The LIGAMENTS which hold the bones together are anterior and posterior inferior tibio-fibular ligaments ; an interosseous ligament, which is continuous above with the interosseous membrane, and below comes into association with the joint cavity ; and a transverse ligament, which stretches posteriorly from the internal malleolus to the inner aspect of the posterior border of the external malleolus. Sometimes there is an actual joint between the two bones, lined with articular cartilage, and the synovial membrane of the ankle-joint extends up between the bones. The ligaments of the ankle-joint form a continuous investment of very varying strength. The anterior ligament is a weak structure, attached above to the lower border of the tibia and to the malleoli, and below to the neck of the astragalus ; and the posterior ligament, also weak, extends from the posterior surface of the tibia to the astragalus, and is strengthened by the transverse ligament already described. Effusions of fluid into the ankle-joint generally show first in front, beneath the extensor tendons, owing to the weakness of the anterior ligament and looseness of the synovial membrane at that point, and if increased cause bulgings of the posterior ligament, which show as fulness, with fluctuation on either side in front of the tendo Achillis. The lateral ligaments are very powerful, particularly the internal, which is triangular in shape, the apex being attached above to the internal malleolus, and the base to the scaphoid in front, then the inferior calcaneo-scaphoid ligament and neck of the astragalus, and behind to the sustentaculum tali and inner surface of the astragalus. It is covered by the tendons of the tibialis posticus and flexor longus digitorum. The external lateral ligament consists of three fasciculi all arising from the external malleolus, of which the anterior extends to the astragalus, the middle to the os calcis (being covered by the tendons of the peronei), and the posterior to the posterior aspect of the astragalus, being attached to a small tubercle of bone, which is occasionally distinct from the astragalus (os trigonum).

The synovial membrane lines the capsule, is lax anteriorly and posteriorly, where it covers pads of fat, and is directly continuous with the inferior tibio-fibular joint, when it exists. The joint is supplied by branches of the anterior tibial and internal saphenous nerves, the former associating it with the sacral segments of the cord, and the latter with the lumbar. The ankle-joint only permits of flexion and extension, save in extreme extension, when very slight lateral movement is just possible, owing to the narrow posterior portion of the astragalus coming in contact with the wider anterior portion of the tibio-fibular cavity. Flexion is limited by the posterior ligament and posterior portions of the lateral ligaments, and by contact of the astragalus with the tibia. Extension is limited by the anterior ligament and anterior portions of the lateral ligaments, and by contact of the astragalus with the tibia. Lateral movements of the foot round an anteroposterior axis take place normally between the astragalus and the os calcis, while those round a vertical axis take place at the midtarsal joint. The position of greatest ease of the joint is that of slight extension, although the capacity is not affected by position.

Sprains of the ankle frequently occur, a forced movement of the foot producing partial tearing of one or other lateral ligament. Sometimes the ligaments withstand the strain, and a portion of one of the malleoli is torn off by it, the condition being called a sprain fracture.

Dislocations of the ankle are generally associated with fracture, and are due to indirect violence applied to the foot. Sometimes, as a result of injury, one of the malleoli may alone be broken, no dislocation resulting. The ankle may be dislocated laterally, antero-posteriorly, or upwards. The outward lateral displacement is by far the most common, and is associated with a fracture of the fýbula some 2 or 3 inches from its lower end, the condition being known as Pott's fracture. Pott's fracture is caused by a violent eversion of the foot, such as is produced by stepping sidewise from a machine in motion, the internal lateral ligament tearing and the astragalus being rotated, and so brought violently against the external malleolus. As the ligaments binding the fibula to the tibia, remain intact, this portion acts as a fulcrum, and, the malleolus being forcibly everted, the lower end of the shaft of the fibula is forcibly inverted, and finally snaps some 3 inches up. Frequently the tip of the internal malleolus is torn off instead of the internal lateral ligament giving way. After the injury the foot is displaced markedly outwards and everted, and there is also a tendency to backward drooping of the heel when the patient is recumbent. Dupuytren's fracture is a somewhat similar condition, which, however, is rare, and only produced by extreme violence. In it, while the fracture of the fibula remains the same, the tibio-fibular articulation gives way, and the everted astragalus is forced up between the two bones, or the lower fragment of the fibula accompanies the astragalus upwards. A simple dislocation of the astragalus upwards between the tibia and fibula without fracture, but with laceration of the tibio-fibular ligaments, rarely occurs, generally from a fall on the feet. Complete inward dislocation of the ankle is rare, is generally caused by a severe twist, and is said to be accompanied by fracture of the external malleolus, and often of the internal malleolus, and even of the astragalus. Complete inward dislocation has occurred, however, without fracture of any part. Antero-posterior dislocations of the astragalus are less common, and are caused by violence with the foot fixed as in jumping from a moving vehicle in the line of its motion. Of the two forms, backward dislocation of the astragalus is much the more common, the articular surface of the tibia resting on the scaphoid and cuneiform bones, and all the ligaments suffering, particularly the anterior and posterior. Here also the fibula may be fractured, and one or both malleoli may be detached.