This is a joint between:
the lower end (1) and medial malleolus (2) of the tibia and the lateral malleolus (3) of the fibulaand
the trochlear surface of the talus
(In this illustration the talus is displaced.)
Stability of the mortice is produced by strong ligamentous ties between the tibia and fibula.These include:
Only a small amount of movement of the lateral malleolus can be felt even when the foot is raised to its maximum.
The facets on the side are triangular (for articulation with the the lateral malleolus) and 'comma-shaped' (for articulation with the medial malleolus).
In dorsiflexion the trochlear surface of the talus slides backwards into the
mortice formed by the malleoli and the joint becomes close-packed, particularily
as the part of the talus going into the mortice is increasingly wider.
By contrast, in plantar flexion the talus slides progressively forwards out of the mortice and movement of the foot as a whole becomes freer.
Plantarflex your foot fully and note how much lateral (side to side) movement the ankle allows in this position.
Now dorsiflex your foot, again noting the degree of lateral movement allowed.
You should find that when dorsiflexed your ankle allows much less movement laterally than when plantarflexed.
The decrease in stability of the ankle when the foot is plantarflexed is
one reason why high heels are unstable, and also why you are less stable when
walking down hill than walking up hill.
When standing erect, the centre of gravity passes in front of the ankle joint (through the midpoint of a line joining the navicular bones.
The tendency to fall forward is normally resisted by activity of the calf muscles (plantarflexors).
Hence Guardsmen (standing to attention on parade) fall forward when they
Stability of the ankle joint is provided by:
Dorsiflexion is limited by tension in the achilles tendon and the posterior part of the collateral ligaments but especially by the bony fit of the joint. At full dorsiflexion, the talus is wedged into the mortice and the joint is close-packed.
Plantar flexion is limited by tension in the tendons lying anterior to the ankle joint and by tension in the anterior parts of the collateral ligaments.