Common Peroneal Nerve Entrapment
Common peroneal nerve entrapment is a focal neuropathy (mononeuropathy) that injures the common trunk and, by extension, affects the innervation of both the deep and the superficial peroneal nerves. The most frequent site of injury or compression is the fibular head, where the nerve courses almost subcutaneously and lies in direct contact with the bony prominence of the fibular head, which—together with fascial bands—may exert significant pressure.
Clinically, the condition often presents with foot drop (difficulty with ankle dorsiflexion) and numbness over the dorsum of the foot. Timely diagnosis is essential, as other conditions—such as L5 radiculopathy or a more proximal lesion of the sciatic nerve—can mimic this presentation. In most cases, we begin with conservative management and targeted offloading of the region; however, when there is persistent or severe neurological deficit or clear evidence of entrapment/mass, surgical decompression may be required.
What is common peroneal nerve entrapment (peroneal neuropathy)?
A focal dysfunction of a nerve that controls critical movements of the lower limb and sensation over the dorsum of the foot.
Common peroneal neuropathy involves the common peroneal nerve, which divides into:
- Deep peroneal nerve (primarily ankle dorsiflexion & toe extension; sensation in the first web space).
- Superficial peroneal nerve (primarily eversion of the foot; sensation over the dorsum of the foot).
When the common peroneal nerve is entrapped or injured, weakness may develop in one or more of these functions. In more severe cases, the characteristic clinical picture of foot drop emerges.
How common is it & who is at higher risk?
It is among the most common focal neuropathies of the lower limb and occurs particularly after trauma or prolonged pressure on the lateral aspect of the knee.
Risk increases in the presence of:
- Sports injuries (direct trauma or traction).
- Knee trauma, dislocation, or severe sprain.
- Fractures involving the fibular region.
- Postoperative states (e.g., ligament reconstruction or knee arthroplasty, especially when correcting valgus alignment).
- Prolonged compression (e.g., habitual leg crossing, prolonged bed rest, tight splints/casts).
- Systemic neuropathies (e.g., diabetes mellitus) that predispose to “secondary” entrapment.
Why is the fibular head a “vulnerable site”?
At this level, the nerve runs very superficially and passes adjacent to bony prominences and tight fascial structures.
At the fibular head, the common peroneal nerve:
- lies almost subcutaneously, with little muscular protection,
- is in contact with a rigid bony prominence,
- passes beneath/through tight fascial bands of the lateral compartment,
- may become functionally “strangled” during movement, leading to edema or local inflammation.
These anatomic features explain why even apparently minor pressure or injury can provoke symptoms.
What are the most common causes (trauma, entrapment, mass effect)?
Injury may result from trauma, true entrapment by fascia, or compression by a local lesion/mass.
- Trauma: contusion, traction, rupture (with/without knee dislocation), fibular head fracture, penetrating injury.
- Focal entrapment: compression as the nerve passes beneath a fascial band in the lateral compartment (often beneath the peroneus longus origin).
- Mass effect: lipomas, nerve sheath tumors, or a ganglion cyst.
- Iatrogenic injury: following knee surgery (especially deformity correction).
What symptoms occur & how does foot drop present?
Symptoms involve both motor weakness and sensory disturbance in a characteristic distribution.
Common symptoms:
- Weakness of ankle dorsiflexion (difficulty lifting the foot upward).
- Difficulty extending the toes.
- Numbness/paresthesia over the dorsum of the foot and/or the first web space.
- Gait instability and toe-walking; patients often adopt a compensatory gait to avoid “dragging” the foot.
- Pain is not the dominant feature but may be present in a minority of cases.
“Foot drop” is not a diagnosis in itself but a clinical sign that mandates identification of the underlying cause (nerve, root, or central nervous system).
How is it distinguished from L5 radiculopathy or sciatic nerve injury?
Accurate differential diagnosis relies on neurological examination, the pattern of weakness, and electrophysiologic testing.
A practical clinical clue is the tibialis posterior muscle, innervated by the tibial nerve and responsible for inversion of the foot:
- In common peroneal neuropathy, inversion is typically preserved.
- In severe L5 radiculopathy, both eversion and inversion may be affected, as muscles not innervated by the peroneal nerve are involved.
Moreover, a more proximal lesion of the sciatic nerve (prior to bifurcation) can mimic fibular head pathology; therefore, evaluation must be systematic.
How is the diagnosis established (clinical exam, EMG/NCV, ultrasound, MRI)?
Clinical examination raises suspicion, while electrophysiologic and imaging studies localize the lesion and define the etiology.
Clinical examination
- Assessment of dorsiflexion and toe extension strength.
- Sensory testing over the dorsum of the foot and first web space.
- Tinel’s sign at the fibular head (may be positive in entrapment, though not fully sensitive/specific).
Electrophysiology (EMG/NCV)
- Nerve conduction studies may show focal slowing at the fibular head (a highly specific finding).
- EMG can demonstrate axonal injury or denervation in muscles innervated by the deep/superficial peroneal nerves.
- Important: inclusion of the short head of the biceps femoris, the only muscle above the knee innervated by the peroneal nerve—if involved, the lesion is likely more proximal.
Imaging
- Ultrasound: useful for evaluating nerve enlargement/compression and detecting cysts.
- MRI: particularly valuable when a mass, ganglion cyst, or iatrogenic injury is suspected.
When is conservative management sufficient and what does it include?
In many cases—especially after traction or contusion—gradual recovery is possible with careful monitoring.
Conservative management may include:
- Avoidance of pressure over the region (e.g., avoiding leg crossing; attention to splints/casts).
- Physical therapy to prevent stiffness and promote functional retraining.
- Ankle–foot orthosis (AFO) in cases of foot drop, to reduce falls and improve gait.
- Follow-up with neurological examination and repeat EMG when indicated.
In cases of true focal entrapment, conservative measures have limited effectiveness. Offloading and time may help, but persistent or worsening weakness warrants reassessment.
When is surgical decompression indicated and what are the goals?
Surgical treatment aims to relieve mechanical compression and maximize the likelihood of neural recovery.
Indications for surgical exploration/decompression:
- Severe entrapment with significant functional impairment.
- Failure of conservative therapy with persistent weakness/sensory deficit.
- Suspected or confirmed mass compressing the nerve (cyst, tumor).
- Selected cases in which recovery from a more proximal injury is functionally “blocked” at the fibular head.
Goals of surgery:
- Decompression of the nerve from fascial bands/structures.
- Neurolysis when indicated, with meticulous handling of nerve fibers.
- If a specific cause is present (e.g., cyst), addressing the underlying pathology to reduce recurrence risk.
What are ganglion cysts and why are they clinically important?
Ganglion cysts represent a distinct cause of compression that requires targeted treatment, not merely nerve decompression.
Ganglion cysts may be associated with degenerative changes of the proximal tibiofibular joint and compress the common peroneal nerve:
- either as an intraneural cyst (via an articular branch),
- or as an extraneural cyst tracking along the nerve.
Management is not simple “drainage”: the objective is to interrupt the communication between the cyst and the joint/articular branch, thereby achieving decompression and reducing the risk of recurrence.
What should I expect—recovery timeline & prognosis?
Prognosis depends on the mechanism of injury and the neurological status at the time treatment is initiated.
In general:
- With traction/contusion, improvement is often gradual and may require weeks to months.
- In focal entrapment, outcomes after decompression are frequently excellent, particularly when residual neurological function is present.
- With traumatic lacerations or severe injuries, recovery may be slower and less predictable.
- In systemic neuropathies or proximal lesions (sciatic nerve), outcomes are more variable and require individualized planning.
Follow-up is based primarily on clinical examination and, when necessary, EMG. Restoration of muscle strength reflects reinnervation and requires time.
Frequently asked questions & when to seek specialized care
Does foot drop always indicate peroneal nerve injury?
No. It may result from peroneal neuropathy, L5 radiculopathy, sciatic nerve injury, or less commonly from central causes. Neurological assessment is therefore essential.
When is electromyography (EMG) necessary?
When precise localization of the lesion is required, to assess severity, and to document potential entrapment at the fibular head.
Can this be due simply to postural compression?
Yes. Prolonged pressure in the region (e.g., habitual leg crossing) can cause symptoms, especially in predisposed individuals. However, acute or progressive weakness warrants immediate evaluation.
When should I seek urgent evaluation?
- Acute foot drop or rapid deterioration in strength.
- Neurological symptoms following trauma or knee surgery.
- Suspicion of a mass or persistent symptoms despite offloading/therapy.
At Neuroknife, assessment includes a structured neurological examination, targeted EMG/NCV, and imaging (ultrasound/MRI) when entrapment or a mass is suspected, enabling us to recommend the safest and most realistic therapeutic strategy.
When should you seek specialized neurosurgical evaluation for a peripheral nerve injury?
If you develop foot drop, persistent weakness, or numbness over the dorsum of the foot—particularly after trauma or knee surgery—prompt evaluation is important to exclude entrapment or another cause requiring targeted treatment.
Early diagnosis can improve the likelihood of functional recovery and reduce the risk of permanent neurological deficit.
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