Swan neck deformity

(A) Posterior view of normal dorsal extensor expansion. The extensor expansion near the proximal interphalangeal joint splits into three parts: a central part, which is inserted into the base of the middle phalanx, and two lateral parts, which converge to be inserted into the base of the distal phalanx. (B) Mallet or baseball finger. The insertion of the extensor expansion into the base of the distal phalanx ruptured; sometimes a flake of bone on the base of the phalanx is pulled off. (C) Boutonniere deformity. The insertion of the extensor expansion into the base of the middle phalanx is ruptured. The arrows indicate the direction of the pull of the muscles and the deformity.