The sciatic nerve splits into the tibial nerve and the common fibular (peroneal) nerve
Mnemonic
“Tibial and common fibular, bound in one sheath until the popliteal fossa”:
- Tibial division (L4 to S3) - supplies the hamstrings (except the short head of biceps), the posterior compartment of the leg and the sole
- Common fibular division (L4 to S2) - supplies the short head of biceps femoris, and then the lateral and anterior compartments
“The short head of biceps femoris is the odd one out”, being the only hamstring supplied by the common fibular division.
The common fibular nerve is the most commonly injured nerve in the lower limb, because it winds around the neck of the fibula where it is subcutaneous. Causes are fibular neck fracture, tight plaster, prolonged squatting or crossing the legs, and it gives foot drop with sensory loss over the dorsum and lateral leg.
Foot drop with an intact ankle jerk suggests a common fibular lesion, whereas an L5 radiculopathy also weakens hip abduction and inversion.
Expansion
The sciatic nerve (L4 to S3) is really two nerves in one sheath:
- Tibial nerve - supplies the posterior thigh, the posterior leg and the sole of the foot
- Common fibular (peroneal) nerve - supplies the anterior and lateral leg and the dorsum of the foot
It leaves the pelvis through the greater sciatic foramen, usually below piriformis.