In Mallet finger, the fingertip curls down and cannot be straightened. It is usually caused by an injury.
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. A common cause is a baseball that hits the fingertip. 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.
The finger is usually painful, swollen, and bruised immediately after the injury. The joint remains bent. The person cannot straighten it. Occasionally, blood collects under the nail (called a subungual hematoma—see Symptoms).