What is the defining feature of a swan neck deformity?

The defining feature of a swan neck deformity is characterized by hyperextension at the proximal interphalangeal (PIP) joint accompanied by flexion at the distal interphalangeal (DIP) joint. This specific positioning creates a shape reminiscent of a swan's neck, hence the name.

In conditions like rheumatoid arthritis, the inherent instability in the PIP joint leads to the described deformity, where the structures of the finger are affected by imbalances in muscle tone and joint mechanics. As a result, the PIP joint becomes hyperextended while the DIP joint is flexed, creating a classic presentation of swan neck deformity.

Recognizing this pattern is crucial for diagnosis and treatment planning for patients with hand deformities, particularly in the context of inflammatory arthritis. Understanding the biomechanics and presentation of such deformities is fundamental in orthotic and prosthetic practice.