The L4 dermatome covers the front of the knee and runs down the inner (medial) side of the lower leg to the medial malleolus and the inner edge of the foot. Much of this skin is supplied by the saphenous nerve. It lies between L3 above and L5 on the outer leg.
| Nerve root | L4 |
|---|---|
| Area | Anterior knee, medial lower leg and the medial border of the foot to the medial malleolus |
| How to test | Light touch and pinprick are compared side to side at the ISNCSCI key sensory point: over the medial malleolus. |
| Nerve Supply | Saphenous nerve (from femoral nerve) |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the L4 dorsal root.
L4 radiculopathy, usually from an L3-L4 disc or a far lateral L4-L5 disc, causes medial leg numbness with weak ankle dorsiflexion and a reduced patellar reflex. Saphenous nerve injury, for example after knee surgery or vein harvesting, gives medial leg numbness without weakness. Dermatome maps differ between sources (Foerster, Keegan and Garrett, and modern evidence-based maps disagree), and neighbouring dermatomes overlap, so borders are approximate.
Compression of the L4 root causing pain along the front of the thigh and inner shin, weak dorsiflexion and a reduced patellar reflex.
Damage to the saphenous nerve during knee arthroscopy or vein harvest leaves numbness on the inner leg without muscle weakness.
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