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
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
Retinacula at the ankle
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.
Flexor retinaculum
The flexor retinaculum is attached anteriorly to the tip of the medial malleolus, distal to which it is continuous with the deep fascia on the dorsum of the foot (Fig. 84.2B). From its malleolar attachment it extends posteroinferiorly to the medial process of the calcaneus and the plantar aponeurosis. Proximally, there is no clear demarcation between its border and the deep fascia of the lower leg, especially the deep transverse layer of the deep fascia. Distally, its border is continuous with the plantar aponeurosis, and many fibres of abductor hallucis are attached to it. The flexor retinaculum converts grooves on the tibia and calcaneus into canals for the tendons, and bridges over the posterior tibial vessels and tibial nerve. As these structures enter the sole they are, from medial to lateral, the tendons of tibialis posterior and flexor digitorum longus, the posterior tibial vessels, the tibial nerve and the tendon of flexor hallucis longus (see Fig. 84.14).
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.
Compartment syndrome in the foot
A compartment syndrome results from an increase in intracompartmental pressure sufficient to impair venous outflow from that compartment. As blood enters at arterial pressure, the compartment pressure increases further until it exceeds arterial pressure, at which point inflow of arterial blood ceases, leading to muscle and nerve ischaemia. Failure to relieve the increased pressure in the compartment surgically results in necrosis of the soft tissues within the compartment. The most common cause of compartment syndrome in the foot is trauma, usually of high-energy (high impact) type: crush injuries, calcaneal fractures and disruption of the tarsometatarsal joints are the usual antecedents associated with compartment syndrome in the foot.
BONE
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).
Fig. 84.16 Coronal section through the left ankle and talocalcaneal joint (seen from behind).
(From Sobotta 2006.)
Directed distally and somewhat inferomedially, the head has a distal surface which is ovoid and convex; its long axis is also inclined inferomedially to articulate with the proximal navicular surface. The plantar surface of the head has three articular areas, separated by smooth ridges. The most posterior and largest is oval, slightly convex and rests on a shelf-like medial calcaneal projection, the sustentaculum tali. Anterolateral to this and usually continuous with it, a flat articular facet rests on the anteromedial part of the dorsal (proximal) calcaneal surface; distally it continues into the navicular surface. Between the two calcaneal facets, a part of the talar head, covered with articular cartilage, is in contact with the plantar calcaneonavicular ligament, which is covered here, superiorly, by fibrocartilage (see Fig. 84.15A,B). When the foot is inverted passively, the dorsolateral aspect of the head is visible and palpable approximately 3 cm distal to the tibia; it is hidden by extensor tendons when the toes are dorsiflexed.
The long axis of the neck, inclined downwards, distally and medially, makes an angle of approximately 150° with that of the body; it is smaller (130–140°) at birth, accounting in part for the inverted foot in young children. The dorsal talonavicular ligament and ankle articular capsule are attached distally to its dorsal surface. Thus the proximal part of this surface lies within the capsule of the ankle joint. The medial articular facet of the talar body and part of the trochlear surface may extend onto the neck. The anterior talofibular ligament is attached on the lateral aspect of the neck, spreading along the adjacent anterior border of the lateral surface. The interosseous talocalcaneal and cervical ligaments are attached to the inferior surface of the neck. A dorsolateral, so-called ‘squatting facet’ is commonly present on the talar neck in those individuals who habitually adopt the squatting position: it articulates with the anterior tibial margin in extreme dorsiflexion and may be double.
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.
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).
No muscles are attached to the talus. However, many ligaments are attached to the bone, and these confer stability to the ankle, subtalar and talocalcaneonavicular joints (Fig. 84.8A–C).
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.
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.
The talus has no muscular attachments and 70% of its surface is covered by articular cartilage: displaced talar neck fractures, where the blood supply to the talar body is interrupted, can therefore result in avascular necrosis and non-union. In general, these complications do not occur in undisplaced fractures. If a subchondral lucency is seen in the talar dome on radiographs at 8 weeks after fracture of the talar neck (Hawkin’s sign), it may be assumed that vascularity to the talar body is intact and that the fracture is likely to heal satisfactorily.
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 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.
The calcaneus is the only tarsal bone that always has two ossification centres (Fig. 84.7). In addition to the main ossification centre, there is a scale-like posterior apophysis that covers most of the posterior, and part of the plantar, surfaces. The main centre appears prenatally in the third month, whereas the posterior apophysis appears in the sixth year in females and the eighth in males, fusing in the 14th and 16th years, respectively.
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.
Muscle and ligament attachments
The navicular tuberosity is the main attachment of tibialis posterior and a groove lateral to it transmits part of the tendon distally to the cuneiforms and middle three metatarsal bases. The plantar calcaneonavicular ligament is attached to a slight projection lateral to the groove and adjacent to the proximal surface. The calcaneonavicular part of the bifurcate ligament is attached to the rough part of the lateral surface (Fig. 84.5B).
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.
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
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
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.
The first metatarsal has two centres of ossification, one in the shaft, the other in the base (unlike the other metatarsals, in which the secondary ossification centre is distal). They appear during the tenth week of prenatal life and the third year of life, respectively (Fig. 84.7) and fuse between the 17th and 20th years. There may be a third centre in the first metatarsal head.
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).
There are two centres of ossification, one in the shaft and one distally in the metatarsal head (Fig. 84.7). Ossification of the shaft starts during the ninth prenatal week and ossification of the metatarsal head starts between the third and fourth years; fusion occurs between the 17th and 20th years.
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.
There are two centres of ossification, one in the shaft and one distally in the metatarsal head (Fig. 84.7). Ossification of the shaft starts during the ninth prenatal week and ossification of the metatarsal head starts between the third and fourth years; fusion occurs between the 17th and 20th years.
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.
There are two centres of ossification, one in the shaft and one distally in the metatarsal head (Fig. 84.7). Ossification of the shaft starts during the ninth prenatal week and ossification of the metatarsal head commences between the third and fourth years; fusion occurs between the 17th and 20th years.
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.
There are three centres of ossification, one at the base in the region of the tuberosity (an apophysis), one in the shaft and one distally in the metatarsal head. Ossification of the shaft starts during the tenth prenatal week and ossification of the metatarsal head starts between the third and fourth years (Fig. 84.7). Fusion of the distal and shaft centres occurs between the 17th and 20th years, the proximal apophysis fuses earlier. An os vesalianum is a rare variant that should not be confused with the basal apophysis. (Fig. 84.12)
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.
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