A mallet finger (Figure 1) results from a separation of the extensor digitorum from its insertion on the distal phalanx. This separation by tearing away (technically, an "avulsion") results from hyperflexion of the distal interphalangeal (DIP) joint. A commonly seen mechanism of injury is a blow against the tip of the finger by a ball being caught. The injury can either cause the avulsion of the tendon off the bone or the avulsion of the tendon with a piece of the bone attached. When there is bony involvement of the distal phalanx, the injury is referred to as a "bony mallet." An injury to the extensor digitorum more proximally, at the level of the middle phalanx, damages the so-called “central slip” and can lead to a boutonnière (meaning "button hole" in French) deformity. In a boutonnière deformity (Figure 2), the head of the proximal phalanx pops through the central slip (like a button through a button hole). This causes the characteristic deformity of flexion at the proximal interphalangeal (PIP) joint and hyperextension at the DIP joint.
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