Fig. 18. Boutonniere injury. The extensor mechanism is disrupted from the central slip attachment at the dorsal base of the middle phalanx. The proximal end migrates proximally, and the pull of the lateral bands of the extensor mechanism opens the hole further. This allows the force of the extensor mechanism to bypass the proximal interphalangeal joint, which remains flexed, and divert it to the distal interphalangeal joint which becomes hyperextended.