A mallet finger is a deformity of the finger caused when the tendon that straightens your finger (the 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 (see Figure 1a and 1b). The force of the blow may even pull away a piece of bone along with the tendon (see Figure 2). The tip of the finger or thumb no longer straightens. This condition is sometimes referred to as baseball finger.