A mallet finger is a deformity of the finger caused when the tendon that straightens your finger (extensor tendon) is damaged. When a ball or other object strikes the tip of the finger or thumb and forcibly bends it, the force tears the tendon that straightens the finger. The force of the blow may even pull away a piece of bone along with the tendon. The finger or thumb is not able to straighten.
This condition is also sometimes referred to as baseball finger.
The key finding with a mallet finger is that the fingertip droops; that is, it cannot straighten on its own power. The finger may be painful, swollen and bruised, especially if there is an associated fracture, but often the only finding is the inability to straighten the tip. Occasionally, blood collects beneath the nail. The nail can even become detached from beneath the skin fold at the base of the nail.
The diagnosis is evident by the appearance of the finger. X-rays are often ordered to see if you have pulled off a piece of bone and to make sure the joint is aligned.
*Source: (c) 2011 American Society for Surgery of the Hand - http://www.assh.org/Public/HandConditions/Pages/Mallet-Finger-Baseball-F...