Thalus

The talus bone (Latin for ankle), astragalus or ankle bone is a bone in the collection of bones in the foot called the tarsus. The tarsus forms the lower part of the ankle joint through its articulations with the lateral and medial malleoli of the two bones of the lower leg, the tibia and fibula. Within the tarsus, it articulates with the calcaneus below and navicular in front within the talocalcaneonavicular joint. Through these articulations, it transmits the entire weight of the body to the foot, provides stabilization which allows people to walk. The second largest of the tarsal bones, it is also one of the bones in the human body with the highest percentage of its surface area covered by articular cartilage. Additionally, it is also unusual in that it has a retrograde blood supply, i.e. arterial blood enters the bone at the distal end. In humans, no muscles attaches to the talus, unlike most bones, and it position is therefore dependent on the position on the neighbouring bones. Structure Though irregular in shape, the talus can be subdivided into three parts. Facing anteriorly, the head carries the articulate surface of the navicular bone, and the neck, the roughened area between the body and the head, has small vascular channels. The body features several prominent articulate surfaces: On its superior side is the trochlea tali, which is semi-cylindrical, and it is flanked by the articulate facets for the two malleoli. The ankle mortise, the fork-like structure of the malleoli, holds these three articulate surfaces in a steady grip, which guarantees the stability of the ankle joint. However, because the trochlea is wider in front than at the back (approximately 5-6 mm) the stability in the joint vary with the position of the foot: with the foot dorsiflexed (toes pulled upward) the ligaments of the joint are kept stretched, which guarantees the stability of the joint; but with the foot plantarflexed (as when standing on the toes) the narrower width of the trochlea causes the stability to decrease. Behind the trochlea is a posterior process with a medial and a lateral tubercle separated by a groove for the tendon of the flexor hallucis longus. Exceptionally, the lateral of these tubercles forms an independent bone called os trigonum or "accessory talus". On the bone's inferior side, three articular surfaces serve for the articulation with the calcaneus, and several variously developed articular surfaces exist for the articulation with ligaments. During the 7-8th intrauterine month an ossification center is formed in the talus. The talus bone lacks a good blood supply. Because of this, healing a broken talus can take longer than most other bones. One with a broken talus may not be able to walk for many months without crutches and will further wear a walking cast or boot of some kind after that.

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