Common Peroneal Nerve
The common fibular (peroneal) nerve is a branch of the sciatic nerve. It supplies the anterior and lateral compartments of the legs, which enable dorsiflexion and ankle eversion respectively. The nerve also has sensory function on the dorsum of the foot and lateral surface of the leg. In this article we will discuss the anatomy of the nerve, as well as its clinical relevance. We will conclude with some review questions to test the reader’s understanding of the article content.
AnatomyThe sciatic nerve (ventral rami of L4-S3) descends along the posterior thigh and divides above the popliteal fossa into the common peroneal (ventral rami L4-S2) and tibial nerves (ventral rami of L4-S3). The tibial nerve continues to descend along the posterior thigh and leg, giving off articular branches to the knee joint, muscular branches to the posterior compartment of the leg and finally divides into the medial and lateral plantar nerves to provide sensorimotor innervation to the foot. The common fibular nerve passes laterally and gives off a small sural communicating branch that combines with a reciprocal communicating branch from the tibial nerve. The nerve that results from this combination is the Sural nerve, which relays sensation from the posterior and lateral surfaces of the distal third of the leg. The common fibular nerve innervates the short head of biceps femoris and passes lateral to its tendon at the level of the knee. It winds around the neck of the fibula to pass in the fibular tunnel between the fibula and the tendon of fibularis longus). It is vulnerable to crush injuries at this location, as it lies just beneath the skin.
The nerve then divides into a small articular branch supplying the knee joint, a superficial and a deep branch. The deep fibular nerve supplies the anterior compartment of the leg, i.e. tibialis anterior (evertor and dorsiflexor of the ankle), extensor hallucis longus (extensor of the big toe and dorsiflexor of the ankle), extensor digitorum longus (extensor of the 2nd to 5th toes and dorsiflexor of the ankle) and fibularis tertius (dorsiflexion and eversion of the foot). The deep division also relays sensation from the first web-space of the foot.
The superficial fibular nerve supplies motor innervation to the lateral compartment of the leg, i.e. fibularis longus and brevis (evertors of the ankle), which originate from the fibula. It also relays sensation from the lateral compartment of the leg (above the sural nerve) and dorsum of the foot (excluding the first dorsal web-space, and fifth toe).
Variant anatomy- In some individuals, the common fibular nerve branches from the sciatic nerve higher than expected, even before the sciatic nerve leaves the bony pelvis. In this case it may pass through the piriformis muscle. The nerve can also descend at the level of the knee in a narrow biceps femoris tunnel between the short head of biceps femoris and the lateral head of gastrocnemius, where it is prone to entrapment. Trifurcation of the common fibular nerve into an articular branch to the knee joint, deep and superficial fibular nerves usually happens at the level of the fibular neck. However, this branching can occur more proximally, between the knee joint and the neck of the fibula or even above the knee joint.
Common Peroneal nerve neuropathy- Many anatomical features predispose the common fibular nerve to entrapment neuropathies around the knee. Factors increasing the risk of external compression of the nerve include the relatively small volume of epineural connective tissue surrounding the nerve, its passage through narrow openings such as the fibular tunnel or the biceps femoris tunnel, its superficial course around the knee or anatomical variations related to its level of branching. In the worst case scenario, this condition can present clinically as foot drop, due to the denervation of the anterior compartment (dorsiflexor compartment) of the leg. It can occur following compression by a tight cast, crush injury, tumours, cysts, etc.