Part six. Anterior compartment of the leg
The front of the leg includes the subcutaneous surface of the tibia on the medial side and the extensor muscular compartment on the anterolateral side.
The cutaneous nerves are derived from the femoral nerve over the tibia and from the common peroneal nerve over the extensor compartment (Fig. 3.46). The saphenous nerve gives off its infrapatellar branch, to supply the subcutaneous periosteum of the upper end of the tibia and the overlying and adjacent prepatellar skin; it then descends just behind the great saphenous vein with which it passes in front of the medial malleolus. It usually bifurcates above the malleolus, and the branches run in front of and behind the vein. The main nerve (anterior branch) often extends on the medial side of the foot as far as the bunion region: the first metatarsophalangeal joint. The lateral cutaneous nerve of the calf, a branch of the common peroneal, supplies deep fascia and skin over the upper parts of the extensor and peroneal compartments and the superficial peroneal nerve replaces it over the rest of these surfaces.
The subcutaneous surface of the tibia has subcutaneous fat in direct contact with the periosteum; the deep fascia here is blended with the periosteum. The great saphenous vein and the saphenous nerve lie in the fat, accompanied by numerous lymphatic vessels which pass up from the foot to the vertical group of superficial inguinal nodes. This is an important part of the course of the great saphenous vein (see p. 113) for here it has most of its deep connections. Along the medial side of the calf behind the medial border of the tibia a variable number of perforating (anastomotic) veins connect the great saphenous with deep veins of the calf (Fig. 3.30). Their location is variable but there is usually one just below and one about 10 cm above the medial malleolus, and another one a little below the middle of the leg. Higher up, there is a perforator just distal to the knee, and a rather long perforator in the lower thigh joining the great saphenous or one of its tributaries to the femoral vein in the adductor canal. The perforators in the leg may connect instead with a superficial longitudinal trunk, the posterior arch vein (Fig. 3.31), which usually joins the great saphenous some way below the knee. When traced deeply through the deep fascia, some of the perforating veins in the leg are seen to join the venae comitantes of the posterior tibial artery, while others join the venous plexus in soleus. The valves in the perforating veins are directed inwards and are found where the veins pierce the deep fascia and also where they join the deep veins. Much of the saphenous blood passes from superficial to deep through the perforators, to be pumped upwards in the deep veins by the contractions of soleus and other calf muscles. If the valves in the perforators become incompetent, the direction of blood flow is reversed and the veins become varicose (Fig. 3.31).
Figure 3.30 Prosection of the left leg in the Anatomy Museum of the Royal College of Surgeons of England: medial aspect. The veins have been injected with blue resin. The medial head of gastrocnemius has been detached from the femur and soleus from the tibia. Both muscles have been reflected posteriorly.
The upper end of the subcutaneous surface of the shaft of the tibia receives the tendons of three muscles that converge from the three constituent parts of the hip bone (Fig. 3.48). They are sartorius (supplied by the femoral, the nerve of the ilium), gracilis (supplied by the obturator, the nerve of the pubis) and semitendinosus (supplied by the sciatic, the nerve of the ischium) in that order from before backwards. The three tendons are separated by a bursa which lies deep to the flattened sartorius tendon.
The deep fascia of the leg covers only muscles, being attached to periosteum at all places where bone is subcutaneous; these include the medial surface of the tibial shaft and malleolus, and the lateral surface of the fibular malleolus and the triangular area above it. The deep fascia thus extends from the anterior border of the tibia around the lateral aspect of the leg to the posterior border of the tibia, enclosing the muscles of the leg. Two intermuscular septa pass from its deep surface to become attached to the fibula. They enclose the peroneal compartment. Between the anterior intermuscular septum and the tibia lies the extensor compartment, while between the posterior intermuscular septum and the tibia posteriorly lies the much more bulky flexor compartment or calf of the leg. On account of the attachment of the deep fascia to the subcutaneous surfaces of the leg bones and the presence of intermuscular septa, any condition that increases the volume of the contents of these tight compartments (e.g. muscle swelling or haemorrhage following trauma) can cause vascular compression and ischaemic damage to nerves and muscles. When such a compartment syndrome is anticipated it should be prevented by an extensive incision of the deep fascia that covers the compartment.
Extensor compartment
This compartment comprises the space between the deep fascia and the interosseous membrane,
bounded medially by the extensor surface of the tibia and laterally by the extensor surface of the fibula and the anterior intermuscular septum. Its contents are muscles: tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius, together with the deep peroneal nerve and anterior tibial vessels.
In its lower extent the deep fascia is thickened to form the superior extensor retinaculum (Fig. 3.32), which is attached to the anterior borders of the tibia and fibula. Deep to the retinaculum lie the tendons of tibialis anterior, extensor hallucis longus, extensor digitorum longus and peroneus tertius, in that order from medial to lateral, in front of the lower end of the tibia. Only the tibialis anterior tendon has a synovial sheath here. The anterior tibial vessels and deep peroneal nerve are also deep to the retinaculum, lying between extensor hallucis longus and extensor digitorum longus, with the vessels medial to the nerve (Fig. 3.32).
Figure 3.32 Left lower leg and dorsum of the foot.
Tibialis anterior
This muscle, which is readily palpable lateral to the tibia, arises from the upper two-thirds of the extensor surface of the tibia, from the interosseous membrane and from the deep fascia overlying it. The muscle descends as a tendon, enclosed in a synovial sheath, through the medial compartments of the superior and inferior extensor retinacula, and is inserted into the medial and inferior surfaces of the medial cuneiform and the adjacent part of the first metatarsal bone (Fig. 3.32).
Nerve supply. By the deep peroneal and recurrent genicular nerves (L4).
Action. Combined dorsiflexion of the ankle joint and inversion of the foot. Tibialis anterior helps to maintain the medial longitudinal arch of the foot (see p. 160).
Test. The foot is dorsiflexed against resistance; the tendon can be seen and felt.
Extensor hallucis longus
This muscle arises from the middle half of the extensor surface of the fibula and the adjacent interosseous membrane. The muscle lies deep at its origin, but emerges between tibialis anterior and extensor digitorum longus in the lower part of the leg, where its tendon crosses in front of the anterior tibial vessels and deep peroneal nerve from their lateral to medial sides. It passes deep to the superior and is slung by the inferior extensor retinacula and proceeds along the medial side of the dorsum of the foot to be inserted into the base of the terminal phalanx of the great toe. It has a separate synovial sheath on the foot (Fig. 3.32).
Nerve supply. By the deep peroneal nerve (L5).
Action. To dorsiflex (anatomically this is to extend) the great toe. Secondarily it is a dorsiflexor of the ankle.
Test. The big toe is dorsiflexed against resistance; the tendon can be seen and felt. This test is a useful index of the L5 anterior ramus and spinal nerve.
Extensor digitorum longus
This muscle arises from the upper three-quarters of the extensor surface of the fibula, a small area on the lateral condyle of the tibia and the interosseous membrane. Its tendon passes deep to the superior extensor retinaculum and then acquires a synovial sheath, which it shares with the tendon of peroneus tertius, and is enclosed in a loop of the interior extensor retinaculum. It divides into four tendons which diverge superficial to extensor digitorum brevis and are inserted into the lateral four toes (Fig. 3.32). Their mode of insertion is the same as that of the extensor digitorum tendons in the hand. A dorsal extensor expansion over the proximal phalanx divides into three slips, the central (middle) slip being inserted into the base of the middle phalanx. The two side slips reunite after being joined by the tendons of the interossei and lumbricals and are inserted into the base of the distal phalanx.
Nerve supply. By the deep peroneal nerve (L5, S1).
Action. To dorsiflex the lateral four toes.
Test. The four lateral toes are dorsiflexed against resistance; the tendons can be seen and felt.
Peronius tertius
Peronius (fibularis) tertius arises from the lower third of the extensor surface of the fibula. The tendon passes deep to the superior extensor retinaculum and through the stem of the inferior retinaculum, where it shares the synovial sheath of extensor digitorum longus, and is inserted into the dorsum of the base of the fifth metatarsal bone and by an extension into the superior surface of that bone (Fig. 3.32).
Nerve supply. By the deep peroneal nerve (L5, S1).
Action. To dorsiflex and evert the foot.
The deep peroneal (deep fibular) nerve arises within peroneus longus, over the neck of the fibula, at the bifurcation of the common peroneal nerve. It spirals around the neck of the fibula deep to the fibres of extensor digitorum longus, and so reaches the interosseous membrane, on the lateral side of the anterior tibial vessels. With them it lies between extensor digitorum longus and tibialis anterior. In the middle of the leg the neurovascular bundle lies on the interosseous membrane between tibialis anterior and extensor hallucis longus. The latter muscle then crosses in front of the bundle, and lies on its medial side. The deep peroneal nerve supplies the four muscles of the extensor compartment of the leg.
The anterior tibial artery, formed at the bifurcation of the popliteal artery in the calf, passes forwards through the upper part of the interosseous membrane near the neck of the fibula, with a companion vein on each side. The artery with its companion veins runs vertically downwards on the interosseous membrane and crosses the lower end of the tibia at the front of the ankle joint, midway between the malleoli, where it changes its name to the dorsalis pedis artery. It gives off an anterior recurrent branch, which pierces tibialis anterior, to the arterial anastomosis around the upper end of the tibia. (An inconstant posterior recurrent branch arises in the popliteal fossa.) The anterior tibial artery supplies the muscles of the extensor compartment and gives malleolar branches to both malleolar regions. The deep peroneal nerve reaches it from the lateral side, runs in front of it in the crowded space of the middle of the leg and returns to its lateral side below. The accompanying anterior tibial veins run, one on each side of the artery, in close contact with it and anastomose by cross channels at frequent intervals.
Tibiofibular joints
The superior tibiofibular joint is a synovial joint between the lateral tibial condyle and the fibular head. The articulating surfaces are almost flat. The capsule is reinforced by anterior and posterior ligaments. The joint cavity may occasionally communicate posteriorly with the bursa deep to the popliteus tendon and thence with the knee joint.
The interosseous membrane consists of strong fibres that slope steeply from the tibia down to the fibula, and are continuous distally with the interosseous tibiofibular ligament.
The inferior tibiofibular joint is a fibrous joint (syndesmosis) between the convex medial surface of the distal end of the fibula and the concave fibular notch of the distal tibia. The bones are held together by anterior and posterior tibiofibular ligaments and are strongly bound by the interosseous tibiofibular ligament, whose fibres occupy the triangular area on each bone at the lower end of the interosseous border (Fig. 3.54).
Figure 3.54 Lower ends of the left and right fibulas, distinguished by the position of the malleolar fossa behind the triangular articular area.
Only slight movements occur at the tibiofibular joints, the fibula rotating laterally a little during dorsiflexion at the ankle.