Part eleven. Ankle and foot joints

Ankle joint

The ankle joint, or talocrural joint, is a synovial joint of the modified hinge variety. The axis of rotation is not fixed but changes between the extremes of plantarflexion and dorsiflexion. The articulating surfaces are covered with hyaline cartilage. The weight-bearing surfaces are the upper trochlear facet of the talus and the inferior facet of the tibia (Fig. 3.39). Stabilizing surfaces are those of the medial and lateral malleoli, which grip the sides of the talus. The joint is enclosed in a capsule lined with synovial membrane. The capsule is attached to the articular margins of all three bones except the anterior part of the talus, where it is fixed some distance in front of the articular margin, on the neck of the bone. Posteriorly the capsule, on its way up to the tibia, is attached also to the posterior tibiofibular ligament.

Figure 3.39 Radiographs of the ankle joint: A anteroposterior projection; B lateral projection.

(Provided by Dr R. Sinnatamby, Addenbrooke's Hospital, Cambridge.)

The synovial membrane is attached to the articular margin of the talus and clothes the intracapsular part of the neck. Elsewhere it is attached to all articular margins and lines the inside of the capsule.

Strong medial and lateral ligaments strengthen the joint. The deltoid ligament, on the medial side, is in two layers. The deep part (Fig. 3.40) is a narrow band extending from the tibial malleolus to the side of the talus. The superficial part is triangular, like a delta. It fans downwards from the borders of the tibial malleolus and its lower margin has a continuous attachment from the medial tubercle of the talus along the edge of the sustentaculum tali and spring ligament to the tuberosity of the navicular (Fig. 3.43).

Figure 3.40 Left ankle and heel from behind.

Figure 3.43 Plantar ligaments of the left foot. On the medial side the deltoid ligament of the ankle joint fuses with the spring ligament.

On the lateral side there are three separate bands, radiating from the lateral malleolus, which are collectively commonly called the lateral ligament. Anterior and posterior bands pass to the talus, the intermediate band to the calcaneus. The anterior talofibular ligament (Fig. 3.41) joins the anterior border of the lateral malleolus to the neck of the talus. The calcaneofibular ligament (Fig. 3.41) extends from the tip of the malleolus down and back to the lateral surface of the calcaneus. The posterior talofibular ligament (Fig. 3.40) lies horizontally between the malleolar fossa of the fibula and the lateral tubercle of the talus. Above it lies the posterior tibiofibular ligament, whose lower part (also called the inferior transverse ligament) is covered by hyaline cartilage and articulates with the talus. In plantarflexion these two ligaments lie edge to edge, but in dorsiflexion they diverge like the blades of an opening pair of scissors.

Figure 3.41 Left ankle from the lateral side.

The blood supply of the capsule and ligaments is derived from anterior and posterior tibial arteries and the peroneal artery, and the nerve supply is by the deep peroneal and tibial nerves.

Movements

The upper facet of the talus, slightly concave from side to side, is convex anteroposteriorly. It is broad in front and narrow behind. In full dorsiflexion the broad anterior articular area of the talus is grasped by the mortice formed by the malleoli and the inferior surface of the tibia. This requires widening of the malleolar gap through slight lateral rotation of the fibula by stretching at the inferior tibiofibular syndesmosis and gliding at the superior tibiofibular joint. In full plantarflexion the smallest area of the talus is in contact with the malleoli and distal tibia, but even in this position inversion and eversion are not possible at the ankle. For all practical purposes the ankle may be regarded as a true hinge joint.

The axis of rotation is not horizontal, but slopes downwards and laterally. It passes through the lateral surface of the talus just below the apex of the triangular articular area and through the medial surface at a higher level, just below the concavity of the comma-shaped articular area. It passes through the malleoli just above their apices. The axis changes during movement, for the upper convexity of the talus is not the arc of a circle, but rather of an ellipse. The obliquity of the axis involves a slight movement resembling inversion in full plantarflexion, and the reverse, resembling slight eversion, in full dorsiflexion, but these apparent movements are not of true inversion and eversion.

From the upright position, with the foot at right angles to the leg, active plantar flexion of about 20° is produced by gastrocnemius and soleus, assisted by the long flexor tendons and the long and short peronei. Active dorsiflexion of about 10° is produced by tibialis anterior, the long toe extensors and peroneus tertius. The degree of passive movements possible is approximately double the above.

Surgical approach

The ankle joint can be exposed from the front between the tendons of extensor hallucis and digitorum longus, avoiding damage to the intervening deep peroneal nerve and anterior tibial vessels. For an approach behind the medial malleolus the tendons of tibialis posterior and flexor digitorum longus are displaced forwards, while on the lateral side the peroneus longus and brevis can be displaced forwards to reach the capsule behind the lateral malleolus. For aspiration the joint can be entered in front of the lateral malleolus and lateral to the tendon of peroneus tertius, or in front of the medial

malleolus medial to tibialis anterior. The joint line should be defined by moving the foot. Arthroscopy utilises the same routes of access to the ankle joint via anterolateral and anteromedial portals.

Tarsal joints

The most important joints in the tarsus are those between the talus, calcaneus and navicular and between the calcaneus and cuboid.

On the undersurface of the talus there are two separate joints. At the back is the talocalcanean joint, where the upper surface of the calcaneus articulates with the undersurface of the talus. In front of this is a more complicated joint, with part of the undersurface of the head of the talus articulating with the upper surface of the sustentaculum tali and body of the calcaneus and the spring ligament, and the front of the head of the talus articulating with the navicular (Fig. 3.42). The whole joint with its single synovial cavity is called talocalcaneonavicular.

Figure 3.42 Bones of the left foot from above, separated at the midtarsal joint. The insertions of all tendons producing inversion and eversion of the foot are in front of this joint.

The talocalcaneonavicular joint is a synovial joint of the ball and socket variety. The ball is the head of the talus (Fig. 3.42) and the socket comprises the navicular and calcaneus and the spring ligament. The posterior surface of the navicular has an articular surface which is concave reciprocally with the anterior convexity of the head of the talus. The anterior end of the upper surface of the calcaneus has a concave facet, and the sustentaculum tali a similar one for articulation with the inferior convexity of the head of the talus. Between these navicular and calcanean surfaces the head of the talus articulates with the fibrocartilaginous upper surface of the spring ligament. All these structures are enclosed in a single capsule.

The talocalcanean joint lies behind the talocalcaneonavicular joint. It is a synovial joint between the concave facet on the undersurface of the talus and the convex facet on the upper surface of the calcaneus (Fig. 3.55).

Figure 3.55 Left talus and calcaneus, as seen when the talus is lifted off the calcaneus and turned over to show its undersurface.

The calcaneocuboid joint is a separate synovial joint between the front of the calcaneus and the back of the cuboid; this and the talonavicular part of the talocalcaneonavicular joint form what is usually called the midtarsal joint (Fig. 3.42). The calcaneocuboid joint is surrounded by a capsule, thickened above and below. The long and short plantar ligaments are accessory ligaments on its plantar surface. Simple gliding movement takes place at this joint during inversion and eversion of the foot.

The short plantar ligament (properly called the plantar calcaneocuboid) is a thick bundle which fills in the adjacent hollows in front of the anterior tubercle of the calcaneus and behind the posterior ridge of the cuboid (Figs 3.43 and 3.57). It is covered over by the long plantar ligament.

The long plantar ligament (Fig. 3.43) is attached to the plantar surface of the calcaneus anterior to its tuberosity, and to the anterior tubercle of that bone. From here it extends forwards to cover the short plantar ligament, and its deeper fibres are attached to the posterior ridge of the cuboid. Its superficial fibres bridge the groove of the cuboid, making a fibrous roof over the peroneus longus tendon, and are attached to the anterior ridge of the cuboid and extend forwards to the bases of the central three metatarsal bones. It is covered by flexor accessorius, and its posterior part is visible in the gap between the medial fleshy and lateral tendinous heads of that muscle.

Various other smaller but stronger interosseous plantar ligaments unite adjacent bones and help to support the arches of the foot.

The plantar calcaneonavicular (spring) ligament is a very strong band that connects the anterior edge of the sustentaculum tali to the plantar surface of the navicular (Fig. 3.43). Its upper surface articulates with the head of the talus, and bears a fibrocartilaginous facet for this purpose. Its lower fibres extend well under the sustentaculum tali and lie almost transversely across the foot. Like ligaments in general, it consists of collagenous tissue and, despite its common name ‘spring ligament’, it is not elastic.

The bifurcate ligament (Fig. 3.41) arises from the upper surface of the calcaneus, under cover of the extensor digitorum brevis muscle at the front of the tarsal sinus. From this origin two limbs diverge from each other. The medial limb is attached to the navicular and the lateral limb is attached to the cuboid.

The tarsal sinus lies obliquely between the talocalcaneonavicular joint and the talocalcanean joint. It is a cylindrical canal that opens anteriorly at its lateral end like the broad end of a funnel. The sinus is occupied by a strong, flat bilaminar interosseous talocalcanean ligament. The central portion of each bony gutter is perforated by vascular foramina. At the lateral end of the sinus is the cervical ligament between the neck of the talus and the upper surface of the calcaneus (Fig. 3.41).

Inversion and eversion of the foot

Inversion and eversion of the foot occur at the subtalar and midtarsal joints, more movement occurring at the former joint. (Adduction and supination of the forefoot accompany inversion; abduction and pronation of the forefoot accompany eversion.) The range of inversion is increased in plantarflexion, and the fully inverted foot is also plantarflexed. The more restricted movement of eversion is linked in an allied manner with dorsiflexion.

The movement of inversion (raising the medial border of the foot) is produced by any muscle that is attached to the medial side of the foot. Tibialis anterior and tibialis posterior are responsible, assisted by extensor and flexor hallucis longus on occasion.

The movement of eversion (raising the lateral border of the foot) is produced by any muscle that is attached to, or pulls upwards upon, the lateral side of the foot. Peroneus longus, brevis, and tertius are responsible.

All the muscles producing inversion and eversion are attached to the fore part of the foot, anterior to the midtarsal joint (Fig. 3.42). The calcaneus and cuboid are firmly connected by the long and short plantar ligaments, which limit the range of mobility at the midtarsal joint; when they and the spring ligament are taut they transmit the rotatory force to the calcaneus. This bone then rotates (i.e. inverts or everts) under the talus, which is firmly wedged against the tibia between the malleoli and cannot therefore be inverted or everted.

The axis of inversion–eversion movement is along an oblique line (Fig. 3.44) passing from the lateral tubercle of the calcaneus upwards, forwards and medially through the neck of the talus and the medial part of the tarsal sinus. The lines of pull of the muscles lie at right angles to this oblique axis. The balanced actions of these muscles combine in different patterns to produce ordered movements of the ankle and tarsal joints.

Forefoot joints

The metatarsus is much more rigid than the metacarpus. The first tarsometatarsal joint (between the medial cuneiform and the first metatarsal) possesses its own capsule and synovial membrane and is capable of some movement in a vertical plane to conform with movements in the medial longitudinal arch of the foot, and it becomes hyperextended in flat foot; but the joint movements in no way compare with those of the carpometacarpal joint of the thumb and no opposition of the big toe is possible. The second tarsometatarsal joint is immobile, the base of the metatarsal being firmly fixed between the anterior ends of the medial and lateral cuneiforms, as it articulates with the intermediate cuneiform. This is a result of the shifting of the axis of the foot towards its medial side, and the second toe forms the line for adduction and abduction of the digits. The immobility of the second metatarsal and the slenderness of its shaft are contributory factors in ‘spontaneous’ fracture (‘march fracture’) of this

bone.

The first metatarsophalangeal joint is the site of hallux valgus. The big toe has no dorsal extensor expansion nor fibrous flexor sheath; its long tendons are held in position by strands of deep fascia. If the phalanges become displaced laterally and the fibrous bands give way, the pull of extensor hallucis longus, like that of extensor hallucis brevis, becomes oblique to the long axis of the toe and tends to increase the deformity.

The interphalangeal joints are similar to those of the hand, with capsules and collateral ligaments (see p. 91).

Supporting mechanisms of the foot

In the erect position the heel, lateral margin of the foot, the ball of the foot (the part underneath the metatarsal heads) and the pads of the distal phalanges touch the ground. The medial margin of the foot arches up between the heel and the ball of the big toe, forming a visible and obvious medial longitudinal arch. The lateral margin of the foot is in contact with the ground, but its constituent bones do not bear with equal pressure on the ground. As on the medial side, so, on the lateral side, there is a bony longitudinal arch extending from the heel to the heads of the metatarsal bones, but the lateral longitudinal arch is much flatter than the medial.

The bones that form the medial longitudinal arch are calcaneus, talus, navicular, the three cuneiform bones and their three metatarsal bones. The pillars of the arch are the tuberosity of the calcaneus posteriorly and the heads of the medial three metatarsal bones anteriorly. The lateral longitudinal arch consists of calcaneus, cuboid and the lateral two metatarsal bones.

The transverse arch is, in reality, only half an arch, being completed as an arch by that of the other foot. It consists of the bases of the five metatarsal bones and the adjacent cuboid and cuneiforms. The heads of all five metatarsal bones lie flat upon the ground, but the first and fifth heads bear more weight than the others.

The factors maintaining the integrity of the arches of the foot are identical with those responsible in any joint of the body, namely, bony, ligamentous and muscular, but their relative importance is different in the three arches.

Medial longitudinal arch

Bony factors do not play a significant role in maintaining the stability of this arch. The head of the talus is supported on the sustentaculum tali, but this is a negligible factor. Ligaments are important, but are unable to maintain the arch entirely on their own. The most important ligament is the plantar aponeurosis, stretching like a tie beam or bowstring between the supporting pillars of the arch. When it is shortened as by extension of the toes, especially the big toe, it draws the pillars together and so heightens the arch. Next in importance is the spring ligament, for it supports the head of the talus. If it stretches it allows the navicular and calcaneus to separate and so the head of the talus, the highest point of the arch, sinks lower between them. Other interosseous ligaments also contribute towards maintaining the arch.

Muscles are indispensable to the maintenance of the medial longitudinal arch. The most important

joint and, indeed, the contraction of soleus and gastrocnemius is the chief factor responsible for propulsion in walking, running and jumping. But the propulsive action of these great muscles of the calf is enhanced by arching of the foot and flexion of the toes. Thus the tendo calcaneus plantarflexes a very mobile, not a rigid, foot. The sequence of events in walking is usually described as four phases: heel-strike, support (stance or weight-bearing), toe-off (push off) and swing (Fig. 3.45). The weight of the foot is taken successively on the heel, lateral border, and the ball of the foot, and the last part to leave the ground is the anterior pillar of the medial longitudinal arch and the medial three digits. In running the heel remains off the ground, the toes and the forefoot taking the thrust of the weight; but the take-off point is still the anterior pillar of the medial longitudinal arch.

Figure 3.45 Phasic action of the pretibial muscle group during walking. The contraction during heel-strike (shock-absorbing) is much greater than that needed for toe clearance during the swingthrough phase.

At the moment of heel-strike, the extensors of the foot and toes contract and then gradually relax to prevent the toes from slapping on the ground. While the heel is rising from the ground the medial toes are gradually extended. The extended toes elongate the flexor hallucis longus and flexor digitorum longus muscles, and this increases the force of their subsequent contraction.

Contraction of the toe flexors, long and short, heightens the medial longitudinal arch and increases the force of the take-off by the pressure of the toes on the ground. Meanwhile, the lumbricals prevent the toes from buckling under when pulled upon by flexor digitorum longus. Contraction of the long flexors also aids plantarflexion at the ankle joint.