The diagnosis is made clinically by noting inability to fully extend the DIP joint.

     PA and lateral radiographs should be obtained to document bony injury and assess articular surface involvement. 

     Look for an avulsion fracture at the extensor tendon attachment on the base on the distal phalanx dorsally.  Negative X rays rule out bony avulsions, but a mallet injury to the tendon is diagnosed clinically and would have normal radiographs.