The L3 dermatome covers the lower front and inner thigh, running down to the inner side of the knee. It lies between the L2 band above and the L4 area on the leg. The key sensory point is just above the knee on the inner side.
| Nerve root | L3 |
|---|---|
| Area | Lower anterior and medial thigh down to the medial knee |
| How to test | Light touch and pinprick are compared side to side at the ISNCSCI key sensory point: on the medial femoral condyle, just above the knee. |
| Nerve Supply | Femoral nerve (intermediate and medial cutaneous nerves of thigh); Obturator nerve (cutaneous branch) |
Carries touch, pain and temperature sensation from this strip of skin to the spinal cord through the L3 dorsal root.
L3 radiculopathy, from an L2-L3 disc or a far lateral L3-L4 disc, gives anterior thigh pain with weak knee extension and a reduced patellar reflex. Femoral neuropathy, for example after pelvic surgery or a haematoma in the iliacus, can look similar but spares hip adduction. 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 L3 root causing pain and numbness down the front of the thigh to the inner knee, quadriceps weakness and a reduced knee jerk.
Damage to the femoral nerve causes anterior thigh numbness and quadriceps weakness, and can mimic an L3 or L4 root problem.
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