Swan-neck deformity

Swan-neck deformity is the inverse pattern of the boutonnière — hyperextension at the PIP joint with flexion at the DIP. In the mallet-related context, untreated terminal tendon disruption produces unopposed flexion at the DIP and, over time, compensatory PIP hyperextension as extensor force is redirected proximally and the volar plate at the PIP attenuates. Swan-neck deformity also arises in rheumatoid arthritis, in volar plate insufficiency from trauma, and in spasticity-related imbalance. The deformity is a late complication of untreated mallet injury and motivates timely splinting even when the acute presentation appears trivial.