[2] This flexion deformity of the proximal interphalangeal joint is due to interruption of the central slip of the extensor tendon such that the lateral slips separate and the head of the proximal phalanx pops through the gap like a finger through a button hole (thus the name, from French boutonnière "button hole").

The distal joint is subsequently drawn into hyperextension because the two peripheral slips of the extensor tendon are stretched by the head of the proximal phalanx (note that the two peripheral slips are inserted into the distal phalanx, while the proximal slip is inserted into the middle phalanx).

This deformity makes it difficult or impossible to extend the proximal interphalangeal joint.

Higher numbers indicate a more severe problem and greater likelihood of a poor outcome.

Usually treated with a splint placing the proximal interphalangeal joint in extension for 4–6 weeks.