Common Peroneal Nerve

1. First, the Common Peroneal Nerve is an important peripheral nerve in the lower limb. It originates from the sciatic nerve, which arises from the lumbar and sacral plexuses. In the posterior thigh, near the popliteal fossa (behind the knee), the sciatic nerve trunk typically divides into two terminal branches: the thicker medial branch is the tibial nerve, while the thinner lateral branch is the Common Peroneal Nerve. Therefore, it can be understood as one of the main "forks" of the sciatic nerve.

2. Next, let's understand its course. After branching from the lateral angle of the popliteal fossa, the common peroneal nerve runs close to the medial side of the tendon of the biceps femoris muscle, along the lateral border of the popliteal fossa, obliquely downward and laterally. It then winds around the posterior and inferior aspect of the fibular head (the prominent bony bump on the outer side below the knee). In this location, it is very superficial and susceptible to injury from compression (e.g., casts, prolonged squatting) or trauma (e.g., fractures of the fibular head/neck). After passing the fibular head, the nerve enters the leg region.

3. After winding around the fibular head, the common peroneal nerve quickly divides into two main branches: the Deep Peroneal Nerve and the Superficial Peroneal Nerve. This is the key step for its functional specialization.

   • Deep Peroneal Nerve: Primarily courses between the muscles of the anterior compartment of the leg (tibialis anterior, extensor hallucis longus, extensor digitorum longus, etc.), descending alongside the anterior tibial vessels. It eventually extends to the dorsum of the foot, innervating a small area of skin.
   • Superficial Peroneal Nerve: Primarily courses between the muscles of the lateral compartment of the leg (fibularis longus and brevis). It then pierces the deep fascia to become subcutaneous at the junction of the middle and lower thirds of the leg.

4. Understanding its course and branches allows us to comprehend its core functions – motor and sensory innervation.

   • Motor Function: The common peroneal nerve overall innervates the anterior compartment muscles and lateral compartment muscles of the leg.
     • Via the Deep Peroneal Nerve, it innervates the anterior compartment muscles, responsible for dorsiflexion of the ankle (pulling the foot upward) and extension of the toes.
     • Via the Superficial Peroneal Nerve, it innervates the lateral compartment muscles, responsible for eversion of the foot (turning the sole outward).
   • Sensory Function:
     • The Superficial Peroneal Nerve is responsible for the skin sensation of the lower lateral part of the leg and most of the dorsum of the foot (except for a small area in the web space between the first and second toes).
     • The sensory innervation area of the Deep Peroneal Nerve is very small, limited to a small patch of skin on the dorsal aspect of the web space between the adjacent sides of the first and second toes.

5. Finally, due to its functions and superficial course, Common Peroneal Nerve injury is very typical in clinical practice. The most common site of injury is at the fibular head/neck. Injury leads to paralysis of the muscles it supplies, resulting in characteristic signs: inability to dorsiflex the foot, evert the foot, or extend the toes. Due to the unopposed action of the antagonist muscles (the posterior compartment muscles, innervated by the tibial nerve), the patient develops a "foot drop" deformity (where the foot hangs down and turns inward). To prevent dragging the toes while walking, the patient adopts a distinctive "high-stepping gait" (walking with exaggerated lifting of the legs, as if stepping over obstacles). The area of sensory loss is mainly located on the lateral leg and the dorsum of the foot.