<unk>  has many of possible causes arising from the DIP, PIP, or even the MCP joints. In all cases, there is a stretching of the volar plate at the PIP joint to allow hyperextension, plus some damage to the attachment of the extensor tendon to the base of the distal phalanx that produces a hyperflexed mallet finger. Duck bill deformity is a similar condition affecting the thumb (which cannot have true<unk>  because it does not have enough joints).
Swan neck deformity
Habit-tic deformity
Planovalgus deformity
Gibbus deformity
Nail disease
Condylar hypoplasia
Barrel chest
Ape hand deformity