Drop foot — the inability to lift (dorsiflex) the front of the foot during the swing phase of gait — causes a characteristic steppage gait pattern and significantly increases fall risk. It results from weakness or paralysis of the muscles that dorsiflex the foot, most commonly from peroneal nerve dysfunction. Understanding the cause is essential because treatment ranges from observation and bracing to nerve decompression or tendon transfer surgery.

Anatomy: The Common Peroneal Nerve

The common peroneal nerve (also called the common fibular nerve) branches from the sciatic nerve at the popliteal fossa and wraps around the fibular head before dividing into the superficial and deep peroneal nerves. The deep peroneal nerve innervates the anterior compartment muscles responsible for ankle dorsiflexion (tibialis anterior, extensor hallucis longus, extensor digitorum longus) and toe extension.

The nerve’s superficial position around the fibular head makes it uniquely vulnerable to external compression and injury.

Causes of Drop Foot

Peroneal Nerve Compression at the Fibular Head

The most common peripheral cause. Compression can result from:

• Prolonged leg crossing (habitual sitting posture)
• Tight casts or knee braces compressing the fibular head
• Prolonged squatting or kneeling
• Weight loss exposing the nerve to external compression
• Baker’s cyst or ganglion at the fibular head

Traumatic Peroneal Nerve Injury

Fibular head fractures, proximal tibiofibular dislocations, and severe lateral ankle sprains can directly injure the peroneal nerve. Total knee replacement and hip replacement are iatrogenic causes.

Lumbar Radiculopathy (L4–L5)

L4 and especially L5 nerve root compression from lumbar disc herniation or spinal stenosis can produce foot drop that mimics peripheral peroneal nerve palsy. Electrodiagnostic studies (EMG/NCS) and MRI of the lumbar spine distinguish radiculopathy from peripheral nerve compression.

Systemic Neurological Conditions

Charcot-Marie-Tooth disease (hereditary motor sensory neuropathy), multifocal motor neuropathy, ALS, and stroke can all produce foot drop through central or peripheral mechanisms.

Diagnosis

Examination assesses dorsiflexor strength (tibialis anterior, extensor hallucis longus), eversion strength (peroneus longus/brevis — superficial peroneal nerve), and inversion strength (posterior tibial nerve — helps localize level). Tinel’s sign at the fibular head supports peripheral peroneal compression.

EMG/NCS localizes the site of nerve injury, quantifies severity (neuropraxia vs. axonotmesis vs. neurotmesis), and provides prognostic information. MRI of the knee and proximal leg identifies compressive lesions. Lumbar spine MRI or CT myelogram evaluates for radiculopathy when proximal compression is suspected.

Treatment

Ankle-Foot Orthosis (AFO)

An AFO maintains the foot in a neutral or slightly dorsiflexed position during ambulation, preventing foot slap, reducing fall risk, and enabling normal gait mechanics. Posterior leaf spring AFOs are lightweight and appropriate for mild-to-moderate drop foot. Carbon fiber and custom-molded AFOs provide more support for complete paralysis.

Peroneal Nerve Decompression

When compressive drop foot has a structural cause (ganglion cyst, fibular head bony prominence, post-traumatic fibrosis), surgical decompression of the common peroneal nerve at the fibular head can produce dramatic recovery — particularly when performed within weeks to months of onset before permanent axonal degeneration occurs.

Functional Electrical Stimulation (FES)

Peroneal nerve stimulators (e.g., WalkAide, Bioness L300) deliver electrical impulses to the peroneal nerve during the swing phase of gait, producing active dorsiflexion. FDA-cleared for foot drop from stroke and multiple sclerosis, and used off-label for peripheral peroneal nerve palsy.

Tendon Transfer Surgery

For permanent, irreversible drop foot, posterior tibial tendon transfer (PTTT) to the dorsum of the foot converts an invertor into a dorsiflexor, restoring functional gait without an AFO. Best results occur in patients with intact posterior tibial and preserved calf muscle function, and in motivated patients who will complete postoperative rehabilitation.

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Dr. Tom Biernacki, DPM

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Dr. Tom Biernacki, DPM

Dr. Tom Biernacki, DPM is a double board-certified podiatrist and foot & ankle surgeon at Balance Foot & Ankle Specialists in Southeast Michigan. With over a decade of clinical experience, he specializes in heel pain, bunions, diabetic foot care, sports injuries, and minimally invasive surgery. Dr. Biernacki is a member of the APMA and ACFAS, and his patient education content on MichiganFootDoctors.com and YouTube has reached over one million views.