Corpus: Talus

The talus is a bone of the skeleton of the foot that belongs to the proximal row of tarsal bones.

2.1. Overview

The talus is a biconvex bone with a compact shape. A high percentage of its surface is covered by cartilage. The bone can be divided into different sections according to its aspect:

• corpus (body),
• collum (neck) and
• caput (head).

2.2. Body of talus

The body of talus has a prominent articular roller of hyaline cartilage on its superior surface, the trochlea talus. It corresponds with the inferior articular facet of the tibia and is slightly indented towards the center in the frontal plane, which corresponds to the protrusion of the lower tibial articular surface and stabilizes the joint.

The lateral surface of the corpus tali is occupied by a triangular cartilage surface, the lateral malleolar facet. It is in contact with the lateral malleolus of the fibula. The talofibular anterior ligament inserts at its anterior edge.

The medial surface is characterised by the pear-shaped medial malleolar facet, which corresponds with the medial malleolus of the tibia. Inferior to the cartilage surface is a rough depression, in which the deep part of the deltoid ligament radiates into the talus.

Together with the articular fork of the tibia and the fibula, these three continuously merging articular surfaces form the talocrural joint. As the anterior trochlea is slightly wider than the malleolar fork, it is securely gripped by the body weight under tension from the tibiofibular syndesmosis when standing. This significantly increases joint stability. The trochlea becomes narrower in the posterior section. When the foot is flexed, the talocrural joint therefore has more play.

The inferior surface of the body of talus carries the deeply concave joint surface to the calcaneus, the posterior articular calcaneal surface. It forms the posterior section of the talotarsal joint. The cartilage surface has an oval shape and runs at an angle of around 45° to the longitudinal axis of the talus.

A depression for the tendon of the flexor hallucis longus muscle, the sulcus tendinis musculi flexoris hallucis longi, runs diagonally downwards and medially over the posterior surface (facies posterior) of the talus. Lateral to the sulcus is a bony projection, the posterior talar process, which is also known as the lateral tuberosity. Sometimes this projection is a separate bone (see below). Medial to the sulcus is a smaller medial tuberosity.

2.3. Neck of talus

The neck part of the talus (collum tali) is the retracted portion of bone that connects the body of the talus with the head of the talus. It is separated from the corpus by the deep sulcus tali and is directed anteriorly and medially. The talar sulcus separates the anterior articular surfaces from the posterior articular facies of the calcaneus. The groove is supplemented by a corresponding groove in the calcaneus (sulcus calcanei) in the anterior lateral direction to form a canal (sinus tarsi) that leads to the interosseous talocalcaneal ligament.

The upper and medial sides of the collum tali are rough and serve as an attachment for ligaments. The lateral surface is smooth and concave and forms the lateral continuation of the talar sulcus. The facies articularis calcanea media, which is involved in the anterior talotarsal joint, is located on the lower surface of the collum tali. In some textbooks, however, it is also listed as part of the corpus tali.

2.4. Head (for navicular)

The head of the talus is also orientated forwards and medially. It bears the anterior facet which is also in articular connection with the calcaneus, and the facies articularis navicularis directed forward to the navicular bone. Between the two articular surfaces is a triangular bone facet that borders on the plantar calcaneonavicular ligament. All the cartilage surfaces mentioned are part of the anterior talotarsal joint.

There are no areas of origin or attachment for muscles on the talus, but there are extensive areas of cartilage. The talus can therefore be compared to a bony meniscus in the ligament of the talocrural and talotarsal joints.

As the highest bone in the foot skeleton, the talus is responsible for transferring the forces exerted by the body weight to the arch of the foot when standing and walking. The talus thus mediates between the statics of the lower leg and the dynamics of the foot. Therefore, special functional requirements must be placed on the form and function of the talus.

The formation of a bone nucleus in the talus takes place between the 7th and 8th fetal month.

In exceptional cases, an independent lateral tubercle (of the posterior talar process) occurs. This piece of bone is then referred to as the os trigonum.

6.1. Malformations

Possible malformations include fusions (synostoses) of varying extent with the neighbouring bones, i.e. the calcaneus or the navicular bone. They are referred to as talocalcaneal or talonavicular coalescence.

Incomplete fusion of the embryonic bone nuclei can result in a talus bipartitus (talar cleft). Another congenital malformation is the spherical talus.

Dislocation of the talonavicular joint and the subtalar joint results in a talus obliquus or talus verticalis, which occurs in congenital flat feet.

6.2. Luxation

Extreme plantar flexion of the foot as part of a rasp trauma can lead to a talus luxation. This causes the joint surfaces in the ankle joint to shift and the talus changes its position in relation to the neighbouring bones.

6.3. Fractures

Bone fractures of the talus (talus fractures) are rare as the bone is very compact. They account for less than 4 % of all foot fractures. The cause may be falls from a great height onto the feet, in which case there are usually concomitant fractures of other bones. The fracture line most frequently runs through the body of the talus. They are categorised according to Hawkins.