Mallet Finger By Danielle Campagne, MD, University of California, San Francisco Reviewed / Revised Mar 2021 | Modified Sep 2022 View Patient Education Symptoms and Signs Diagnosis Treatment Topic Resources Mallet finger 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.