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.
Superficial muscles
Gastrocnemius
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.
Soleus
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).
Plantaris
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.
