The peroneal nerve is one of the branches of the sciatic nerve. It receives most of its innervation from the L4, L5, and S1 nerves. The common peroneal nerve (aka: common fibular nerve) wraps around the outside of the knee over the head of the fibula. It then branches into two separate nerves: the superficial peroneal nerve and the deep peroneal nerve.
The superficial peroneal nerve innervates two muscles: peroneus longus and brevis (aka: fibularis longus and brevis). These muscles allow you to evert (ie: move your foot outwards) and plantarflex (ie: help you step on the gas pedal) the foot. This nerve also provides the sensation to the outside half (ie: lateral half) of the lower leg, as well as the top of most of the foot (ie: the dorsum of the foot).
The deep peroneal nerve innervates three muscles in the leg: tibialis anterior, extensor digitorum longus, and extensor hallucis longus. Tibialis anterior allows you to dorsiflex (ie: lift your foot off the ground) and invert (ie: bringing your big to closer to the middle of the body) your foot. Extensor digitorum longus helps you extend your toes (ie: the opposite of curling them), as well as evert, and dorsiflex your foot. The third muscle, extensor hallucis longus, allows you to extend your big toe.
The deep peroneal nerve also innervates two muscles in the foot: extensor digitorum brevis (also helps to extend the toes) and extensor hallucis brevis (also helps to extend the big toe).
Importance in Disease
The peroneal nerve is most frequently compressed over the fibular head. Compression typically affects the deep peroneal nerve rather than the common or superficial nerve; however, all of the nerves may be involved.
When the deep peroneal nerve is compressed, the foot is unable to dorsiflex secondary to dysfunction of the tibialis anterior muscle. Additionally, the patient is unable to extend the toes. Sensation may be decreased on a small patch of skin between the big toe and second toe; pain in the area of the lateral lower leg may also be present.
When the superficial peroneal nerve is compressed, the peroneus longus and brevis muscles are affected. Dysfunction of these muscles prevents the patient from everting their foot. Sensation over the lateral half of the lower leg and top of the foot may also be decreased.
Placing the nerve under passive, or active, stretch by placing the patient’s foot in inversion will often reproduce the symptoms. Percussing the nerve over the fibular head (Tinel’s test) can reproduce the symptoms.
It is important to distinguish a peroneal nerve palsy from a herniated L4-L5 disc. A patient with a herniated L4-L5 disc – causing an L5 radiculopathy – will not only have difficulty dorsiflexing the foot and toes (secondary to dysfunction of the L5 component of the peroneal nerve), but will also have difficulty inverting the foot (secondary to dysfunction of the L5 component of the tibial nerve).
Wait a second! This still shouldn’t make sense if you are actually thinking about it, and this is where things get tricky… Both the anterior tibialis (deep peroneal nerve innervated) and the posterior tibialis (tibial nerve innervated) help invert the foot. So how do we know that the weakness in inversion is related to an L5 disc herniation, a peroneal nerve palsy, or a tibial nerve palsy? You need to have the patient attempt to invert the foot while plantarflexing! The posterior tibialis is an inverter and plantarflexor of the foot so if dorsiflexion is weak (L5 –> deep peroneal nerve –> anterior tibialis) and the patient cannot invert (L5 –> tibial nerve –> posterior tibialis OR L5 –> deep peroneal nerve –> anterior tibialis) well while the foot is plantarflexed you probably have an L5 radiculopathy and not a peroneal nerve palsy.
To make it a bit easier clinically… A foot drop WITH inversion weakness is most likely an L5 radiculopathy (most commonly from a herniated disc) because you are getting the invertors for both the deep peroneal nerve (tibialis anterior muscle) AND the tibial nerve (tibialis posterior muscle), which both have get fibers from the L5 nerve root. However, injury to the sciatic nerve proximally in the leg could also give you this, but it would be associated with significant plantarflexion weakness too (due to gastroc weakness from S1)! So therefore a foot drop with eversion weakness and toe extension weakness is also suspicious for a peroneal nerve injury.
Overview
The peroneal nerve splits at the level of the knee from the sciatic nerve. It then further divides into the superficial and deep peroneal nerves. The superficial branch controls the evertors of the foot (peroneus longus and brevis) and provides sensation over the lateral aspect of the lower leg and top of the foot. The deep branch controls the dorsiflexors of the foot, and the extensor muscles of the toes. The common peroneal, and either of its branches, is most commonly compressed near the fibular head.
More Important Anatomy to Master…
- Radial Nerve and the Saturday Night Palsy
- An Overview of the Sciatic Nerve
- The Median Nerve: Anatomy, Function, and Clinical Relevance
References and Resources
- Greenberg MS. Handbook of Neurosurgery. Sixth Edition. New York: Thieme, 2006. Chapter 25.
- Prakash, Bhardwaj AK, Devi MN. Sciatic nerve division: a cadaver study in the Indian population and review of the literature. Singapore Med J. 2010 Sep;51(9):721-3.
- Yuen EC, So YT. Sciatic neuropathy. Neurol Clin. 1999 Aug;17(3):617-31, viii.
- Simon RP, Aminoff MJ, Greenberg DA. Clinical Neurology, Seventh Edition (LANGE Clinical Medicine). Seventh Edition. New York: McGraw Hill, 2009.
- Netter FH. Atlas of Human Anatomy: with Student Consult Access (Netter Basic Science). Fifth Edition. Philadelphia: Saunders Elsevier, 2010.
- Bickley LS, Szilagyi PG. Bates’ Guide to Physical Examination and History Taking. Ninth Edition. New York: Lippincott Williams and Wilkins, 2007.