A patient diagnosed with rheumatoid arthritis presents with bilateral thumb deformities described as metacarpophalangeal flexion with interphalangeal hyperextension. Which deformity is MOST consistent with this presentation?

• A. Swan neck
• B. Boutonniere
• C. Dupuytren
• D. Mallet finger

Answer: (B)

Boutonniere deformity is caused by disruption of the extensor mechanism, specifically affecting how the finger or thumb extends across its joints. When the central slip (or the analogous extensor structures) is weakened or torn, the proximal joint sits in flexion while the distal joint tends to hyperextend. In the thumb this pattern shows up as metacarpophalangeal flexion with interphalangeal hyperextension, which matches the description given.

This contrasts with other common deformities: swan neck features hyperextension at the proximal interphalangeal joint with flexion at the distal joint; Dupuytren’s contracture involves progressive palmar fascia tightening leading to flexion of the MCP and PIP joints; mallet finger is an isolated flexion at the distal interphalangeal joint due to an extensor tendon rupture. The pattern described is most consistent with a boutonniere deformity.