Chapter 17: The ankle and foot
The word ankle refers to the angle between the leg and the foot. The foot functions in support and in locomotion, whereas the hand is a tactile and grasping organ. The toes are numbered from one to five, beginning with the great toe, or hallux. Thus, the pre-axial digit in either the hand or the foot is numbered one. The terms abduction and adduction of the toes are used with reference to an axis through the second toe. Thus, abduction of the big toe is a medial movement, away from the second toe. The tendons around the ankle (similar to those at the wrist) are bound down by retinacula (see fig. 17-4).
The fascia on the sole of the foot is a strong sheet termed the plantar aponeurosis, which acts as a mechanical tie. It extends anteriorly from the calcaneal tuberosity and divides into five processes, each of which is anchored at a metatarsophalangeal joint. Fascial "spaces" are situated superior to the plantar aponeurosis, and the big and little toes have special compartments. Synovial sheaths are found (1) anterior to the ankle (fig. 17-1) for the tendons of the (a) tibialis anterior, (b) extensor hallucis longus, and (c) extensor digitorum longus and fibularis tertius; (2) posterior to the medial malleolus for the tendons of the (a) tibialis posterior, (b) flexor digitorum longus, and (c) flexor hallucis longus; and (3) posterior to the lateral malleolus for the tendons of the fibularis longus and brevis. Some further sheaths are found in the sole and in relation to the toes.
Muscles of foot (table 17-1)
The extensor digitorum brevis is the only muscle on the dorsum of the foot. It and the extensor hallucis longus tendon can be felt and sometimes seen when dorsiflexing the proximal phalanges against resistance, and these actions are used to test the integrity of the fifth lumbar nerve (L5) or the fibular nerve.
The muscles of the sole are collectively important in posture and locomotion, and they provide strong support for the arches of the foot during movement. They may be considered in three groups: for the big toe (medially), the central portion of the sole, and the little toe (laterally). They may also be considered, however, in four layers (table 17-1 and fig. 17-2).
The flexor digitorum brevis resembles the flexor digitorum superficialis of the upper limb, in that each muscle has four tendons (omitting the first digit), which are perforated by the long flexor tendons and then divide to be inserted into the sides of the middle phalanges.
Although classified as dorsal and plantar, this is a relative position and both groups of interossei are actually more plantar. The lumbricals and interossei are arranged in a manner basically similar to those of the hand but are organized around the axis of the second toe as compared with that of the third finger. There are, however, some structural and functional differences between the interossei of the hand and of the feet. For example, the interossei of the foot are not inserted into the extensor aponeuroses, and they probably strengthen the metatarsal arch by holding the metatarsals together.
The plantar reflex is a superficial reflex that consists of (plantar) flexion of the toes when the skin of the sole is stroked slowly along its lateral border. In infants before they walk and in patients with certain disorders of the motor pathways of the brain and spinal cord, however, similar stimulation of the sole results in a slow dorsiflexion of the great toe and a slight spreading of the other toes. This response is known as the Babinski sign.
The medial plantar nerve, the larger terminal branch of the tibial nerve, arises posterior to the medial malleolus, deep to the flexor retinaculum and the abductor hallucis (see fig. 17-4A). It runs anteriorly between the abductor hallucis and the flexor digitorum brevis and supplies these muscles (see fig. 15-2) as well as the skin on the medial side of the foot. It ends as plantar digital nerves that supply the flexor hallucis brevis, the first lumbrical, and the skin of the medial toes, including their nail beds. The medial plantar nerve is comparable to the median nerve in the hand.
The lateral plantar nerve arises posterior to the medial malleolus. It runs anteriorly and laterally, deep to the flexor digitorum brevis, and divides into superficial and deep branches. It supplies the quadratus plantae and abductor digiti minimi muscles, as well as the lateral side of the sole. The superficial branch supplies the flexor digiti minimi brevis muscle and gives rise to plantar digital nerves to the lateral toes. The deep branch turns medially and supplies the interossei, lumbricals 2 to 4, and the adductor hallucis. The lateral plantar nerve is comparable to the ulnar nerve in the hand.
Vessels of foot (fig. 17-3)
The medial plantar artery, one of the terminal branches of the posterior tibial artery, arises deep to the flexor retinaculum and the abductor hallucis muscle. It runs anteriorly with its companion nerve and gives digital branches to the medial toes (fig. 17-4A).
The lateral plantar artery, with its companion nerve, runs anteriorly and laterally, deep to the flexor digitorum brevis muscle. It then turns medially and forms the plantar arch, which lies between the third and fourth layers of the muscles of the sole. The arch gives off a series of metatarsal and digital arteries.
The dorsal artery of the foot, variable in size and course, is the continuation of the anterior tibial artery at a point midway between the malleoli (fig. 17-4C). This artery extends to the posterior end of the first intermetatarsal space. The dorsal artery of the foot is important clinically in assessing peripheral circulation. Its pulsations should be sought, and can generally be felt, between the tendons of the extensor hallucis longus and extensor digitorum longus (fig. 17-4C). The artery is crossed by the inferior extensor retinaculum and extensor hallucis brevis. It lies successively on the capsule of the ankle joint, the head of the talus, the navicular, and the intermediate cuneiform. Its branches form an arterial network on the dorsum of the foot. The tendon of the extensor hallucis longus crosses either the anterior tibial artery or the dorsal artery of the foot and comes to lie on the medial side of the latter. The dorsal artery of the foot ends in a deep plantar branch, which passes to the sole between the heads of the first dorsal interosseus and completes the plantar arch.
A strong fibrous union exists between the lower ends of the tibia and fibula. It consists of an interosseous ligament, strengthened in its anterior and posterior aspects by anterior and posterior tibiofibular ligaments, and a transverse ligament from the malleolar fossa of the fibula to the posterior aspect of the tibia. A recess of the ankle joint often extends upward into the lower portion of the syndesmosis.
The ankle, or talocrural, joint is a hinge joint between (1) the tibia and fibula, which form a socket, and (2) the trochlea of the talus (see fig. 12-29). The jooint capsule is thickened on each side by a strong ligament. The medial, or deltoid, ligament (fig. 17-5) runs from the medial malleolus to the talus, navicular, and calcaneus. It is crossed by the tendons, vessels, and nerves that have entered the foot by passing posterior to the medial malleolus. The lateral ligaments (fig. 17-5) consists of (1) the anterior talofibular ligament, between the neck of the talus and the lateral malleolus; (2) the calcaneofibular ligament; and (3) the posterior talofibular ligament, between the talus and the malleolar fossa. The medial and lateral ligaments prevent anterior and posterior slipping of the talus. They may be torn in injuries to the ankle, with the lateral ligaments being significantly weaker and more liable to sprain. If the ligaments do not yield, one or both malleoli may be broken off in dislocations of the ankle joint. The shape of the surfaces at the ankle is such that, aided by the ligaments of the tibiofibular syndesmosis, the malleoli grip the talus tightly in dorsiflexion. Therefore, most sprains of the ankle occur with the ankle in some degree of flexion.
The ankle joint allows dorsiflexion and plantar flexion (fig. 17-6) around an axis that passes approximately through the malleoli. The range of movement varies. The triceps surae and fibularis longus muscles plantar-flex the foot. The tibialis anterior and extensor digitorum longus muscles dorsiflex the foot (see fig. 16-5). It should be appreciated that, physiologically, plantar flexion of the foot and toes is an extensor response, whereas dorsiflexion of the foot and toes is a flexor response.
The talus moves with the foot during dorsiflexion and plantar flexion. However, during inversion and eversion, which occur at intertarsal joints, the talus moves very little. The most important intertarsal joints are the subtalar, the talocalcaneonavicular, and the calcaneocuboid. The last two constitute the transverse tarsal, or midtarsal, joint. The transverse tarsal joint can be represented by a line from the posterior aspect of the tuberosity of the navicular to the midpoint between the lateral malleolus and the tuberosity of the fifth metatarsal. The other intertarsaljoints are the cuneocuboid, intercuneiform, and cuneonavicular, all of which are plane joints.
The subtalar joint (figs. 12-36 and 17-7) is a separate talocalcanean articulation lying posterior to the tarsal canal. The talocalcaneonavicular joint, a part of the transverse tarsal joint, lies in front of the tarsal canal. It resembles a ball-and-socket joint in that the head of the talus fits into a socket formed by the navicular on the anterior side, the calcaneus on the inferior side, and the plantar calcaneonavicular ligament in between (fig. 17-7). This band, frequently termed the spring ligament (figs. 17-5 and 17-8), connects the sustentaculum tali with the navicular, and the tibialis posterior tendon lies immediately inferior to it. The other part of the transverse tarsal joint is the calcaneocuboid, which resembles a limited saddle joint. A strong bifurcate ligament extends from the floor of the tarsal sinus (on the supeior aspect of the calcaneus) to the navicular and cuboid bones. (The tarsal sinus is the expanded anterolateral end of the tarsal canal, which runs obliquely between the talus and calcaneus.)
The foot may be disarticulated at the ankle joint (Syme's amputation) or at the transverse tarsal joint (Chopart's amputation).
The tension that develops during the support of body weight is taken up by strong ligaments on the plantar aspect of the tarsus (figs. 17-8 and 17-9). The long plantar ligament extends from the plantar aspect of the calcaneus to the tuberosity of the cuboid. The short plantar ligament, also extending from the calcaneus to the cuboid, is more deeply placed. We have already discussed the plantar calcaneonavicular (spring) ligament that is involved in supporting the talus.
The chief movements of the foot distal to the ankle joint are inversion and eversion. In inversion, the sole of the foot is directed medially. In eversion, the sole is turned so that it faces laterally (see fig. 17-6). Inversion and eversion occur mainly at the subtalar and transverse tarsal joints. The axes of movement at these articulations are situated obliquely with reference to the standard anatomical planes. Hence, each movement is a combination of two or more primary movements. Inversion comprises supination, adduction, and plantar flexion. Eversion involves pronation, abduction, and dorsiflexion. Usage is unfortunately variable, but supination and pronation of the foot generally refer to medial and lateral rotation about an anteroposterior axis. Abduction and adduction refer to movements of the anterior part of foot about a vertical axis. The tibialis posterior and anterior muscles invert the foot. The fibularis and extensor digitorum longus muscles evert the foot (see fig. 16-5).
The tarsometatarsal and intermetatarsal joints are plane articulations that allow gliding. The medial cuneiform and first metatarsal have an independent joint cavity (see fig. 12-31A). The second metatarsal fits into a socket formed by the three cuneiforms (see fig. 12-32). The metatarsophalangeal joints are ellipsoid, and the interphalangeal joints are hinge, but the ligamentous arrangements of both are similar. Collateral ligaments are present, as are fibrous or fibrocartilaginous pads termed plantar ligaments (cf. palmar ligaments of fingers). The pads are interconnected by the deep transverse metatarsal ligament, which helps to hold the metatarsal heads together. The metatarsophalangealjoints allow flexion, extension, abduction, and adduction. The interphalangeal joints permit flexion and extension. The metatarsophalangeal joint ofthe big toe is specialized. Two grooves on the plantar aspect of the head of the first metatarsal articulate with the sesamoids that are embedded in the plantar ligament (fig. 17-9). The sesamoids are attached to the plantar aponeurosis and anchored to the phalanx. The sesamoids of the big toe take the weight of the body, especially during the latter part of the stance phase of walking. The sesamoid mechanism is deranged in bunions and in hallux valgus. A bunion is a swelling medial to the joint, and it is due to bursal thickening. In hallux valgus the big toe is displaced laterally because of angulation at the metatarsophalangeal joint.
The arches of the foot (figs. 17-9, 17-10, 17-11 and 17-12) are the longitudinal and the transverse. The longitudinal arch is formed medially by the calcaneus, talus, navicular, cuneiforms, and first to third metatarsals and laterally by the calcaneus, cuboid, and fourth and fifth metatarsals. The transverse, or metatarsal, arch is formed by the navicular, cuneiforms, cuboid, and first to fifth metatarsals. These osseous arches depend on the mechanical arrangement of the bones, but they are supported by ligaments and, during movement, by muscles, especially the invertors and evertors. The arches develop during fetal life, but they are masked by a fat pad, which makes the sole convex in the newborn. The medial arch can be recognized in the footprints of most adults, but the extent of contact between the sole and the ground does not necessarily indicate precisely the height of the bony arches. The term "flatfoot" (pes planus) is used for several conditions, including a simple depression of the longitudinal arch, which in many individuals is not pathological (fig. 17-10B). The converse is pes cavus, in which the longitudinal arch is very high (fig. 17-10C). The term "clubfoot" (talipes) is used for a foot that appears twisted out of shape or position. The commonest variety of congenital clubfoot comprises plantar flexion, supination, and adduction (talipes equinovarus).
Inman, V. T., The Joints of the Ankle, Williams & Wilkins, Baltimore, 1976. Biomechanical studies of the ankle and subtalar joints.
Jones, F. W., Structure and Function as Seen in the Foot, 2nd ed., Bailliere, Tindall, and Cox, London, 1949. A very readable classic.
17-1 What is the plantar reflex?
17-2 What is the Babinski sign?
17-3 How far distally do (a) the femoral, (b) the obturator, and (c) the sciatic cutaneous territories extend?
17-4 In terms of dermatomes, which spinal nerves supply the hand and the foot?
17-5 Which vessels are used in seeking a pulse at the ankle and foot?
17-6 Which structures are situated between the medial malleolus and the heel?
17-7 Which structures cross the anterior aspect of the ankle joint?
17-8 Which structures are situated posterior to the lateral malleolus?
17-9 What is the deltoid ligament of the ankle joint?
17-10 What is the clinical importance of the lateral ligament of the ankle joint?
17-11 Which are the most important intertarsal joints and which important movements occur at them?
17-12 Which is the most frequent type of clubfoot?
17-13 What is the significance of the adjectives varus and valgus?
Figure 17-1 The synovial tendon sheaths of the foot and ankle. The more distal and somewhat less constant sheaths of the extensor digitorum longus and brevis have been omitted.
Figure 17-2 Muscles of the sole of the foot, shown in successive layers from inferior to superior. A, The first, or most superficial, layer. B, The second layer. C, The third layer, including the peroneus longus, the insertion of which belongs to the fourth layer. See fig. 17-8 for the fourth layer.
Figure 17-3 The arteries of the sole and dorsum of the foot.
Figure 17-4 The structures on (A) the medial, (B) the lateral, and (C) the anterior portions of the ankle. The various retinacula are shown, but the synovial sheaths (see fig. 17-1) are not indicated. The posterior tibial artery is situated (in A) between the medial malleolus and the calcaneal tendon. The dorsal artery of the foot is found (in C) between the digitorum and hallucis tendons. The pulsations of these arteries are sought in clinical examinations of the lower limb.
Figure 17-5 The ligaments of the ankle joint. The medial view shows the medial ligament, which forms a dense, almost continuous deltoid ligament. The ligaments on the lateral side, however, are usually separated from one another. Note the sinus tarsi in the lateral view.
Figure 17-6 Movements of the foot and ankle. Dorsiflexion and plantar flexion are shown as in walking up and down hill. Movement occurs at the ankle joint. Eversion and inversion are shown as in standing sideways on a hill. Movement occurs at the tarsal joints, the talus remaining fixed. (Based on Mollier.)
Figure 17-7 The facets of the ankle, subtalar, and talocalcaneonavicular joints. A, Diagram of the talus from above to show the three-surfaced trochlea that fits into the mortise formed by the lower ends of the tibia and fibula. B, Diagram of the calcaneus from above to show the posterior facet (P) for the subtalar joint, separated by the canalis and sinus tarsi from the middle (M) and anterior (A) facets of the talocalcaneonavicular joint. The socket of this latter joint is completed by the spring ligament and the concavity of the navicular. C, Diagram of the talus from below to show its corresponding facets for the subtalar and calcaneonavicular joints. Cf. fig. 12-36. A broad arrow in A emphasizes that the head of the talus is directed anteromedially.
Figure 17-8 The tendons and ligaments of the foot, plantar aspect. Note the widespread insertion of the tibialis posterior. The fibularis longus tendon crosses the sole obliquely to reach the medial cuneiform, to which the tibialis anterior is also attached: the two muscles thus form a sling or stirrup.
Figure 17-9 Schematic representation of the plantar aponeurosis and the long plantar ligament.
Figure 17-10 Footprints. A, Normal. B, Flatfoot. C, High longitudinal arch.
Figure 17-11 A, The three main points of weight bearing in the foot. B, The medial part of the longitudinal arch. The arrows indicate the distribution of weight that tends to flatten the arch. The connection between the calcaneus and the metatarsal represents schematically the ligamentous support of the arch. (Based on Mollier.)
Figure 17-12 The bony components of the longitudinal arches. The third figure shows both arches.