Foot drop is caused by an injury to?

Appeared in: AIIMS BBSR SNO - 2019

Explanation

Foot drop is the inability to perform dorsiflexion, which is lifting the front part of the foot towards the shin.
This movement is controlled by the muscles in the anterior compartment of the lower leg.
The common peroneal nerve (and its deep branch) innervates these muscles.
Therefore, injury or compression of the common peroneal nerve is the most frequent cause of foot drop.
As shown in the image, this nerve is superficial as it wraps around the fibular head, making it susceptible to injury.

Option A: The tibial nerve controls plantar flexion (pointing the foot down) and toe flexion. Injury to this nerve would result in an inability to stand on tiptoes, not foot drop.
Option C: The obturator nerve innervates the adductor muscles of the thigh. Its injury affects hip adduction (moving the leg inward), not movements of the foot or ankle.
Option D: The sciatic nerve is the parent nerve of both the common peroneal and tibial nerves. While a severe, high sciatic nerve injury can cause foot drop, it would also cause other deficits like weakness in knee flexion and loss of plantar flexion. The common peroneal nerve is the most specific and direct cause of isolated foot drop.

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 at risk, such as those with leg casts, splints, prolonged immobility, or after knee/hip surgery.
Early detection involves checking for the ability to dorsiflex the foot and assessing for sensation on the top of the foot. A simple test is to ask the patient to 'walk on their heels'.
Preventive measures include proper positioning to avoid pressure on the lateral knee, using footboards or high-top sneakers for immobile patients, and performing regular range-of-motion exercises.

How to Approach the Question

First, identify the key term in the question: 'Foot drop'.
Recall the definition of foot drop: the inability to lift the front of the foot (dorsiflexion).
Access your knowledge of lower limb neuroanatomy. Which nerve controls the muscles responsible for dorsiflexion?
Systematically evaluate the options: The tibial nerve controls plantar flexion. The obturator nerve controls thigh adduction. The sciatic nerve is a larger nerve that branches; while its injury can cause foot drop, it's a less specific answer. The common peroneal nerve specifically controls dorsiflexion.
Conclude that the common peroneal nerve is the most direct and common cause of foot drop.

Concept Tested & Keywords

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

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

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