Expansion
L4 medial leg, L5 dorsum of the foot and big toe, S1 lateral foot and little toe
Mnemonic
Landmarks worth memorising, working down the limb:
- L1 - the inguinal ligament
- L2, L3 - the anterior thigh
- L4 - the medial malleolus and medial leg, so “down to the floor on the big toe side”
- L5 - the dorsum of the foot and the big toe
- S1 - the lateral malleolus, the little toe and the sole
- S2, S3 - the back of the thigh
- S4, S5 - the perianal “saddle” area
“L4 on the floor, L5 the big toe is alive, S1 is the small one” links the three that matter most in a suspected disc prolapse. Saddle anaesthesia in S4 and S5 is the red flag for cauda equina syndrome.
Expansion
- L3 - front of the thigh and the knee
- L4 - medial leg and medial malleolus (down to the floor on the inside)
- L5 - dorsum of the foot and the big toe
- S1 - lateral border of the foot, little toe and heel
Memory hook: L4 to the floor (medial), L5 the largest (big toe), S1 the smallest (little toe).