Root Origin
- Spinal nerve roots: L4, L5, S1, S2 (via the sciatic nerve, posterior division)
- Plexus: Sacral plexus (through the sciatic nerve)
- Type: Mixed (motor and sensory)
Course
- Sciatic nerve division. The common peroneal nerve separates from the tibial nerve at the superior angle of the popliteal fossa, typically two-thirds of the way down the posterior thigh. In the thigh, it supplies only one muscle — the short head of the biceps femoris (the only hamstring innervated by the peroneal division).
- Lateral popliteal fossa. After the division, the nerve follows the medial border of the biceps femoris tendon, traveling laterally and superficially along the lateral border of the popliteal fossa. It gives off the lateral sural cutaneous nerve (lateral proximal leg sensation) and a communicating branch that joins the tibial contribution to form the sural nerve.
- Fibular head and neck. The nerve wraps around the neck of the fibula, passing between the fibula and the peroneus longus muscle. Here it lies directly on bone with only skin and a thin layer of fascia covering it — no protective muscle. This is the most vulnerable point of any nerve in the lower extremity and the most common site of injury.
- Division into terminal branches. As the nerve enters the peroneus longus, it divides into its two terminal branches: the deep peroneal nerve and the superficial peroneal nerve.
Motor Distribution
Thigh (Before Division from Sciatic)
| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/biceps-femoris-short-head | Knee flexion, lateral tibial rotation | The ONLY hamstring supplied by the peroneal division; isolated short head weakness localizes to the peroneal branch of the sciatic nerve |
Below the Knee — Via Deep Peroneal Nerve
See anatomy/nerves/deep-peroneal-nerve for full details.| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/tibialis-anterior | Dorsiflexion, inversion | Loss = foot drop; the critical muscle for clearing the foot during gait |
| anatomy/muscles/extensor-digitorum-longus | Extends toes 2-5, assists dorsiflexion | Anterior compartment |
| anatomy/muscles/extensor-hallucis-longus | Extends great toe, assists dorsiflexion | L5 myotome test |
| anatomy/muscles/peroneus-tertius | Dorsiflexion, eversion | Not always present |
| anatomy/muscles/extensor-digitorum-brevis | Extends toes 2-4 | Only intrinsic muscle on the dorsum of the foot |
Below the Knee — Via Superficial Peroneal Nerve
See anatomy/nerves/superficial-peroneal-nerve for full details.| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/peroneus-longus | Eversion, plantar flexion of first ray | Stabilizes the medial longitudinal arch from below |
| anatomy/muscles/peroneus-brevis | Eversion, plantar flexion | Attaches to the base of the 5th metatarsal |
Sensory Distribution
- Lateral sural cutaneous nerve. Branches in the popliteal fossa. Supplies the lateral proximal leg below the knee.
- Via deep peroneal nerve. First web space only — the small patch of skin between the great toe and the second toe.
- Via superficial peroneal nerve. Anterolateral leg and dorsum of the foot (except the first web space).
- Clinical significance: The combination of foot drop (motor) plus numbness over the anterolateral leg and dorsal foot (sensory) is the hallmark presentation of common peroneal nerve palsy at the fibular head. Foot drop alone without sensory loss suggests a more central lesion (L4-L5 radiculopathy) or anterior compartment syndrome.
Entrapment Sites
1. Fibular Head and Neck
- Location: Where the nerve wraps around the neck of the fibula, between the fibula and the peroneus longus
- Structure: The nerve lies directly on bone with minimal soft tissue protection. External compression, traction, or direct trauma easily damages it at this point.
- Causes: Habitual leg crossing (compression of the fibular head against the opposite knee), tight casts or braces, tight high boots, prolonged lateral knee compression during sleep or anesthesia, knee dislocation or proximal fibular fracture, rapid weight loss (loss of protective fat pad).
- Presentation: Foot drop (inability to dorsiflex the foot) — the foot slaps during gait because the ankle dorsiflexors cannot lift the foot during swing phase. Steppage gait — the patient lifts the knee higher than normal to clear the dropped foot. Numbness over the anterolateral leg and dorsal foot. Eversion weakness (superficial peroneal branch).
- MT relevance: Foot drop is a dramatic presentation that demands clear clinical reasoning. If foot drop develops gradually in a patient who habitually crosses their legs, the cause is external compression at the fibular head — counsel the patient to stop crossing their legs and the nerve usually recovers in weeks. If foot drop develops acutely with back pain, think L4-L5 disc herniation and refer urgently. If accompanied by bowel/bladder changes, suspect cauda equina — refer immediately.
2. Fibular Tunnel (Peroneus Longus Entrapment)
- Location: Where the nerve passes through the fibrous arch of the peroneus longus origin as it enters the lateral compartment
- Structure: A fibrous band at the peroneus longus origin can compress the nerve, particularly in runners and athletes with lateral compartment hypertrophy
- Presentation: Similar to fibular head compression but may be more insidious in onset and related to activity. Pain over the lateral knee that radiates down the anterolateral leg.
- MT relevance: If lateral knee pain in a runner is accompanied by dorsal foot numbness, consider peroneal nerve compression at the fibular tunnel rather than IT band or lateral meniscus pathology.
Clinical Tests
| Test | Procedure | Positive Finding | What It Tells You |
|---|---|---|---|
| Resisted ankle dorsiflexion (L4) | Patient dorsiflexes the ankle against resistance. | Weakness or inability to dorsiflex. | Tibialis anterior function — common peroneal nerve (deep branch) or L4-L5 myotome. Inability to dorsiflex = foot drop. |
| Resisted great toe extension (L5) | Patient extends the great toe against resistance. | Weakness. | Extensor hallucis longus — the most sensitive myotome test for L5. Weakness here with dorsiflexion weakness confirms either common peroneal nerve or L5 root involvement. |
| Resisted eversion | Patient everts the foot against resistance. | Weakness of eversion. | Peroneus longus and brevis — superficial peroneal branch. Eversion weakness with dorsiflexion weakness localizes to the common peroneal nerve (both branches affected). Eversion weakness alone suggests isolated superficial peroneal involvement. |
| Tinel's at fibular head | Tap over the common peroneal nerve at the fibular head/neck. | Tingling radiating into the anterolateral leg or dorsal foot. | Nerve irritability at the fibular head — the most common compression site. |
| Dorsal foot sensation | Light touch over the dorsum of the foot and first web space. | Decreased sensation. | Superficial peroneal (dorsal foot) and deep peroneal (first web space) sensory territories. Loss confirms common peroneal distribution. |
Clinical Notes
- Foot drop is the defining clinical sign. Inability to dorsiflex the foot creates a characteristic "steppage gait" — the patient lifts the knee excessively high to clear the dropped foot during swing phase, then the foot slaps down on initial contact. This is visible from across a room. Any patient who reports "tripping over my foot" or whose shoe catches on the ground should be assessed for dorsiflexion strength.
- The leg-crossing question. When a patient presents with foot drop, ask about leg-crossing habits. Habitual crossing of the affected leg over the opposite knee compresses the peroneal nerve directly against the fibular head. This is the most common cause of peroneal nerve palsy in outpatient practice and is completely reversible by eliminating the habit. Recovery time: 2-12 weeks for neurapraxia.
- Weight loss and peroneal palsy. Rapid weight loss — from illness, surgery, or intentional dieting — removes the small fat pad that protects the nerve at the fibular head. Patients who have lost significant weight quickly and develop foot drop likely have compression neuropathy from loss of this protective tissue. Hospital patients are particularly at risk: immobility + weight loss + lateral positioning = peroneal palsy.
- Differentiating peroneal palsy from L5 radiculopathy. Both produce foot drop and dorsal foot numbness. The key differentiators: (1) Eversion — peroneal palsy weakens eversion (peroneus longus/brevis are peroneal nerve); L5 radiculopathy spares eversion because the peroneals are primarily S1. (2) Inversion — L5 radiculopathy may weaken inversion (tibialis posterior is tibial nerve, L5); peroneal palsy spares inversion. (3) Back pain and SLR — present in radiculopathy, absent in peroneal palsy. (4) Ankle reflex — normal in both (S1-S2 through the tibial nerve).
- Protect the fibular head during treatment. When positioning a patient side-lying, ensure the fibular head of the lower leg is not compressed against the table. Use a pillow between the knees. During prone positioning for hamstring or calf work, avoid bolsters or table edges pressing against the lateral knee. These are preventable iatrogenic compression injuries.
Related Nerves
- anatomy/nerves/tibial-nerve — The other terminal branch of the sciatic nerve. Together, the tibial and common peroneal nerves supply everything below the knee. Tibial takes posterior/plantar; peroneal takes anterior/lateral.
- anatomy/nerves/deep-peroneal-nerve — Terminal branch supplying the anterior compartment (dorsiflexors) and first web space sensation. Isolated deep peroneal entrapment is less common than common peroneal palsy at the fibular head.
- anatomy/nerves/superficial-peroneal-nerve — Terminal branch supplying the lateral compartment (evertors) and dorsal foot sensation.
- anatomy/nerves/sciatic-nerve — Parent trunk. The sciatic nerve's peroneal division is more vulnerable than its tibial division — in many sciatic nerve injuries (piriformis syndrome, posterior hip dislocation), the peroneal division is preferentially affected, producing foot drop as the primary finding.
Key Takeaways
- The most commonly injured nerve in the lower extremity — wraps around the fibular neck with minimal soft tissue protection, making it vulnerable to external compression from leg crossing, casts, and surgical positioning.
- Foot drop (inability to dorsiflex) with steppage gait is the hallmark — ask about leg-crossing habits as the most common reversible cause.
- Differentiating from L5 radiculopathy: peroneal palsy weakens eversion (peroneal nerve muscles) but spares inversion (tibial nerve); L5 radiculopathy may affect both.
- Protect the fibular head during patient positioning — side-lying and prone positions can compress the nerve against the table.