🦴 Lumbar Radiculopathy
Disc prolapse, the traversing root rule, L4/L5/S1 myotomes, reflexes, and dermatomes — with an interactive foot map and the L5 vs peroneal nerve trap.
The Anatomy of a “Slipped Disc”
The intervertebral disc is like a jam doughnut — a tough outer ring (anulus fibrosus) containing a soft, gelatinous, high-pressure centre (nucleus pulposus). Under degeneration or trauma, the anulus tears and the pressurised nucleus herniates outward. Understanding where it herniates and which nerve it hits is the foundation of the entire topic.
Why Posterolateral?
Herniations favour the posterolateral direction for two anatomical reasons: the anulus fibrosus is naturally thinner posterolaterally, and crucially, this region lacks the reinforcing support provided by the broad anterior and posterior longitudinal ligaments, which run down the midline and protect the disc from central herniation.
The Traversing vs. Exiting Root — The Most Important Concept
In the lumbar spine, the nerve root exits the spinal canal high up in the intervertebral foramen, right under the pedicle of the vertebra above. This means it has already cleared the disc space before the herniation site.
The traversing root (the next root down) is still travelling through the lateral recess when it passes the posterolateral disc — it is directly in the line of fire of a standard paracentral herniation.
Therefore: name the disc, number the root one level lower.
L4/L5 disc → L5 root. L5/S1 disc → S1 root. L3/L4 disc → L4 root.
L4 Radiculopathy
Classic disc: L3/L4 | Mnemonic: L4 hits the Floor (medial malleolus)
L3/L4 disc prolapse is the classic cause of a depressed knee jerk on examination. Always test and compare both sides.
The L4 Mnemonic — “L4 hits the Floor”
L4 sensory territory reaches the medial malleolus — the bony prominence on the inner ankle, which is literally the part of the foot/ankle that hits the floor first when you roll your foot inward. The medial side. Medial malleolus = medial = L4.
L4 in Context — What to Look For Clinically
- Pain radiating from the lower back, into the anterior thigh, medial knee, and medial leg (not going past the ankle in most cases)
- Reduced or absent knee jerk — always compare bilaterally
- Weakness of knee extension — ask the patient to extend the knee against your hand resistance while sitting
- The hip flexors (L2/L3) are spared — the deficit starts at the quadriceps
L5 Radiculopathy
Classic disc: L4/L5 | Mnemonic: L5 = Lift the 5 toes (dorsiflexion)
This is the most commonly tested fact about L5. Normal knee jerk (L4) + normal ankle jerk (S1) with a foot drop = L5 lesion. Reflexes are intact but the patient has foot drop.
The Boss-Level Trap — L5 Radiculopathy vs Common Peroneal Nerve Palsy
Both cause foot drop (loss of dorsiflexion and eversion). This is one of the most commonly tested clinical distinctions in orthopaedics and neurology:
🧪 Test: Ask the patient to invert their foot (sole turns inward).
✅ L5 radiculopathy → inversion is also WEAK (Tibialis Posterior is L4/L5, innervated via the tibial nerve, which comes from the L5 root)
✅ Common Peroneal Nerve (CPN) palsy at fibular neck → inversion is NORMAL (tibial nerve and its L5 branches are not affected by peroneal nerve compression)
Foot drop + weak inversion = spinal lesion (L5 root). Foot drop + normal inversion = peripheral nerve lesion (CPN at fibular neck).
Other clues for CPN palsy: a history of prolonged leg crossing, recent knee surgery, plaster cast at the fibular neck, or weight loss exposing the fibular head.
The Reflexes Trap — L5 is the “Reflex-Free” Root
In the exam: if a question describes a patient with sciatica + foot drop + normal knee jerk + normal ankle jerk, the answer is L5. The absence of a reliable reflex makes L5 the only root that can present with a significant motor deficit (foot drop) without any reflex change. This absence is itself a diagnostic clue — not an oversight.
S1 Radiculopathy
Classic disc: L5/S1 | Mnemonic: S1 = Stand on 1 leg on tiptoes (plantar flexion)
The ankle jerk is the S1 reflex. A depressed or absent ankle jerk in a patient with buttock-to-heel sciatica is the pathognomonic finding of L5/S1 disc prolapse and S1 radiculopathy.
The S1 Clinical Test — Tiptoe Walking
The gastrocnemius/soleus complex is enormously powerful — a single isolated calf squeeze can miss mild S1 weakness because the examiner’s arm cannot overpower the calf. The correct clinical test is to ask the patient to repeatedly rise onto tiptoes on the affected leg alone (single-leg heel raises). Normally, 10+ repetitions are possible. With S1 weakness, the patient either cannot rise at all or fatigues within 2–3 repetitions with the heel visibly dropping lower on each attempt.
Interactive Foot Map & Full Comparison
Click a nerve root in the legend to highlight its dermatome on the foot. The mnemonic: L4 = medial (Floor side), L5 = dorsum (Large 5 toes), S1 = lateral (Small 1 / little toe side).
Click to highlight dermatome:
“L4 hits the Floor”
“L5 = Large 5 toes”
“S1 = Small 1 / little toe”
Click any entry to show/hide that dermatome. Click again to toggle back on.
Full Comparison Table
| Feature | L4 | L5 | S1 |
|---|---|---|---|
| Classic disc level | L3/L4 | L4/L5 | L5/S1 |
| Myotome / Key muscle | Quadriceps femoris | Tibialis anterior + EHL | Gastrocnemius + soleus |
| Motor deficit | Weak knee extension | Foot drop + weak big toe extension | Weak plantar flexion — can’t rise on tiptoes |
| Reflex lost | Knee jerk (patellar) | ⚠️ NONE — reflexes intact | Ankle jerk (Achilles) |
| Dermatome | Medial lower leg & medial malleolus | Dorsum of foot & 1st web space | Lateral border of foot, little toe, sole |
| Foot mnemonic | “L4 hits the Floor” (medial malleolus) | “Large 5 toes” (dorsum) | “Small 1” (little toe / lateral) |
| Action mnemonic | “L3/L4 kick the door” (knee jerk) | “L5 = Lift the 5 toes” | “S1 = Stand on 1 leg tiptoe” |
🧠 Mnemonics — All in One Place
Paracentral disc → hits the traversing root (ONE level below)
L3/L4 disc → L4 root | L4/L5 disc → L5 root | L5/S1 disc → S1 root
Far-lateral (foraminal) disc → hits the EXITING root (same level)
Exception: a far lateral disc hits the root at the same level — L4/L5 far lateral → L4 root. Always specify disc location in clinical communication.
“S1, S2 — buckle my shoe” → Ankle Jerk (Achilles)
S1 = Achilles tendon reflex. You buckle your shoe at the ankle level. Absent ankle jerk = S1 lesion (L5/S1 disc).
“L3, L4 — kick the door” → Knee Jerk (Patellar)
L3/L4 = Patellar reflex. Kicking uses knee extension. Absent knee jerk = L4 lesion (L3/L4 disc).
L5 has NO reflex — this absence IS the finding
Normal knee jerk + normal ankle jerk + foot drop = L5 radiculopathy. Don’t be fooled by normal reflexes.
L4 = “kick the door” — Knee extension (Quadriceps)
Kicking requires knee extension. L4/Quadriceps. Absent knee jerk + medial leg numbness = L4.
L5 = “Lift the 5 toes” — Dorsiflexion + big toe extension
Lifting the toes = dorsiflexion = tibialis anterior + EHL. L5 weakness = foot drop. Dorsum of foot numb.
S1 = “Stand on 1 leg tiptoe” — Plantar flexion (Gastrocnemius)
Rising on tiptoes = plantar flexion = gastrocnemius/soleus = S1. Test with single-leg heel raises. Absent ankle jerk = S1.
L4 hits the Floor — Medial malleolus & medial lower leg
The medial side of the foot/ankle is the part closest to the floor (inner side). L4 = medial = Floor side.
L5 = Large 5 — Dorsum + big toe + 1st web space
The big toe (the largest = L5) and the entire top of the foot. The 1st web space is the specific exam question spot.
S1 = Small 1 — Little toe + lateral border + sole
The little toe (the smallest = S1) and the outer edge of the foot. Lateral border + sole.
Foot drop + WEAK inversion = L5 root (spine)
Tibialis Posterior (invertor) is L4/L5, innervated by tibial nerve from L5 root. L5 radiculopathy weakens inversion.
Foot drop + NORMAL inversion = CPN palsy (at fibular neck)
The Common Peroneal Nerve does not supply inversion (tibial nerve function). Inversion intact = the tibial nerve (and L5 root) is working = the lesion is peripheral, at the fibular neck.
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