A mallet finger happens when the extensor tendon to the distal joint of the finger is stretched or torn. The extensor tendon is on the top of the finger. The distal joint is the last joint near the tip of the finger. This injury sometimes includes a small fracture of the finger.

Bones, Tendons, and Muscles of the Hand
Hand anatomy
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The most common cause of a mallet finger is when the tip of a finger is forcibly jammed into flexion or hit against a solid object. It occurs often in sports, such as baseball and basketball, when a ball hits the end of an extended finger. This injury causes the tendon on the back of the finger to be partially or completely torn. The tendon can no longer fully extend the distal joint.