What is the defining feature of a swan neck deformity?

A swan neck deformity is characterized by hyperextension at the proximal interphalangeal (PIP) joint and flexion at the distal interphalangeal (DIP) joint. This specific positioning results in a curvature of the fingers that resembles the neck of a swan, hence the name of the deformity.

In this condition, the volar plate at the PIP joint becomes lax or elongated, allowing hyperextension to occur, while the flexor tendons may become tight or shortened, causing the DIP joint to flex. This combination of joint positions leads to the classic presentation of the swan neck deformity, making the understanding of joint mechanics and soft tissue involvement critical for assessing and managing this condition in clinical practice.

The features of the deformity not only illustrate the anatomical changes but also highlight the functional impairments that can arise, impacting hand function and necessitating appropriate orthotic or surgical interventions for management.