The Calcaneus (os calcis) is the largest of the tarsal bones. It is situated at the lower and back part of the foot, serving to transmit the weight of the body to the ground, and forming a strong lever for the muscles of the calf. It is irregularly cuboidal in form, having its long axis directed forward and lateralward; it presents for examination six surfaces.
The superior surface extends behind on to that part of the bone which projects backward to form the heel. This varies in length in different individuals, is convex from side to side, concave from before backward, and supports a mass of fat placed in front of the tendo calcaneus. In front of this area is a large usually somewhat oval-shaped facet, the posterior articular surface, which looks upward and forward; it is convex from behind forward, and articulates with the posterior calcaneal facet on the under surface of the talus. It is bounded anteriorly by a deep depression which is continued backward and medialward in the form of a groove, the calcaneal sulcus. In the articulated foot this sulcus lies below a similar one on the under surface of the talus, and the two form a canal (sinus tarsi) for the lodgement of the interosseous talocalcaneal ligament. In front and to the medial side of this groove is an elongated facet, concave from behind forward, and with its long axis directed forward and lateralward. This facet is frequently divided into two by a notch: of the two, the posterior, and larger is termed the middle articular surface; it is supported on a projecting process of bone, the sustentaculum tali, and articulates with the middle calcaneal facet on the under surface of the talus; the anterior articular surface is placed on the anterior part of the body, and articulates with the anterior calcaneal facet on the talus. The upper surface, anterior and lateral to the facets, is rough for the attachment of ligaments and for the origin of the Extensor digitorum brevis.
The inferior or plantar surface is uneven, wider behind than in front, and convex from side to side; it is bounded posteriorly by a transverse elevation, the calcaneal tuberosity, which is depressed in the middle and prolonged at either end into a process; the lateral process, small, prominent, and rounded, gives origin to part of the Abductor digiti quinti; the medial process, broader and larger, gives attachment, by its prominent medial margin, to the Abductor hallucis, and in front to the Flexor digitorum brevis and the plantar aponeurosis; the depression between the processes gives origin to the Abductor digiti quinti. The rough surface in front of the processes gives attachment to the long plantar ligament, and to the lateral head of the Quadratus plantae while to a prominent tubercle nearer the anterior part of this surface, as well as to a transverse groove in front of the tubercle, is attached the plantar calcaneocuboid ligament.
The lateral surface is broad behind and narrow in front, flat and almost subcutaneous; near its center is a tubercle, for the attachment of the calcaneofibular ligament. At its upper and anterior part, this surface gives attachment to the lateral talocalcaneal ligament; and in front of the tubercle it presents a narrow surface marked by two oblique grooves. The grooves are separated by an elevated ridge, or tubercle, the trochlear process (peroneal tubercle), which varies much in size in different bones. The superior groove transmits the tendon of the Peronaeus brevis; the inferior groove, that of the Peronaeus longus.
The medial surface is deeply concave; it is directed obliquely downward and forward, and serves for the transmission of the plantar vessels and nerves into the sole of the foot; it affords origin to part of the Quadratus plantae. At its upper and forepart is a horizontal eminence, the sustentaculum tali, which gives attachment to a slip of the tendon of the Tibialis posterior. This eminence is concave above, and articulates with the middle calcaneal articular surface of the talus; below, it is grooved for the tendon of the Flexor hallucis longus; its anterior margin gives attachment to the plantar calcaneo-navicular ligament, and its medial, to a part of the deltoid ligament of the ankle-joint.
The anterior or cuboid articular surface is of a somewhat triangular form. It is concave from above downward and lateralward, and convex in a direction at right angles to this. Its medial border gives attachment to the plantar calcaneo-navicular ligament.
The posterior surface is prominent, convex, wider below than above, and divisible into three areas. The lowest of these is rough, and covered by the fatty and fibrous tissue of the heel; the middle, also rough, gives insertion to the tendo calcaneus and Plantaris; while the highest is smooth, and is covered by a bursa which intervenes between it and the tendo calcaneus.
The calcaneus articulates with two bones: the talus and cuboid.