The ankle joint, also known as the talocrural joint, is a synovial joint that connects the bones of the leg, the fibula and tibia, with the talus of the foot. It is a complex hinge joint composed of two articulations.
It is often described as a tenon and mortise joint, as the tibia and fibula act as a mortise and form a notch in which the body of the talus fits, acting as the tenon.
The main action of the ankle joint is to allow dorsiflexion and plantar flexion of the foot, as well as some degree of pronation and supination with subtalar and midtarsal joints. The joint also acts as a shock absorber as the heel strikes the ground during the first phases of gait.
|Type||Synovial hinge joint; uniaxial|
|Articular surfaces||Tibiotarsal joint: distal end of tibia, medial malleolus of the tibia, lateral malleolus of fibula, body of talus|
|Ligaments||Anterior talofibular, posterior talofibular, calcaneofibular, deltoid (tibiocalcaneal, tibionavicular, tibiotalar parts), fibular collateral ligaments|
|Innervation||Deep fibular (peroneal), tibial and sural nerves|
|Blood supply||Anterior tibial, posterior tibial and fibular arteries|
|Movements||Dorsiflexion, plantar flexion|
This article will discuss the anatomy and function of the ankle joint.
- Articular surfaces
- Joint capsule
- Blood supply
- Muscles acting on the ankle joint
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.
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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.
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.
Lateral collateral ligament
The lateral collateral ligament is a strong compound ligament that reinforces the lateral aspect of the ankle joint. It is comprised of three distinct bands:
- Anterior talofibular ligament: a weak, flat band that originates on the lateral malleolus of the fibula and extends anteromedially to the lateral side of the neck of the talus.
- Posterior talofibular ligament: a strong band that extends medially and posteriorly from the distal part of the lateral malleolar fossa of the fibula to the lateral tubercle of the talus. It is also connected to the medial malleolus by a tibial slip of fibres.
- Calcaneofibular ligament: a long band that originates from the apex of the lateral malleolus of the fibula, and extends posteroinferiorly to attach on a tubercle on the lateral aspect of the calcaneus.
Medial collateral ligament (deltoid ligament)
The medial collateral ligament is a strong, triangular band that reinforces the medial aspect of the ankle joint. This ligament is important to stabilize the ankle joint in eversion and prevent dislocations of the joint (over-eversion). The medial collateral ligament has a proximal attachment on the apex and borders of the medial malleolus. From here, the ligament fans out and inserts onto the talus, calcaneus, and navicular bones. The medial collateral ligament can be quite variable containing 3-6 parts ranging from superficial to deep. However, the ligament is most commonly organized into three continuous parts based on the distal attachment points:
- Tibionavicular ligament: composes the superficial fibers of the medial collateral ligament that descend from the medial malleolus to attach distally on the navicular tuberosity. Upon the attachment point, the ligament blends with the medial margin of the plantar calcaneonavicular ligament.
- Tibiocalcaneal ligament: composes the intermediate part of the medial collateral ligament that passes almost vertically downwards from the medial malleolus to attach onto the sustentaculum tali.
Tibiotalar ligament: comprises the deep part of the medial collateral ligament. It consists of an anterior and posterior portion. They both originate on the apex of the medial malleolus, after which the anterior tibiotalar ligament descends to attach onto the the medial tubercle of the talus, while the posterior tibiotalar ligament attaches onto the non-articular posterior part of the medial talar surface.
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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.
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
|Plantar flexion||Gastrocnemius, soleus, flexor digitorum longus, flexor hallucis longus, fibularis longus, tibialis posterior|
|Dorsiflexion||Tibialis anterior, extensor digitorum longus, extensor hallucis longus, fibularis tertius|
|Inversion||Tibialis anterior, tibialis posterior|
|Eversion||Fibularis longus, fibularis tertius, fibularis brevis|
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.
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.
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