Posterior Compartment of the Leg Anatomy
The compartment extends from the popliteal fossa above to the ankle below. Gastrocnemius and soleus account for its bulk (Fig. 6.41) and the characteristic contour of the calf. Their common tendon is conspicuous as it passes towards the ankle. Deeper are tibialis posterior and the two long flexors of the toes, flexor hallucis longus and flexor digitorum longus, whose tendons all pass distally into the foot. The posterior tibial artery and its venae comitantes and the tibial nerve pass distally between soleus and the long flexors to enter the foot, where they supply structures in the sole.
Superficial structures and deep fascia
The small (short) saphenous vein begins on the lateral side of the foot as a continuation of the dorsal venous arch. Passing behind the lateral malleolus (Fig. 6.42) it ascends in the midline of the calf and usually terminates by piercing the fascial roof of the popliteal fossa (Fig. 6.37). The vein has frequent communication with the great saphenous vein and important communicating (perforating) veins, which pierce the investing deep fascia to link up with the deep veins of the calf, particularly just above the ankle.
The sural nerve is formed principally from the cutaneous branch of the tibial nerve and descends from the popliteal fossa to pierce the deep fascia in the proximal part of the calf. Here it is joined by the sural communicating branch of the common fibular nerve and continues distally with the small saphenous vein posteroinferior to the lateral malleolus to reach the foot (Fig. 6.42). The nerve supplies skin over the lower twothirds of the calf and on the lateral side of the ankle and foot.
The deep fascia is continuous proximally with the popliteal fascia, and distally near the heel is thickened in two places. The first forms a thick band stretching between the tibia and fibula over which the tendo calcaneus passes, separated by a bursa. The second thickening, the flexor retinaculum, bridges the gap between the medial malleolus and the medial surface of the calcaneus, completing a fibroosseous tunnel similar to the carpal tunnel of the wrist. It is the gateway into the sole of the foot for the tendons of the deep muscles of the compartment and the neurovascular bundle. The tibial nerve may be compressed in the tunnel, causing pain in the heel and foot.
The muscle has medial and lateral heads attached to the respective femoral condyles. The medial head is the larger and its fibres extend more distally. The two heads share a common aponeurosis, which overlies that of the subjacent soleus (Fig. 6.43). The long parallel fibres of gastrocnemius provide a large range of movement and are used especially during walking and running. Because of its femoral attachments, it is a weak flexor of the knee joint.
This muscle attaches to the soleal line on the posterior border of the tibia, from where it arches across to the posterior aspect of the head and upper shaft of the fibula (Figs 6.44 & 6.45). Its short multipennate fibres are continually in action during standing.
The aponeuroses of gastrocnemius and soleus form the tendo calcaneus (Achilles), which attaches to the middle third of the posterior surface of the calcaneus, and via this tendon the two muscles are the principal flexors of the foot at the ankle joint. The tendon, separated from the upper part of the calcaneus, is easily palpable and is used clinically to obtain the stretch reflex or ankle jerk (S1 & S2).
This muscle attaches to the lower end of the femur close to the lateral head of gastrocnemius. Its long thin tendon passes deep to the medial head of gastrocnemius (Fig. 6.45) between the aponeuroses of gastrocnemius and soleus to insert into the posterior surface of the calcaneus medial to the tendo calcaneus. It is a weak flexor of the knee and ankle joints.
Gastrocnemius, soleus and plantaris are supplied by the tibial nerve in the popliteal fossa (Fig. 6.40). Soleus is further supplied by the tibial nerve as it passes beneath the soleal arch.
At the lower border of popliteus, the popliteal artery divides into two terminal branches, the anterior tibial artery (p. 290) and the larger posterior tibial artery. The latter, accompanied by its venae comitantes, passes deep to the soleal arch and descends on the fascia of tibialis posterior (Fig. 6.41) between flexor digitorum longus and flexor hallucis longus. Near the ankle, the artery lies on the distal end of the tibia and the capsule of the ankle joint, where its pulsations are easily felt. Passing beneath the flexor retinaculum, it gives calcaneal branches to the superficial tissues of the heel and then divides into medial and lateral plantar arteries. It provides branches to the muscles in the calf and also a large nutrient artery to the tibial shaft.
The most conspicuous branch, the fibular (peroneal) artery, arises just beyond the soleal arch (Fig. 6.46) and passes distally and laterally deep to flexor hallucis longus and upon the fascia of tibialis posterior. It gives muscular branches and a nutrient branch to the fibula. The fibular artery terminates behind the lateral malleolus as lateral calcaneal branches supplying the tissues of the heel. In the lower part of the leg, a communicating artery links the fibular and posterior tibial arteries.
A perforating branch of the fibular artery passes forwards through the interosseous membrane into the anterior compartment of the leg. When the anterior tibial artery is small, this perforating artery may substitute for it distally, continuing into the foot as the dorsalis pedis artery.
The tibial nerve enters the compartment from the popliteal fossa by passing beneath the soleal arch. Usually, the posterior tibial artery lies medially as it descends through the calf on the fascia of tibialis posterior and on the distal end of the tibia (Fig. 6.46). Similar to the artery, as it enters the foot it often terminates beneath the flexor retinaculum as two branches, the medial and lateral plantar nerves. It supplies soleus and the deep leg muscles and gives sensory branches to skin over the ball of the heel.
Lying deep to soleus are flexor digitorum longus, flexor hallucis longus and tibialis posterior (Figs 6.41 & 6.46). Proximally, their attachments are limited by the origin of soleus from the tibia and fibula. Their tendons enter the foot beneath the flexor retinaculum.
Flexor digitorum longus arises from the posterior surface of the tibia (Fig. 6.44) and from the fascia covering tibialis posterior. In the foot, the tendon divides into four slips, which are attached to the bases of the terminal phalanges of the lateral toes. The muscle flexes the toes and assists in plantar flexion of the ankle joint.
Flexor hallucis longus is larger than flexor digitorum longus (Fig. 6.46) and has extensive attachments to the posterior surface of the shaft of the fibula (Fig. 6.44), the posterior intermuscular septum and the fascia covering tibialis posterior. Its muscle fibres characteristically extend almost as far as the ankle (Fig. 6.46) and give way to a tendon, which passes beneath the flexor retinaculum to insert into the base of the distal phalanx of the great toe. It is a powerful flexor of the hallux, especially in forward propulsion of the foot at the takeoff point during walking. Plantar flexion at the ankle joint is also assisted.
Tibialis posterior is the deepest muscle in the compartment and attaches to the posterior surface of the tibia (Fig. 6.44), to the medial surface of the fibula and to the upper twothirds of the interosseous membrane. Passing under the flexor retinaculum, where it is the most medial structure, the tendon enters the foot to attach principally to the tuberosity of the navicular and by small slips to the other tarsal bones. Together with tibialis anterior its action is to invert the foot. It is also a weak plantar flexor of the ankle joint.
All three deep muscles of the calf are supplied by the tibial nerve. Further details of the tendons in the sole of the foot are given on p. 282.