This is due to rupture of the extensor tendon at its attachment to the finger close to the base of the nail. It is normally caused by stubbing the tip of the finger.
Symptoms of Mallet Finger
It is not possible to actively extend (straighten) the far joint of the finger. Occasionally an x-ray may reveal a small fragment of bone has been pulled off with the tendon.
Treatment of Mallet Finger
Immobilisation of the far joint of the finger in a splint fully extended (straight) for a continuous period of at least 6 weeks. Surgery is rarely required.