tibial

Peroneal (Fibular) Nerve and the Dropped Foot

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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.

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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.

An Overview of the Sciatic Nerve

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The sciatic nerve is a collection of nerve fibers that exit the spinal column between the 4th lumbar and 3rd sacral levels (L4-S3).

In the upper leg, the sciatic nerve gives off branches to the hamstring muscles. These muscles, which include the semimembranous, semitendinous, biceps femoris and part of the adductor magnus, are powerful flexors of the knee joint.

After sending off branches to the hamstrings, the sciatic nerve hits the back of the knee. At the popliteal fossa the sciatic splits into two distinct nerves: the common peroneal nerve and the tibial nerve.

The first nerve, the common peroneal (aka: common fibular), wraps around the outside of the knee and over the head of the fibula. It then branches into two separate nerves: the superficial peroneal and deep peroneal nerves.

The superficial peroneal nerve innervates two muscles: peroneus longus and brevis (aka: fibularis longus and brevis). These muscles allow you to evert (ie: allow you to lift your "pinky" toe higher than your "big" toe) 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 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: bring your "big" toe higher than your "pinky" toe) 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).

Peroneal Nerve and its Branches
Branch Muscles
Superficial peroneal – Peroneus longus
– Peroneus brevis
Deep peroneal
(leg branches) 		– Tibialis anterior
– Extensor digitorum longus
– Extensor hallucis longus
Deep peroneal
(foot branches)		 – Extensor digitorum brevis
– Extensor hallucis brevis

The other branch of the sciatic nerve, the tibial nerve, dives deep into the lower part of the leg where it acts on muscles in the calf and foot. In the calf it innervates the gastrocnemius (commonly called the "calf" muscle), soleus, popliteus, plantaris, flexor digitorum longus, flexor hallucis longus, and tibialis posterior. The gastrocnemius, soleus, and tibialis posterior are important plantarflexors of the foot (ie: allow you to "step on the gas pedal"). Flexor digitorum longus and flexor hallucis help flex (ie: curl) the toes.

The tibial nerve also sends branches into the foot. It further branches into the medial plantar and lateral plantar nerves, which innervate numerous muscles (see table below) in the foot itself.

The tibial nerve branches in the foot, namely the medial plantar and lateral plantar nerves also provide sensation to the sole of the foot.

Tibial Nerve and its Branches
Branch Muscles
Tibial nerve (leg branches) – Gastrocnemius
– Soleus
– Popliteus
– Plantaris
– Flexor digitorum longus
– Flexor hallucis longus
– Tibialis posterior

Tibial nerve (foot branches) -> medial plantar nerve

– Abductor hallucis
– Flexor digitorum brevis
– First lumbrical
Tibial nerve (foot branches) ->
lateral plantar nerve 		– Flexor digiti minimi
– Adductor hallucis
– Interossei
– 2nd, 3rd, 4th lumbricals
– Abductor digiti minimi

Importance in Disease

The most well known problem of the sciatic nerve is a condition termed "sciatica". Sciatica is not a disease or disorder in itself, but rather a symptom of some underlying spine or nerve pathology. It is most often due to compression of one or more of the nerve roots that contribute axons to the sciatic nerve (L4-S3).

Compression of the nerve roots can be caused by many different pathologies, of which the most common is a herniated disc. Other causes include spinal stenosis and spondylolisthesis. Rarely, the nerve itself is compressed at some point as it travels down the leg.

A common symptom of sciatica is a severe and sharp pain that starts in the lower back and shoots down the buttock and back of the leg. If the involved nerve roots are severely compressed, weakness may also occur. This can cause a "foot drop" (ie: an inability to lift your foot off the ground).

The sciatic nerve may also get compressed as it passes through the piriformis muscle in the pelvis. When this occurs weakness of the hamstrings, lower leg, and foot muscles occurs; in addition, sensation of the outside of the lower leg, the calf, and the sole of the foot may also be affected.

Overview

The sciatic nerve is really two nerves that split at the level of the knee. The two main branches are the common peroneal (aka: common fibular) and tibial nerves, each of which branch again to innervate different muscles in the lower leg and foot. The most common pathologic problem with the sciatic nerve is a constellation of symptoms termed "sciatica". Sciatica is caused, most commonly, by compression of the nerve roots (L4-S3) in the spine that give rise to the sciatic nerve.

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References and Resources