Foot Drop After a Closed Fibular Neck Fracture: Localizing and Timing the Lesion
A 47-year-old cyclist is struck by a car and sustains a closed fracture of the right fibular neck. Two days later he cannot dorsiflex or evert the right foot and reports numbness over the dorsum of the foot and the first web space. Ankle plantar flexion and inversion are both 5/5. Sensation over the lateral heel and the lateral border of the foot is intact. The resident concludes that 'the tibial nerve is spared, so this is a clean common fibular lesion' and plans observation alone. Bedside anterior compartment pressure is 12 mmHg. Three weeks later there is still no active dorsiflexion and no sensory improvement.
1. Which two bedside motor findings outside the fibular nerve's territory would most reliably separate an L5 radiculopathy from this mononeuropathy, and what is the anatomical basis for each?
Read the worked answer
Two findings decide it:
- Hip abduction. Gluteus medius and minimus are supplied by the superior gluteal nerve (L4–S1, mainly L5). No branch of the fibular nerve reaches the hip, so abduction weakness localizes the lesion proximal to the fibular nerve - root, plexus, or sciatic nerve.
- Ankle inversion. Tibialis posterior (tibial nerve, L4–L5), with help from flexor digitorum and hallucis longus, is the reliable inverter here. Tibialis anterior is fibular-innervated and is weak in both lesions, so it cannot discriminate.
Sensory patterns overlap too much to be decisive: the deep fibular nerve supplies the first web space and the superficial fibular nerve the dorsum, but an L5 radicular loss can mimic both. Intact lateral heel sensation does not exclude a common fibular lesion, because the sural nerve derives most of its fibers from the tibial (medial sural cutaneous) contribution. In this patient, preserved hip abduction and inversion favor a fibular mononeuropathy and argue against a root lesion.
2. How should the 12 mmHg compartment pressure be interpreted, and what clinical findings would override it?
Read the worked answer
12 mmHg is a normal resting value in an awake, normotensive patient and is not an indication for fasciotomy - but it is one number, from one compartment, at one moment, and it should never substitute for the examination.
- Interpretation is by delta pressure (diastolic pressure minus compartment pressure); a delta under about 30 mmHg is concerning regardless of the absolute value. Absolute values above roughly 30 mmHg are traditionally treated as compartment syndrome.
- With a fibular neck fracture the anterior and lateral compartments are at risk; the deep posterior compartment can also be involved.
- Findings that override the number: pain out of proportion to the injury, pain on passive stretch of the toe extensors, a tense compartment, progressive sensory loss in the first web space, and new weakness.
- If the patient cannot be examined reliably (sedation, head injury), serial or continuous pressure monitoring matters more.
So: do not take this leg to the operating room for 12 mmHg, and do not stop examining it because of 12 mmHg.
3. No recovery at three weeks - what does electrodiagnostic testing show at this point, and how should the timing shape the decision about surgical exploration?
Read the worked answer
- Wallerian degeneration is complete by roughly 7–10 days, so at three weeks a distal study will show an absent or markedly reduced compound muscle action potential from extensor digitorum brevis, with fibrillation potentials and positive sharp waves beginning to appear in paralysed muscles at about 2–3 weeks.
- At three weeks, however, testing cannot reliably separate axonotmesis from neurotmesis, and a pure neurapraxia should already be showing early recovery by 3–6 weeks.
- Practical consequence: for a closed fracture with a presumed contusion, serial examination is appropriate, with repeat electrodiagnostic study at about three months. Exploration is reserved for an open or penetrating injury, an expanding mass or hematoma, or no recovery by 3–4 months.
- The absence of improvement at three weeks therefore changes nothing yet. It is the trajectory over the next several weeks, not a single snapshot, that drives the decision.