The Tarsus

The tarsus is composed of seven bones, referred to collectively as the tarsal bones. The tarsal bones may be divided into two-groups: the proximal region consisting of the calcaneus, the talus, and the navicular; and a distal row (named from medial to lateral) consisting of the first, second, and third cuneiform bones and the cuboid.

The calcaneus (os calcis) or heel bone is the largest of the tarsal bones. Its large posterior end forms the heel, and is marked by an expanded portion, the calcaneal tuberosity.

The talus (astragalus) is the second largest of the tarsal bones. It consists of a body, a head, and a neck. The superior aspect of the body presents an articular surface, called the trochlea. The head is the rounded anterior end which is received into the posterior concavity of the navicular bone.

The navicular bone is somewhat boat-shaped. It is situated on the medial side of the foot between the talus posteriorly and the cuneiform bones anteriorly. Posteriorly, it presents an oval, concave surface for articulation with the rounded head of the talus.

The cuboid bone is a cube-shaped bone. It is situated on the lateral side of the foot in front of the calcaneus and behind the fourth and fifth metatarsal bones.

The cuneiform bones are placed at the anterior portion of the tarsus lying side by side between the navicular bone and the bases of the first three metatarsals. The first cuneiform bone is the largest. The smaller second cuneiform bone is placed with its base (broad end) directed superiorly and its apex (thin end) directed downwards. The third cuneiform bone is the second largest. Like the second cuneiform, its base is directed superiorly and its apex inferiorly.

The Metatarsus

The foot proper is formed by the metatarsus. The metatarsal bones are numbered one through five, from the medial to the lateral side. Each metatarsal bone consists of a shaft, or body, and two extremities. The base or proximal extremity is wedge-shaped and the head, or distal extremity, is rounded. The first metatarsal, which provides attachment for the great toe, is the strongest and shortest of the metatarsal bones. It serves as the main support of the body when in the walking position. The large rounded head that forms the “ball of the foot” presents two grooves on its inferior, or plantar surface, on which glide two sesamoid bones in the tendon of the flexor hallucis brevis muscle. The heads of the second and third metatarsals generally extend beyond the first.

The Digits

The digits, or toes, are composed of 14 phalanges. There are two phalanges in the great toe and three in each of the other toes (the proximal, the middle, and the distal). Each phalanx consists of a shaft and two extremities. The proximal extremity of each proximal phalanx presents a concave facet for articulation with the head of the corresponding metatarsal. The distal end presents a trochlear articular surface for articulation with the middle phalanx. The distal end of each distal phalanx is flattened and presents the ungual tuberosity for the support of the toenail.

The Foot As a Whole

The bones of the foot are so arranged and adapted to each other that they form two distinct arches, the longitudinal arch and the transverse arch. These arches form a firm basis of support for the human body in the standing position, give elasticity to the step, and accommodate the plantar blood vessels, nerves, tendons, and muscles. Abnormally high arches are called pes cavus. The longitudinal arch is the principal one and can be seen when the foot is viewed from the medial, or inner side. It consists of an anterior pillar formed by the heads of the metatarsals, a posterior pillar formed by the calcaneus, and a keystone formed by the talus. The three cuneiform bones and the cuboid (with the proximal ends of the metatarsal bones) form the transverse arch. The “top” of the foot is referred to as the dorsal aspect, the “bottom” as the plantar aspect. The term “march fracture” is defined as a fracture of the distal portion of the second and third metatarsals without a history of trauma.

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