Tibial and common peroneal nerves: skin supply
The sciatic nerve gives no skin sensation itself: the tibial nerve supplies the sole, and the common peroneal nerve the lateral leg and top of the foot.
Remember
The sciatic nerve has no skin territory of its own, so trace numbness to a branch: the sole points to the tibial nerve, and the lateral leg and top of the foot point to the common peroneal nerve.
Memory aid
picture the sciatic nerve as a cable that splits into two wires, the tibial nerve running to the sole and the common peroneal nerve running to the outer leg and the top of the foot
Think of one cable that splits into two wires, where each wire is a branch with its own skin area.
The sciatic nerve is best pictured as the tibial and common peroneal (also called common fibular) nerves travelling together in one sheath. It usually divides just above the popliteal fossa (the hollow behind the knee). The trunk (the part before it divides) has no direct sensory function, so the skin territories are named after its branches. A trunk injury can still cause numbness, because the nerves carrying sensation for both branches run together inside it, so it would be expected to affect the skin territories of both.
The tibial nerve supplies the skin of the sole of the foot and the heel. It runs down the back of the leg and passes behind the medial malleolus (the bony bump on the inner side of the ankle), where it divides into the medial and lateral plantar nerves. The medial plantar nerve covers the medial two thirds of the sole, and the lateral plantar nerve covers the lateral third.
The common peroneal nerve supplies the skin of the lateral side of the lower leg and the dorsum (the upper surface) of the foot. One of its branches, the superficial fibular nerve, also supplies the top of the foot.
Some look-alikes are worth separating. The sural nerve is formed from branches of both the tibial and common fibular nerves and supplies the posterolateral leg (the outer part, towards the back) and the lateral edge of the foot. The saphenous nerve is a sensory branch of the femoral nerve (whose branches supply the skin of the front of the thigh), and the saphenous nerve itself supplies the skin of the medial leg and medial foot. The posterior femoral cutaneous nerve (cutaneous means relating to the skin) supplies the back of the thigh.
Key facts
- Sciatic nerve
- has no skin branches of its own; its skin supply arrives only through the two branches into which it divides
- Tibial nerve
- supplies skin of the sole of the foot and the heel
- Common peroneal nerve
- supplies skin of the lateral leg and the dorsum (top) of the foot
- Plantar nerves
- the medial plantar nerve supplies the medial two thirds of the sole and the lateral plantar nerve the lateral third
- Sural nerve
- is formed from tibial and common fibular branches and supplies the posterolateral leg (the outer part, towards the back) and the lateral edge of the foot
- Pattern to match
- a numb sole with a spared lateral leg is a tibial nerve pattern
Exam traps
- Numbness over the sole alone does not point to the sciatic nerve, because the sciatic trunk has no direct sensory function and an injury to it would be expected to affect the skin territories of both of its branches.
- The dorsum (top) of the foot does not belong to the tibial nerve, because it is mainly supplied by the superficial fibular nerve, a branch of the common fibular nerve.
- Swapping the territories of the two branches is a common slip: the tibial nerve supplies the sole, and the common peroneal nerve supplies the lateral leg and the top of the foot, not the other way round.
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