Structure
The foot is divided into three parts: the hindfoot (talus and calcaneus), the midfoot (navicular, cuboid and three cuneiforms) and the forefoot (metatarsals and phalanges). The great toe has two phalanges; the other toes have three.
| Bone | Position | Features to know |
|---|---|---|
| Talus | Top of the hindfoot, under the tibia | Body with the trochlea (dome), a neck, and a rounded head; no muscle attachments |
| Calcaneus | Heel | Largest tarsal; posterior tuberosity for the calcaneal tendon; sustentaculum tali on the medial side; fibular trochlea laterally |
| Navicular | Medial midfoot, in front of the talar head | Concave back surface; medial tuberosity for tibialis posterior |
| Cuboid | Lateral midfoot, in front of the calcaneus | Plantar groove for fibularis (peroneal) longus; articulates with the fourth and fifth metatarsals |
| Cuneiforms (medial, intermediate, lateral) | Between the navicular and the first three metatarsals | Intermediate is shortest, so the second metatarsal base is recessed |
The talus
The dome is covered in cartilage and sits in the ankle mortise. The neck points forwards and medially and is the narrowest, weakest part. The head articulates with the navicular in front and rests below on the calcaneus and the spring (plantar calcaneonavicular) ligament.
The calcaneus
The upper surface carries three facets for the talus: posterior, middle (on the sustentaculum tali) and anterior. Between the posterior and middle facets runs a groove that, with a matching groove on the talus, forms the tarsal sinus and canal. Flexor hallucis longus runs in a groove under the sustentaculum.
Joints and arches
The tarsal bones meet at three clinically named joint lines, and together they form the arches of the foot.
- Subtalar joint: between the talus and calcaneus, with the interosseous talocalcaneal ligament in the tarsal canal. It is where inversion and eversion mostly take place.
- Transverse tarsal (Chopart) joint: the talonavicular and calcaneocuboid joints side by side. It adds to inversion and eversion and lets the forefoot adapt to uneven ground.
- Tarsometatarsal (Lisfranc) joint: the cuneiforms and cuboid meeting the metatarsal bases, keyed by the recessed base of the second metatarsal.
The medial longitudinal arch runs through the calcaneus, talus, navicular, cuneiforms and first three metatarsals, with the talar head at its summit. The lateral arch is lower, through the calcaneus, cuboid and fourth and fifth metatarsals. The transverse arch lies across the cuneiforms, cuboid and metatarsal bases. The arches are held by the spring ligament, the long and short plantar ligaments, the plantar aponeurosis, and the tendons of tibialis posterior and fibularis longus.
Blood supply
The talus has a precarious blood supply because most of its surface is cartilage. Vessels enter mainly through the neck and the tarsal canal: the artery of the tarsal canal from the posterior tibial artery, the artery of the tarsal sinus from the dorsalis pedis and fibular arteries, and deltoid branches on the medial side. The body is supplied from front to back, so a fracture through the neck can cut it off.
The navicular has a relatively poor supply in its central third, which is where stress fractures occur and heal slowly.
Clinical relevance
Tarsal injuries are uncommon but disabling, and several have named patterns.
- Calcaneal fracture: follows a fall from a height onto the heels. Böhler's angle, measured on a lateral radiograph, is reduced or reversed. Examine the other heel and the thoracolumbar spine.
- Talar neck fracture: from forced dorsiflexion, as on a car's footwell. The Hawkins classification grades displacement; the more displaced, the higher the risk of avascular necrosis of the body. A thin lucent line under the dome at around six to eight weeks (Hawkins sign) suggests the blood supply has returned.
- Navicular stress fracture: in running and jumping athletes, with pain on the dorsum of the midfoot.
- Köhler disease: avascular necrosis of the navicular in young children, which usually recovers.
- Lisfranc injury: disruption of the tarsometatarsal joint, easily missed; a small fleck of bone between the first and second metatarsal bases is a clue.
- Cuboid 'nutcracker' fracture: the cuboid is crushed between the calcaneus and the fourth and fifth metatarsals when the forefoot is forced outwards.
- Sever disease: traction apophysitis of the calcaneal tuberosity in active children.
On the specimen
The station shows the tarsal bones, with markers on the calcaneus, three parts of the talus, the navicular and the cuboid. Start with the talus: the smooth, pulley-shaped dome on top is the trochlea; the narrow waist in front of it is the neck; the rounded knob at the front is the head.
The calcaneus is the large bone below and behind, recognised by its tuberosity and the shelf of the sustentaculum tali on the medial side. The navicular lies medially in front of the talar head, and its tuberosity is felt on the inner border of the foot. The cuboid lies laterally in front of the calcaneus and has a deep groove underneath for fibularis longus. To side a loose talus, hold the head forwards: it points medially, and the larger, triangular lateral facet faces the fibula.