What causes a type I swan-neck deformity in rheumatoid arthritis (RA)?

Updated: Jun 11, 2021
  • Author: Michael Neumeister, MD, FRCSC, FACS; Chief Editor: Joseph A Molnar, MD, PhD, FACS  more...
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Swan-neck deformity can arise at the PIP or DIP joint; in either case, it can lead to the classic appearance of PIP joint hyperextension with DIP joint flexion. Patients with type I deformity maintain the ability to actively flex the PIP joint. When the deformity originates at the PIP joint, it is caused by stretching of the capsule secondary to active synovitis or rupture of the flexor digitorum superficialis tendon, removing the restraint to PIP joint hyperextension.

If the synovitis involves the DIP joint, the deformity begins with stretching or rupture of the terminal tendon attachment of the extensor mechanism to the distal phalanx, resulting in a mallet deformity. (See also the article Mallet Finger.) This subsequently causes extensor mechanism imbalance, with relative overpull of the central slip; these problems, together with laxity of the PIP joint's volar plate, result in PIP joint hyperextension.

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