A 40-year-old woman with seropositive rheumatoid arthritis notices inability to actively straighten her left middle finger. On examination the PIP joint rests in flexion while the DIP joint sits in hyperextension, and she cannot actively extend the PIP joint. Which structure has failed?

• A Central slip of the extensor tendon at its insertion into the base of the middle phalanx ✓
• B Volar plate of the PIP joint
• C Flexor digitorum superficialis insertion
• D Terminal extensor tendon insertion into the distal phalanx

Explanation

This is boutonniere deformity. Rupture of the central slip lets the lateral bands slide volar to the axis of the PIP joint, converting them into flexors of the PIP while their pull now extends the DIP, giving flexed PIP with hyperextended DIP. The best distractor, terminal tendon rupture, produces the opposite picture, a mallet finger with flexed DIP. Swan neck deformity involves volar plate laxity with PIP hyperextension and DIP flexion, the reverse of this patient.

Reference: Campbell's Operative Orthopaedics, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.