Which muscle tear causes foot drop?

Foot Drop: Causative Muscle Tear

Foot drop is caused by rupture of the tibialis anterior tendon, which is the primary dorsiflexor of the foot. 1

Anatomical Basis

The tibialis anterior muscle is the key muscle responsible for ankle dorsiflexion (lifting the foot upward against gravity). When this tendon ruptures completely, patients lose the ability to dorsiflex the foot, resulting in the characteristic foot drop presentation.

Clinical Presentation

A patient with tibialis anterior tendon rupture typically presents with:

• Painful swelling on the dorsum (top) of the foot 1
• Complete inability to lift the foot (0/5 power in dorsiflexion) 2
• Abnormal gait pattern with exaggerated knee and hip flexion to compensate for the dropped foot 2
• Muscle wasting in the anterior compartment of the leg if chronic 2

Important Clinical Caveat

While tibialis anterior tendon rupture is the direct muscular cause of foot drop, you must rule out neurological causes, particularly common peroneal nerve (CPN) injury or lumbar nerve root compression, as these are actually more common causes of foot drop than isolated tendon rupture 2, 3, 4. The common peroneal nerve innervates the tibialis anterior and other dorsiflexor muscles.

Key Distinguishing Features:

• Tendon rupture: Palpable gap or swelling at the tendon, localized to anterior ankle/foot
• Nerve injury: May have sensory changes along CPN distribution, tenderness at fibular neck, or radicular symptoms if spinal origin 2, 4

Diagnostic Approach

1. Palpate the anterior ankle and dorsum of foot for tendon discontinuity or focal swelling
2. Test motor strength specifically for ankle dorsiflexion (0/5 suggests complete rupture)
3. Assess sensation along the dorsum of the foot to rule out nerve involvement
4. MRI is definitive for confirming tendon rupture and excluding other pathology 2

The provided evidence about ankle sprains 5, 6, 7, 8 and ligament injuries 9 is not relevant to foot drop, as lateral ankle ligament injuries do not cause foot drop—they cause ankle instability and inversion weakness, not dorsiflexion weakness.