Isolated subluxation of the extensor pollicis longus tendon. A cause of 'boutonnière' deformity of the thumb.

Isolated ulnar subluxation of the extensor pollicis longus (EPL) tendon occurs when there is damage to the radial side of the extensor hood on the dorsum of the metacarpophalangeal joint of the thumb. The thumb collapses into a pseudoboutonnière deformity because the EPL tendon subluxes and comes to lie palmar to the centre of rotation of the metacarpophalangeal joint to act as a flexor instead of an extensor. The extensor pollicis brevis tendon which lies in a deeper plane to the EPL is unaffected. We describe four cases and propose conservative treatment as being effective in recent injuries.