Calcaneus

The calcaneus articulates superiorly with the talus bone at three surfaces: the anterior, middle, and posterior talar articular facets. The joint formed by the calcaneus's posterior facet and the talus's inferior surface is called the subtalar or talocalcaneal joint. Meanwhile, the talus's inferior surfaces that articulate with the anterior and middle facets on the calcaneus form the talocalcaneonavicular joint. 

The middle talar articular facet sits on the superior surface of a shelf-like projection of bone called the sustentaculum tali. In contrast, the inferior surface of the sustentaculum tali has a groove for the flexor hallucis longus tendon to pass through. Located between the middle and posterior facets is the calcaneal sulcus. Together with the talar sulcus superior to it, these sulci form the tarsal canal that leads anterolaterally to the tarsal sinus.

The primary functions of the calcaneus are to help support the body's weight and aid in the movement of multiple joints. As the heel strikes the ground while walking, the axial load of the body is carried down through the hip, leg, and ankle before being transferred to the calcaneus. Ultimately, this load is supported by the lateral and medial processes of the calcaneal tuberosity. As the body continues to move through a step, the arches of the foot begin to support the body's weight. The calcaneus helps form the two longitudinal arches along the plantar surface of the foot (e.g. the medial and lateral arches), which help to absorb shock in a spring-like manner as the body’s weight is carried through the step motion.

The calcaneus also plays a significant role in plantar flexion. Plantar flexion is an ankle joint movement resulting in a downward pointing of the foot. The superficial muscles in the posterior compartment of the lower leg (e.g., the gastrocnemius, plantaris, and soleus muscles) join at their inferior aspects to form the calcaneal tendon, or Achilles tendon, which attaches to the calcaneal tuberosity. As these muscles contract, they pull the posterior aspect of the calcaneus upward, causing the foot to rotate downward from the ankle into plantar flexion. This motion is used in everyday movements, such as walking, running, driving, or riding a bicycle.

As one of the bones of the subtalar joint, the calcaneus also plays a role in the eversion and inversion movements of the foot. Eversion is the rotation of the foot outward at the ankle joint, whereas inversion is the rotation of the foot inward. These movements are possible because the articulation between the calcaneus and talus lies on an oblique axis.

Though most heel pain results from inflammation of tissues surrounding the calcaneus (e.g., tendons, bursae, and fasciae), injuries causing calcaneal fracture can occur. The most common calcaneal fractures are intraarticular, meaning that one or more fractures extend into the articulation surface of a bone and damage a joint. The most common joint affected is the subtalar joint, and it results from a large axial load increase on the calcaneus, such as when a person lands after a jump or falls from a tall height. As the load transfers from the talus to the calcaneus, the calcaneus can be crushed, causing a comminuted fracture by breaking the bone into several small pieces. This type of calcaneal fracture is often severe, usually results in great pain for the individual, and often requires surgical repair. Before surgery, the foot and ankle can be immobilized with a pressure dressing. Even after withholding weight bearing for 8-12 weeks post-operatively, one may experience chronic pain with weight bearing, inversion, and eversion due to long-term arthritis in the affected joint. Complications that may occur from intraarticular fractures include swelling, severe pain, acute compartment syndrome, and fracture blisters.

Extraarticular fractures, or fractures that do not extend into a joint, are a less common type of calcaneal fracture. These fractures typically result from falls or twisting of the joints. They include fractures limited to the bone tissue, such as within the bone body, at one of the bone’s processes, or avulsion fractures at tendon or ligament attachment sites. An avulsion fracture occurs when a force applied along a tendon or ligament pulls a small piece of bone away from its surface. Initial treatment of extraarticular fractures typically includes a compression dressing, rest, application of ice, elevation, and avoiding any weight bearing on the affected foot. Some extraarticular fractures require a walking boot or cast for four to six weeks, while others may take eight to ten weeks to heal. After cast or boot removal, ankle range of motion exercises at home with increasing weight bearing can help the individual return to usual activity. Additional physical therapy may be necessary for some individuals.

Finally, stress fractures of the calcaneus occur with repeated loading over time or a recent increase in activity level, such as when runners train on hard surfaces. Treatment of stress fractures can range from reducing activity and wearing heel inserts for minor cases to using crutches and avoiding weight bearing for more severe cases.