Sole of the Foot Anatomy
The sole of the foot contains the plantar aponeurosis, intrinsic muscles, tendons originating from muscles in the leg, and plantar vessels and nerves. The tendons, accompanied by the vessels and nerves, enter the sole deep to the flexor retinaculum (Fig. 6.48) in the tarsal tunnel, between the medial aspect of the calcaneus and the medial malleolus. The skin of the sole is thick and heavily keratinized and is firmly attached to the deep fascia by fibrous septa which traverse and loculate the subcutaneous fat. In the heel, it forms a weight bearing fat pad.
The plantar aponeurosis, the deep fascia of the sole covering the superficial layer of muscles, is especially thick in its central portion. Posteriorly, the aponeurosis is attached to the medial and lateral calcaneal tubercles. Anteriorly, it widens and diverges into five digital slips (Fig. 6.47), which attach to the fibrous flexor sheaths and plantar metatarsal plates (Fig. 6.96). These plates give the aponeurosis indirect attachment to the bases of all the proximal phalanges, providing an important support for the longitudinal arches of the foot (Figs 6.98–6.102). Inflammation of the aponeurosis (plantar fasciitis) is a cause of pain in the foot.
The muscles and tendons in the sole of the foot are arranged in four layers.
This layer comprises three muscles, which span the length of most of the foot and attach to the deep surface of the plantar aponeurosis. Abductor hallucis (Fig. 6.49) lies on the medial side of the foot and arises from the medial calcaneal tubercle and the adjacent flexor retinaculum. Its tendon passes to the great toe with the medial tendon of flexor hallucis brevis and attaches to the medial side of the base of the proximal phalanx. It is supplied by the medial plantar nerve.
Flexor digitorum brevis lies centrally (Fig. 6.49) and attaches to the medial calcaneal tubercle (Fig. 6.50). It has four tendons, which pass forwards to enter the fibrous flexor sheaths of the lateral four toes. Each tendon splits to allow the passage of the long flexor tendon (Figs 6.49 & 6.51), then reunites to attach to the flexor surface of the middle phalanx. Flexor digitorum brevis aids flexion of the proximal parts of the toes and is supplied by the medial plantar nerve. Abductor digiti minimi traverses the lateral border of the sole (Fig. 6.49) and is attached posteriorly to the medial and lateral calcaneal tubercles. Entering the little toe, its tendon attaches to the lateral side of the proximal phalanx. The muscle is supplied by the lateral plantar nerve.
This layer includes the tendons of two long flexor muscles, which arise in the leg, namely flexor hallucis longus and flexor digitorum longus. Attached to the tendon of flexor digitorum longus (Fig. 6.52) are quadratus plantae (flexor accessorius) and the four lumbricals.
The tendon of flexor hallucis longus grooves the posterior surface of the talus and curves forwards inferior to the sustentaculum tali and the spring (plantar calcaneonavicular) ligament. It passes deep to the tendon of flexor digitorum longus, to which it is usually attached. The tendon of flexor hallucis longus continues forwards inferior to the head of the first metatarsal, where it passes between the sesamoid bones in the two tendons of the flexor hallucis brevis (see below) and enters the fibrous flexor sheath of the great toe to reach its attachment to the base of the distal phalanx (Fig. 6.52). The actions and innervation of flexor hallucis longus are described on p. 279.
The tendon of flexor digitorum longus passes forwards medial to the sustentaculum tali and divides into four tendons, one for each of the lateral four toes (Figs 6.51 & 6.52). Each tendon enters its fibrous flexor sheath and passes through the flexor digitorum brevis tendon before attaching to the base of the terminal phalanx. The actions and innervations of flexor digitorum longus are considered on p. 279.
Quadratus plantae (flexor accessorius) (Figs 6.51 & 6.52) is quadrangular and is anchored posteriorly to the medial and lateral tubercles of the calcaneus. Anteriorly, the muscle attaches to the tendons of flexor digitorum longus. By pulling on these tendons, it can flex the toes irrespective of the position of the ankle. Its nerve supply is from the lateral plantar nerve.
The lumbricals are four small muscles attached proximally to the tendons of flexor digitorum longus (Fig. 6.52). Distally, each slender tendon winds around the medial side of the appropriate digit to attach to the extensor expansion and base of the proximal phalanx. The lumbricals extend the lateral four toes but flex the metatarsophalangeal joints. The first lumbrical is supplied by the medial plantar nerve and the remainder by the lateral plantar nerve.
Deep intrinsic muscles
The three muscles in this layer are located in the anterior part of the foot (Fig. 6.53), adjacent to the metatarsals. Flexor hallucis brevis and adductor hallucis lie medially and are attached to the great toe, while flexor digiti minimi brevis lies laterally and acts on the little toe.
Flexor hallucis brevis lies along the medial side of the foot and is attached posteriorly to the cuboid and the three cuneiforms. Passing forwards on either side of the flexor hallucis longus tendon (Fig. 6.52), the muscle gives rise to two short tendons, which lie inferior to the first metatarsophalangeal joint. Each tendon contains a sesamoid bone (Figs 6.101 & 6.102) and enters the great toe to attach to the appropriate side of the base of the proximal phalanx. Flexor hallucis brevis flexes the proximal phalanx and is innervated by the medial plantar nerve.
Adductor hallucis (Fig. 6.53) consists of an oblique head which is attached to the bases of the second, third and fourth metatarsals, and a transverse head which lies across the anterior part of the sole, attached to the deep transverse metatarsal ligaments and the lateral three metatarsophalangeal joints. The two heads converge and attach to the lateral tendon and sesamoid of flexor hallucis brevis. Adductor hallucis assists flexor hallucis brevis and may help to support the transverse arch of the foot. It is supplied by the lateral plantar nerve.
Flexor digiti minimi brevis lies along the plantar surface of the fifth metatarsal (Fig. 6.53), attaching posteriorly to the base of the bone and anteriorly to the lateral side of the base of the proximal phalanx of the little toe. The muscle flexes the proximal phalanx and is innervated by the lateral plantar nerve.
Lying deeply in the sole, attached to the metatarsals, the three plantar interosseous muscles (Fig. 6.54) attach by single heads to the plantar borders of the third, fourth and fifth metatarsals. The four dorsal interossei (Fig. 6.55) are bicipital and attach to contiguous surfaces of the shafts of the metatarsals. The tendons pass forwards dorsal to the deep transverse metatarsal ligaments and attach to the bases of the proximal phalanges and dorsal extensor expansions. These muscles are supplied by the lateral plantar nerve. They assist extension of the phalanges and flexion of the metatarsophalangeal joints. Their attachments indicate that plantar interossei should adduct and dorsal interossei should abduct the toes, but these movements are often poorly developed, especially in the shod foot.
The tendons of fibularis (peroneus) longus and tibialis posterior (Fig. 6.56) pass deeply into the sole to reach their attachments on tarsal and metatarsal bones. The fibularis longus tendon enters the lateral side of the sole and is retained in a groove inferior to the cuboid by the long plantar ligament. The tendon passes forwards and medially to reach the lateral aspect of the base of the first metatarsal and adjacent medial cuneiform (compare with tibialis anterior, p. 288). The tendon of tibialis posterior enters the medial aspect of the foot deep to the flexor retinaculum (Figs 6.48 & 6.56), attaches to the tuberosity of the navicular and sends additional slips to other tarsal bones and the metatarsal bases. These two tendons may help to support the medial longitudinal arch. The other actions of fibularis longus and tibialis posterior and their innervation are considered on pp 279, 290.
Deep to the flexor retinaculum, the tibial artery divides into medial and lateral plantar branches (Fig. 6.51). Passing forwards, the medial plantar artery runs deep to the superficial intrinsic muscles, gives off plantar cutaneous branches, which pass between abductor hallucis and flexor digitorum brevis, and terminates by supplying digital branches to the medial (and occasionally lateral) side of the great toe. The larger lateral plantar artery passes for wards and laterally (Fig. 6.54) deep to the superficial intrinsic muscles towards the base of the fifth metatarsal. Here the artery inclines medially and deeply to form the plantar arch, lying on the interossei and bases of the metatarsals. A perforating branch between the first and second metatarsals usually links the arch with the dorsalis pedis artery (p. 290). From the arch, plantar metatarsal arteries (Fig. 6.54) pass distally to the webs of the toes where they divide into digital branches. Venae comitantes accompany medial and lateral plantar arteries and their branches.
Cutaneous branches of the tibial nerve supply the skin over the heel. Deep to the flexor retinaculum the tibial nerve divides into medial and lateral branches, which supply their respective sides of the sole (Figs 6.48 & 6.51). The larger medial plantar nerve accompanies the corresponding artery and gives cutaneous branches to the medial three and onehalf digits, including the nail beds. The nerve also supplies abductor hallucis, flexor hallucis brevis, flexor digitorum brevis and the first lumbrical.
The lateral plantar nerve accompanies the corresponding artery (Fig. 6.51), supplying quadratus plantae and abductor digiti minimi and giving cutaneous branches to the sole. Near the base of the fifth metatarsal it divides into superficial and deep branches, which supply the remaining muscles of the sole. The deep branch accompanies the plantar arch and terminates in adductor hallucis. In addition to muscular branches, the superficial branch also gives cutaneous branches to the lateral one and onehalf toes.