What nerve is injured in foot drop after a fibular neck fracture?

The common peroneal nerve wraps around the neck of the fibula and is vulnerable to injury here. It innervates dorsiflexion (deep peroneal branch) and eversion (superficial peroneal branch). Injury produces foot drop with the foot inverted and plantarflexed at rest, and sensory loss over the dorsum of the foot.

What is Trendelenburg gait and which nerve causes it?

Trendelenburg gait results from superior gluteal nerve injury (L4-S1), which innervates the gluteus medius and minimus. When the patient stands on the affected side, the contralateral pelvis drops because the abductors cannot stabilize it. The lesion is ipsilateral to the weight-bearing leg.

What is meralgia paresthetica?

Meralgia paresthetica is entrapment of the lateral femoral cutaneous nerve (L2-L3) at the inguinal ligament near the ASIS. It causes burning pain and paresthesias restricted to the anterolateral thigh with NO motor deficit, since this is a pure sensory nerve. Associated with obesity, tight belts, and pregnancy.

Lower Extremity Nerve Injuries: Sciatic, Peroneal, Femoral (clinical practice Preparation)

Anatomy

The Lower Extremity Nerve Map

The lumbosacral plexus (L2-S3) generates six nerves. Each has a unique anatomic pinch point and a unique injury signature. Tap a nerve to see its territory.

L2-L4

Femoral Nerve

Motor: Quadriceps (knee extension), iliopsoas (hip flexion)
Sensory: Anterior thigh + medial leg (saphenous)
Reflex: Patellar
Injury cause: Retroperitoneal hematoma, pelvic fracture, psoas abscess
Presentation: Quad weakness, absent patellar reflex, medial leg numbness

L2-L4

Obturator Nerve

Motor: Adductors (longus, brevis, gracilis, magnus)
Sensory: Medial thigh
Injury cause: Pelvic operations, obturator hernia
Presentation: Weak hip adduction, medial thigh numbness, normal reflexes

L4-S3

Sciatic Nerve

Motor: Hamstrings + all below knee (splits into peroneal + tibial)
Sensory: Posterior knee + all below knee except medial leg
Injury cause: Herniated disc, posterior hip dislocation, piriformis syndrome
Presentation: Foot drop + lost plantarflexion + posterior thigh weakness

L4-S2

Common Peroneal

Motor: Dorsiflexion (deep) + eversion (superficial)
Sensory: Dorsum of foot (except 1st webspace)
Injury cause: Fibular neck fracture, tight cast
Presentation: Foot dropPED - inverted + plantarflexed at rest; steppage gait

L4-S3

Tibial Nerve

Motor: Plantarflexion, toe flexion, foot invertors
Sensory: Sole of foot
Injury cause: Baker cyst (proximal), tarsal tunnel syndrome (distal)
Presentation: Can't stand on TIPtoes; sole numbness; Tinel at medial malleolus

L4-S1

Superior Gluteal

Motor: Gluteus medius + minimus + tensor fasciae latae
Sensory: None
Injury cause: IM injection in superomedial gluteal region, hip arthroplasty
Presentation: Trendelenburg gait - contralateral pelvis drops during stance

Three mnemonics you cannot afford to forget:
PED = Peroneal Everts and Dorsiflexes. If injured, foot dropPED.
TIP = Tibial Inverts and Plantarflexes. If injured, can't stand on TIPtoes.
Trendelenburg = ipsilateral superior gluteal nerve; contralateral hip drops.

Clinical Challenge

Nerve Locator

Read the clinical scenario. Identify the nerve before choosing. The anatomy is the answer.

Patient cannot dorsiflex the foot. Sole sensation is normal.

Tap to reveal nerve

Common peroneal nerve. Dorsiflexion is the deep peroneal branch. Normal sole = tibial territory is intact. Foot is inverted and plantarflexed at rest.

Patient cannot adduct the hip. Medial thigh is numb. Patellar reflex is normal.

Tap to reveal nerve

Obturator nerve (L2-L4). Adductors = obturator. Normal patellar reflex excludes femoral. Medial thigh = obturator sensory territory. Pelvic surgery risk.

Pelvis drops on the left when the patient stands on the right leg.

Tap to reveal nerve

Right superior gluteal nerve. The lesion is on the WEIGHT-BEARING side. The right abductors cannot hold the pelvis level when the left foot lifts off.

Burning anterolateral thigh pain. No motor deficit. No reflex changes.

Tap to reveal nerve

Lateral femoral cutaneous nerve (L2-L3). Pure sensory. No motor fibers. Meralgia paresthetica from entrapment at the ASIS/inguinal ligament. Obesity and tight belts.

Nerve Locator · Question 1 of 3

A patient has foot drop - the foot is inverted and plantarflexed at rest. Plantarflexion strength is 4/5. The sole of the foot has normal sensation. Which of the following is the most likely injured nerve?

Common peroneal nerve. The key: plantarflexion is INTACT (tibial nerve is fine) and sole sensation is INTACT (also tibial). Only dorsiflexion and eversion are lost - that is precisely the common peroneal territory (PED). The sciatic nerve injury would have knocked out plantarflexion too.

Nerve Locator · Question 2 of 3

A patient has absent patellar reflex, 2/5 knee extension, and numbness over the anterior thigh and medial leg. Hip extension, plantar flexion, and Achilles reflex are all intact. Which nerve is most likely affected?

Femoral nerve (L2-L4). The triad: quadriceps weakness (knee extension) + absent patellar reflex (L4 arc via femoral) + sensory loss over anterior thigh and medial leg (saphenous branch). Intact Achilles rules out sciatic and S1. The obturator innervates adductors, not extensors. The lateral femoral cutaneous nerve is pure sensory with no reflex arc.

Nerve Locator · Question 3 of 3

A patient cannot flex the knee OR plantar flex the foot. Knee extension is 5/5. Sensation is absent over the sole and posterior calf. What is the most likely injured nerve?

Sciatic nerve (L4-S3). The sciatic is the trunk BEFORE it splits. Lost knee flexion (hamstrings) + lost plantarflexion (tibial division) + lost dorsiflexion (peroneal division) = all of these together point proximal, to the main sciatic trunk. If this were purely the peroneal nerve, plantarflexion would be intact.

All three localized correctly. The anatomy is the answer every time.

Anatomy Reference

Clinical Anatomy Images

Tap any image to view full size. Source: Wikimedia Commons public domain anatomical plates.

Gray's Anatomy lower extremity nerves anterior view

LE nerves - anterior view

Sciatic nerve anatomy

Tibial nerve to tarsal tunnel

Clinical Pearls

How the Board Tests This

From the Attending

The board has one move with lower extremity nerve injuries: they give you a mechanism, then show you the wrong territory.

Fibular neck fracture? They draw you toward L4-L5 disc herniation because dorsiflexion fails in both. The separator: disc herniation gives you back pain, a positive straight leg raise, and medial foot numbness. Peroneal nerve injury gives you dorsal foot numbness, no back pain, no reflex change at the knee.

Hip arthroplasty with Trendelenburg? They draw you toward femoral nerve (they both involve the hip). The separator: femoral nerve injury drops the patellar reflex and weakens the quad. Superior gluteal injury leaves the quad strong - the pelvis just can't stay level.

Medial malleolus Tinel sign? The board wants you to say S1 radiculopathy because the sole is numb. The separator: S1 radiculopathy kills the Achilles reflex. Tarsal tunnel syndrome leaves it intact. The Tinel sign is never present in radiculopathy.

Know the separator. That is the only question they can ask.

Root	Sensory territory	Motor loss	Reflex
L2-L3	Anterior/medial thigh	Hip flexion (partial)	None
L4	Medial leg, medial malleolus	Dorsiflexion weakness	Patellar
L5	Lateral leg, dorsal foot, great toe	Dorsiflexion, hip abduction	None (or medial hamstring)
S1	Lateral malleolus, lateral foot, sole	Plantarflexion weakness	Achilles
S2-S4	Posterior thigh, perineum, perianal	Sphincter (S2-S4)	Bulbocavernosus

Board Walkthrough

Five Original Vignettes

Each vignette contains exactly one final question and a tap-to-reveal teaching chain after answering. Shuffle with the remix button.

VIGNETTE 1 OF 5

All five vignettes complete. The nerve map is yours.