What is the anatomy of the lower leg affected by foot drop?

Updated: Mar 23, 2020
  • Author: James W Pritchett, MD; Chief Editor: Vinod K Panchbhavi, MD, FACS  more...
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Answer

Fibers from the dorsal branches of the ventral rami of L4-S1 are found in the peroneal nerve, which is paired with the tibial nerve to constitute the sciatic nerve. The sciatic nerve leaves the pelvic cavity at the greater sciatic foramen, just inferior to the piriformis. It bifurcates to form the peroneal and tibial nerves either in the distal third of the thigh or at the midthigh level.

The peroneal nerve crosses laterally to curve over the posterior rim of the fibular neck to the anterior compartment of the lower leg, dividing into superficial and deep branches. The superficial branch travels between the two heads of the peronei and continues down the lower leg to lie between the peroneal tendon and the lateral edge of the gastrocnemius. It then branches to the ankle anterolaterally to supply sensation to the dorsum of the foot (see the image below).

Common and superficial peroneal nerves, branches, Common and superficial peroneal nerves, branches, and cutaneous innervation.

The deep branch divides just after rounding the fibular neck. Its initial branch supplies the tibialis anterior, and the remaining branches supply the EDL, the EHL, and a small sensory patch at the first dorsal web space (see the image below).

Deep peroneal nerve, branches, and cutaneous inner Deep peroneal nerve, branches, and cutaneous innervation.

The peroneal nerve is susceptible to injury all along its course. In that it is part of the sciatic nerve, its funiculi are relatively isolated from those of the tibial nerve. Therefore, trauma to the sciatic nerve may affect only one of its divisions. The funiculi of the peroneal nerve also are larger and have less protective connective tissue than those of the tibial nerve, making the peroneal nerve more susceptible to trauma. In addition, the peroneal nerve has fewer autonomic fibers; thus, in any injury, motor and sensory fibers bear the brunt of the trauma.

The peroneal nerve runs a more superficial course than the tibial nerve does, especially at the fibular neck, and this relatively exposed position makes it vulnerable to direct insult. Its close adherence to the periosteum of the proximal fibula renders it susceptible to injury during surgical procedures in this area.


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