Part ten. Sole of the foot

The skin of the sole is supplied from the medial and lateral plantar nerves by branches that perforate the plantar aponeurosis along each edge of its strong central portion. Blood vessels accompany all the cutaneous nerves of the sole. The plantar surfaces of the toes are supplied by digital branches of the medial (three and a half toes) and lateral (one and a half toes) plantar nerves. The plantar digital nerves also supply the nail bed and surrounding skin. The median and ulnar nerve distributions to the fingers are identical.

The subcutaneous tissue in the sole, as in the palm, differs from that of the rest of the body in being more fibrous. Fibrous septa divide the tissue into small loculi which are filled with a rather fluid fat under tension, so that the cut tissue bulges. This makes a shock-absorbing pad, especially over the heel. The septa anchor the skin to the underlying plantar aponeurosis and limit the mobility of the skin.

T h e plantar aponeurosis (fascia) is composed of dense collagen fibres arranged mainly longitudinally. It arises posteriorly from the medial process of the calcaneus and fans out over the sole (Fig. 3.36). It divides into five bands, one for each toe. Just distal to the metatarsal heads, these are connected by transverse fibres, the superficial transverse metatarsal ligament. The digital slips bifurcate for the passage of the flexor tendons and are inserted around the edges of the fibrous flexor sheaths and into the deep transverse metatarsal ligaments that unite the plantar ligaments of adjacent metatarsophalangeal joints. From each edge of the plantar aponeurosis a septum penetrates the sole, separating the flexor digitorum brevis from the abductors of big and little toes. The septa are attached to the first and fifth metatarsal bones. The abductors of the big and little toes, lying along the margins of the sole, are covered by deep fascia that is much thinner than the central plantar aponeurosis.

Figure 3.36 Left plantar aponeurosis.

The muscles of the sole are arranged in four layers. The superficial layer consists of three short muscles that cover the sole, beneath the plantar aponeurosis. The second layer consists of long

tendons to the digits, and their connections. The third layer consists of the short muscles of the great and little toes; it is confined to the metatarsal region of the foot. The fourth layer consists of both plantar and dorsal interossei and it includes also the tendons of peroneus longus and tibialis posterior. The segmental nerve supply of all the muscles is essentially S2.

First layer

Three short muscles lie side by side along the sole of the foot. The central of these, flexor digitorum brevis, is the counterpart of flexor superficialis in the upper limb, and is represented in the palm by the four tendons of that muscle.

Flexor digitorum brevis arises from the medial process of the calcaneus and the deep surface of the central part of the plantar aponeurosis. It divides into four tendons which pass to the lateral four toes. Each tendon enters the fibrous flexor sheath on the plantar aspect of its digit, divides and spirals around the long flexor tendon, and partially reunites in a chiasma before dividing again to be inserted into the sides of the middle phalanx. This is an identical arrangement with that of the flexor digitorum superficialis.

Nerve supply. By the medial plantar nerve.

Action. To flex the toes with equal effect in any position of the ankle joint.

Abductor hallucis arises from the medial process of the calcaneus and from the flexor retinaculum. It runs along the medial margin of the foot (Fig. 3.36) to be inserted into the medial side of the base of the proximal phalanx of the great toe.

Nerve supply. By the medial plantar nerve.

Action. It abducts the great toe.

Abductor digiti minimi arises from both medial and lateral processes of the calcaneus, deep to the origin of flexor digitorum brevis. It lies along the lateral margin of the foot (Fig. 3.36). Its tendon is inserted into the lateral side of the base of the proximal phalanx of the fifth toe and the tubercle of the fifth metatarsal bone.

Nerve supply. By a branch from the main trunk of the lateral plantar nerve.

Action. It abducts the little toe.

Second layer

This consists of the long flexor tendons and their connections in the sole.

The tendon of flexor hallucis longus passes forward like a bowstring beneath the medial longitudinal arch of the foot (Fig. 3.44). It is crossed inferiorly by the tendon of flexor digitorum longus, to the medial two of whose divisions it gives off a strong slip. It next lies in a groove between the two sesamoids beneath the head of the first metatarsal bone, and finally is inserted into the base of the distal phalanx of the big toe (Fig. 3.37). It is invested by a synovial sheath throughout its whole course in the foot.

The tendon of flexor digitorum longus enters the sole on the medial side of the tendon of flexor hallucis longus. At its point of division into its four tendons of insertion it crosses superficial to the tendon of flexor hallucis longus, which gives a strong slip to the tendons for the second and third toes. It also receives the insertion of flexor accessorius (Fig. 3.37). The four tendons pass forwards in the sole deep to those of flexor digitorum brevis and after giving off the lumbricals they enter the fibrous sheaths of the lateral four toes. Each tendon perforates the tendon of flexor digitorum brevis and passes on to be inserted into the base of the distal phalanx.

Flexor accessorius arises by a large medial head from the medial surface of the calcaneus and by a small lateral head that arises from the lateral border of the plantar surface of the calcaneus; they converge on the muscle belly. The posterior part of the long plantar ligament is visible in the triangular interval between the two heads. The muscle belly is inserted into the tendon of flexor digitorum longus as it subdivides into tendons for the digits.

Nerve supply. By the main trunk of the lateral plantar nerve.

Action. By pulling on the tendons of flexor digitorum longus, it provides a means of flexing the lateral four toes in any position of the ankle joint. It also straightens the pull of the long flexor tendons on the toes.

The lumbrical muscles arise from the tendons of flexor digitorum longus, and pass forward on the medial (big toe) sides of the metatarsophalangeal joints of the lateral four toes (Fig. 3.37). (In the hand the lumbricals are on the thumb side of the corresponding joints.) Their tendons lie on the plantar surfaces of the deep transverse ligament of the metatarsal heads and pass dorsally to be inserted into the extensor expansions.

Nerve supply. As in the hand, a lumbrical supplied by the medial plantar (cf. median) nerve is unicipital; one supplied by the lateral plantar (cf. ulnar) nerve is bicipital. In the foot only the first lumbrical is supplied by the medial plantar nerve; it arises by a single head from its own tendon. The lateral three lumbricals are supplied by the lateral plantar nerve (deep branch) and each arises by two heads from the adjoining sides of the tendons.

Action. The lumbricals maintain extension of the digits at the interphalangeal joints while the flexor digitorum longus tendons are flexing the toes, so that in walking and running the toes do not buckle under.

Third layer

This consists, like the first layer, of three muscles but they are shorter and confined to the metatarsal region of the foot. Two act on the big toe, one on the little toe.

Flexor hallucis brevis lies against the undersurface of the metatarsal bone of the first toe. It arises from the cuboid, the lateral cuneiform and the tendon of tibialis posterior. The belly of the muscle splits into two parts which are inserted, each by way of a sesamoid bone, into the medial and lateral sides of the base of the plantar surface of the proximal phalanx of the great toe. The medial insertion blends with that of abductor hallucis, the lateral with that of adductor hallucis.

Nerve supply. By the medial plantar nerve.

Action. To flex the proximal phalanx of the big toe.

Adductor hallucis has two heads. The large oblique head arises anterior to flexor hallucis brevis, from the long plantar ligament where it roofs over the peroneus longus tendon, and from the bases of the second, third and fourth metatarsal bones. The slender transverse head has no bony origin; it arises from the deep transverse ligament and from the plantar ligaments of the lateral four metatarsophalangeal joints. The two heads unite in a short tendon which is inserted, with the lateral insertion of flexor hallucis brevis, into the lateral side of the plantar surface of the base of the proximal phalanx of the big toe (Fig. 3.37).

Nerve supply. By the deep branch of the lateral plantar nerve.

Action. The muscle draws the big toe towards the axis of the metatarsus and assists in maintaining the transverse arch.

Flexor digiti minimi brevis arises from the base of the fifth metatarsal bone and the adjoining fibrous sheath of peroneus longus. The muscle belly lies along the undersurface of the fifth metatarsal bone. Its tendon is inserted into the base of the proximal phalanx medial to the insertion of abductor digiti minimi (Fig. 3.37).

Nerve supply. By the superficial branch of the lateral plantar nerve.

Action. To assist in flexing the little toe.

Fourth layer

The fourth layer of muscles consists of the interossei in the intermetatarsal spaces. The tendons of tibialis posterior and peroneus longus, lying deeply against the undersurface of the tarsus, are conveniently included in this layer.

Interosseous muscles

The actions of the interossei of the foot (Fig. 3.38), as of the hand, are indicated by the mnemonic ‘PAD and DAB’: the plantar adduct and the dorsal abduct, but with the important difference that the longitudinal axis of the foot has shifted medially, and lies along the second metatarsal bone and the phalanges of the second toe (which is usually the longest toe). As in the hand, a plantar interosseous arises from the metatarsal bone of its own toe, while the bulkier dorsal interosseous arises by two heads from the two metatarsals between which it lies.

The three plantar adducting muscles adduct towards the second toe. The first toe has its own adductor mechanism (oblique and transverse heads of adductor hallucis) so that only the lateral three toes require adducting interossei. The first, second and third plantar interossei, each arising from the metatarsal bone of its own digit, are inserted by tendons into the medial sides of the third, fourth and fifth digits. The three tendons pass dorsal to the deep transverse ligament of the metatarsal heads and are inserted chiefly into the bases of the proximal phalanges, and also into the dorsal extensor expansion.

The four dorsal abducting muscles abduct away from the line of the second toe. The big and little toes each possess an abductor muscle. The second requires an abducting muscle on each side while the

third and fourth require a single muscle each to abduct the digit laterally, away from the second toe. Each arises from both bones of its own intermetatarsal space. The first is inserted on the big toe side of the second toe, the second, third and fourth on the little toe side of the second, third and fourth toes. The tendons are inserted chiefly into the bases of the proximal phalanges, and each also gives an extension to the dorsal extensor expansion.

Nerve supplies. All interossei are supplied by the lateral plantar nerve. Those of the fourth space are supplied by the superficial branch, all the remainder by the deep branch.

Action. The adducting and abducting actions of the interossei are of little significance in the human foot. It is more important that they assist the lumbricals in extending the interphalangeal joints (through the extensor expansions), and they flex the metatarsophalangeal joints (by their attachments to the proximal phalanges).

The tendon of peroneus longus crosses the sole obliquely (Fig. 3.38). It lies against the posterior ridge of the groove, under the cuboid. At the lateral margin of the foot it contains a sesamoid fibrocartilage which may be ossified. As it crosses the groove of the cuboid it is held in position by the long plantar ligament, which is attached to the anterior and posterior ridges as it bridges the groove. Emerging from this tunnel the tendon proceeds to its insertion at the base of the first metatarsal and adjoining area on the lateral surface of the medial cuneiform. A synovial sheath accompanies it throughout its course.

The tendon of tibialis posterior lies above the sustentaculum tali and spring ligament and is inserted mainly into the tuberosity of the navicular (Fig. 3.44). From its insertion many bands of fibres are traceable to other parts of the foot; usually described as insertions of the muscle, they are rather in the nature of ligaments. They pass to the sustentaculum tali, all three cuneiforms, the floor of the groove in the cuboid and the bases of the second, third and fourth metatarsals (Fig. 3.38).

Vessels and nerves

These are derived from the posterior tibial neurovascular bundle in the calf. The posterior tibial artery and tibial nerve divide, each into medial and lateral plantar branches, under cover of the flexor retinaculum. On the medial and lateral borders of the sole the artery is more marginal than the nerve and each artery is accompanied by a pair of venae comitantes. Where they cross, the nerve is nearer the skin. The plantar arteries and nerves lie between the first and second layers, inferior to the long tendons.

The medial plantar artery is smaller than its fellow; it gives rise to no plantar arch and its digital supply is restricted practically to the big toe. In the hand there are two palmar arches, a superficial from the ulnar artery, and a deep from the radial artery; but in the foot there is only one plantar arch, derived from the lateral plantar artery.

The medial plantar nerve (Fig. 3.37) supplies abductor hallucis, flexor digitorum brevis, flexor hallucis brevis and the first lumbrical; in addition it gives off digital cutaneous branches that supply the medial three and a half toes. Its most lateral cutaneous branch communicates with the neighbouring lateral plantar digital branch across the plantar surface of the fourth metatarsophalangeal joint, where pressure on the nerve may give rise to the painful condition known as metatarsalgia.

The lateral plantar artery crosses the sole obliquely deep to the first layer of the sole, towards the base of the fifth metatarsal bone. It gives off a branch that accompanies the superficial branch of the lateral plantar nerve, but its main trunk accompanies the deep branch of the nerve to form the plantar arch. The plantar arch curves convexly forwards, across the bases of the fourth, third and second metatarsals and is joined in the proximal part of the first intermetatarsal space by the dorsalis pedis artery.

From the convexity of the plantar arch plantar metatarsal arteries run forwards and bifurcate to supply the four webs and digits. Perforating arteries from the plantar arch and its metatarsal arteries reinforce the dorsal metatarsal arteries.

The veins accompanying the perforating arteries take most of the blood from the sole and from the interosseous muscles to the dorsal venous arch. The plantar muscles act as a ‘sole pump’ compressing these veins; this aids the ‘soleal pump’ of the posterior compartment of the calf (see p. 148).

The lateral plantar nerve crosses the sole obliquely medial to the lateral plantar artery (Fig. 3.37). Its course and distribution have similarities with those of the ulnar nerve in the hand. It supplies flexor accessorius and abductor digiti minimi and sends perforating branches through the plantar aponeurosis to supply skin on the lateral side of the sole. Near the base of the fifth metatarsal bone it divides into superficial and deep branches. The superficial branch supplies the fourth cleft and communicates with the medial plantar nerve and, by a lateral branch, supplies the skin of the lateral side and distal dorsum of the little toe. Unlike the superficial branch of the ulnar nerve, this branch supplies three muscles, namely flexor digiti minimi brevis and the two interossei of the fourth space (third plantar and fourth dorsal). The deep branch lies within the concavity of the plantar arch and ends by sinking into the deep surface of the oblique head of adductor hallucis. It gives off branches to the remaining interossei, to the transverse head of adductor hallucis and to the three lateral (bicipital) lumbricals.