The base of each proximal phalanx articulates with the head of the related metatarsal.
The head of each distal phalanx is nonarticular and flattened into a crescent-shaped plantar tuberosity under the plantar pad at the end of the digit.
In each toe, the total length of the phalanges combined is much shorter than the length of the associated metatarsal.
The ankle joint is synovial in type and involves the talus of the foot and the tibia and fibula of the leg (Fig. 6.102).
The ankle joint mainly allows hinge-like dorsiflexion and plantarflexion of the foot on the leg.
The distal end of the fibula is firmly anchored to the larger distal end of the tibia by strong ligaments. Together, the fibula and tibia create a deep bracket-shaped socket for the upper expanded part of the body of the talus:
The roof of the socket is formed by the inferior surface of the distal end of the tibia.
The medial side of the socket is formed by the medial malleolus of the tibia.
The longer lateral side of the socket is formed by the lateral malleolus of the fibula.
The articular surfaces are covered by hyaline cartilage.
The articular part of the talus is shaped like a short half-cylinder tipped onto its flat side with one end facing lateral and the other end facing medial. The curved upper surface of the half-cylinder and the two ends are covered by hyaline cartilage and fit into the bracket-shaped socket formed by the distal ends of the tibia and fibula.
When viewed from above, the articular surface of the talus is much wider anteriorly than it is posteriorly. As a result, the bone fits tighter into its socket when the foot is dorsiflexed and the wider surface of the talus moves into the ankle joint than when the foot is plantarflexed and the narrower part of the talus is in the joint. The joint is therefore most stable when the foot is dorsiflexed.
The articular cavity is enclosed by a synovial membrane, which attaches around the margins of the articular surfaces, and by a fibrous membrane, which covers the synovial membrane and is also attached to the adjacent bones.
The ankle joint is stabilized by medial (deltoid) and lateral ligaments.
The medial (deltoid) ligament is large, strong (Fig. 6.103), and triangular in shape. Its apex is attached above to the medial malleolus and its broad base is attached below to a line that extends from the tuberosity of the navicular bone in front to the medial tubercle of the talus behind.
The medial ligament is subdivided into four parts based on the inferior points of attachment:
The part that attaches in front to the tuberosity of the navicular and the associated margin of the plantar calcaneonavicular ligament (spring ligament), which connects the navicular bone to the sustentaculum tali of the calcaneus bone behind, is the tibionavicular part of the medial ligament.
The tibiocalcaneal part, which is more central, attaches to the sustentaculum tali of the calcaneus bone.
The posterior tibiotalar part attaches to the medial side and medial tubercle of the talus.
The fourth part (the anterior tibiotalar part) is deep to the tibionavicular and tibiocalcaneal parts of the medial ligament and attaches to the medial surface of the talus.
The lateral ligament of the ankle is composed of three separate ligaments, the anterior talofibular ligament, the posterior talofibular ligament, and the calcaneofibular ligament (Fig. 6.104):
The anterior talofibular ligament is a short ligament, and attaches the anterior margin of the lateral malleolus to the adjacent region of the talus.
The posterior talofibular ligament runs horizontally backward and medially from the malleolar fossa on the medial side of the lateral malleolus to the posterior process of the talus.