(See also Overview of Hand Disorders.)
The most common cause of swan-neck deformity is rheumatoid arthritis. Other causes include untreated mallet finger, looseness (laxity) of the fibrous plate inside the hand at the base of the fingers or of the finger ligaments, chronic muscle spasm affecting the hands caused by nerve damage (called muscle spasticity), other types of arthritis, a ruptured finger tendon, and misalignment in the healing of a fracture of the middle bone of the finger. Normal bending of the finger may become impossible. The deformity can therefore result in considerable disability.
When the Fingers Are Abnormally Bent
True swan-neck deformity does not affect the thumb, which has one less joint than the other fingers. However, in a variant of swan-neck deformity, called duck-bill, Z (zigzag) type, or 90°-angle deformity, the top joint of the thumb is severely overstraightened with a bending in of the joint at the base of the thumb to form a 90° angle. If duck-bill deformity and swan-neck deformity of one or more fingers occur together, the ability to pinch can be seriously reduced.
A doctor makes the diagnosis of swan-neck deformity by examining the hand and finger.
Treatment of swan-neck deformity is aimed at correcting the underlying disorder when possible.
Mild deformities that have not yet developed scarring may be treated with finger splints (ring splints), which correct the deformity while still allowing a person to use the hand.
Problems with the ability to pinch can be greatly improved by surgically realigning the joints or by fusing the thumb or finger joints together (called interphalangeal arthrodesis) into positions that allow for optimal function.