Bone and Joints
The ankle joint is the articulation of the leg bones (the tibia and fibula) and the talus (the superior-most bone of the foot). This joint is formed as the tibial shaft flares out distally to form the medial malleolus. This malleolus serves as a medial buttress of the ankle and as the proximal point of origin of the deltoid ligament. The distal articular surface of the tibia (the plafond) is not perfectly flat but instead has a slight central ridge that corresponds to a central depression in the talar dome. This shape increases the congruity and stability of the joint. The fibula also flares out distally to form the lateral malleolus, the anchor point for the lateral ankle ligaments and a buttress preventing lateral displacement of the talus. The distal tibia and fibula thus form a “mortise,” an inverted U, which contains the dome of the talus. The talar dome accordingly comprises the floor of the ankle joint and articulates with the tibial plafond superiorly, the medial malleolus medially, and the lateral malleolus laterally.
The architecture of the foot can be divided into three parts: the hindfoot, the mid- foot, and the forefoot (Figs. 35 and 36).
The hindfoot consists of the talus and calcaneus and serves as the link between the ankle and the remainder of the foot. The undersurface of the talar body consists of articular cartilage that contacts the calcaneus to form the subtalar joint. Much of the talus is covered by articular cartilage with no direct muscle attachments and few areas for the entry of blood vessels into the bone. In the presence of severe fractures or dislocations, this limited vascular supply predisposes the talus to osteonecrosis. The body of the calcaneus is its largest portion and bears large compressive loads during weight bearing. The sus- tentaculum tali, a dense bony projection off the medial side of the calcaneal body, helps support the talus.
The midfoot is made up of the tarsal bones, namely, the navicular; the cuboid; and the medial, middle, and lateral cuneiforms. The midfoot meets the hindfoot at the talo- navicular and calcaneocuboid joints, which collectively are known as the transverse tarsal joint, or Chopart’s joint. The distal extent of the midfoot is the tarsometatarsal joint, or the Lisfranc joint. Medially, the navicular serves as the insertion for the posterior tibialis tendon. The cuboid, the most lateral bone, has a shallow groove on its lateral side in which the peroneus longus tendon is housed as it turns from the lateral side of the foot toward the plantar aspect.
The forefoot bones include the metatar- sals, the phalanges, and two accessory bones beneath the hallux, the medial and lateral sesamoids. The metatarsal bones link the midfoot with the toes. The great toe, or hal- lux, has two phalanges (proximal and distal) and one interphalangeal joint. The four lesser toes each have three phalanges (proximal, middle, and distal), with two intervening joints, the proximal interphalangeal (PIP) joint and the distal interphalangeal (DIP) joint. The phalanges are short tubular bones. The concave proximal phalanx base articu- lates with the metatarsal head.
The sesamoid bones beneath the metatarsophalangeal joint of the great toe have a function similar to that of the patella and increase the mechanical advantage of the muscles that flex that metatarsophalangeal joint. Because of their position at the ball of the foot, the sesamoids undergo substantial mechanical force and are often the site of overuse conditions or injuries.
An extensive array of ligaments stabilize the ankle (Fig. 37).
Medially, the deltoid ligament resists eversion (tilting up of the lateral foot) as well as rotation of the talus within the mortise. The lateral ankle ligaments resist inver- sion of the ankle joint (tilting up of the medial foot). The lateral ligament complex consists of the anterior talofibular, calcaneofibular, and posterior talofibular ligaments (Fig. 38).
The calcaneofibular ligament resists varus tilting of the joint when the ankle is in the neutral position (neither plantar flexed nor dorsiflexed). When the ankle is in the neutral position, the anterior talofibular ligament resists anterior subluxation of the talus. In plantar flexion, the anterior talofibular ligament resists inversion because in that position the ligament is oriented more vertically.
The syndesmotic ligaments (syndesmosis) stabilize the tibia to the fibula above the level of the ankle joint. This complex is composed of four major structures that resist separation (or diastasis) of the tibia from the fibula.
The subtalar joint is stabilized primarily by ligaments directly between the talus and calcaneus. The motions at the subtalar joint are inversion and eversion. The midfoot demonstrates a complex variety of motion, allowing the foot to accommodate uneven surfaces or terrain. These motions include dorsiflexion, plantar flexion, abduction, adduction, and rotation. The metatarsophalangeal joints in the forefoot are stabilized by medial and lateral collateral ligaments, a weak dorsal joint capsule, and a strong plan- tar ligament complex (the “plantar plate”). The PIP and DIP joints have a similar ligamentous structure that, along with the bony anatomy, provides stability.
Muscles of the foot and ankle are divided into two groups: intrinsic muscles, which have their bellies in the foot; and extrinsic muscles, which are based in the compartments of calf, with only their tendons cross- ing the ankle. The muscles of the superficial posterior compartment (gastrocnemius and soleus) attach to the calcaneus and are powerful plantar flexors of the ankle. The muscles of the deep posterior compartment cross the ankle behind the medial malleolus deep to the flexor retinaculum. The posterior tibialis inverts the ankle and provides dynamic support of the arch of the foot. The flexor hallucis longus and flexor digitorum longus tendons insert into the distal phalanges of the great toe and lesser toes; flex the toes; and, to a small degree, contribute to ankle plantar flexion.
The anterior compartment muscles (tibialis anterior, extensor hallucis longus, and ex- tensor digitorum longus) are extensors of the ankle. They cross the ankle anteriorly and pass deep to the extensor retinaculum to exit onto the dorsum of the foot. The tibialis anterior tendon inserts on the medial cuneiform and the base of the first metatarsal and is the primary extensor of the ankle. The extensor hallucis longus and extensor digitorum longus extend the toes.
The lateral compartment of the leg contains the peroneus longus and peroneus brevis muscles, which are important dynamic stabilizers of the lateral ankle and protect against inversion sprains. These two muscles cross the ankle behind the lateral malleolus in a shallow groove that is covered by the peroneal retinaculum. The peroneus brevis tendon inserts on the base of the fifth metatarsal, whereas the peroneus longus tendon curves around a groove in the cuboid and travels deep into the plantar aspect of the foot to insert on the base of the first metatarsal.
The intrinsic muscles are housed in the foot itself. The extensor hallucis brevis and extensor digitorum brevis muscles arise from the lateral hindfoot, and their tendons travel across the dorsum of the foot. The extensor hallucis brevis inserts on the proximal phalanx of the great toe and extends the metatarsophalangeal joint. The extensor digitorum brevis tendons extend the metatarsophalangeal joint and are innervated by a branch from the deep peroneal nerve.
On the plantar side, the plantar fascia originates from the calcaneus and inserts on the plantar plate of the metatarsophalangeal joint, which helps support the longitudinal arch of the foot. Branches of the tibial nerve innervate the plantar intrinsic foot muscles. These muscles, which abduct the toes and assist with flexion and extension, are similar in function to the intrinsic muscles of the hand.
Nerves and Blood Vessels
Five nerves supply the lower legs, ankles, and feet: the tibial, saphenous, sural, superficial peroneal, and deep peroneal nerves (Fig. 39).
The saphenous and sural nerves are purely sensory nerves, whereas the other three are mixed sensory and motor nerves. At the level of the ankle, the tibial nerve travels posterior to the medial malleolus and runs deep to the flexor retinaculum in the tarsal tunnel. It then trifurcates into the medial calcaneal, medial plantar, and lateral plantar nerve branches. High in the calf, the common peroneal nerve divides into two branches, the superficial and deep peroneal nerves. The deep peroneal nerve runs within the anterior compartment of the leg with the anterior tibial vessels and innervates the muscles there. It sends a sen- sory branch onto the dorsum of the foot with the dorsalis pedis artery, which terminates in the web space between the great toe and the second toe. The superficial peroneal nerve runs within the lateral muscle compartment of the leg, innervating the peroneus longus and brevis muscles. The nerve continues distally to supply sensation to most of the dorsum of the foot.
The ankle and foot have three major arteries providing vascular inflow: the posterior tibial, anterior tibial, and peroneal arteries (Fig. 40).
These three branches arise in the proximal leg at the “trifurcation” of the popliteal artery. (This is not a true trifurcation as the anterior tibial splits off first.) The posterior tibial artery pulse is palpable posterior to the medial malleolus. The anterior tibial artery crosses the ankle and courses on the dorsum of the foot, at which point it is renamed the dorsalis pedis artery. The peroneal artery runs within the deep posterior compartment but supplies the peroneal muscles and the lateral foot and ankle. As in the hand, these vessels have extensive anastomoses; therefore, collateral flow is often sufficient when an iso- lated arterial injury occurs.