The calcaneus, also known as the heel bone, is found at the back of the foot near the ankle, just below talus, tibia, and fibula bones of the lower leg. The calcaneus is the largest bone in the foot, it projects posterior to the tibia and fibula and acts as a short lever for the calf muscles (gastrocnemius and soleus) which insert onto its posterior surface via the Achilles tendon. It plays an important role in weight bearing and stability.
The calcaneus is an irregular bone, cuboid in shape whose superior surface can be divided into three areas - the posterior, middle and anterior aspects. The posterior aspect is rough and concavo-convex in shape. This convexity supports the fibroadipose tissue (Kager's fat pad) between the calcaneal tendon and the ankle joint. The middle aspect carries the posterior talar facet which is oval and convex in shape while the anterior aspect is partly articular. A rough depression - the sulcus calcanei or calcaneal sulcus - narrows into a groove on the medial side connecting the sinus tarsi with the talus. The front of the calcaneus features many curves to accommodate the talus and the many different tarsal bones, which lead to the metatarsals and phalanges. The back of the calacaneus is not as complex, featuring a tuberosity and a medial process.
The calcaneus provides insertion points for the abductor hallucis and the abductor digit minimi muscles. At the back of the heel, the Achilles tendon inserts into the rough area located on the superior side. This tendon, as well as other ligaments and muscles, is necessary for standing and walking. Therefore a broken or fractured calcaneus will cause difficulty in standing and walking. There are two articulations (with three facets) with the talus and one saddle-shaped articulation with the cuboid. The sustentaculum tali is a horizontal shelf that arises from the anteromedial portion of the calcaneus. The superior surface is concave and articulates with the middle calcaneal surface of the talus. The inferior surface has a groove for the tendon of flexor hallucis longus. The sustentaclulum tali bears the greatest weight per area and also has a lot of soft tissue structures attaching around it. The lateral wall of the calcaneus is thin and has attachments for the calcaneofibular ligament and the osseus reflection of the peroneal tendons. On the medial aspect, the bone is thicker and there is close proximity to the toe flexors and neurovascular structures.
Muscle and ligament attachments
As the calcaneus is the largest of the bones in the foot it provides a large surface area for the complex attachment of many muscles and ligaments. The calcaneal sulcus provides attachments for the interosseous, talocalcaneal and cervical ligaments and also for the medial root of the inferior extensor retinaculum. There is also a non-articular area on the calcaneus, distal to the posterior talar facet which provides an attachment surface for extensor digitorum brevis, the main band of the inferior extensor retinaculum and the stem of the bifurcate ligament. There is a prominent medial margin on the medial process of the calcaneal tuberosity which provides attachments for the superficial part of the flexor retinaculum and distally the plantar aponeurosis. Some muscles also attach there including abductor hallucis and flexor digitorum brevis. The abductor digiti minimi attaches mainly to the lateral process however does extend medially onto the medial process. There is a rough region between these processes which extends to the anterior tubercle distally which provides an attachment surface for the long plantar ligament. Near this is the attachment of the head of the flexor accessories muscle. The short plantar ligament is attached to the tubercle and the area distal to it. Plantaris attaches to the posterior surface near the calcaneal tendon. The fibular tendons cross on the lateral surface and the calcaneofibular ligament attaches just proximal to the fibular trochlear. Many structures also attach around the sustentaculum tali. Its plantar surface is grooved by the tendon of flexor hallucis longus and this gives attachments to the deep part of the flexor retinaculum. The plantar calcaneonavicular ligament attaches to the medial margin of the sustentaculum tali. Proximal to this are the attachments of the tendon of tibialis posterior, superficial fibres of the deltoid ligament and medial talocalcaneal ligaments. On the margin of the sustentaculum tali are the attachments of the tendon of flexor digitorum longus and the large medial head of flexor accessorius is attached distal to the groove for flexor hallucis longus.
Role with the talus and cuboid
The calcaneus articulates with two bones: the talus and the cuboid. There are three articular surfaces for the talus. The largest is the slightly convex oval surface for the body of the talus. Anteromedial to this is the medial articular surface for the talus on the dorsal aspect of the sustentaculum tali. Anterolateral to this surface is the small anterior facet for the talus, which may sometimes blend with the medial facet. The latter two facets are shallowly concave. The distal end forms an obliquely directed, oval, articular surface that is convex in the transverse plane and concave in the vertical plane. This surface articulates with the cuboid bone.
The calcaneus is the most commonly fractured tarsal bone and accounts for about 2% of all fractures in the body, and 60% of all tarsal fractures. Calcaneal fractures can be divided broadly into two types: accounting for 25-30% of cases are extra-articular fractures, which can be either calcaneal tuberosity avulsion or extra-articular Lover's (‘Casanova’) fractures and intra-articular fractures which account for 70-75% of cases and are also known as intra-articular Lover's fractures. It is interesting to note that if bilateral calcaneal fractures are present, the spine should also be evaluated because injuries caused by events such as jumping from a second storey window may cause problems further up into the spine.
Plantar fasciitis, a condition which indirectly affects the calcaneus, is the pain caused by degenerative irritation at the insertion of the plantar fascia. The plantar fascia inserts on the medial process of the calcaneal tuberosity. It is the most common cause of heel pain, and the typical presentation is sharp pain localized at the anterior aspect of the calcaneus.