Ankle joint

Articular surfaces

The ankle joint is a complex of articulations between the distal ends of the tibia and its medial malleolus, lateral malleolus of the fibula, and the trochlear surface of the talus. All of the articular surfaces of the ankle joint are covered with hyaline cartilage.

There are three articulations in the ankle joint:

• The distal end of the tibia articulates with the trochlea of the talus, a pulley-shaped rounded superior articular surface. The trochlea of the talus is convex in the parasagittal plane and slightly concave in the transverse plane. The distal end of the tibia is reciprocally shaped so that its congruent with the talar surface.
• The medial malleolus, a bony projection of the distal end of the tibia, articulates with the medial surface of the talus.
• The lateral malleolus, an enlargement of the distal end of the fibula, articulates with the lateral aspect of the talus. The lateral malleolus of the fibula is positioned more distally and posteriorly than the medial malleolus of the tibia.

The malleoli of the tibia and fibula, together with the inferior transverse tibiofibular ligament, form a rectangular socket (mortise) into which the trochlea of the talus fits into. This is why the ankle joint is sometimes referred to as a mortise joint.

Master the ankle joint anatomy with our video tutorials, quizzes, labelled diagrams, and articles:

Joint capsule

Proximally, the capsule of the ankle joint is attached to the margins of the articular surfaces of the medial malleolus of the tibia and lateral malleolus of the fibula. Distally, the capsule is attached to the margins of the talar trochlear surface. The joint capsule is relatively weak and thin, but strengthened medially and laterally by strong collateral ligaments which are discussed below. The internal surface of the capsule is lined by synovial membrane which projects into the distal tibiofibular joint up to the interosseous tibiofibular ligament.

Ligaments

Due to its heavy load-bearing nature, the ankle joint must be stabilized, but to the extent in which it is still permitted full mobility. The ligaments that stabilize the ankle joint are the medial and lateral collateral ligaments.

Innervation

The nervous supply of the ankle joint is derived from roots L4 to S2 by branches from the deep fibular (peroneal) nerve, as well as from the tibial and sural nerves.

Blood supply

The ankle joint receives arterial blood supply from the anterior and posterior tibial and fibular arteries. These arteries form an anastomosis around the malleoli that gives off anterior medial and lateral malleolar branches to supply the ankle joint. Venous blood is drained through the corresponding veins.

Movements

Being a hinge joint, the ankle joint only allows plantar flexion (flexion) and dorsiflexion (extension), which occur on the transverse (medial-lateral) axis that passes through the talus in the sagittal plane. The degree of movement in the ankle joint amounts to approximately 30-50° of plantar flexion, and about 20° of dorsiflexion.

Plantarflexion is restricted by the muscles of the anterior compartment of the leg, mainly by tibialis anterior; the anterior part of the medial collateral ligament, the anterior talofibular ligament and the anterior part of the joint capsule. Dorsiflexion is restricted by the muscles of the posterior compartment of the leg, mainly by gastrocnemius and soleus; the posterior part of the medial collateral ligament, the calcaneofibular ligament and the posterior portion of the joint capsule.

Plantar flexion and dorsiflexion at the ankle joint are rarely performed alone, but rather in conjunction with movements at the subtalar and midtarsal joints. This means that plantar flexion is most commonly followed by adduction and inversion allowing supination of the foot, while dorsiflexion is followed by abduction and eversion allowing pronation of the foot.

In the erect position, the ankle joint represents the 90° angle made between the foot and the leg. This is the neutral, or the loose packed position for the ankle joint, at about 0-10° plantar flexion. Since the weight of the body acts through a vertical line passing anteriorly to the joint, a strong muscular support is required posterior to the joint to maintain stability and forward inclination or collapse during standing. This support is mainly brought by the soleus and gastrocnemius muscles. The close packed position of the ankle joint is at maximum dorsiflexion, the position in which the stability of the ankle joint is the greatest.

Muscles acting on the ankle joint

The primary muscles that perform plantar flexion are gastrocnemius and soleus. Other significant plantar flexors are the flexor digitorum longus, flexor hallucis longus, fibularis longus and tibialis posterior. A common denominator of these muscles is the fact that they all enter the foot posterior to the medial and lateral malleoli.

Dorsiflexion of the foot in the ankle joint is produced by tibialis anterior, extensor digitorum longus, extensor hallucis longus and fibularis tertius, that cross the ankle joint anteriorly.

Inversion of the ankle is produced by the main dorsiflexor of the foot, the tibialis anterior, as well as tibialis posterior, that acts as a plantar flexor. Eversion of the foot is produced by the dorsiflexor fibularis tertius, and plantar flexors fibularis longus and fibularis brevis.