Mallet finger

[Tracie Lynn]

In Mallet finger, the fingertip curls down and cannot be straightened. It is usually caused by a tendon injury.
(See also Overview of Sprains and Other Soft-Tissue Injuries and Finger Fractures.)
Mallet finger usually results when the tendon that attaches bone to muscle in the fingertip is torn. This tendon (called the extensor tendon) is used to straighten the fingertip. Usually, the tendon is torn when a force causes the fingertip to bend more than it normally does. A common cause is a baseball that hits the fingertip and jams it. Thus, mallet finger is sometimes called baseball finger.
One or more fingers may be affected. Sometimes the joint is also dislocated.
When the tendon tears, it may pull a piece of bone from the finger (called an avulsion fracture). When an avulsion fracture occurs, the cartilage at the end of the affected bone (the joint surface) is also fractured.

[Lanell]

Mallet finger is a flexion deformity of the fingertip caused by avulsion of the extensor tendon, with or without fracture, from the proximal end of the distal phalanx.
(See also Overview of Sprains and Other Soft-Tissue Injuries.)
The usual mechanism is forced flexion of the distal phalanx, typically when hit with a ball. The extensor tendon may avulse part of the proximal aspect of the distal phalangeal bone (see figure Mallet finger). The avulsed part involves the articular surface.