| Tarsus | |
|---|---|
 Right foot bones, seen from below (left) and above (right). 7 bones constituting the tarsus: | |
 Bones constituting the tarsus. (same color scheme as above) | |
| Details | |
| Part of | Foot |
| Identifiers | |
| Latin | ossa tarsi |
| MeSH | D013639 |
| TA98 | A02.5.09.001 |
| TA2 | 1447 |
| FMA | 24491 |
| Anatomical terms of bone | |
In thehuman body, thetarsus (pl.:tarsi) is a cluster of seven articulatingbones in eachfoot situated between the lower end of thetibia and thefibula of the lower leg and themetatarsus. It is made up of the midfoot (cuboid, medial, intermediate, and lateralcuneiform, andnavicular) and hindfoot (talus andcalcaneus).
The tarsus articulates with the bones of the metatarsus, which in turn articulate with theproximal phalanges of the toes. The joint between the tibia and fibula above and the tarsus below is referred to as theankle joint proper.
In humans the largest bone in the tarsus is the calcaneus, which is the weight-bearing bone within the heel of the foot.
Thetalus bone or ankle bone is connected superiorly to the two bones of the lower leg, the tibia and fibula, to form theankle joint or talocrural joint; inferiorly, at thesubtalar joint, to the calcaneus or heel bone. Together, the talus and calcaneus form the hindfoot.[1]
The five irregular bones of the midfoot—thecuboid,navicular, and threecuneiform bones—form thearches of the foot which serves as a shock absorber. The midfoot is connected to the hind- and forefoot by muscles and theplantar fascia.[1]
The complex motion of the subtalar joint occurs in three planes and produces subtalarinversion andeversion. Along with the transverse tarsal joint (i.e. talonavicular and calcaneocuboid joint), the subtalar joint transforms tibial rotation into forefootsupination andpronation. The axis of rotation in the joint is directed upward 42 degrees from the horizontal plane and 16 degrees medially from the midline of the foot. However, together, the subtalar facets form a screw orArchimedean spiral (right-handed in the right foot) about which subtalar motion occurs. So, during subtalar inversion, the calcaneus also rotates clockwise and translates forward along the axis of the screw. Average subtalar motion is 20-30 degrees inversion and 5-10 degrees eversion. Functional motion during thegait cycle is 10-15 degrees (the heel strikes the ground in slight inversion followed by quick eversion).[2]
Thetalonavicular andcalcaneocuboid joints (i.e. between the talus and navicular bones, and the calcaneus and cuboid bones) form the so-calledtransverse tarsal joint orChopart's joint. It has two axes of motion. Inversion and eversion occur about a longitudinal axis oriented 15 degrees upward from the horizontal plane and 9 degrees medially from the longitudinal axis of the foot. Flexion and extension occur primarily about an oblique axis oriented 52 degrees upward from the horizontal plane and 57 degrees anteromedially (forward-inward).In vitro talonavicular motion is 7 degrees flexion-extension and 17 degrees pronation-supination; while calcaneocuboid motion is 2 degrees flexion-extension and 7 degrees pronation-supination.[2]
The motions of the subtalar and transverse talar joints interact to make the foot either flexible or rigid. With the subtalar joint in eversion, the two joints of the transverse joint are parallel, which make movements in this joint possible. With the subtalar joint in inversion, the axes of the transverse joint are convergent, movements in this joint are thus locked and the midfoot rigid.[2]
In primitivetetrapods, such asTrematops, the tarsus consists of three rows of bones. There are three proximal tarsals, thetibiale,intermedium, andfibulare, named for their points of articulation with the bones of the lower limb. These are followed by a second row of four bones, referred to as the centralia (singular:centrale), and then a row of five distal tarsals, each articulating with a single metatarsal. In the great majority of tetrapods, including all of those alive today, this simple pattern is modified by the loss and fusion of some of the bones.[3]
In reptiles and mammals, there are normally just two proximal tarsals, the calcaneus (equivalent to the amphibian fibulare) and the talus (probably derived from a fusion of multiple bones). In mammals, including humans, the talus forms a hinge joint with the tibia, a feature especially well developed in theartiodactyls. The calcaneus is also modified, forming a heel for the attachment of theAchilles tendon. Neither of these adaptations is found in reptiles, which have a relatively simple structure to both bones.[3]
The fifth distal tarsal disappears relatively early in evolution, with the remainder becoming the cuneiform and cuboid bones. Reptiles usually retain two centralia, while mammals typically have only one (the navicular).[3]
In birds, the tarsus has disappeared, with the proximal tarsals having fused with the tibia, the centralia having disappeared, and the distal bones having fused with the metatarsals to form a singletarsometatarsus bone, effectively giving the leg a third segment.[3]
