What deformity is characterized by the PIP joint being flexed and DIP joint being extended?

The deformation characterized by the proximal interphalangeal (PIP) joint being flexed while the distal interphalangeal (DIP) joint is extended is known as Boutonnière deformity. This condition often arises due to an injury, typically a rupture or laceration of the central slip of the extensor tendon, which impairs the ability of the tendon to extend the PIP joint. As a result, the PIP joint cannot straighten, leading to the characteristic flexion of that joint, while the DIP joint remains extended, creating a "buttonhole" appearance.

In contrast, other conditions involve different joint mechanics or different positions of the joints. For instance, Swan neck deformity features hyperextension of the PIP joint and flexion of the DIP joint. Jersey finger pertains to an inability to flex the DIP joint, commonly due to a rupture of the flexor tendon, which presents differently. Mallet finger is identified by the inability to extend the DIP joint while the PIP joint is typically in a neutral position, which again differs from the behavior observed in Boutonnière deformity. Therefore, understanding the mechanics of the joints involved is crucial in identifying these distinct deformities accurately.