Achilles Tendon Rupture: Thompson Test, Anatomy, and the Missed Diagnosis
Special Tests · 7 min read · 2026-05-02
The Most Missed Tendon Rupture
Up to one in four acute Achilles tendon ruptures are missed at the first medical encounter. The reason is almost always the same: the patient can still plantarflex the ankle, the examiner concludes "the tendon is intact," and a calf strain is diagnosed. The Thompson test exists precisely to prevent this error.
Anatomy You Need
The Achilles tendon is the conjoined tendon of gastrocnemius and soleus, inserting on the posterior calcaneus. It is the strongest and thickest tendon in the human body, but has a notorious avascular zone 2 to 6 cm proximal to its insertion — the watershed area where most ruptures occur.
The plantarflexors of the ankle are not limited to the Achilles. The tibialis posterior, peroneals, flexor digitorum longus, and flexor hallucis longus can all weakly plantarflex the ankle. This is precisely why a patient with a complete Achilles rupture can often still move their foot downwards — the secondary plantarflexors compensate.
The Mechanism
A sudden eccentric load on a dorsiflexed ankle, classically in a "weekend warrior" male aged 30 to 50 starting a sprint or jump. Patients describe being "kicked" or "shot" in the back of the calf, often turning around expecting to find someone behind them.
The Thompson Test
With the patient prone and feet hanging off the end of the bed, squeeze the calf muscle.
- Normal: passive plantarflexion of the foot
- Achilles rupture: no plantarflexion — the foot remains motionless
The test works because squeezing the gastrocnemius bellies should pull on an intact Achilles and tilt the foot. With a complete rupture, the connection is broken and the foot does not move regardless of how strongly the patient could otherwise plantarflex.
Sensitivity: ~96% for complete rupture when performed correctly.
Other Findings
- Palpable gap in the tendon 2 to 6 cm above the calcaneus
- Bruising along the posterior leg and into the heel
- Reduced resting tension of the foot — comparing both feet prone, the affected ankle hangs in more dorsiflexion
- Matles test (knee flexion test): with the patient prone and knees flexed to 90°, the affected foot drops into more dorsiflexion than the unaffected side
Distinguishing From a Calf Strain (Gastrocnemius Tear)
Common Pitfall
The most common error is asking the patient to plantarflex the foot and concluding the tendon is intact when they can. Active plantarflexion is preserved in most complete Achilles ruptures because of secondary plantarflexors. The Thompson test bypasses voluntary motion and tests the integrity of the gastrocnemius–Achilles–calcaneus chain directly.
Clinical Pearl
Fluoroquinolone antibiotics (ciprofloxacin, levofloxacin) are independently associated with Achilles tendon rupture, with risk persisting for months after the course. Always ask about recent antibiotic use in any patient with Achilles pain or rupture, and consider this in patients on corticosteroids.
Summary
Achilles ruptures are missed because patients can still move their foot. The Thompson test removes voluntary motion from the equation. A positive Thompson test, palpable gap, and asymmetric resting foot position together are nearly pathognomonic. Suspect, then test — never assume.