Ankle anatomy. (a) Anterior view. (b) Lateral view. (c) Medial view. (Reprinted with permission from the Philip Peng Educational Series)
The ankle joint is formed from three primary articulations: the tibiotalar joint, the subtalar joint, and the distal tibiofibular joint (Fig. 24.1). The tibiotalar joint is formed by the articulation between the tibia and fibula with the talus. It is a hinged synovial joint, which allows for flexion and extension. The subtalar joint is formed by the articulation of the talus and the calcaneous. The subtalar joint has an anterior and posterior component. The tibiotalar and subtalar joints communicate in approximately 10–20% of people.
There are five peripheral nerves which innervate the foot: the superficial peroneal nerve (SPN), deep peroneal nerve (DPN), saphenous nerve (SaN), tibial nerve (TB), and sural nerve (SuN).
The SPN and DPN are branches of the common peroneal nerve . The SPN provides sensation to the majority of dorsum of the foot, apart from the webspace of the first and second toe, which is supplied by the DPN (Fig. 24.2). The SPN usually emerges below the knee in the middle third of the leg in the fascial plane between the peroneus muscles and the extensor digitorum longus muscle (Fig. 24.3). The terminal branches are sensory. Above the level of the ankle joint the DPN lies lateral to the anterior tibial artery between the extensor digitorum longus and extensor hallucis longus tendons.
On the medial aspect of the ankle, the SaN provides sensation to the region around the medial malleolus and the medial aspect of the foot (Fig. 24.4). The SaN runs adjacent to the greater saphenous vein in the distal lower extremity. It commonly bifurcates 3 cm above the medial malleolus into an anterior and posterior division.
The TN provides sensation to the plantar aspect of the foot. The TN runs in the tarsal tunnel adjacent to the posterior tibial artery (Fig. 24.5). The TN gives off a calcaneal branch proximal to the medial malleolus and then continues to terminate in the medial and lateral branches.
The SuN provides sensation to the lateral aspect of the distal leg and the lateral aspect of the foot. In the distal leg, the SuN runs adjacent to the lesser saphenous vein between the Achilles tendon and the peroneus tendons (Fig. 24.6).
Injection of the tibiotalar and subtalar joints can be valuable for diagnostic and therapeutic purposes. Joint injections with local anesthetic can help to clarify the articular contribution of pain and may be used for surgical planning for arthrodesis. Joint injections can also provide symptomatic relief for osteoarthritis, rheumatoid arthritis, and ankle impingement. Perineural injections around the foot and ankle nerves may be offered for patients with neuropathic pain, particularly in the context of peripheral nerve injury or entrapment.
Ultrasound Scan for Ankle Nerves
Sural and Superficial Peroneal Nerve
Position: Supine, tilted contralateral side, affected leg nondependent position
Probe: Linear 6–15 MHz
The author suggests to place the probe initially at the lower one-third of the leg over the fibula (F) and extensor digitorum longus (EDL) (Fig. 24.7 upper panel). SPN is deep to the crural fascial (bold arrow). The SPN (arrow) can be located at the junction of the intermuscular septum (between the peroneus muscle and EDL as indicated by the arrow heads) and the crural fascia.
By moving the probe in the caudal direction, the SPN (arrow) can be seen within the crural fascia (bold arrows) in the middle figure (Fig. 24.7 middle panel).
Further caudal movement of the probe, the SPN (arrows) is now superficial to the crural fascia (bold arrows) (Fig. 24.7 lower panel). PB and PL, peroneus brevis and longus; ∗∗, PB tendon; F, fibula.
Placing the probe between the lateral malleolus and Achilles tendon (TA), a fascia plane (bold arrows) can be appreciated (Fig. 24.8). Sural nerve (arrow) can be seen adjacent to the lesser saphenous vein (V) in this fascia plane.
Deep Peroneal Nerve
Position: Supine, knee flexed, foot on the examination table
Probe: Linear 6–15 MHz
By placing the probe in short axis to the tendons just above the ankle joint, the most prominent tendon will be the tibialis anterior tendon (orange circle) (Fig. 24.9). Deep to the extensor hallucis longus (EHL) and extensor digitorum longus (EDL) tendon, the anterior tibial artery is seen. The DPN is in the fascia plane with the vessel.