Bones of the Foot: Tarsals, Metatarsals and Phalanges
The human foot is a very complex and highly developed structure. The bones of the foot provide mechanical support for the soft tissues, helping the foot withstand the weight of the body.
The bones of the foot can be divided into three categories:
- Tarsals – A set of seven irregularly shaped bones. They are situated proximally in the foot, in the ankle area.
- Metatarsals – These bones connect the phalanges to the tarsals. There are five in number – one for each digit.
- Phalanges – The bones of the toes. Each toe has three phalanges – a proximal, intermediate and distal (except the big toe, which only has two phalanges).
The tarsal bones of the foot are organised into three rows; proximal, intermediate and distal.
The proximal tarsal bones are the talus and the calcaneus. They form the bony framework around the proximal ankle and heel area.
The talus is the most superior of the tarsal bones. It has three articulations:
- Superiorly: Ankle joint – between the talus and the bones of the leg (the tibia and fibula).
- Inferiorly: Subtalar joint – between the talus and calcaneus.
- Anteriorly: Talonavicular joint – between the talus and the navicular.
The main function of the talus is to transmit forces from the tibia to the heel bone (known as the calcaneus). Whilst numerous ligaments attach to the talus, it is not a site of muscle attachment or origin.
The calcaneus lies underneath the talus, and has two articulations:
- Superiorly: Subtalar joint – between the calcaneus and the talus.
- Anteriorly: Calcaneocuboid joint – between the calcaneus and the cuboid.
It is thick and sturdy, acting to transmit forces from the talus to the ground. The posterior aspect of the calcaneus is marked by calcaneal tuberosity, to which the Achilles tendon attaches.
The intermediate row of tarsal bones contains one bone, the navicular (given its name because it is shaped like a boat). It articulates with the talus posteriorly, the cuneiform bones anteriorly, and the cuboid bone laterally.
On the plantar surface of the navicular, there is a tuberosity for the attachment of the tibialis posterior tendon.
In the distal row, there are four tarsal bones – the cuboid and the three cuneiforms. These bones articulate with the metatarsals of the foot.
The cuboid is the most lateral bone in the distal row, articulating with the calcaneus posteriorly, and two metatarsals anteriorly. As the name suggests, it is shaped like a cube. The inferior surface of the cuboid is marked by a groove for the fibularis longus muscle.
The three cuneiforms (lateral, intermediate and medial) are wedge shaped bones. They articulate with the navicular posteriorly, and the metatarsals anteriorly. The shape of the bones helps form a transverse arch across the foot.
Clinical Relevance: Fractures of the Talus and Calcaneus
The talus and the calcaneus sit in the proximal part of the foot and ankle, and are involved in transmitting forces from the body to the ground. They are the most frequently fractured of all the tarsal bones.
Talar fractures occur in two places – the neck of the talus, or the body:
- Neck fractures are caused by excessive dorsiflexion of the foot. The neck of the talus is pushed against the tibia. In this type of fracture, the blood supply to the talus may be disturbed, leading to avascular necrosis of the bone.
- Body fractures usually occur from jumping from a height.
In any fracture of the talus, the malleoli of the leg bones act to hold the fragments together, so there is little displacement of the fracture pieces.
The calcaneus is often fractured in a ‘crush‘ type injury. The most common mechanism of damage is falling onto the heel from a height – the talus is driven into the calcaneus. The bone can break into several pieces – known as a comminuted fracture. Upon x-ray, the calcaneus will appear shorter and wider.
Even after treatment, a calcaneal fracture can cause further problems. The sub-talar joint is usually disrupted, causing the joint to become arthritic. The patient will experience pain upon inversion and eversion – which can make walking on uneven ground particularly painful.
The metatarsals are located in the midfoot, between the tarsals and phalanges. They are numbered I-V (medial to lateral).
Each metatarsal has a similar structure. They consist of a distal head and proximal base, which are joined by a shaft of bone. They have three or four articulations:
- Proximally: Tarsometatarsal joint – between the metatarsal bases and the cuneiforms or cuboid bones.
- Laterally: Intermetatarsal joint(s) – between the metatarsal and the adjacent metatarsals.
- Distally: Metatarsophalangeal joint – between the metatarsal head and the proximal phalanx.
Clinical Relevance: Fractures of the Metatarsal Bones
Metatarsal fractures can occur by three mechanisms.
The most common method of fracture is a direct blow to the foot – usually from a heavy object dropping onto the foot.
Another type of metatarsal injury is a stress fracture, an incomplete fracture caused by repeated stress to the bone. It is common in athletes, and occurs most frequently to metatarsals II, III and IV.
The metatarsals can also be fractured by excessive inversion of the foot. If the foot is violently inverted, the fibularis brevis muscle can avulse (‘tear off’) the base of metatarsal V.
The phalanges are the bones of the toes. Most toes have three phalanges – proximal, intermediate and distal. The great toe only has proximal and distal phalanges.
Each phalanx consists of a body, a proximal extremity and a distal extremity.