Duplicate extensor tendons of the thumb mimicking rupture of the extensor pollicis longus tendon.

We report a patient with duplicate extensor pollicis longus tendons, who complained of inability to extend the thumb fully because of impingement of the musculotendinous junction of the duplicate tendons. Extension of the thumb recovered completely after Lister's tubercle was excised and the extensor retinaculum was released. Selective windowing with highlighting of the soft tissue in the axial T1-weighted magnetic resonance image at the level of the distal radioulnar joint showed two tendons in the third extensor compartment.