Ankle and foot

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CHAPTER 84 Ankle and foot

The ankle joint (talocrural joint) is a diarthrodial articulation involving the distal tibia and fibula and the body of the talus: it is the only example in the human body of a true mortise joint. The human foot is a complex structure adapted to allow orthograde bipedal stance and locomotion and is the only part of the body which is in regular contact with the ground. There are 28 separate bones in the human foot, including the sesamoid bones of the first metatarsophalangeal joint and 31 joints, including the ankle joint.

SKIN AND SOFT TISSUE

SKIN

Vascular supply and lymphatic drainage

The skin around the ankle is supplied by anterior lateral and anterior medial malleolar arteries from the anterior tibial artery, medial malleolar branches from the posterior tibial artery, and fasciocutaneous perforators from the anterior and posterior tibial and fibular arteries. The main blood supply to the medial side of the heel is from the medial calcaneal branches of the lateral plantar artery passing through the flexor retinaculum; the skin of the lateral side of the heel is supplied by calcaneal branches of the fibular artery and the lateral tarsal artery. The arterial supply to the skin of the foot is rich and is derived from branches of the dorsalis pedis (the direct continuation of the anterior tibial artery), posterior tibial and fibular arteries. The skin covering the dorsum of the foot is supplied by the dorsalis pedis artery, and by its continuation, the first dorsal metatarsal artery, with smaller contributions from the anterior perforating branch of the fibular artery and the marginal anastomotic arteries on the medial and lateral borders of the foot. The plantar skin is supplied by perforating branches of the medial and lateral plantar arteries (the terminal branches of the posterior tibial artery). The skin of the forefoot is supplied by cutaneous branches of the common digital arteries.

Cutaneous venous drainage is via dorsal and plantar venous arches, which drain into medial and lateral marginal veins. On the plantar aspect, a superficial venous network forms an intradermal and subdermal mesh that drains to the medial and lateral marginal veins. Branches that accompany the medial and lateral plantar arteries arise from a deep venous network. Uniquely within the lower limb, venous flow in the foot is bidirectional. However, when valves are present, flow is from the plantar to the superficial dorsal system. From here, blood leaves the foot in the superficial and deep veins of the lower limb.

Superficial lymphatic drainage is via vessels that accompany the long saphenous vein medially and the short saphenous vein laterally and drain via the inguinal lymph nodes. Deep lymphatic vessels accompany the dorsalis pedis, posterior tibial and fibular arteries and pass via the popliteal lymph nodes.

Cutaneous innervation

The skin covering the ankle and foot is supplied by the fourth and fifth lumbar and first sacral spinal nerves (see Figs 79.18, 79.20). Innervation of the dorsum of the foot is provided medially by the saphenous nerve, centrally by the superficial fibular nerve and laterally by the sural nerve; the deep fibular nerve supplies the dorsum of the first web space. Dorsal branches of the medial and lateral plantar nerves supply the nail beds. The plantar aspect of the foot is supplied by the medial and lateral plantar nerves, which arise as terminal branches of the tibial nerve. The medial plantar nerve supplies sensation to the plantar aspect of the hallux, the second, the third and the medial half of the fourth toes. The lateral plantar nerve supplies the remaining lateral aspect of the fourth and the entire fifth toe. The heel is innervated by calcaneal branches of the tibial nerve. Injury to any of these nerves can lead to painful neuromata and loss of protective sensation. The sural nerve is especially prone to neuroma formation.

SOFT TISSUES

The tendons that cross the ankle joint are all deflected to some degree from a straight course, and must therefore be held down by retinacula and enclosed in synovial sheaths.

Retinacula at the ankle

In the vicinity of the ankle joint, the tendons of the muscles of the leg are bound down by localized, band-shaped thickenings of the deep fascia termed retinacula which collectively serve to prevent bowstringing of the underlying tendons. There are superior and inferior extensor retinacula, superior and inferior fibular retinacula, and a flexor retinaculum.

Extensor retinacula

Superior extensor retinaculum

The superior extensor retinaculum binds down the tendons of tibialis anterior, extensor hallucis longus, extensor digitorum longus and fibularis tertius immediately proximal to the anterior aspect of the talocrural joint (see Fig. 83.7; Fig. 84.1). The anterior tibial vessels and deep fibular nerve pass deep to the retinaculum and the superficial fibular nerve passes superficially. The retinaculum is attached laterally to the distal end of the anterior border of the fibula and medially to the anterior border of the tibia. Its proximal border is continuous with the fascia cruris, and dense connective tissue connects its distal border to the inferior extensor retinaculum. The tendon of tibialis anterior is the only extensor tendon that possesses a synovial sheath at the level of the superior extensor retinaculum.

Inferior extensor retinaculum

The inferior extensor retinaculum is a Y-shaped band lying anterior to the talocrural joint (see Fig. 83.7; Fig. 84.2A,B). The stem of the Y is at the lateral end, where it is attached to the upper surface of the calcaneus, in front of the sulcus calcanei. The band passes medially, forming a strong loop around the tendons of fibularis tertius and extensor digitorum longus (Fig. 84.2A). From the deep surface of the loop, a band passes laterally behind the interosseous talocalcaneal ligament and the cervical ligament and is attached to the sulcus calcanei. At the medial end of the loop, two diverging limbs extend medially to complete the ‘Y’ shape of the retinaculum. The proximal of the two limbs consists of two layers. The deep layer passes deep to the tendons of extensor hallucis longus and tibialis anterior, but superficial to the anterior tibial vessels and deep fibular nerve, to reach the medial malleolus. The superficial layer crosses superficial to the tendon of extensor hallucis longus and then adheres firmly to the deep one; in some cases it continues superficial to the tendon of tibialis anterior, before blending with the deep layer. The distal limb extends downwards and medially and blends with the plantar aponeurosis. It is superficial to the tendons of extensor hallucis longus and tibialis anterior, the dorsalis pedis artery and the terminal branches of the deep fibular nerve.

Synovial sheaths at the ankle

Anterior to the ankle, the sheath for tibialis anterior extends from the proximal margin of the superior extensor retinaculum to the interval between the diverging limbs of the inferior retinaculum (Figs 84.1, 84.2A,B). A common sheath encloses the tendons of extensor digitorum longus and fibularis tertius, starting just above the level of the malleoli, and reaching to the level of the base of the fifth metatarsal bone (Figs 84.1, 84.2A). The sheath for extensor hallucis longus starts at a level just distal to that for extensor digitorum longus and extends as far as the base of the first metatarsal bone (Figs 84.1, 84.2A,B).

Posteromedial to the ankle, the sheath for tibialis posterior starts approximately 4 cm above the medial malleolus and ends just proximal to the attachment of the tendon to the tuberosity of the navicular (Fig. 84.2B). The sheath for flexor hallucis longus starts at the level of the medial malleolus, and extends distally as far as the base of the first metatarsal bone (Fig. 84.2B). Occasionally, as a result of overuse, particularly in ballet dancers where balance on the tips of the toes en pointe involves sustained extreme plantar flexion of the ankle and first toe in the weight-bearing position, a fibrous nodule may develop in the tendon, just proximal to the tendon sheath. This may result in the thickened tendon being caught intermittently in the sheath, causing pain and ‘triggering’ of the great toe, a condition referred to as hallux saltans. Surgical opening of the sheath may be required. In athletes, the muscle belly of flexor hallucis longus may be abnormally large and may extend more distally than usual; it can also catch at the opening of the sheath. The sheath for flexor digitorum longus starts slightly above the level of the medial malleolus and ends at the level of the navicular (Fig. 84.2B).

Posterolateral to the ankle, the tendons of fibularis longus and brevis are enclosed in a sheath that is single proximally but double distally (Fig. 84.2A). From the tip of the lateral malleolus it extends for about 4 cm both proximally and distally.

Plantar fascia

The plantar fascia or aponeurosis is composed of densely compacted collagen fibres orientated mainly longitudinally, but also transversely (Fig. 84.3). Its medial and lateral borders overlie the intrinsic muscles of the hallux and fifth toe respectively, while its dense central part overlies the long and short flexors of the digits.

The central part is the strongest and thickest. The fascia is narrow posteriorly, where it is attached to the medial process of the calcaneal tuberosity proximal to flexor digitorum brevis, and traced distally it becomes broader and somewhat thinner. Just proximal to the level of the metatarsal heads it divides into five bands, one for each toe. As these five digital bands diverge below the metatarsal shafts, they are united by transverse fibres (Fig. 84.3). Proximal, plantar and a little distal to the metatarsal heads and the metatarsophalangeal joints, the superficial stratum of each of the five bands is connected to the dermis by skin ligaments (retinacula cutis). These ligaments reach the skin of the ball of the foot proximal to, and in the floors of, the furrows that separate the toes from the sole: Dupuytren’s disease may involve these ligaments resulting in contractures of the affected digits. The deep stratum of each digital band of the aponeurosis yields two septa that flank the digital flexor tendons and separate them from the lumbricals and the digital vessels and nerves. These septa pass deeply to fuse with the interosseous fascia, the deep transverse metatarsal ligaments (which run between the heads of adjacent metatarsals), the plantar ligaments of the metatarsophalangeal joints, and the periosteum and fibrous flexor sheaths at the base of each proximal phalanx. Pads of fat develop in the webs between the metatarsal heads and the bases of the proximal phalanges; they cushion the digital nerves and vessels from adjoining tendinous structures and extraneous plantar pressures. Just distal to the metatarsal heads, a plantar interdigital ligament (superficial transverse metatarsal ligament) blends progressively with the deep aspect of the superficial stratum of the plantar aponeurosis where it enters the toes (Fig. 84.3). The central part of the plantar aponeurosis thus provides an intermediary structure between the skin and the osteoligamentous framework of the foot via numerous cutaneous retinacula and deep septa that extend to the metatarsals and phalanges. The central part is also continuous with the medial and lateral parts: at the junctions, two intermuscular septa, medial and lateral, extend in oblique vertical planes between the medial, intermediate and lateral groups of plantar muscles to reach bone. Thinner horizontal intermuscular septa, derived from the vertical intermuscular septa, pass between the muscle layers.

The lateral part of the plantar aponeurosis, which covers abductor digiti minimi, is thin distally and thick proximally, where it forms a strong band, sometimes containing muscle fibres, between the lateral process of the calcaneal tuberosity and the base of the fifth metatarsal bone. It is continuous medially with the central part of the aponeurosis, and with the fascia on the dorsum of the foot around its lateral border. The medial part of the plantar aponeurosis, which covers abductor hallucis, is thin. It is continuous proximally with the flexor retinaculum, medially with the fascia dorsalis pedis, and laterally with the central part of the plantar aponeurosis.

Fascial compartments of the foot

There are four main compartments of the plantar aspect of the foot (Jones 1949) (Fig. 84.4). The medial compartment contains abductor hallucis and flexor hallucis brevis, and is bounded inferiorly and medially by the medial part of the plantar aponeurosis and its medial extension, laterally by an intermuscular septum, and dorsally by the first metatarsal. The central compartment contains flexor digitorum brevis, the lumbricals, flexor accessorius and adductor hallucis, and is bounded by the plantar aponeurosis inferiorly, the osseofascial tarsometatarsal structures dorsally and intermuscular septa medially and laterally. The lateral compartment contains abductor digiti minimi and flexor digiti minimi brevis, and its boundaries are the fifth metatarsal dorsally, the plantar aponeurosis inferiorly and laterally, and an intermuscular septum medially. The interosseous compartment contains the seven interossei and its boundaries are the interosseous fascia and the metatarsals.

The dorsal aspect of the foot effectively contains a single compartment which is occupied by the extensor tendons and extensor digitorum brevis, and which is roofed by the deep dorsal fascia (see below).

BONE

Functionally, the skeleton of the foot may be divided into tarsus, metatarsus and phalanges. With regard to nomenclature of the surfaces of the foot, the terms ‘plantar’ and ‘dorsal’ are used, to denote the inferior and superior surfaces respectively. The terms ‘proximal’ and ‘distal’ are used with the same significance as in limbs generally. Rotation of the limb buds that occurs in the early stages of the development of the limbs results in a laterally directed thumb in the hand, and a medially directed great toe in the foot.

DISTAL TIBIA

The distal end of the tibia has anterior, medial, posterior, lateral and distal surfaces, and projects inferomedially as the medial malleolus (see Figs 83.2A,B; 83.3A,B). The distal surface articulates with the talus and is wider anteriorly than posteriorly. It is concave sagittally and slightly convex transversely and continues medially into the malleolar articular surface. The medial malleolus is short and thick and has a smooth lateral surface with a crescentic facet that articulates with the medial surface of the talar body. The distal end of the tibia, including its ossification, is described in detail in Chapter 83.

DISTAL FIBULA

The distal end of the fibula or lateral malleolus projects distally and posteriorly relative to the medial malleolus (see Figs 83.2A,B; 83.3A,B). Its lateral aspect is subcutaneous, the posterior surface has a broad groove with a prominent lateral border, and the anterior surface is rough and somewhat rounded and articulates with the anteroinferior aspect of the tibia. The medial surface has a triangular articular facet, vertically convex with its apex directed distally. It articulates with the lateral talar surface. Behind the facet is a rough malleolar fossa. The distal end of the fibula, including its ossification, is described in detail in Chapter 83.

TARSUS

The seven tarsal bones occupy the proximal half of the foot (Figs 84.5A,B; 84.6). The tarsus and carpus are homologous, but the tarsal elements are larger, reflecting their role in supporting and distributing body weight. As in the carpus, tarsal bones are arranged in proximal and distal rows, but medially there is an additional single intermediate tarsal element, the navicular. The proximal row is made up of the talus and calcaneus; the long axis of the talus is inclined anteromedially and inferiorly, its distally directed head is medial to the calcaneus and at a higher level. The distal row contains, from medial to lateral, the medial, intermediate and lateral cuneiforms and the cuboid. Collectively these bones display an arched transverse alignment that is dorsally convex. Medially, the navicular is interposed between the head of the talus and the cuneiforms. Laterally, the calcaneus articulates with the cuboid.

The tarsus and metatarsus are arranged to form intersecting longitudinal and transverse arches. Hence thrust and weight are not transmitted from the tibia to the ground (or vice versa) directly through the tarsus, but are distributed through the tarsal and metatarsal bones to the ends of the longitudinal arches. For the purposes of description, each tarsal bone is arbitrarily considered to be cuboidal in form, with six surfaces. The ossification sites and dates are summarized in Fig. 84.7.

Talus

The talus is the link between the foot and leg, through the ankle joint (see Figs 84.16 and 84.18).

Body

The body is cuboidal, covered dorsally by a trochlear surface articulating with the distal end of the tibia. It is anteroposteriorly convex, gently concave transversely, widest anteriorly and, therefore, sellar. The triangular lateral surface is smooth and vertically concave for articulation with the lateral malleolus. Superiorly, it is continuous with the trochlear surface; inferiorly its apex is a lateral process. Proximally, the medial surface is (posterosuperiorly) covered by a comma-shaped facet, which is deeper in front and articulates with the medial malleolus. Distally, this surface is rough and contains numerous vascular foramina. The small posterior surface features a rough projection termed the posterior process. The process is marked by an oblique groove between two tubercles which lodges the tendon of flexor hallucis longus. The lateral tubercle is usually larger; the medial is less prominent and immediately behind the sustentaculum tali (Fig. 84.8A). The plantar surface articulates with the middle one-third of the dorsal calcaneal surface by an oval concave facet, its long axis directed distolaterally at an angle of approximately 45° with the median plane. The medial edge of the trochlear surface is straight, but its lateral edge inclines medially in its posterior part and is often broadened into a small elongated triangular area which is in contact with the posterior tibiofibular ligament in dorsiflexion.

image

Fig. 84.8 Skeleton of the foot. A, Foot bones. B, Calcaneus. C, Talus.

(From Drake, Vogl, Mitchell, Tibbitts and Richardson 2008.)

The posterior talofibular ligament is attached to the lateral tubercle of the posterior process. Its attachment extends up to the groove, or depression, between the process and posterior trochlear border. The posterior talocalcaneal ligament is attached to the plantar border of the posterior process. The groove between the tubercles of the process contains the tendon of flexor hallucis longus and continues distally into the groove on the plantar aspect of the sustentaculum tali. The medial talocalcaneal ligament is attached below to the medial tubercle, whereas the most posterior superficial fibres of the deltoid ligament are attached above the tubercle. The deep fibres of the deltoid ligament are attached still higher to the rough area immediately below the comma-shaped articular facet on the medial surface (Fig. 84.8A,C).

Vascular supply

The talar blood supply is rather tenuous because of the lack of muscle attachments. The first comprehensive account of talar blood supply was provided by Wildenauer in 1950. The extraosseous blood supply is via the posterior tibial, dorsalis pedis and fibular arteries (Fig. 84.9). The ‘artery of the tarsal canal’ arises from the posterior tibial artery approximately 1cm proximal to the origin of the medial and lateral plantar arteries (Fig. 84.10) and passes anteriorly between the sheaths of flexor digitorum longus and flexor hallucis longus to enter the tarsal canal in which it lies anteriorly, close to the talus. (The ‘tarsal canal’ is the term that is commonly used to describe the tunnel-shaped medial end of the sinus tarsi.) Branches from the arterial network in the tarsal canal enter the talus. The artery continues through the tarsal canal into the lateral part of the tarsal sinus, where it anastomoses with the artery of the tarsal sinus, forming a vascular sling under the talar neck. A branch of the artery of the tarsal canal known as the deltoid branch passes deep to the deltoid ligament and supplies part of the medial aspect of the talar body. Sometimes it arises from the posterior tibial artery; rarely, it arises from the medial plantar artery. In talar fractures it may be the only remaining arterial supply to the talus to maintain the viability of the talar body. The dorsalis pedis artery supplies branches to the superior aspect of the talar neck and also gives off the artery of the tarsal sinus. This large vessel is always present and anastomoses with the artery of the tarsal canal. The artery of the tarsal sinus receives a contribution from the anterior perforating branch of the fibular artery and supplies direct branches to the talus. The fibular artery provides small branches which form a plexus of vessels posteriorly with branches of the posterior tibial artery, however, the contribution that the fibular artery makes to the talar blood supply is thought to be insignificant.

The intraosseous blood supply of the talar head comes medially from branches of the dorsalis pedis and laterally via vessels that arise from the anastomosis between the arteries of the tarsal canal and tarsal sinus. The middle one-third of the talar body, other than its most superior aspect, and the lateral one-third, other than its posterior aspect, are supplied mainly by the anastomotic arcade in the tarsal canal. The medial one-third of the talar body is supplied by the deltoid branch of the artery of the tarsal canal.

Ossification

A single ossification centre appears prenatally at 6 months (Fig. 84.7). The posterior process (Stieda’s process) is a separate bone in 5% of individuals and arises from a separate ossification centre, which appears between 8 and 11 years. In athletes and dancers, it may be susceptible to impingement against the posterior tibia, resulting in pain and sometimes requires surgical removal. Another accessory bone (although rare) is the os supratalare, which lies on the dorsal aspect of the talus; it rarely measures more than 4 mm in length.

Calcaneus

The calcaneus is the largest tarsal bone and projects posterior to the tibia and fibula as a short lever for muscles of the calf attached to its posterior surface. It is irregularly cuboidal, its long axis being inclined distally upwards and laterally (Fig 84.8A,B). The superior or proximal surface is divisible into three areas. The posterior one-third is rough and concavo-convex; the convexity is transverse and supports fibroadipose tissue (Kager’s fat pad) between the calcaneal tendon and ankle joint. The middle one-third carries the posterior talar facet, which is oval and convex anteroposteriorly. The anterior one-third is partly articular; distal (anterior) to the posterior articular facet, a rough depression, the sulcus calcanei, narrows into a groove on the medial side and completes the sinus tarsi with the talus. (The sinus tarsi is a conical hollow bounded by the talus medially, superiorly and laterally, with the superior surface of the calcaneus below. Its medial end is narrow and tunnel-shaped, and is often referred to as the tarsal canal.) Distal and medial to this groove, an elongated articular area covers the sustentaculum tali and extends distolaterally on the body of the bone. This facet is often divided into middle and anterior talar facets by a non-articular interval at the anterior limit of the sustentaculum tali (the incidence of this subdivision varies with sex, race and occupation). Rarely, all three facets on the upper surface of the calcaneus are fused into one irregular area. A detailed analysis of patterns of anterior talar articular facets in a series of 401 Indian calcanei revealed four types. Type I (67%) showed one continuous facet on the sustentaculum extending to the distomedial calcaneal corner; type II (26%) presented two facets, one sustentacular and one distal calcaneal; type III (5%) possessed only a single sustentacular facet; and type IV (2%) showed confluent anterior and posterior facets.

The anterior surface is the smallest, and is an obliquely set concavo-convex articular facet for the cuboid. The posterior surface is divided into three regions: a smooth proximal (superior) area separated from the calcaneal tendon by a bursa and adipose tissue; a middle area, which is the largest, limited above by a groove and below by a rough ridge for the calcaneal tendon; a distal (inferior) area inclined downwards and forwards, vertically striated, which is the subcutaneous weight-bearing surface.

The plantar surface is rough, especially proximally as the calcaneal tuberosity, the lateral and medial processes of which extend distally, separated by a notch. The medial process is longer and broader (Fig. 84.8B). Further distally, an anterior tubercle marks the distal limit of the attachment of the long plantar ligament.

The lateral surface is almost flat. It is proximally deeper and palpable on the lateral aspect of the heel distal to the lateral malleolus. Distally, it presents the fibular trochlea (Fig. 84.8A,B), which is exceedingly variable in size and palpable 2 cm distal to the lateral malleolus when well developed. It bears an oblique groove for the tendon of fibularis longus and a shallower proximal groove for the tendon of fibularis brevis. About 1 cm or more behind and above the fibular trochlea, a second elevation may exist for attachment of the calcaneofibular part of the lateral ligament.

The medial surface is vertically concave, and its concavity is accentuated by the sustentaculum tali, which projects medially from the distal part of its upper border (Fig. 84.8B). Superiorly, the process bears the middle talar facets and inferiorly a groove which is continuous with that on the talar posterior surface for the tendon of flexor hallucis longus (Fig. 84.8A,B). The medial aspect of the sustentaculum tali can be felt immediately distal to the tip of the medial malleolus; occasionally it is also grooved by the tendon of flexor digitorum longus.

Muscle and ligament attachments

The interosseous talocalcaneal and cervical ligaments and the medial root of the inferior extensor retinaculum are attached in the calcaneal sulcus. The non-articular area distal to the posterior talar facet is the site of attachment of extensor digitorum brevis (in part), the principal band of the inferior extensor retinaculum and the stem of the bifurcate ligament.

Abductor hallucis and the superficial part of the flexor retinaculum and, distally, the plantar aponeurosis and flexor digitorum brevis, are all attached to the medial process of the calcaneal tuberosity at its prominent medial margin. Abductor digiti minimi is attached to the lateral process, extending medially to the medial process. The long plantar ligament is attached to the rough region between the processes proximally, and extends to the anterior tubercle distally. The short plantar ligament is attached to the tubercle and the area distal to it. The lateral tendinous head of flexor accessorius is attached distal to the lateral process near the lateral margin of the long plantar ligament. Plantaris is attached to the posterior surface near the medial side of the calcaneal tendon. The anterior part of the lateral surface is crossed by the fibular tendons, but is largely subcutaneous. The calcaneofibular ligament is attached 1–2 cm proximal to the fibular trochlea, usually to a low, rounded elevation.

The dorsal surface of the sustentaculum tali is part of the talocalcaneonavicular joint; its plantar surface is grooved by the tendon of flexor hallucis longus and margins of the groove give attachment to the deep part of the flexor retinaculum. The plantar calcaneonavicular ligament is attached distally to the medial margin of the sustentaculum, which is narrow, rough and convex. A slip from the tendon of tibialis posterior, and superficial fibres of the deltoid ligament and medial talocalcaneal ligaments, are attached proximally. Distal to the attachment of the deltoid ligament, the tendon of flexor digitorum longus is related to the margin of the sustentaculum and may groove it. The large medial head of flexor accessorius is attached distal to the groove for flexor hallucis longus.

Navicular

The navicular articulates with the talar head proximally and with the cuneiform bones distally (Figs 84.8A,B; 84.11). Its distal surface is transversely convex and divided into three facets (the medial being the largest) for articulation with the cuneiforms. The proximal surface is oval and concave and articulates with the talar head. The dorsal surface is rough and convex. The medial surface is also rough and projects proximally as a prominent tuberosity, palpable approximately 2.5 cm distal and plantar to the medial malleolus. The plantar surface, rough and concave, is separated from the tuberosity medially by a groove. The lateral surface is rough, irregular and often bears a facet for articulation with the cuboid.

The facet for articulation with the medial cuneiform is roughly triangular, its rounded apex is medial and its ‘base’, facing laterally, is often markedly curved; the articular facets for the intermediate and lateral cuneiforms are also triangular, with plantar facing apices. The facet for the lateral cuneiform may approach a wide crescent or a semicircle rather than a triangle (Fig. 84.11). Dorsal talonavicular, cuneonavicular and cubonavicular ligaments are attached to the dorsal navicular surface.

Ossification

The navicular ossification centre appears during the third year (Fig. 84.7). It is sometimes affected by avascular necrosis between the ages of 4 and 7 years (Köhler’s disease). An accessory navicular bone, which is considered an anatomic variant, occurs in approximately 5% of individuals. It arises from a separate ossification centre in the region of the navicular tuberosity. There are three distinct types of accessory navicular. Type I is probably a sesamoid bone within the plantar aspect of the tendon of tibialis posterior at the level of the inferior calcaneonavicular ligament. In type II, the accessory bone is separated from the body of the navicular by a synchondrosis. Type III is commonly called ‘the cornuate navicular,’ where the accessory bone is united to the navicular by a bony ridge, and may represent the possible end stage of type II. An accessory navicular may be the source of pain in athletes. Type II is the most commonly symptomatic variant: it has been suggested that the pull of the tendon of tibialis posterior, the degree of foot pronation, and the location of the accessory bone in relation to the undersurface of the navicular may produce tension, shear, and/or compression forces on the synchondrosis.

Rarely, the navicular is bipartite and it arises from two distinct centres of ossification. This can lead to premature degeneration within the talocalcaneonavicular joint (Muller–Weiss disease). Occasionally, a small bone is found within the talocalcaneonavicular joint on its dorsal aspect. Referred to as an os talonaviculare dorsale, it represents either a separate accessory bone or a fractured osteophyte of the proximal dorsal aspect of the navicular.

Cuboid

The cuboid, the most lateral bone in the distal tarsal row, lies between the calcaneus proximally and the fourth and fifth metatarsals distally (Fig. 84.11). Its dorsolateral surface is rough for the attachment of ligaments. The plantar surface is crossed distally by an oblique groove for the tendon of fibularis longus and bounded proximally by a ridge that ends laterally in the tuberosity of the cuboid, the lateral aspect of which is faceted for a sesamoid bone or cartilage that is frequently found in the tendon of fibularis longus. Proximal to its ridge, the rough plantar surface extends proximally and medially because of the obliquity of the calcaneocuboid joint, making its medial border much longer than the lateral. The lateral surface is rough; the groove for fibularis longus extends from a deep notch on its plantar edge. The medial surface, which is much more extensive and partly non-articular, bears an oval facet for articulation with the lateral cuneiform, and proximal to this another facet (sometimes absent) for articulation with the navicular: the two form a continuous surface separated by a smooth vertical ridge. The distal surface is divided vertically into a medial quadrilateral articular area for the fourth metatarsal base and a lateral triangular area, its apex lateral, for the fifth metatarsal base. The proximal surface, triangular and concavo-convex, articulates with the distal calcaneal surface; its medial plantar angle projects proximally and inferior to the distal end of the calcaneus.

Cuneiforms

The wedge-like cuneiform bones articulate with the navicular proximally and with the bases of the first to third metatarsals distally; the medial cuneiform is the largest, the intermediate the smallest. The dorsal surfaces of the intermediate and lateral cuneiforms form the base of the wedge. The wedge is reversed in the medial cuneiform, which is a prime factor in shaping the transverse arch. The proximal surfaces of all three form a concavity for the distal surface of the navicular. The medial and lateral cuneiforms project distally beyond the intermediate cuneiform and so form a recess for the second metatarsal base.

Medial cuneiform

The medial cuneiform (Figs 84.5A,B; 84.11) articulates with the navicular and first metatarsal base. It has a rough, narrow dorsal surface. The distal surface is a reniform facet for the first metatarsal base, its ‘hilum’ being lateral. The proximal surface bears a piriform facet for the navicular, which is concave vertically and dorsally narrowed. The medial surface, rough and subcutaneous, is vertically convex; its distal plantar angle carries a large impression which receives the principal attachment of the tendon of tibialis anterior (Fig. 84.5B). The lateral surface is partly non-articular; there is a smooth right-angled strip along its proximal and dorsal margins for the intermediate cuneiform. Its distal dorsal area is separated by a vertical ridge from a small, almost square, facet for articulation with the dorsal part of the medial surface of the second metatarsal base. Plantar to this, the medial cuneiform is attached to the medial side of the second metatarsal base by a strong ligament. Proximally, an interosseous intercuneiform ligament connects this surface to the intermediate cuneiform. The distal and plantar area of the surface is roughened by attachment of part of the tendon of fibularis longus (Fig. 84.5B).

Intermediate cuneiform

The intermediate cuneiform articulates proximally with the navicular and distally with the second metatarsal base (Figs 84.5A; 84.11). It has a narrow, plantar surface that receives a slip from the tendon of tibialis posterior. The distal and proximal surfaces are both triangular articular facets and articulate with the second metatarsal base and the navicular, respectively. The medial surface is partly articular: it articulates via a smooth, angled strip that is occasionally double with the medial cuneiform along its proximal and dorsal margins. The lateral surface is also partly articular: along its proximal margin a vertical strip, usually indented, abuts the lateral cuneiform. Strong interosseous ligaments connect non-articular parts of both surfaces to the adjacent cuneiforms.

Lateral cuneiform

The lateral cuneiform is between the intermediate cuneiform and cuboid, and also articulates with the navicular and, distally, with the third metatarsal base (Figs 84.5A; 84.11). Like the intermediate cuneiform, its dorsal surface, which is rough and almost rectangular, is the base of a wedge. The plantar surface is narrow and receives a slip from tibialis posterior and sometimes part of flexor hallucis brevis. The distal surface is a triangular articular facet for the third metatarsal base. The proximal surface is rough on its plantar aspect, but its dorsal two-thirds articulate with the navicular by a triangular facet. The medial surface is partly non-articular and has a vertical strip, indented by the intermediate cuneiform, on its proximal margin; on its distal margin, a narrower strip (often two small facets) articulates with the lateral side of the second metatarsal base. The lateral surface, also partly non-articular, bears a triangular or oval proximal facet for the cuboid; a semilunar facet on its dorsal and distal margin articulates with the dorsal part of the medial side of the fourth metatarsal base. Non-articular areas of the medial and lateral surfaces receive intercuneiform and cuneocuboid ligaments, respectively, which are important in the maintenance of the transverse arch.

Tarsal coalition

Tarsal coalition is a hereditary condition in which there is a fibrous, cartilaginous or osseous union of two or more tarsal bones, and is believed to arise as a result of a failure of segmentation of primitive mesenchyme. Harris & Beath (1948) were the first to recognize an association between tarsal coalitions and ‘peroneal (fibular) spastic flat foot’. The two most common examples are talocalcaneal and calcaneonavicular coalitions, which usually present with symptoms early in the second decade of life. They are often, but not invariably, associated with flat feet (see below). A talonavicular coalition is rare, but when present is often associated with a ‘ball and socket’ ankle joint. Surgical resection of tarsal coalitions may eradicate associated pain but seldom improves the range of movement.

METATARSALS

The five metatarsal bones lie in the distal half of the foot and connect the tarsus and phalanges. Like the metacarpals, they are miniature long bones, and have a shaft, proximal base and distal head. Except for the first and fifth, the shafts are long and slender, longitudinally convex dorsally, and concave on their plantar aspects. Prismatic in section, they taper distally. Their bases articulate with the distal tarsal row and with adjacent metatarsal bases. The line of each tarsometatarsal joint, except the first, inclines proximally and laterally, metatarsal bases being oblique relative to their shafts. The heads articulate with the proximal phalanges, each by a convex surface that passes farther on to its plantar aspect, where it ends on the summits of two eminences. The sides of the heads are flat, with a depression surmounted by a dorsal tubercle for a collateral ligament of the metatarsophalangeal joint.

On occasion, an os intermetatarseum is encountered between the medial cuneiform and the bases of the first and second metatarsal bones and represents a rare accessory bone in this region.

Individual metatarsals

First metatarsal

The first metatarsal (Fig. 84.5A,B; 84.11) is the shortest and thickest, and has a strong shaft, of marked prismatic form. The base sometimes has a lateral facet or ill-defined smooth area as a result of contact with the second metatarsal. Its large proximal surface, usually indented on the medial and lateral margins, articulates with the medial cuneiform. Its circumference is grooved for tarsometatarsal ligaments and, medially, part of the tendon of tibialis anterior is attached; its plantar angle has a rough, oval, lateral prominence for the tendon of fibularis longus. The medial head of the first dorsal interosseous is attached to the flat lateral surface of the shaft. The large head has a plantar elevation, the crista, which separates two grooved facets (of which the medial is larger), on which sesamoid bones glide.

Second metatarsal

The second metatarsal is the longest (Figs 84.5A,B; 84.11). Its cuneiform base bears four articular facets. The proximal one, concave and triangular, is for the intermediate cuneiform. The dorsomedial one, for the medial cuneiform, is variable in size and usually continuous with that for the intermediate cuneiform. Two lateral facets, dorsal and plantar, are separated by non-articular bone, each divided by a ridge into distal demifacets, which articulate with the third metatarsal base, and a proximal pair (sometimes continuous) for the lateral cuneiform. The areas of these facets vary, particularly the plantar facet, which may be absent. An oval pressure facet, caused by contact with the first metatarsal, may appear on the medial side of the base, plantar to that for the medial cuneiform. Because of its length, its steep inclination, and the position of its base recessed in the tarsometatarsal joint, it is at risk of stress overload; perhaps this is why it is a common site for stress fractures in athletes and an avascular phenomenon in its head (Freiberg’s infraction).

Third metatarsal

The third metatarsal (Figs 84.5A,B; 84.11) has a flat triangular base, articulating proximally with the lateral cuneiform, medially with the second metatarsal, via dorsal and plantar facets, and laterally, via a single facet, with the dorsal angle of the fourth metatarsal. The medial plantar facet is frequently absent. The third tarsometatarsal joint is relatively immobile and predisposes the third metatarsal to stress fracture.

Fourth metatarsal

The fourth metatarsal is smaller than the third (Figs 84.5A,B; 84.11). Its base has proximally, an oblique quadrilateral facet for articulation with the cuboid; laterally, a single facet for the fifth metatarsal; medially, an oval facet for the third metatarsal. The latter is sometimes divided by a ridge, in which case the proximal part articulates with the lateral cuneiform.

Fifth metatarsal

The fifth metatarsal has a tuberosity (styloid process) on the lateral side of its base (Figs 84.5A,B; 84.11). The base articulates proximally with the cuboid by a triangular, oblique surface, and medially with the fourth metatarsal. The tuberosity can be seen and felt midway along the lateral border of the foot; in acute inversion it may be fractured. The metaphysial–diaphysial junction of the fifth metatarsal base is prone to traumatic or stress fractures, and these have a tendency to delayed and non-union, and often require surgical fixation. It is believed that fractures at this level damage the nutrient artery and the extraosseous plexus, resulting in compromised vascularity of the fracture site and consequent poor fracture healing.

PHALANGES OF THE FOOT

In general, the phalanges of the foot resemble those of the hand: there are two in the hallux, and three in each of the other toes (Fig. 84.5A,B). On occasion there are only two phalanges in the little toe and, rarely, this is the case with the other lesser toes. The phalanges of the toes are much shorter than their counterparts in the hand and their shafts, especially those of the proximal set, are compressed from side to side. In the proximal phalanges, the compressed shaft is convex dorsally, with a plantar concavity. The base is concave for articulation with a metatarsal head and the head is a trochlea for a middle phalanx. Middle phalanges are small and short, but broader than their proximal counterparts. Distal phalanges resemble those in the fingers, but are smaller and flatter. Each has a broad base for articulation with a middle phalanx and an expanded distal end. A rough tuberosity on the plantar aspect of the latter supports the pulp of the toe, and provides a weight-bearing area.

Muscle attachments

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