Chapter 16: The leg
Anterior and posterior intermuscular septa form from inward extensions of the fascia of the leg, thereby giving rise to three compartments that allow for very little expansion: (1) the anterior, or extensor, compartment (sometimes refered to as the cural region), consisting of muscles supplied by the common or deep fibular (peroneal) nerve or by both; (2) lateral, or fibular (peroneal), compartment, with muscles supplied by the superficial fibular nerve; and (3) posterior, or flexor, compartment (sometimes referred to as the sural region), having muscles supplied by the tibial nerve (fig. 16-1). The posterior compartment is subdivided by the deep transverse fascia of the leg, which runs between the medial border of the tibia and the posterior border of the fibula. The interosseous membrane, which connects the interosseous borders of the tibia and fibula, is almost sagittal in the upper part of the leg.
Anterior leg (table 16-1)
The muscles.
The muscles of the anterior leg are the tibialis anterior, extensor digitorum longus, fibularis tertius, and extensor hallucis longus (figs. 16-1 and 17-4C). They arise from bone, the strong investing fascia, and the interosseous membrane. The muscles of the anterior leg are supplied by the common or deep fibular nerve or by both, and they dorsiflex the foot. The tendons of these muscles, surrounded by their synovial sheaths, are bound down by fascial thickenings known as the superior and inferior extensor retinacula.
The deep fibular nerve.
The deep fibular (peroneal) nerve (see fig. 15-7) (at one time known as the anterior tibial nerve and then the deep peroneal nerve) is one of the terminal branches of the common fibular nerve. It continues around the neck of the fibula and descends on the interosseous membrane in company with the anterior tibial artery. It supplies the tibialis anterior, extensor digitorum longus, extensor hallucis longus, fibularis tertius, and extensor digitorum brevis muscles, and it gives off dorsal digital nerves to the first two toes. Damage to the deep fibular nerve may result in footdrop and a "steppage" gait, owing to paralysis of the dorsiflexors of the foot and the extensors of the toes. This gait is characterized by lifting the knee high in order to allow the drooping toes to clear the floor.
The anterior tibial artery.
The anterior tibial artery, the smaller division of the popliteal artery, passes through the fibrous arch of the tibialis posterior and through a defect in the superior part of the interosseous membrane to meet its companion nerve (fig. 16-2). It descends on the anterior side of the interosseous membrane and, on the dorsum of the foot, is typically continued as the dorsal artery of the foot (see fig. 17-4C). The pulsations of the anterior tibial artery are often palpable between the two malleoli and lateral to the extensor hallucis longus tendon. Its branches supply adjacent muscles and contribute to the anastomoses around the knee and ankle joints.
Lateral side of leg (table 16-2)
The fibularis muscles.
The fibularis (peroneus) longus and brevis muscles lie between the anterior and posterior intermuscular septa, and they arise from these septa, as well as from fascia and the fibula, itself (figs. 16-1, 16-3, and 16-5). The fibularis longus and brevis are supplied by the superficial fibular nerve, and they evert the foot. They have a common synovial sheath as they descend posterior to the lateral malleolus, and they are bound down by the superior and inferior fibular retinacula (see fig. 17-4B). The peroneus longus traverses a notch in the cuboid and then contains either a sesamoid bone or a fibrocartilaginous thickening. The tendon crosses the sole of the foot to reach the medial cuneiform and the base of the first metatarsal opposite the more medial insertion of the tibialis anterior (see fig. 17-2C), with which it forms a stirrup. The fibularis brevis lies deep to the longus and inserts on the tuberosity of the fifth metatarsal at the lateral side of the foot.
The superficial fibular nerve.
The superficial fibular nerve is one of the terminal branches of the common fibular nerve. It descends anterior to the fibula (see fig. 15-7). It supplies the fibularis longus and brevis (and sometimes the extensor digitorum brevis) and provides cutaneous branches to usually all five toes. Section of the superficial fibular nerve may result in impairment and loss of eversion.
Posterior leg (tables 16-3 and 16-4)
The muscles.
The superficial muscles of the posterior leg are the gastrocnemius and soleus (fig. 16-4), which together are sometimes termed the triceps surae, and the plantaris. The deep muscles are the popliteus, flexor digitorum longus, flexor hallucis longus, and tibialis posterior (figs. 16-1, 16-2, 16-3 and 16-5). The last three are separated from the superficial group by the deep transverse fascia of the leg and arise from the interosseous membrane as well as from the leg bones. All the muscles of the posterior leg are supplied by the tibial nerve, and they plantar-flex the foot.
The gastrocnemius has two large heads that arise from the condyles of the femur (fig. 15-9). Bursae underlie the heads, and the lateral head may contain a sesamoid bone known as the fabella (see fig. 12-21B). At about the middle of the leg, the heads end in a common aponeurosis, which unites with the underlying tendon of the soleus to form the calcaneal tendon (fig. 16-5), frequently referred to as Achilles' tendon. * The calcaneal tendon is inserted into the back of the calcaneus (see fig. 12-35). The soleus presents a tendinous arch between the fibula and tibia. The arch lies posterior to the popliteal vessels and tibial nerve. The triceps surae plantar-flexes the foot and is an important muscle in both posture and locomotion. It is used in walking, running, jumping, and dancing. The ankle jerk is a twitch of the triceps surae induced by tapping the calcaneal tendon. The reflex center is generally in the S1 segment of the spinal cord.
The popliteus muscle descends from the femur laterally to insert on the tibia medially (see fig. 15-10). Hence it rotates the tibia medially or the femur laterally (when the tibia is fixed in place). The popliteus arises also from the lateral meniscus, which it pulls posteriorly at the beginning of flexion. The muscle is active in crouching, when it probably helps to prevent the femur from sliding anteriorly on the tibia. The flexor digitorum longus resembles the flexor digitorum profundus of the upper limb, in that each muscle has four tendons (omitting one for the great toe), gives origin to lumbricals, has fibrous and synovial sheaths, is anchored to the phalanges by vincula, and is inserted into the distal phalanges (see fig. 17-2B). Although it arises laterally from the fibula, the flexor hallucis longus goes to the medially positioned great toe. This means that its tendon must cross those of the flexor digitorum longus. The tibialis posterior, more deeply placed, has an extensive origin from the interosseous membrane, fibula, and tibia. It is inserted into all the tarsal bones except the talus, but chiefly into the tuberosity of the navicular. The tibialis posterior is the principal inverter of the foot. The long flexors and tibialis posterior, together with their synovial sheaths and the posterior tibial vessels and tibial nerve, are limited posterior to the medial malleolus by an indistinct fascial thickening, the flexor retinaculum (see fig. 17-4A).
The tibial nerve.
The tibial nerve, which lies on the popliteus muscle and under cover of the gastrocnemius, passes anterior to the tendinous arch of the soleus and descends on the deep muscles of the leg (see figs. 15-8 and 15-10). Deep to the flexor retinaculum, it divides into the medial and lateral plantar nerves. In addition to the gastrocnemius and plantaris muscles, the tibial nerve supplies the soleus, flexor digitorum longus, flexor hallucis longus and tibialis posterior muscles, as well as the skin of the heel and sole. Injury to the tibial nerve is followed by sensory loss in the sole of the foot, which interferes with posture and locomotion. The extent of motor loss depends on the level of the lesion, but it may involve the muscles of the calf and the small muscles of the foot. Plantar flexion may be lost and inversion (produced by the tibialis posterior) may be impaired.
The posterior tibial artery.
The posterior tibial artery, the larger division of the popliteal artery, begins at the lower border of the popliteus muscle (see figs. 16-2 and 16-4) where it and the anterior tibial artery are the terminal branches of the popliteal artery. It descends on the deep muscles of the leg and is covered by the soleus and gastrocnemius muscles. Deep to the flexor retinaculum at the ankle, it divides into the medial and lateral plantar arteries, which provide the chief blood supply to the foot (see fig. 17-4A). The pulsations of the posterior tibial artery are often palpable between the medial malleolus and the calcaneal tendon. Palpation of this pulse is an important part of the physical exam. In its course through the leg, the posterior tibial artery supplies: (1) branches to adjacent muscles; (2) a large nutrient artery to the tibia; (3) branches to the anastomoses around the knee and ankle, and; (4) the fibular artery. The fibular artery (see fig. 16-2) arises from the posterior tibial artery inferior to the lower border of the popliteus muscle and descends along the medial crest of the fibula. It gives off a number of small branches, including its terminal calcaneal ones. Its perforating branch, which reaches the anterior aspect of the leg by piercing the interosseous membrane, anastomoses with branches of the dorsal artery of the foot. If the anterior tibial artery is small or absent, the fibular is large and, by means of its perforating branch, may replace the dorsal artery of the foot.
Questions
16-1 What are the main compartments of the leg?
16-2 Which nerves accompany the arteries in the leg?
16-3 What are the chief (a) invertors and (b) evertors of the foot?
16-4 What is the triceps surae?
16-5 Where is the reflex center for the ankle jerk?
16-6 What is the action of the popliteus muscle?
16-7 What are the chief effects of damage to the tibial nerve?
Figure legends
Figure 16-1 Horizontal section through the middle of the leg. In A, the nerves and vessels are identified. B shows the extensors (supplied by the deep fibular nerve) anteriorly, the fibular muscles (supplied by the superficial fibular nerve) laterally, and the flexors (supplied by the tibial nerve) posteriorly. The posterior compartment contains superficial and deep muscles, separated by the deep transverse fascia of the leg. E.D.L., extensor digitorum longus; E.H.L., extensor hallucis longus; F.D.L., flexor digitorum longus; F.H.L., flexor hallucis longus; P.L., fibularis longus; T.A., tibialis anterior; T.P., tibialis posterior.
Figure 16-2 The divisions of the popliteal artery and their relations to the tibialis posterior. (Based on Shellshear and Macintosh.)
Figure 16-3 Origins of the deep muscles of the calf. Note the fibrous arch in the tibialis posterior for the anterior tibial vessels (asterisk). Note also the arch in the tibialis posterior for the nutrient artery to the tibia (two asterisks). (Based on Shellshear and Macintosh.)
Figure 16-4 The soleus muscle superimposed on the structures shown in fig. 16-2. Note that its apex is at the fibula. Note also from the inset that the soleus ascends from the soleal line and then turns downward. Thus, the upper border of the muscle is superior to the soleal line. (Based on Shellshear and Macintosh.)
Figure 16-5 Horizontal section through the lower end of the tibia and fibula. The tendons of the muscles of the three compartments of the leg (see fig. 16-1) are shown, as are also the neurovascular bundles related to the hallucis and digitorum tendons (deep fibular nerve and anterior tibial artery; posterior tibial artery and tibial nerve). The synovial sheaths (see fig. 17-1) are not represented. The plantaris muscle is seen joining the medial side of the calcaneal tendon. Some important muscular functions are indicated. Dorsiflexion and plantar flexion take place at the ankle joint, whereas inversion and eversion occur at more distal joints. (See fig. 17-7.) (Modified from Castaing and Soutoul.)
* Achilles is said to have been held by his heels when dipped in the river Styx to make him invulnerable, and, at the siege of Troy, he was mortally wounded by an arrow in his heel: for several centuries the calcaneal tendon was known as Achilles' tendon.