A 42-year-old woman with a 2-year history of seropositive rheumatoid arthritis presents with progressive deformity of her right index finger. On examination, the structure marked **A** (PIP joint flexion deformity) is noted, with compensatory hyperextension at the DIP joint. She reports morning stiffness lasting 90 minutes and symmetric swelling of her MCPs and PIPs bilaterally. Which of the following best explains the mechanism of the deformity marked **A**?

Why "Disruption of the central slip of the extensor tendon with palmar migration of the lateral bands" is right

The boutonnière deformity, characterized by flexion at the PIP joint (A) with compensatory hyperextension at the DIP joint, is the pathognomonic hand deformity of chronic rheumatoid arthritis. The mechanism is well-established in the ACR/EULAR guidelines and rheumatology literature: chronic synovitis and pannus formation at the PIP joint erode and disrupt the central slip of the extensor tendon. Once the central slip is compromised, the lateral bands (which normally assist in PIP extension) migrate palmarly, converting them from extensors to flexors of the PIP joint. This results in the characteristic flexion posture at A and compensatory DIP hyperextension as the intact terminal tendon attempts to extend the finger. Early recognition and PIP extension splinting can prevent fixed deformity.

Why each distractor is wrong

ACR/EULAR Rheumatoid Arthritis Management Guidelines 2021; Smolen et al. Ann Rheum Dis. 2021