A 52-year-old woman with a 10-year history of rheumatoid arthritis presents with progressive deformity of her right index finger. On examination, she demonstrates flexion at the proximal interphalangeal joint with compensatory hyperextension at the distal interphalangeal joint, creating a characteristic "buttonhole" appearance. The structure marked **A** in the diagram shows the PIP joint flexion component of this deformity. Which of the following best explains the pathomechanics of the PIP joint flexion in boutonnière deformity?

Explanation

## Why Option 1 is correct The hallmark pathomechanics of boutonnière deformity is disruption of the central slip of the extensor tendon at its insertion on the middle phalanx. This allows the lateral bands to slip volarly (palmar to the PIP joint axis), which converts them from extensors of the PIP joint into flexors. The PIP head then buttonholes through the gap between the lateral bands, creating the characteristic flexion deformity marked as **A** in the diagram. In rheumatoid arthritis, chronic synovitis erodes the central slip insertion, initiating this cascade. This is the foundational anatomical-mechanical principle underlying boutonnière deformity (Harrison 21e Ch 363; Green's Operative Hand Surgery 8e). ## Why each distractor is wrong - **Option 2**: While chronic synovitis does erode cartilage and bone in RA, direct erosion alone does not explain the specific mechanism of PIP flexion in boutonnière. The deformity is driven by the shift in the lateral band vector from dorsal to volar, not by joint destruction per se. This confuses the underlying cause (synovitis) with the mechanical pathophysiology. - **Option 3**: Flexor digitorum superficialis contracture is not the primary mechanism of boutonnière deformity. FDS contracture is more associated with swan-neck deformity (the opposite deformity: PIP hyperextension + DIP flexion), where volar plate and FDS attenuation play a role. - **Option 4**: Volar plate attenuation and MCP volar subluxation are features of swan-neck deformity, not boutonnière. Swan-neck is the opposite deformity pattern and arises from different pathomechanics in RA. **High-Yield:** Boutonnière = PIP flexion + DIP hyperextension from central slip disruption allowing lateral band volar slip; Swan-neck = opposite (PIP hyperextension + DIP flexion) from volar plate/FDS attenuation. [cite: Harrison 21e Ch 363; Green's Operative Hand Surgery 8e]