The talus bone, 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. 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 attach to the talus, unlike most bones, and its position therefore
depends on the position of 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 the stability in the
joint vary with the position of the foot: with the foot dorsiflexed the ligaments of
the joint are kept stretched, which guarantees the stability of the joint; but with the foot
plantarflexed 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. Evolution
The talus apparently derives from the fusion of three separate bones in the feet of primitive
amphibians; the tibiale, articulating with tibia, the intermedium, between the bases
of the tibia and fibula, and the fourth centrale, lying in the mid-part of the tarsus. These
bones are still partially separate in modern amphibians, which therefore do not have a
true talus. The talus forms a considerably more flexible joint in mammals than it does
in reptiles. This reaches its greatest extent in artiodactyls, where the distal surface
of the bone has a smooth keel to allow greater freedom of movement of the foot, and thus
increase running speed. Additional images
See also Squatting facets
Knucklebones, a dice game using astragali Shagai
Notes
References Platzer, Werner. Color Atlas of Human Anatomy,
Vol. 1: Locomotor System. Thieme. ISBN 3-13-533305-1. Thieme Atlas of Anatomy: General Anatomy and
Musculoskeletal System. Thieme. 2006. ISBN 1-58890-419-9. External links
Anatomy of the talus by Maurice Laude, Laboratory of Anatomy and Organogenesis, Amiens Medical
School Fractures of the Talus at mdmercy.com
lljoints at The Anatomy Lesson by Wesley Norman Illustration at orthoinfo.aaos.org