Applied anatomy of the lower leg, ankle and foot

Definitions

The functional terminology in the foot is very misleading, because the same terms are often used for very different things. The following definitions and explanations are used here:

• Plantiflexion–dorsiflexion indicate movement at the ankle. Plantiflexion is movement downwards towards the ground, and dorsiflexion upwards towards the tibia. There is also a plantiflexion–dorsiflexion movement at the midtarsal joint.

• Varus–valgus: these movements occur at the subtalar joint. In valgus, the calcaneus is rotated outwards on the talus; in varus, the calcaneus is rotated inwards on the talus.

• Abduction–adduction: these take place at the midtarsal joints. Abduction moves the forefoot laterally and adduction moves the forefoot medially on the midfoot.

• Pronation–supination are also movements at the midtarsal joints. In pronation, the forefoot is rotated big toe downwards and little toe upwards. In supination, the reverse happens: the big toe rotates upwards and the little toe downwards. Sometimes the terms internal rotation and external rotation are used to indicate supination or pronation.

• Inversion–eversion: these are combinations of three movements. In inversion, the varus movement at the subtalar joint is combined with an adduction and supination movement at the midtarsal joint. During eversion, a valgus movement at the subtalar joint is combined with an abduction and pronation movement at the midtarsal joint.

The leg

The bones (tibia and fibula) together with the interosseous membrane and fasciae divide the leg into three separate compartments: anterolateral, lateral and posterior. They contain the so-called extrinsic foot muscles. Each compartment has its own blood supply and innervation.

The ankle and foot

The 26 bones of the foot create an architectural vault, supported by three arches and resting on the ground at three points, which lie at the corners of an equilateral triangle (Fig. 2). Ligaments bind the bones to provide the static stability of the foot. The dynamic stability of the vault is achieved by the intrinsic and extrinsic muscles.

Fig 2 The three arches of the foot.

Three functional segments can be distinguished: forefoot, midfoot and ankle (Fig. 3). Each has its particular functions and specific disorders (see Standring, Fig. 84.8A). The posterior segment (ankle) is the connection between foot and leg. It lies directly under the tibia and fibula and is connected to them by strong ligaments. This segment contains the talus (in the mortice between the fibula and the tibia) and the calcaneus.

Fig 3 The three segments of the foot: 1, phalanges; 2, metatarsals; 3–5, medial, intermediate and lateral cuneiforms; 6, navicular; 7, cuboid; 8, talus; 9, calcaneus.

The middle segment (midfoot) contains the five tarsal bones and is the keystone in the plantar vault. Excessive stresses falling on the vault therefore influence the midfoot in particular.

The anterior segment (five metatarsals and 14 phalangeal bones) takes the whole body weight during the ‘heel-off’ phase in walking or running. This specific function of the forefoot underlies pressure disorders at the plantar surface of the metatarsals and toes.

The posterior segment

The talus is the mechanical keystone at the ankle (Fig. 4). It distributes the body weight backwards towards the heel and forwards towards the midfoot (medial arch of the plantar vault). The talus has no muscular insertions but is entirely covered by articular surfaces and ligamentous reinforcements. It is also crossed by the tendons of the extrinsic muscles of the foot.

Fig 4 The talus distributes the body weight to the heel and midfoot.

Ankle joint

The superior surface and the two sides of the body of the talus are gripped between fibular and tibial malleoli, forming the ankle mortice (see Standring, Fig. 84.16). Stability of this tibiofibular mortice is provided by the anterior and posterior tibiofibular ligaments and the interosseous ligament. Because the body of the talus is wedge shaped, with the wider portion anterior, no varus–valgus movement will be possible with an ankle held in dorsiflexion, unless the malleoli or the tibiofibular ligaments are damaged (Fig. 5).

Fig 5 The body of the talus is wedge shaped with a wider anterior portion.

The normal movement of the ankle joint is plantiflexion–dorsiflexion, through an axis that passes transversely through the body of the talus. The lateral end of this axis goes through the lateral malleolus (Fig. 6). Hence, a normal plantiflexion–dorsiflexion movement does not influence the tightness of the calcaneofibular ligament. At the medial end, however, the axis is placed under the tip of the medial malleolus and thus under the malleolar point of attachment of the medial ligaments. Here, the posterior fibres of the deltoid ligament will become taut on dorsiflexion and the anterior fibres on plantiflexion (Fig. 7).

Fig 6 Axis of plantiflexion–dorsiflexion (X–X′): 1, lateral malleolus; 2, talus; 3, calcaneus.

Fig 7 The anterior fibres of the deltoid ligament become taut during plantiflexion (a) and the posterior fibres during dorsiflexion (b).

Subtalar joint

The subtalar or talocalcaneal joint consists of two separate parts, divided by the tarsal canal (Fig. 8), which is funnel shaped with the wide portion at its lateral end. The lateral end of the canal is easily palpated in front of the fibular malleolus between talus and calcaneus, especially when the foot is inverted. The canal runs posteromedially, to have its medial opening just behind and above the sustentaculum tali. In the canal a strong ligament, the interosseous talocalcaneal ligament, binds the two bones (see Standring, Fig. 84.15).