Mallet Finger

History: 45 yo with trauma to 3rd digit.

AP radiographs of suspected digit injuries are very high on the list of imaging studies that will bold face lie to you. If all you have is an AP,  politely request that the ordering provider order a diagnostic exam. 

Touch more superimposition of the DIP joint than it's fellow 2nd and 4th digits.

Focal flexion of the DIP joint in the setting of neutral PIP joint is abnormal.  For all those reading this with atraumatic digits, try doing it right now - takes an impressive and sustained effort and is near impossible to do with a single digit beside the index finger. 

MALLET FINGER is the post-traumatic involuntary flexion of a DIP joint 2/2 to avulsion of the extensor digitorum tendon insertion with subsequent unopposed tension from the flexor digitorum profundus causing the flexed posture.   

The actual tissue injury can be radiographically occult with only the subsequent abnormal flexion telling the tale. 

Same external appearance can arise from a spectrum of bony involvement too.

From a tiny bone avulsion related to the extensor tendon slip insertion (we were kind enough to straighten this digit unnecessarily)

To a dorsal base intra-articular fracture.  This person punched a wall, as young men are known to do.   

As always, appreciate the illustrations in the reference below.  Make the extensor digitorum tendon and finger normal to abnormal to normal as many times that keeps you entertained. 

And remember, AP imaging of the hand for digital injuries is essentially non-diagnostic!

https://radiopaedia.org/articles/mallet-finger?lang=us