(See also Overview and Evaluation of Hand Disorders.)
A swan-neck deformity consists of hyperextension of the proximal interphalangeal (PIP) joint, flexion of the distal interphalangeal (DIP) joint, and sometimes flexion of the metacarpophalangeal (MCP) joint (see Figure: Boutonnière and swan-neck deformities.).
Although characteristic in rheumatoid arthritis, swan-neck deformity has several causes, including untreated mallet finger, laxity of the ligaments of the volar aspect of the PIP joint (eg, as can occur after rheumatic fever or in SLE as Jaccoud arthropathy), spasticity of intrinsic hand muscles, rupture of the flexor tendon of the PIP joint, and malunion of a fracture of the middle or proximal phalanx. The inability to correct or compensate for hyperextension of the PIP joint makes finger closure impossible and can cause severe disability.
Treatment of swan-neck deformity is aimed at correcting the underlying disorder when possible (eg, correcting the mallet finger or any bony malalignment, releasing spastic intrinsic muscles). Mild deformities in patients with RA may be treated with a functional ring splint.
True swan-neck deformity does not affect the thumb, which has only one interphalangeal joint. However, severe hyperextension of the interphalangeal joint of the thumb with flexion of the MCP joint can occur; this is called a duck bill, Z (zigzag) type, or 90°-angle deformity. With simultaneous thumb instability, pinch is greatly impaired. This deformity can usually be corrected by interphalangeal arthrodesis along with tendon reconstruction at the MCP joint.