Which finding indicates a rupture of the flexor digitorum profundus tendon in a jersey finger injury?

• A. Inability to flex the distal phalanx
• B. Inability to flex the proximal interphalangeal joint
• C. Inability to extend the distal phalanx
• D. Pain with passive extension of the finger

Answer: (A)

A rupture of the flexor digitorum profundus tendon shows up as an inability to actively flex the distal phalanx. The FDP tendon inserts on the distal phalanx and is the primary muscle that flexes the DIP joint, so when it is torn or avulsed, the fingertip cannot bend at the DIP. The middle and proximal joints can still flex because the flexor digitorum superficialis handles PIP flexion, which is usually preserved in this injury. This is classically seen in jersey finger when the finger is forcefully extended while the DIP is actively flexed. In contrast, inability to flex the proximal interphalangeal joint would suggest FDS involvement, and inability to extend the distal phalanx points to an extensor tendon injury (mallet finger); pain with passive extension is less specific.