The peroneal nerve is one of the branches of the sciatic nerve. 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 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 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: 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).
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 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.
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 is also decreased.
It is important to distinguish a peroneal nerve palsy from a herniated L5 disc. A patient with a herniated L4-L5 disc - causing an L5 radiculopathy - will not only have difficulty dorsiflexing the foot (which also happens in a peroneal nerve palsy), but will also have difficulty inverting the foot (ie: bringing the big toe higher than the pinky toe). Patients with peroneal nerve dysfunction should still be able to invert the foot.
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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