The interphalangeal joints are hinge joints that allow mainly flexion and extension. They are reinforced by medial and lateral collateral ligaments and by plantar ligaments (Fig. 6.109).
Tarsal tunnel, retinacula, and arrangement of major structures at the ankle
The tarsal tunnel is formed on the posteromedial side of the ankle by: a depression formed by the medial malleolus of the tibia, the medial and posterior surfaces of the talus, the medial surface of the calcaneus, and the inferior surface of the sustentaculum tali of the calcaneus; and an overlying flexor retinaculum (Fig. 6.110).
The flexor retinaculum is a strap-like layer of connective tissue that spans the bony depression formed by the medial malleolus, the medial and posterior surfaces of the talus, the medial surface of the calcaneus, and the inferior surface of the sustentaculum tali (Fig. 6.110). It attaches above to the medial malleolus and below and behind to the inferomedial margin of the calcaneus.
The retinaculum is continuous above with the deep fascia of the leg and below with the deep fascia (plantar aponeurosis) of the foot.
Septa from the flexor retinaculum convert grooves on the bones into tubular connective tissue channels for the tendons of the flexor muscles as they pass into the sole of the foot from the posterior compartment of the leg (Fig. 6.110). Free movement of the tendons in the channels is facilitated by synovial sheaths, which surround the tendons.
Two compartments on the posterior surface of the medial malleolus are for the tendons of the tibialis posterior and flexor digitorum longus muscles. The tendon of the tibialis posterior is medial to the tendon of the flexor digitorum longus.
Immediately lateral to the tendons of the tibialis posterior and flexor digitorum longus, the posterior tibial artery with its associated veins and the tibial nerve pass through the tarsal tunnel into the sole of the foot. The pulse of the posterior tibial artery can be felt through the flexor retinaculum midway between the medial malleolus and the calcaneus.
Lateral to the tibial nerve is the compartment on the posterior surface of the talus and the undersurface of the sustentaculum tali for the tendon of the flexor hallucis longus muscle.
Two extensor retinacula strap the tendons of the extensor muscles to the ankle region and prevent tendon bowing during extension of the foot and toes (Fig. 6.111):
A superior extensor retinaculum is a thickening of deep fascia in the distal leg just superior to the ankle joint and attached to the anterior borders of the fibula and tibia.
An inferior retinaculum is Y-shaped, attached by its base to the lateral side of the upper surface of the calcaneus, and crosses medially over the foot to attach by one of its arms to the medial malleolus, whereas the other arm wraps medially around the foot and attaches to the medial side of the plantar aponeurosis.
The tendons of the extensor digitorum longus and fibularis tertius pass through a compartment on the lateral side of the proximal foot. Medial to these tendons, the dorsalis pedis artery (terminal branch of the anterior tibial artery), the tendon of the extensor hallucis longus muscle, and finally the tendon of the tibialis anterior muscle pass under the extensor retinacula.
Fibular (peroneal) retinacula bind the tendons of the fibularis longus and fibularis brevis muscles to the lateral side of the foot (Fig. 6.112):
A superior fibular retinaculum extends between the lateral malleolus and the calcaneus.