This condition is a result of tendon imbalance in the finger or thumb. In the finger, it causes a characteristic deformity in which the middle finger joint (called the PIP joint) hyperextends, and the fingertip joint (called the DIP joint) bends downward. When viewed from the side, the finger looks like the outstretched neck of a swan.

swan-neck-deformity-1In the Thumb
In the thumb, the condition causes the thumb tip joint (called the IP joint) to bend and the middle thumb joint (called the MCP joint) to hyperextend, resulting in the characteristic swan-neck appearance. If the IP joint bends but the MCP joint does not hyperextend, this is sometimes called a duck-bill thumb deformity.

This deformity is most commonly caused by rheumatoid arthritis, which can weaken or rupture the stabilizing tissues around the joints and can alter the joints’ mechanics. It may also be caused by a hyperextension injury to the PIP joint, a mallet finger injury in a patient with loose ligaments, muscle imbalance, an improperly-healed finger fracture, or cerebral palsy.