What is the cause of a boutonniere deformity?

A boutonniere deformity occurs primarily due to an interruption or rupture of the central slip of the extensor hood. This central slip, a part of the extensor mechanism, is responsible for extending the middle phalanx at the proximal interphalangeal (PIP) joint. When this structure is compromised, the lateral bands of the extensor mechanism may also migrate volarly, leading to a characteristic deformity where the PIP joint becomes flexed while the distal interphalangeal (DIP) joint is hyperextended. This results in the classic appearance of a boutonniere deformity.

Understanding this mechanism is crucial for recognizing how the anatomy of the finger is affected in such conditions. The other options do not accurately describe the underlying pathology, as stretching of the volar plate primarily relates to other types of injuries, while palmar fibromatosis and flexor tendinitis involve different anatomical structures and conditions entirely. Thus, the interruption of the central slip is specifically linked to the boutonniere deformity, making it the correct choice.