar fossa) on its inner back surface for the PTFL. The calcaneus is identified by the sustentaculum tali, the shelf on its medial side.
Tell the navicular from the cuboid by position and shape: the navicular sits medially and has a cup for the talar head, while the cuboid sits laterally with a groove underneath for fibularis longus. The medial cuneiform is the biggest of the three wedges. The first metatarsal is the stubbiest metatarsal, with two grooves under its head for the sesamoids.