What is a common cause of swan neck deformity?

• A. Carpal tunnel syndrome
• B. MP subluxation or hyperextension of PIP joint (volar plate weakness or injury)
• C. Humerus fracture
• D. Distal phalanx avulsion

Answer: (B)

Swan neck deformity arises when the extensor mechanism around the finger loses its normal restraint at the PIP joint, allowing the PIP to hyperextend. The volar plate at the PIP normally checks hyperextension; if it’s weak or torn, the PIP drifts into hyperextension and the extensor mechanism shifts in a way that the DIP is driven into flexion. This pattern is most commonly due to volar plate weakness or injury at the PIP, sometimes with metacarpophalangeal joint subluxation contributing to the imbalance.

Other options don’t fit the pattern: carpal tunnel is a nerve compression issue with no direct impact on finger joint alignment; a humerus fracture is in the upper arm and doesn’t create this finger deformity; a distal phalanx avulsion causes a mallet finger, which presents as DIP flexion with an unaffected or differently aligned PIP.