The talus or ankle bone is an irregularly shaped bone which forms the link between the foot and the leg through the ankle joint. It is the second largest and most proximal tarsal bone consisting of a cuboid body, a distally directed neck capped by a convex, oval head, a proximolateral facet for the fibular malleolus, and a proximal trochlea for the tibia. The talus plays an important role in the stability of the ankle as, although it has no muscular attachments, it has many ligamentous attachments.
The talus is part of a group of bones in the foot which are collectively referred to as the tarsus. The talus articulates with four bones - the tibia, fibula, calcaneus and navicular. Within the tarsus, it articulates with the calcaneus below and the navicular in front within the talocalcaneonavicular joint. Through these articulations, it transmits the entire weight of the body to the foot.
There is a dorsal pulley-like trochlear surface which articulates with the distal end of the tibia. A lateral extension of this surface forms an articular facet with the fibular malleolus. There is also a convexly oval head which articulates distally with the proximal end of the navicular bone.
The inferior surface presents two articular areas, the posterior and middle calcaneal surfaces, separated from one another by a deep groove, the sulcus tali. The middle calcaneal articular surface is small, oval in form and slightly convex; it articulates with the upper surface of the sustentaculum tali of the calcaneus. This creates the subtalar joint which provides shock absorption and facilitates the movements of inversion and eversion.
The talus also articulates with the tibia and fibula to create the talocrural joint. The talocrural joint is a hinge-type synovial joint and is the main joint of the ankle. Its functions are to aid stability and allow dorsiflexion and plantarflexion of the foot for locomotion. The fibrous capsule is thin anteriorly and posteriorly but it is supported on each side by strong collateral ligaments. It is attached superiorly to the borders of the articular surfaces of the tibia and malleoli. It is attached inferiorly to the talus close to the superior articular surface, except anteroinferiorly, where it is attached to the dorsum of the neck of the talus.
The head of the talus has a convex surface and carries the articular surface of the navicular bone. The inferior surface of the head contains two articular areas, each separated by smooth ridges. The medial, situated in front of the middle calcaneal facet, is convex, triangular, or semi-oval in shape, and rests on the plantar calcaneonavicular ligament; the lateral, named the anterior calcaneal articular surface, is somewhat flattened, and articulates with the facet on the upper surface of the anterior part of the calcaneus.
The plantar calcaneonavicular ligament which is covered superiorly by fibrocartilage is in contact with a part of the talar head, which sits between the two calcaneal facets, covered with articular cartilage. When the foot is inverted passively, part of the head is visible and palpable approximately 3 cm distal to the tibia; however it is hidden by the extensor tendons when the toes are dorsiflexed.
The neck of the talus is the narrow region between head and body. It presents many rough surfaces for the attachment of ligaments. It contains a deep sulcus tali, which after articulation of talus and calcanues, forms a roof to the sinus tarsi. The sinus tarsi is a small cavity located on the outside of the ankle between the talus and calcaneus bones. This cavity contains numerous anatomical structures including ligaments and a joint capsule. This sinus is occupied by interosseous talocalcaneal and cervical ligaments.
The body of the talus is cuboidal in shape and articulates with the distal end of the tibia. It is covered dorsally by a trochlear surface, which also articulates with the distal end of the tibia. The lateral surface of the talus is triangular, smooth and concave for articulation with the lateral malleolus. The medial surface is covered by comma-shaped facets for articulation with medial malleolus and distally the surface contains numerous vascular foramina. The posterior surface of the body of the talus is rough and is marked by a groove for the tendon of flexor hallucis longus.
This groove lies between the lateral and medial tubercles. The lateral tubercle is larger and articulates with the sustentaculum tali. The lateral tubercle of the posterior process forms an attachment for the posterior talofibular ligament, and the plantar border of the posterior process forms an attachment for the posterior talocalcaneal ligament.
The medial talocalcaneal ligament is attached below to the medial tubercle whereas the most posterior superficial fibres of the deltoid ligament attach above the tubercle. The deep fibres of the deltoid ligament attach slightly higher. There are a large number of ligaments attached to the talus since it is the centrepiece between three joints - the ankle, the subtalar and the talocalcaneonavicular joints. This makes the talus an important bone in the stability of the ankle.
Muscle & Ligamentous attachments
No muscles are attached to the talus but many ligaments are attached to the bone, creating stability in the ankle, subtalar and calcaneonavicular joints. On the lateral side, the joint is held together by the posterior talofibular and anterior talofibular ligaments.
On the medial side it is held together by a massive ligament, the deltoid ligament, which attaches not only to a broad area on the talus but also to the adjoining bones below and in front. The ligaments of the ankle joint ensure that the talus cannot rock from side to side, or move backwards or forwards, relative to the tibia and fibula.
70% of the surface of the talus is covered by articular cartilage. As a result of this, displaced talar neck fractures, where the blood supply to the talar body is interrupted, can result in avascular necrosis and non-union. The blood supply to the talus is can be seriously compromised by a fracture-dislocation scenario. The posterior tibial artery, which has branches medially, the dorsalis pedis, which has branches anteriorly, and the peroneal artery, which has branches laterally, are all connected through a sling of vessels that lie within the sinus tarsi which makes them vunerable to rupture. These complications however do not tend to occur in undisplaced fractures.