Which condition can cause a swan-neck deformity of the fingers?

• A. Mallet Finger
• B. Rheumatoid Arthritis
• C. Boutonnière Deformity
• D. Central Slip Injury

Answer: (B)

Swan-neck deformity happens when the finger extensor mechanism becomes unbalanced, so the PIP joint hyperextends while the DIP joint flexes. Chronic inflammation from rheumatoid arthritis weakens the volar plate and the supporting ligaments around the PIP, allowing the PIP to drift into hyperextension and altering tendon tension to pull the DIP into flexion. This pattern is characteristic of RA, making it the best answer.

Mallet finger, by contrast, injures the terminal extensor tendon at the DIP and causes DIP flexion, not a swan neck. Boutonnière deformity arises from injury to the central slip, producing PIP flexion with DIP hyperextension. Central slip injury underlies Boutonnière deformity, not swan-neck.