Foot drop: inability to dorsiflex the foot at the ankle, due to paralysis of the dorsiflexors of the anterior compartment of the leg, most commonly from common peroneal nerve injury.
Nerve Supply of the Dorsiflexors
The deep peroneal nerve, one of the two terminal branches of the common peroneal nerve, supplies all muscles of the anterior compartment of the leg, the dorsiflexors of the foot.
Injury proximal to this division therefore paralyses the dorsiflexors and produces foot drop.
Clinical relevance
Site vulnerable to injury: common peroneal nerve is extremely vulnerable to injury as it winds around the posterolateral aspect of the neck of the fibula.
Causes at this site: direct trauma, fracture of the neck of the fibula, or a tightly applied plaster cast.
High-stepping (steppage) gait: dorsiflexor paralysis means the patient cannot clear the toes in a normal stride, so the foot is raised higher than normal to prevent the toes hitting the ground.
The patient also cannot stand on the heel, since the dorsiflexors and evertors are both paralysed.
Foot slap: due to paralysis of the dorsiflexors and evertors, the foot cannot be lowered with control; putting it down suddenly on the ground produces an audible slapping sound.
Foot drop occurs alongside loss of toe extension (extensor digitorum longus, extensor hallucis longus) and loss of eversion (peroneus longus, peroneus brevis) when the common peroneal nerve injury is complete.
Cutaneous sensory loss over the anterolateral leg and dorsum of the foot may accompany these motor deficits in a complete common peroneal nerve injury.