Peroneal nerve lesions
The common peroneal nerve is the smaller terminal branch of the sciatic nerve which supplies muscles which act on the ankle joint.
This nerve is easily damaged because it runs down in the popliteal fossa, then winds laterally around the neck of the fibula. It can be compressed by a plaster cast, in compartment syndrome, by lying unconscious with the leg externally rotated or it may be stretched when the knee is forced into varus with lateral ligament injuries.
It has two terminal branches, the superficial and deep peroneal nerves. The superficial nerve supplies peroneus longus and peroneus brevis, which plantarflex and evert the foot, and the skin on the lower, lateral side of the leg and foot. The deep nerve supplies muscles which dorsiflex the ankle and a small area of skin on the dorsum of the foot around the first web space.
Common peroneal nerve injury: Drop foot, both dorsiflexion of the ankle and eversion of the foot are weak but not plantarflexion (gastrocnemius and soleus are much more powerful plantarflexors of the foot). Sensation is lost over the front and outer leg and the dorsum of the foot.
Superficial branch injury: Foot eversion is lost, but dorsiflexion is intact. Sensation is lost over the outer side of the leg and foot.
Deep branch injury: This tends to occur in anterior compartment syndrome. There is weakened dorsiflexion and a small area of sensory loss on the dorsum of the foot.
Most cases resolve spontaneously if due to compression. Compartment syndrome however requires emergency decompression. If the nerve is cut or torn, it should be repaired. A splint can be worn to keep the foot in a neutral position. If nerve damage is permanent, tendon transfers or arthrodesis of the foot can help.