Ankle Joint Anatomy
The ankle joint is a synovial hinge
joint between the lower ends of the tibia and fibula and the upper part of the
talus (Fig. 6.84), and all the articular surfaces are
covered by hyaline cartilage. The
proximal articular surface comprises the distal end of the tibia and the medial
and lateral malleoli, which together form a deep socket (Fig. 6.85), completed posteriorly by the posterior
tibiofibular ligament (see below). The socket is wider anteriorly than
posteriorly and is completely congruous with the upper part of the talus, which
is reciprocally wedgeshaped (Figs 6.84 & 6.86). The articular surface on the lateral side of
the talus is more extensive than that on the medial side.
The fibrous capsule attaches to the
margins of the articular surfaces, but anteriorly extends forwards onto the
neck of the talus (Fig. 6.86). The capsule is thin anteriorly and posteriorly,
but is reinforced on each side by ligaments. Synovial membrane lines the
capsule internally and covers the intracapsular part of the neck of the talus.
Ligaments
The posterior tibiofibular ligament
spans the gap between the distal ends of the tibia and fibula, contributing to
the articular socket posteriorly (Fig. 6.85). There are two collateral ligaments. The
medial (deltoid) ligament (Fig. 6.89) is attached by its apex to the tip of the
medial malleolus. Its deeper fibres descend to the margin of the articular
surface on the medial side of the talus and its longer superficial fibres
attach to the tuberosity of the navicular, the medial border of the spring
(plantar calcaneonavicular) ligament and the sustentaculum tali.
The lateral ligament has three
components: the anterior and posterior talofibular and the calcaneofibular ligaments
(Figs 6.87 & 6.88). All attach to the lateral malleolus. The
anterior talofibular ligament passes forwards to the lateral side of the neck
of the talus, the posterior talofibular ligament medially to the posterior
tubercle of the talus, and the calcaneofibular downwards and backwards to the
side of the calcaneus. Tearing of the medial and/or lateral collateral
ligaments occurs with fractures of one or both malleoli.
They are known by the clinical
eponym of Pott’s fracture.
Movements
Only extension (dorsiflexion) and
flexion (plantar flexion) occur at the ankle joint, around a transverse axis
between the malleoli. Plantar flexion is produced by soleus and gastrocnemius,
assisted by tibialis posterior, flexors hallucis longus and digitorum longus,
and fibularis longus and brevis. Extension (dorsiflexion) is produced by
tibialis anterior, extensors hallucis longus and digitorum longus, and
fibularis tertius.
Stability
The joint is very stable due to the
wedge shape of the articulating surfaces and the strong collateral ligaments.
During standing and walking, body weight tends to dis place the tibiofibular
socket forwards so that it becomes closely packed against the wider anterior
part of the talus, which further enhances stability during dorsiflex ion.
Excessive forward displacement of the tibia and fibula on the talus is
prevented by the posterior fibres of the medial (deltoid) ligament and by the
calcaneofibular and posterior talofibular ligaments. However, in plantar
flexion the narrow part of the talus articulates with the wider anterior part
of the socket, allowing some sidetoside movement. In this position, forced
inversion of the foot may damage the anterior talofibular ligament, one form of
sprained ankle.
Branches of the anterior and
posterior tibial arteries, including the fibular, anastomose at the level of
the malleoli and supply the joint. Innervation is from the deep fibular and
tibial nerves.
Relations
The sides of the joint, the bony
malleoli, are superficial (Fig. 6.90) and easily palpable. Passing subcutaneously
in front of the joint are branches of the superficial fibular nerve and, just
anterior to the medial malleolus, the saphenous nerve and great saphenous vein.
On a deeper plane are the tendons of tibialis anterior and extensor hallucis
longus, the dorsalis pedis artery, the deep fibular nerve, and the tendons of
extensor digitorum longus and fibularis tertius (Fig. 6.91).
Posteriorly, the tendo calcaneus
(Achilles) lies separated from the joint capsule by a bursa and pad of fat.
Behind the medial malleolus are the tendons of tibialis posterior, flexor
digitorum longus and flexor hallucis longus (Fig. 6.90), accompanied by the tibial nerve and
posterior tibial artery (Fig. 6.91). Passing below the medial malleolus, they
enter the foot beneath the flexor retinaculum. Passing superficially behind the
lateral malleolus are the small saphenous vein and sural nerve and, more
deeply, the tendons of fibularis longus and brevis (Fig. 6.91).
