Foot drop is caused by an injury to?

Appeared in: AIIMS BBSR SNO - 2019

Explanation

Foot drop is the inability to lift the front part of the foot, a movement known as dorsiflexion.
This condition is most commonly caused by injury or compression of the common peroneal nerve.
The common peroneal nerve innervates the muscles in the anterior compartment of the leg, which are responsible for dorsiflexing the ankle and extending the toes.
The nerve is particularly vulnerable to injury as it wraps superficially around the head of the fibula on the outside of the knee, as shown in the provided image.

Option A: Injury to the tibial nerve causes weakness in plantar flexion (pointing the foot down) and inversion of the foot.
Option C: The obturator nerve supplies the adductor muscles of the thigh. An injury here affects the ability to move the leg inward toward the midline.
Option D: The sciatic nerve is a large nerve that branches into the common peroneal and tibial nerves. While a severe, high sciatic nerve injury can cause foot drop, it would also typically cause weakness in plantar flexion (tibial nerve function) and hamstring weakness (knee flexion).

Clinical Relevance

Nursing practice connection: Knowing Etiology of Foot Drop helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Nurses must frequently assess for foot drop in patients with leg casts, splints, prolonged bed rest, or those who have undergone knee or hip surgery, as pressure on the fibular head is a common cause.
Patient education on avoiding habitual leg crossing and ensuring proper positioning are key preventive nursing interventions.
What if? If a patient with foot drop also has weakness in plantar flexion (pointing toes down), the injury is likely more proximal, involving the sciatic nerve or even the lumbosacral plexus, and requires a broader neurological assessment and urgent reporting.

How to Approach the Question

First, identify the key clinical sign in the question: 'foot drop'.
Recall the definition of foot drop, which is the inability to perform dorsiflexion (lifting the foot upwards).
Next, access your knowledge of lower limb neuroanatomy. Correlate the major nerves with their primary motor functions.
Systematically evaluate the options: The tibial nerve controls plantar flexion, the obturator nerve controls thigh adduction, and the sciatic nerve is a parent nerve to both tibial and peroneal. The common peroneal nerve specifically controls dorsiflexion.
Conclude that an injury to the common peroneal nerve is the most direct and common cause of isolated foot drop.

Concept Tested & Keywords

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E6 Nursing Fundamentals Potter Perry 12e Part 4 p. 132-134

E6 Medicine Harrison 22e Part 2 p. 1518-1520

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