After a muscle test of the ankle/foot region the following findings were noted:
a. tibial nerve at popliteal fossa
b. tibial nerve at medial malleolus
c. superficial peroneal nerve at fibular neck
d. deep peroneal nerve just after it's bifurcation from the
common peroneal nerve
e. deep peroneal nerve at malleoli
f. lateral plantar nerve at tuberosity of 5th metatarsal
The answer is: e. deep peroneal nerve at malleoli
The significant finding in this problem is a paralysis of the extensor digitorum longus which is innervated by the deep peroneal nerve. In fact this is the terminus of the motor component of the deep peroneal nerve. While the tibialis anterior, extensor digitorum and extensor hallucis longus are innervated by the deep peroneal nerve, their motor points are in the proximal two thirds of the anterior compartment. The plantar intrinisics are innervated by the medial and lateral plantar nerves which branch from the tibial nerve in or slightly distal to the tarsal tunnel inferior to the medial malleolus. After emerging from under cover of the pre-tibial muscles in the anterior compartment of the leg the deep peroneal nerve crosses the dorsum of the foot and move laterally to the belly of the extensor digitorum brevis on the dorsolateral calcaneus. The key to understanding the solution to this problem is to know not only the innervation of each muscle but also the location of the motor point. If the lesion was located at the bifurcation point of the deep peroneal nerve from the common peroneal nerve at the level of the fibular neck all muscles innervated by the deep peroneal nerve would be paralyzed. Because the pre-tibial muscles are intact and the extensor digitorum brevis is compromised the lesion would have to be somewhere between the anterior leg and dorsal foot. Given the available choices the lesion would have to be at the level of the malleoli.